Tenth planet? Artists concept of the view from Eris with Dysnomia in the background, looking back towards the distant sun. Credit: Robert Hurt (IPAC)
Ask a person what Dysnomia refers to, and they might venture that it’s a medical condition. In truth, they would be correct. But in addition to being a condition that affects the memory (where people have a hard time remembering words and names), it is also the only known moon of the distant dwarf planet Eris.
In fact, the same team that discovered Eris a decade ago – a discovery that threw our entire notion of what constitutes a planet into question – also discovered a moon circling it shortly thereafter. As the only satellite that circles one of the most distant objects in our Solar System, much of what we know about this ball of ice is still subject to debate.
Discovery and Naming:
In January of 2005, astronomer Mike Brown and his team discovered Eris using the new laser guide star adaptive optics system at the W. M. Keck Observatory in Hawaii. By September, Brown and his team were conducting observations of the four brightest Kuiper Belt Objects – which at that point included Pluto, Makemake, Haumea, and Eris – and found indications of an object orbiting Eris.
Provisionally, this body was designated S/2005 1 (2003 UB³¹³). However, in keeping with the Xena nickname that his team was already using for Eris, Brown and his colleagues nicknamed the moon “Gabrielle” after Xena’s sidekick. Later, Brown selected the official name of Dysnomia for the moon, which seemed appropriate for a number of reasons.
For one, this name is derived from the daughter of the Greek god Eris – a daemon who represented the spirit of lawlessness – which was in keeping with the tradition of naming moons after lesser gods associated with the primary god. It also seemed appropriate since the “lawless” aspect called to mind actress Lucy Lawless, who portrayed Xena on television. However, it was not until the IAU’s resolution on what defined a planet – passed in August of 2006 – that the planet was officially designated as Dysnomia.
Size, Mass and Orbit:
The actual size of Dysnomia is subject to dispute, and estimates are based largely on the planet’s albedo relative to Eris. For example, the IAU and Johnston’s Asteroids with Satellites Database estimate that it is 4.43 magnitudes fainter than Eris and has an approximate diameter of between 350 and 490 km (217 – 304 miles)
However, Brown and his colleagues have stated that their observations indicate it to be 500 times fainter and between 100 and 250 km (62 – 155 miles) in diameter. Using the Herschel Space Observatory in 2012, Spanish astronomer Pablo Santo Sanz and his team determined that, provided Dysnomia has an albedo five times that of Eris, it is likely to be 685±50 km in diameter.
Eris and its moon, Dysnomia, as imaged by the W.M. Keck Observatory in Hawaii. Credits:NASA/ESA and M. Brown/Caltech
In 2007, Brown and his team also combined Keck and Hubble observations to determine the mass of Eris, and estimate the orbital parameters of the system. From their calculations, they determined that Dysnomia’s orbital period is approximately 15.77 days. These observations also indicated that Dysnomia has a circular orbit around Eris, with a radius of 37350±140 km. In addition to being a satellite of a dwarf planet, Dysnomia is also a Kuiper Belt Object (KBO) like Eris.
Composition and Origin:
Currently, there is no direct evidence to indicate what Dysnomia is made of. However, based on observations made of other Kuiper Belt Objects, it is widely believed that Dysnomia is composed primarily of ice. This is based largely on infrared observations made of Haumea (2003 EL61), the fourth largest object in the Kuiper Belt (after Eris, Pluto and Makemake) which appears to be made entirely of frozen water.
Astronomers now know that three of the four brightest KBOs – Pluto, Eris and Haumea – have one or more satellites. Meanwhile, of the fainter members, only about 10% are known to have satellites. This is believed to imply that collisions between large KBOs have been frequent in the past. Impacts between bodies of the order of 1000 km across would throw off large amounts of material that would coalesce into a moon.
Artist’s concept of Kuiper Belt Object Eris and its tiny satellite Dysnomia. The Hubble Space Telescope and Keck Observatory took images of Dysnomia’s movement from which astronomer Mike Brown (Caltech) precisely calculated Eris to be 27 percent more massive than Pluto. Credit: NASA/ESA/Adolph Schaller (for STScI)
This could mean that Dysnomia was the result of a collision between Eris and a large KBO. After the impact, the icy material and other trace elements that made up the object would have evaporated and been ejected into orbit around Eris, where it then re-accumulated to form Dysnomia. A similar mechanism is believed to have led to the formation of the Moon when Earth was struck by a giant impactor early in the history of the Solar System.
Since its discovery, Eris has lived up to its namesake by stirring things up. However, it has also helped astronomers to learn many things about this distant region of the Solar System. As already mentioned, astronomers have used Dysnomia to estimate the mass of Eris, which in turn helped them to compare it to Pluto.
While astronomers already knew that Eris was bigger than Pluto, but they did not know whether it was more massive. This they did by measuring the distance between Dysnomia and how long it takes to orbit Eris. Using this method, astronomers were able to discover that Eris is 27% more massive than Pluto is.
With this knowledge in hand, the IAU then realized that either Eris needed to be classified as a planet, or that the term “planet” itself needed to be refined. Ergo, one could make that case that it was the discovery of Dysnomia more than Eris that led to Pluto no longer being designated a planet.
Global Color Mosaic of Triton, taken by Voyager 2 in 1989. Credit: NASA/JPL/USGS
The planets of the outer Solar System are known for being strange, as are their many moons. This is especially true of Triton, Neptune’s largest moon. In addition to being the seventh-largest moon in the Solar System, it is also the only major moon that has a retrograde orbit – i.e. it revolves in the direction opposite to the planet’s rotation. This suggests that Triton did not form in orbit around Neptune, but is a cosmic visitor that passed by one day and decided to stay.
And like most moons in the outer Solar System, Triton is believed to be composed of an icy surface and a rocky core. But unlike most Solar moons, Triton is one of the few that is known to be geologically active. This results in cryovolcanism, where geysers periodically break through the crust and turn the surface Triton into what is sure to be a psychedelic experience!
Discovery and Naming:
Triton was discovered by British astronomer William Lassell on October 10th, 1846, just 17 days after the discovery of Neptune by German astronomer Johann Gottfried Galle. After learning about the discovery, John Herschel – the son of famed English astronomer William Herschel, who discovered many of Saturn’s and Uranus’ moons – wrote to Lassell and recommended he observe Neptune to see if it had any moons as well.
New Horizons image of Neptune and its largest moon, Triton, taken by the LORRI instrument on June 23, 2010. Credit: NASA
Lassell did so and discovered Neptune’s largest moon eight days later. Thirty-four years later, French astronomer Camille Flammarion named the moon Triton – after the Greek sea god and son of Poseidon (the equivalent of the Roman god Neptune) – in his 1880 bookAstronomie Populaire. It would be several decades before the name caught on however. Until the discovery of the second moon Nereid in 1949, Triton was commonly known simply as “the satellite of Neptune”.
Size, Mass and Orbit:
At 2.14 × 1022 kg, and with a diameter of approx. 2,700 kilometers (1,680 miles) km, Triton is the largest moon in the Neptunian system – comprising more than 99.5% of all the mass known to orbit the planet. In addition to being the seventh-largest moon in the Solar System, it is also more massive than all known moons in the Solar System smaller than itself combined.
With no axial tilt and an eccentricity of virtually zero, the moon orbits Neptune at a distance of 354,760 km (220,438 miles). At this distance, Triton is the farthest satellite of Neptune, and orbits the planet every 5.87685 Earth days. Unlike other moons of its size, Triton has a retrograde orbit around its host planet.
Most of the outer irregular moons of Jupiter and Saturn have retrograde orbits, as do some of Uranus’s outer moons. However, these moons are all much more distant from their primaries, and are rather small in comparison. Triton also has a synchronous orbit with Neptune, which means it keeps one face aimed towards the planet at all times.
As Neptune orbits the Sun, Triton’s polar regions take turns facing the Sun, resulting in seasonal changes as one pole, then the other, moves into the sunlight. Such changes were observed in April of 2010 by astronomers using the European Southern Observatory’s Very Large Telescope.
Another all-important aspect of Triton’s orbit is that it is decaying. Scientists estimate that in approximately 3.6 billion years, it will pass below Neptune’s Roche limit and will be torn apart.
Composition:
Triton has a radius, density (2.061 g/cm3), temperature and chemical composition similar to thatof Pluto. Because of this, and the fact that it circles Neptune in a retrograde orbit, astronomers believe that the moon originated in the Kuiper Belt and later became trapped by Neptune’s gravity.
Another theory has it that Triton was once a dwarf planet with a companion. In this scenario, Neptune captured Triton and flung its companion away when the giant gas moved further out into the solar system, billions of years ago.
Also like Pluto, 55% of Triton’s surface is covered with frozen nitrogen, with water ice comprising 15–35% and dry ice (aka. frozen carbon dioxide) forming the remaining 10–20%. Trace amounts of methane and carbon monoxide ice are believed to exist there as well, as are small amounts of ammonia (in the form of ammonia dihydrate in the lithosphere).
Triton’s density suggests that its interior is differentiated between a solid core made of rocky material and metals, a mantle composed of ice, and a crust. There is enough rock in Triton’s interior for radioactive decay to power convection in the mantle, which may even be sufficient to maintain a subterranean ocean. As with Jupiter’s moon of Europa, the proposed existence of this warm-water ocean could mean the presence of life beneath the icy crusts.
Atmosphere and Surface Features:
Triton has a considerably high albedo, reflecting 60–95% of the sunlight that reaches it. The surface is also quite young, which is an indication of the possible existence of an interior ocean and geological activity. The moon has a reddish tint, which is probably the result of the methane ice turning to carbon due to exposure to ultraviolet radiation.
Triton is considered to be one of the coldest places in the Solar System. The moon’s surface temperature is approx. -235°C while Pluto averages about -229°C. Scientists say that Pluto may drop as low as -240°C at the furthest point from the Sun in its orbit, but it also gets much warmer closer to the Sun, giving it a higher overall temperature average.
It is also one of the few moons in the Solar System that is geologically active, which means that its surface is relatively young due to resurfacing. This activity also results in cryovolcanism, where water ammonia and nitrogen gas burst forth from the surface instead of liquid rock. These nitrogen geysers can send plumes of liquid nitrogen 8 km above the surface of the moon.
Triton (lower left) compared to the Moon (upper left) and Earth (right), to scale. Credit: NASA/JPL/USGS
Because of the geological activity constantly renewing the moon’s surface, there are very few impact craters on Triton. Like Pluto, Triton has an atmosphere that is thought to have resulted from the evaporation of ices from its surface. Like its surface ices, Triton’s tenuous atmosphere is made up of nitrogen with trace amounts of carbon monoxide and small amounts of methane near the surface.
This atmosphere consists of a troposphere rising to an altitude of 8km, where it then gives way to a thermosphere that reaches out to 950 km from the surface. The temperature of Triton’s upper atmosphere, at 95-100 K (ca.-175 °C/-283 °F) is higher than that at the surface, due to the influence of solar radiation and Neptune’s magnetosphere.
A haze permeates most of Triton’s troposphere, thought to be composed largely of hydrocarbons and nitriles created by the action of sunlight on methane. Triton’s atmosphere also has clouds of condensed nitrogen that lie between 1 and 3 km from the surface.
Observations taken from Earth and by the Voyager 2 spacecraft have shown that Triton experiences a warm summer season every few hundred years. This could be the result of a periodic change in the planet’s albedo (i.e. its gets darker and redder) which could be caused by either frost patterns or geological activity.
Using the CRIRES instrument on ESO’s Very Large Telescope, a team of astronomers has been able to see that the summer is in full swing in Triton’s southern hemisphere. Credit: ESO
This change would allow more heat to be absorbed, followed by an increase in sublimation and atmospheric pressure. Data collected between 1987 and 1999 indicated that Triton was approaching one of these warm summers.
Exploration:
When NASA’s Voyager 2 made a flyby of Neptune in August of 1989, the mission controllers also decided to conduct a flyby of Triton – similar to Voyager 1‘s encounter with Saturn and Titan. When it made its flyby, most of the northern hemisphere was in darkness and unseen by Voyager.
Because of the speed of Voyager’s visit and the slow rotation of Triton, only one hemisphere was seen clearly at close distance. The rest of the surface was either in darkness or seen as blurry markings. Nevertheless, the Voyager 2 spacecraft managed to capture several images of the moon and spotted geysers of liquid nitrogen blasting out of two distinct features on the surface.
In August of 2014, in anticipation of New Horizons impending encounter with Pluto, NASA restored these photos and used them to create the first global color map of Triton. Produced by Paul Schenk, a scientist at the Lunar and Planetary Institute in Houston, the map was also used to make a movie (shown below) that recreated the historic Voyager 2 encounter in time for the 25th anniversary of the event.
Yes, Triton is indeed an unusual moon. Aside from its rather unique characteristics (retrograde motion, geological activity) the moon’s landscape is likely to be an amazing sight. For anyone standing on the surface, surrounded by colorful ices, plumes of nitrogen and ammonia, a nitrogen haze and Neptune’s big blue disc hanging on the sky, the experience would seem like something akin to a hallucination.
In the end, it is too bad that the Solar System will one day be saying good-bye to this moon. Because of the nature of its orbit, the moon will eventually fall into Neptune’s gravity well and break up. At which point, Neptune will have a huge ring like Saturn, until those particles crash into the planet as well.
That too would be something to behold. One can only hope that humanity will still be around in 3.6 billion years to witness it!
Europa’s water exists in a layer around the planet, encased in a layer of ice. Could there be life down there?
Hooray! Welcome to the 200th official episode of the Guide To Space!
First off, thank you. Thank you for watching, liking, sharing, subscribing and being a patron of our show. Yes, you. Thank you.
So to celebrate, a few weeks ago we invited the members of the Weekly Space Hangout Crew Google+ Community to suggest topics for episodes, and the winner would receive a precious iron-nickel meteorite. Congratulations Andres Munoz, this meteorite is for you.
This episode, chosen by Andres, is for everyone.
The search for life in the Solar System is about the hunt for water. Wherever we find liquid water on Earth, we find life. I’m talking everywhere. In the most briny, salty pools in Antarctica, in the hottest hot springs in Yellowstone, under glaciers, and kilometers deep underground.
So we go searching for liquid water in the Solar System.
You might be surprised to learn that Jupiter’s moon Europa has the most water in the entire Solar System. If you took all the water on Earth, collected it into a big sphere, it would measure almost 1,400 kilometers across.
Europa’s water would measure nearly 1,800 kilometers.All that water exists in a layer around Europa, encased in a layer of ice. How thick? We don’t know.
Is there life down there? We don’t know.You can say there might be, and it wouldn’t be untrue. However, if you say there isn’t, that’s way less interesting for clickbait purposes. Whenever we don’t know the answers to fundamental and intriguing questions like that, it’s time to send a mission.
Good news! An actual mission to Europa is in the works right now. In 2015, NASA approved the development of an orbiter mission to Europa. If all goes well, and nothing gets cancelled…
And nothing will get cancelled, right? Right? I heard Firefly. Which one of you said Firefly?!?
According to the plan, a spacecraft will launch in the 2020s, carrying 9 instruments to Europa. Most will be familiar cameras, mass spectrometers, and the like, to study the surface of Europa to a high level of resolution. Over the course of 45 flybys, the spacecraft will get down as close as 25 kilometers and capture it with incredible resolution.
A “colorized” image of Europa from NASA’s Galileo spacecraft, whose mission ended in 2003. The whiteish areas are believed to be pure water ice. Credit: NASA/JPL-Caltech/SETI Institute
Perhaps the most exciting, and controversial instrument on board the new Europa Orbiter mission will be its ice-penetrating radar. Mission planners battled over installing a radar this sophisticated, as it will be an enormous drain on the orbiter’s power.
This for us is incredibly exciting. It will allow the spacecraft to map out the depth and thickness of Europa’s icy exterior. Is it thick or thin? Are there pockets of water trapped just below the surface, or is it tough shell that goes on for dozens of kilometers?
The worst case scenario is that the shell goes thicker than the radar can reach, and we won’t even know how far it goes.
Whatever happens, the Europa orbiter will be a boon to science, answer outstanding questions about the moon and the chances of finding life there.
We’re just getting started. What we really want to send is a lander. Because of the intense radiation from Jupiter, the Sun, and space itself, the surface of the ice on Europa would be sterilized. But dig down a few centimeters and you might find life that’s protected from the radiation.
A future Europa lander might be equipped with a heated drill attached to a tether. The lander would be have with a heat-generated radioisotope thermoelectric generator, like most of NASA’s big, outer Solar System spacecraft.
Artist illustration of a Europa probe. Image credit: NASA/JPL
But in addition to using it for electricity, it’ll use the raw heat to help a tethered drill to grind through the ice a few meters and sample what’s down there.
Drilling more than a few meters is probably the stuff of science fiction. Russian scientists in Antarctica drilled for almost two decades to get through 4,000 meters of ice above Lake Vostok. Imagine trying to get through 100 kilometers of the stuff, on a distant world, with a robot.
But, since I’ve talked about moving the Sun, and terraforming the Moon, maybe I shouldn’t put any bounds on my imagination. Nuclear-powered Europa submarines will get us swimming with the singing Europan space whales in no time.
Europa is the best place to search the Solar System for life, and I’m excited to see what the upcoming Europa Orbiter mission turns up. And I’m even more excited about the possibility of any future lander missions.
It was a lot of fun wrapping my brain around a topic chosen by the fans. What topic would you like us to cover next? I’ve got a whole pocket of meteorites here. Put it in the comments below.
First, I want to thank everyone. It’s been a crazy race getting up to 200 episodes, but it’s been a blast all the way through. Thanks again for all your support and here’s to 200 more!
A radar view of Venus taken by the Magellan spacecraft, with some gaps filled in by the Pioneer Venus orbiter. Credit: NASA/JPL
As the morning star, the evening star, and the brightest natural object in the sky (after the Moon), human beings have been aware of Venus since time immemorial. Even though it would be many thousands of years before it was recognized as being a planet, its has been a part of human culture since the beginning of recorded history.
Because of this, the planet has played a vital role in the mythology and astrological systems of countless peoples. With the dawn of the modern age, interest in Venus has grown, and observations made about its position in the sky, changes in appearance, and similar characteristics to Earth have taught us much about our Solar System.
Size, Mass, and Orbit:
Because of its similar size, mass, proximity to the Sun, and composition, Venus is often referred to as Earth’s “sister planet”. With a mass of 4.8676×1024 kg, a surface area of 4.60 x 108 km², and a volume of 9.28×1011 km3, Venus is 81.5% as massive as Earth, and has 90% of its surface area and 86.6% of its volume.
Venus orbits the Sun at an average distance of about 0.72 AU (108,000,000 km/67,000,000 mi) with almost no eccentricity. In fact, with its farthest orbit (aphelion) of 0.728 AU (108,939,000 km) and closest orbit (perihelion) of 0.718 AU (107,477,000 km), it has the most circular orbit of any planet in the Solar System.
Size comparison of Venus and Earth. Credit: NASA/JPL/Magellan
When Venus lies between Earth and the Sun, a position known as inferior conjunction, it makes the closest approach to Earth of any planet, at an average distance of 41 million km (making it the closest planet to Earth). This takes place, on average, once every 584 days. The planet completes an orbit around the Sun every 224.65 days, meaning that a year on Venus is 61.5% as long as a year on Earth.
Unlike most other planets in the Solar System, which rotate on their axes in an counter-clockwise direction, Venus rotates clockwise (called “retrograde” rotation). It also rotates very slowly, taking 243 Earth days to complete a single rotation. This is not only the slowest rotation period of any planet, it also means that a sidereal day on Venus lasts longer than a Venusian year.
Composition and Surface Features:
Little direct information is available on the internal structure of Venus. However, based on its similarities in mass and density to Earth, scientists believe that they share a similar internal structure – a core, mantle, and crust. Like that of Earth, the Venusian core is believed to be at least be partially liquid because the two planets have been cooling at about the same rate.
One difference between the two planets is the lack of evidence for plate tectonics, which could be due to its crust being too strong to subduct without water to make it less viscous. This results in reduced heat loss from the planet, preventing it from cooling and the possibility that internal heat is lost in periodic major resurfacing events. This is also suggested as a possible reason for why Venus has no internally generated magnetic field.
The internal structure of Venus – the crust (outer layer), the mantle (middle layer) and the core (yellow inner layer). Credit: Wikipedia Commons
Venus’ surface appears to have been shaped by extensive volcanic activity. Venus also has several times as many volcanoes as Earth, and has 167 large volcanoes that are over 100 km across. The presence of these volcanoes is due to the lack of plate tectonics, which results in an older, more preserved crust. Whereas Earth’s oceanic crust is subject to subduction at its plate boundaries, and is on average ~100 million years old, the Venusian surface is estimated to be 300-600 million years of age.
There are indications that volcanic activity may be ongoing on Venus. Missions performed by the Soviet space program in 1970s and more recently by the European Space Agency have detected lightning storms in Venus’ atmosphere. Since Venus does not experience rainfall (except in the form of sulfuric acid), it has been theorized that the lightning is being caused by a volcanic eruption.
Other evidence is the periodic rise and fall of sulfur dioxide concentrations in the atmosphere, which could be the result of periodic, large volcanic eruptions. And finally, localized infrared hot spots (likely to be in the range of 800 – 1100 K) have appeared on the surface, which could represent lava freshly released by volcanic eruptions.
The preservation of Venus’ surface is also responsible for its impact craters, which are impeccably preserved. Almost a thousand craters exist, which are evenly distributed across the surface and range from 3 km to 280 km in diameter. No craters smaller than 3 km exist because of the effect the dense atmosphere has on incoming objects.
3-D perspective of the Venusian volcano, Maat Mons generated from radar data from NASA’s Magellan mission.
Essentially, objects with less than a certain amount of kinetic energy are slowed down so much by the atmosphere that they do not create an impact crater. And incoming projectiles less than 50 meters in diameter will fragment and burn up in the atmosphere before reaching the ground.
Atmosphere and Climate:
Surface observations of Venus have been difficult in the past, due to its extremely dense atmosphere, which is composed primarily of carbon dioxide with a small amount of nitrogen. At 92 bar (9.2 MPa), the atmospheric mass is 93 times that of Earth’s atmosphere and the pressure at the planet’s surface is about 92 times that at Earth’s surface.
Venus is also the hottest planet in our Solar System, with a mean surface temperature of 735 K (462 °C/863.6 °F). This is due to the CO²-rich atmosphere which, along with thick clouds of sulfur dioxide, generates the strongest greenhouse effect in the Solar System. Above the dense CO² layer, thick clouds consisting mainly of sulfur dioxide and sulfuric acid droplets scatter about 90% of the sunlight back into space.
The surface of Venus is effectively isothermal, which means that their is virtually no variation in Venus’ surface temperature between day and night, or the equator and the poles. The planet’s minute axial tilt – less than 3° compared to Earth’s 23° – also minimizes seasonal temperature variation. The only appreciable variation in temperature occurs with altitude.
The highest point on Venus, Maxwell Montes, is therefore the coolest point on the planet, with a temperature of about 655 K (380 °C) and an atmospheric pressure of about 4.5 MPa (45 bar).
Another common phenomena is Venus’ strong winds, which reach speeds of up to 85 m/s (300 km/h; 186.4 mph) at the cloud tops and circle the planet every four to five Earth days. At this speed, these winds move up to 60 times the speed of the planet’s rotation, whereas Earth’s fastest winds are only 10-20% of the planet’s rotational speed.
Venus flybys have also indicated that its dense clouds are capable of producing lightning, much like the clouds on Earth. Their intermittent appearance indicates a pattern associated with weather activity, and the lightning rate is at least half of that on Earth.
Historical Observations:
Although ancients peoples knew about Venus, some of the cultures thought it was two separate celestial objects – the evening star and the morning star. Although the Babylonians realized that these two “stars” were in fact the same object – as indicated in the Venus tablet of Ammisaduqa, dated 1581 BCE – it was not until the 6th century BCE that this became a common scientific understanding.
Many cultures have identified the planet with their respective goddess of love and beauty. Venus is the Roman name for the goddess of love, while the Babylonians named it Ishtar and the Greeks called it Aphrodite. The Romans also designated the morning aspect of Venus Lucifer (literally “Light-Bringer”) and the evening aspect as Vesper (“evening”, “supper”, “west”), both of which were literal translations of the respective Greek names (Phosphorus and Hesperus).
Venus approaches the Sun in a 2012 transit visible from Earth. Credit: NASA
The transit of Venus in front of the Sun was first observed in 1032 by the Persian astronomer Avicenna, who concluded that Venus is closer to Earth than the Sun. In the 12th century, the Andalusian astronomer Ibn Bajjah observed two black spots in front of the sun, which were later identified as the transits of Venus and Mercury by Iranian astronomer Qotb al-Din Shirazi in the 13th century.
Modern Observations:
By the early 17th century, the transit of Venus was observed by English astronomer Jeremiah Horrocks on December 4th, 1639, from his home. William Crabtree, a fellow English astronomer and friend of Horrocks’, observed the transit at the same time, also from his home.
When the Galileo Galilei first observed the planet in the early 17th century, he found it showed phases like the Moon, varying from crescent to gibbous to full, and vice versa. This behavior, which could only be possible if Venus’ orbited the Sun, became part of Galileo’s challenge to the Ptolemaic geocentric model and his advocacy of the Copernican heliocentric model.
The atmosphere of Venus was discovered in 1761 by Russian polymath Mikhail Lomonosov, and then observed in 1790 by German astronomer Johann Schröter. Schröter found when the planet was a thin crescent, the cusps extended through more than 180°. He correctly surmised this was due to the scattering of sunlight in a dense atmosphere.
Artist’s impression of the surface of Venus Credit: ESA/AOES
In December 1866, American astronomer Chester Smith Lyman made observations of Venus from the Yale Observatory, where he was on the board of managers. While observing the planet, he spotted a complete ring of light around the dark side of the planet when it was at inferior conjunction, providing further evidence for an atmosphere.
Little else was discovered about Venus until the 20th century, when the development of spectroscopic, radar, and ultraviolet observations made it possible to scan the surface. The first UV observations were carried out in the 1920s, when Frank E. Ross found that UV photographs revealed considerable detail, which appeared to be the result of a dense, yellow lower atmosphere with high cirrus clouds above it.
Spectroscopic observations in the early 20th century also gave the first clues about the Venusian rotation. Vesto Slipher tried to measure the Doppler shift of light from Venus. After finding that he could not detect any rotation, he surmised the planet must have a very long rotation period. Later work in the 1950s showed the rotation was retrograde.
Radar observations of Venus were first carried out in the 1960s, and provided the first measurements of the rotation period, which were close to the modern value. Radar observations in the 1970s, using the radio telescope at the Arecibo Observatory in Puerto Rico revealed details of the Venusian surface for the first time – such as the presence of the Maxwell Montes mountains.
Exploration of Venus:
The first attempts to explore Venus were mounted by the Soviets in the 1960s through the Venera Program. The first spacecraft, Venera-1 (also known in the west as Sputnik-8) was launched on February 12th, 1961. However, contact was lost seven days into the mission when the probe was about 2 million km from Earth. By mid-may, it was estimated that the probe had passed within 100,000 km (62,000 miles) of Venus.
The Mariner 1 and 2 spacecrafts made their way to Venus. Mariner 2 was the first successful Venus Flyby on . Credit: NASA/JPL
The United States launched the Mariner 1 probe on July 22nd, 1962, with the intent of conducting a Venus flyby; but here too, contact was lost during launch. The Mariner 2 mission, which launched on December 14th, 1962, became the first successful interplanetary mission and passed within 34,833 km (21,644 mi) of Venus’ surface.
Its observations confirmed earlier ground-based observations which indicated that though the cloud tops were cool, the surface was extremely hot – at least 425 °C (797 °F). This put an end all speculation that the planet might harbor life. Mariner 2 also obtained improved estimates of Venus’s mass, but was unable to detect either a magnetic field or radiation belts.
The Venera-3 spacecraft was the Soviets second attempt to reach Venus, and their first attempted to place a lander on the planet’s surface. The spacecraft cash-landed on Venus on March 1st, 1966, and was the first man-made object to enter the atmosphere and strike the surface of another planet. Unfortunately, its communication system failed before it was able to return any planetary data.
On October 18th, 1967, the Soviets tried again with the Venera-4 spacecraft. After reaching the planet, the probe successfully entered the atmosphere and began studying the atmosphere. In addition to noting the prevalence of carbon dioxide (90-95%), it measured temperatures in excess of what Mariner 2 observed, reaching almost 500 °C. Due to the thickness of Venus’ atmosphere, the probe descended slower than anticipated, and its batteries ran out after 93 minutes when the probe was still 24.96 km from the surface.
The Mariner 10 spacecraft. Credit: NASA/JPL
One day later, on October 19th, 1967, Mariner 5 conducted a fly-by at a distance of less than 4000 km above the cloud tops. Originally built as a backup for the Mars-bound Mariner 4, the probe was refitted for a Venus mission after Venera-4‘s success. The probe managed to collect information on the composition, pressure and density of the Venusian atmosphere, which was then analyzed alongside the Venera-4 data by a Soviet-American science team during a series of symposiums.
Venera-5 and Venera-6 were launched in January of 1969, and reached Venus on 16th and 17th of May. Taking into account the extreme density and pressure of Venus’ atmosphere, these probes were able to achieve a faster descent and reached an altitude of 20 km before being crushed – but not before returning over 50 minutes of atmospheric data.
The Venera-7 was built with the intent of returning data from the planet’s surface, and was construed with a reinforced descent module capable of withstanding intense pressure. While entering the atmosphere on December 15th, 1970, the probe crashed on the surface, apparently due to a ripped parachute. Luckily, it managed to return 23 minutes of temperature data and the first telemetry from the another planet’s surface before going offline.
The Soviets launched three more Venera probes between 1972 and 1975. The first landed on Venus on July 22nd, 1972, and managed to transmit data for 50 minutes. Venera-9 and 10 – which entered Venus’ atmosphere on October 22nd and October 25th, 1975, respectively – both managed to send back images of Venus’ surface, the first images ever taken of another planet’s landscape.
Images of Venusian surface taken by the Venera 10 lander on October 25th, 1977. Credit: Russian Space Web/Donald Mitchell
On November 3rd, 1973, the United States had sent the Mariner 10probe on a gravitational slingshot trajectory past Venus on its way to Mercury. By February 5th, 1974, the probe passed within 5790 km of Venus, returning over 4000 photographs. The images, which were the best to date, showed the planet to be almost featureless in visible light; but revealed never-before-seen details about the clouds in ultraviolet light.
By the late seventies, NASA commenced the Pioneer Venus Project, which consisted of two separate missions. The first was the Pioneer Venus Orbiter, which inserted into an elliptical orbit around Venus on December 4th, 1978, where it studied its atmosphere and mapped the surface for a period of 13 days. The second, the Pioneer Venus Multiprobe, released a total of four probes which entered the atmosphere on December 9th, 1978, returning data on its composition, winds and heat fluxes.
Four more Venera lander missions took place between the late 70s and early 80s. Venera 11 and Venera 12 detected Venusian electrical storms; and Venera 13 andVenera 14 landed on the planet on March 1st and 5th, 1982, returning the first color photographs of the surface. The Venera program came to a close in October 1983, when Venera 15 and Venera 16 were placed in orbit to conduct mapping of the Venusian terrain with synthetic aperture radar.
In 1985, the Soviets participated in a collaborative venture with several European states to launch the Vega Program. This two-spacecraft initiative was intended to take advantage of the appearance of Halley’s Comet in the inner Solar System, and combine a mission to it with a flyby of Venus. While en route to Halley on June 11th and 15th, the two Vega spacecraft dropped Venera-style probes supported by balloons into the upper atmosphere – which discovered that it was more turbulent than previously estimated, and subject to high winds and powerful convection cells.
The first color pictures taken of the surface of Venus by the Venera-13 space probe. Credit: NASA
NASA’s Magellan spacecraft was launched on May 4th, 1989, with a mission to map the surface of Venus with radar. In the course of its four and a half year mission, Magellan provided the most high-resolution images to date of the planet and was able to map 98% of the surface and 95% of its gravity field. In 1994, at the end of its mission, Magellan was sent to its destruction into the atmosphere of Venus to quantify its density.
Venus was observed by the Galileo and Cassini spacecraft during flybys on their respective missions to the outer planets, but Magellan was the last dedicated mission to Venus for over a decade. It was not until October of 2006 and June of 2007 that the MESSENGER probe would conduct a flyby of Venus (and collect data) in order to slow its trajectory for an eventual orbital insertion of Mercury.
The Venus Express, a probe designed and built by the European Space Agency, successfully assumed polar orbit around Venus on April 11th, 2006. This probe conducted a detailed study of the Venusian atmosphere and clouds, and discovered an ozone layer and a swirling double-vortex at the south pole before concluding its mission in December of 2014.
Future Missions:
The Japan Aerospace Exploration Agency (JAXA) devised a Venus orbiter – Akatsuki (formerly “Planet-C”) – to conduct surface imaging with an infrared camera, studies on Venus’ lightning, and to determine the existence of current volcanism. The craft was launched on May 20th, 2010, but the craft failed to enter orbit in December 2010. Its main engine is still offline, but its controllers will attempt to use its small attitude control thrusters to make another orbital insertion attempt on December 7th, 2015.
Artist’s concept of the Venus in Situ explorer mission, which could be deployed to Venus by 2022. Credit: NASA
In late 2013, NASA launched the Venus Spectral Rocket Experiment, a sub-orbital space telescope. This experimented is intended to conduct ultraviolet light studies of Venus’s atmosphere, for the purpose of learning more about the history of water on Venus.
The European Space Agency’s (ESA) BepiColombo mission, which will launch in January 2017, will perform two flybys of Venus before it reaches Mercury orbit in 2020. NASA will launch the Solar Probe Plusin 2018, which will perform seven Venus flybys during its six-year mission to study the Sun.
Under its New Frontiers Program, NASA has proposed mounting a lander mission to Venus called the Venus In-Situ Explorer by 2022. The purpose will be to study Venus’ surface conditions and investigate the elemental and mineralogical features of the regolith. The probe would be equipped with a core sampler to drill into the surface and study pristine rock samples not weathered by the harsh surface conditions.
The Venera-D spacecraft is a proposed Russian space probe to Venus, which is scheduled to be launched around 2024. This mission will conduct remote-sensing observations around the planet and deploy a lander, based on the Venera design, capable of surviving for a long duration on the surface.
Because of its proximity to Earth, and its similarity in size, mass and composition, Venus was once believed to hold life. In fact, the idea of Venus being a tropical world persisted well into the 20th century, until the Venera and Mariner programs demonstrated the absolute hellish conditions that actually exist on the planet.
Nevertheless, it is believed that Venus may once have been much like Earth, with a similar atmosphere and warm, flowing water on its surface. This notion is supported by the fact that Venus sits within the inner edge of the Sun’s habitable zone and has an ozone layer. However, owing to the runaway greenhouse effect and the lack of a magnetic field, this water disappeared many billions of years ago.
Still, there are those who believed that Venus could one day support human colonies. Currently, the atmospheric pressure near to the ground is far too extreme for settlements to be built on the surface. But 50 km above the surface, both the temperature and air pressure are similar to Earth’s, and both nitrogen and oxygen are believed to exist. This has led to proposals for “floating cities” to be built in the Venusian atmosphere and the exploration of the atmosphere using Airships.
In addition, proposals have been made suggesting the Venus should be terraformed. These have ranged from installing a huge space-shade to combat the greenhouse effect, to crashing comets into the surface to blow the atmosphere off. Other ideas involve converting the atmosphere using calcium and magnesium to sequester the carbon away.
Much like proposals to terraform Mars, these ideas are all in their infancy and are hard-pressed to address the long-term challenges associated with changing the planet’s climate. However, they do show that humanity’s fascination with Venus has not diminished over time. From being a central to our mythology and the first star we saw in the morning (and the last one we saw at night), Venus has since gone on to become a subject of fascination for astronomers and a possible prospect for off-world real estate.
But until such time as technology improves, Venus will remain Earth’s hostile and inhospitable “sister planet”, with intense pressure, sulfuric acid rains, and a toxic atmosphere.
Mars, otherwise known as the “Red Planet”, is the fourth planet of our Solar System and the second smallest (after Mercury). Named after the Roman God of war, its nickname comes from its reddish appearance, which has to do with the amount of iron oxide prevalent on its surface. Every couple of years, when Mars is at opposition to Earth (i.e. when the planet is closest to us), it is most visible in the night sky.
Because of this, humans have been observing it for millennia, and its appearance in the heavens has played a large role in the mythology and astrological systems of many cultures. And in the modern era, it has been a veritable treasure trove of scientific discoveries, which have informed our understanding of our Solar System and its history.
Size, Mass and Orbit:
Mars has a radius of approximately 3,396 km at its equator, and 3,376 km at its polar regions – which is the equivalent of roughly 0.53 Earths. While it is roughly half the size of Earth, it’s mass – 6.4185 x 10²³ kg – is only 0.151 that of Earth’s. It’s axial tilt is very similar to Earth’s, being inclined 25.19° to its orbital plane (Earth’s axial tilt is just over 23°), which means Mars also experiences seasons.
At its greatest distance from the Sun (aphelion), Mars orbits at a distance of 1.666 AUs, or 249.2 million km. At perihelion, when it is closest to the Sun, it orbits at a distance of 1.3814 AUs, or 206.7 million km. At this distance, Mars takes 686.971 Earth days, the equivalent of 1.88 Earth years, to complete a rotation of the Sun. In Martian days (aka. Sols, which are equal to one day and 40 Earth minutes), a Martian year is 668.5991 Sols.
Composition and Surface Features:
With a mean density of 3.93 g/cm³, Mars is less dense than Earth, and has about 15% of Earth’s volume and 11% of Earth’s mass. The red-orange appearance of the Martian surface is caused by iron oxide, more commonly known as hematite (or rust). The presence of other minerals in the surface dust allow for other common surface colors, including golden, brown, tan, green, and others.
As a terrestrial planet, Mars is rich in minerals containing silicon and oxygen, metals, and other elements that typically make up rocky planets. The soil is slightly alkaline and contains elements such as magnesium, sodium, potassium, and chlorine. Experiments performed on soil samples also show that it has a basic pH of 7.7.
Although liquid water cannot exist on Mars’ surface, owing to its thin atmosphere, large concentrations of ice water exist within the polar ice caps – Planum Boreum and Planum Australe. In addition, a permafrost mantle stretches from the pole to latitudes of about 60°, meaning that water exists beneath much of the Martian surface in the form of ice water. Radar data and soil samples have confirmed the presence of shallow subsurface water at the middle latitudes as well.
Like Earth, Mars is differentiated into a dense metallic core surrounded by a silicate mantle. This core is composed of iron sulfide, and thought to be twice as rich in lighter elements than Earth’s core. The average thickness of the crust is about 50 km (31 mi), with a maximum thickness of 125 km (78 mi). Relative to the sizes of the two planets, Earth’s crust (averaging 40 km or 25 mi) is only one third as thick.
Artist rendition of the formation of rocky bodies in the solar system – how they form and differentiate and evolve into terrestrial planets. Image credit: NASA/JPL-Caltech
Current models of its interior imply that the core region measures between 1,700 – 1850 kilometers (1,056 – 1150 mi) in radius, consisting primarily of iron and nickel with about 16–17% sulfur. Due to its smaller size and mass, the force of gravity on the surface of Mars is only 37.6% of that on Earth. An object falling on Mars falls at 3.711 m/s², compared to 9.8 m/s² on Earth.
The surface of Mars is dry and dusty, with many similar geological features to Earth. It has mountain ranges and sandy plains, and even some of the largest sand dunes in the Solar System. It also has the largest mountain in the Solar System, the shield volcano Olympus Mons, and the longest, deepest chasm in the Solar System: Valles Marineris.
The surface of Mars has also been pounded by impact craters, many of which date back billions of years. These craters are so well preserved because of the slow rate of erosion that happens on Mars. Hellas Planitia, also called the Hellas impact basin, is the largest crater on Mars. Its circumference is approximately 2,300 kilometers, and it is nine kilometers deep.
Mars also has discernible gullies and channels on its surface, and many scientists believe that liquid water used to flow through them. By comparing them to similar features on Earth, it is believed these were were at least partially formed by water erosion. Some of these channels are quite large, reaching 2,000 kilometers in length and 100 kilometers in width.
Mars’ Moons:
Mars has two small satellites, Phobos and Deimos. These moons were discovered in 1877 by the astronomer Asaph Hall and were named after mythological characters. In keeping with the tradition of deriving names from classical mythology, Phobos and Deimos are the sons of Ares – the Greek god of war that inspired the Roman god Mars. Phobos represents fear while Deimos stands for terror or dread.
Phobos and Deimos, photographed here by the Mars Reconnaissance Orbiter, are tiny, irregularly-shaped moons that are probably strays from the main asteroid belt. Credit: NASA
Phobos measures about 22 km (14 mi) in diameter, and orbits Mars at a distance of 9234.42 km when it is at periapsis (closest to Mars) and 9517.58 km when it is at apoapsis (farthest). At this distance, Phobos is below synchronous altitude, which means that it takes only 7 hours to orbit Mars and is gradually getting closer to the planet. Scientists estimate that in 10 to 50 million years, Phobos could crash into Mars’ surface or break up into a ring structure around the planet.
Meanwhile, Deimos measures about 12 km (7.5 mi) and orbits the planet at a distance of 23455.5 km (periapsis) and 23470.9 km (apoapsis). It has a longer orbital period, taking 1.26 days to complete a full rotation around the planet. Mars may have additional moons that are smaller than 50- 100 meters (160 to 330 ft) in diameter, and a dust ring is predicted between Phobos and Deimos.
Scientists believe that these two satellites were once asteroids that were captured by the planet’s gravity. The low albedo and the carboncaceous chondrite composition of both moons – which is similar to asteroids – supports this theory, and Phobos’ unstable orbit would seem to suggest a recent capture. However, both moons have circular orbits near the equator, which is unusual for captured bodies.
Another possibility is that the two moons formed from accredit material from Mars early in its history. However, if this were true, their compositions would be similar to Mars itself, rather than similar to asteroids. A third possibility is that a body impacted the Martian surface, who’s material was ejected into space and re-accreted to form the two moons, similar to what is believed to have formed the Earth’s Moon.
Atmosphere and Climate:
Planet Mars has a very thin atmosphere which is composed of 96% carbon dioxide, 1.93% argon and 1.89% nitrogen along with traces of oxygen and water. The atmosphere is quite dusty, containing particulates that measure 1.5 micrometers in diameter, which is what gives the Martian sky a tawny color when seen from the surface. Mars’ atmospheric pressure ranges from 0.4 – 0.87 kPa, which is equivalent to about 1% of Earth’s at sea level.
Because of its thin atmosphere, and its greater distance from the Sun, the surface temperature of Mars is much colder than what we experience here on Earth. The planet’s average temperature is -46 °C (-51 °F), with a low of -143 °C (-225.4 °F) during the winter at the poles, and a high of 35 °C (95 °F) during summer and midday at the equator.
The planet also experiences dust storms, which can turn into what resembles small tornadoes. Larger dust storms occur when the dust is blown into the atmosphere and heats up from the Sun. The warmer dust filled air rises and the winds get stronger, creating storms that can measure up to thousands of kilometers in width and last for months at a time. When they get this large, they can actually block most of the surface from view.
Trace amounts of methane have also been detected in the Martian atmosphere, with an estimated concentration of about 30 parts per billion (ppb). It occurs in extended plumes, and the profiles imply that the methane was released from specific regions – the first of which is located between Isidis and Utopia Planitia (30°N260°W) and the second in Arabia Terra (0°N310°W).
Artist’s concept of what Mars atmosphere appears like today. Credit: NASA
It is estimated that Mars must produce 270 tonnes of methane per year. Once released into the atmosphere, the methane can only exist for a limited period of time (0.6 – 4 years) before it is destroyed. Its presence despite this short lifetime indicates that an active source of the gas must be present.
Several possible sources have been suggested for the presence of this methane, ranging from volcanic activity, cometary impacts, and the presence of methanogenic microbial life forms beneath the surface. Methane could also be produced by a non-biological process called serpentinization involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars.
The Curiosity rover has made several measurements for methane since its deployment to the Martian surface in August of 2012. The first measurements, which were made using its Tunable Laser Spectrometer (TLS), indicated that there were less than 5 ppb at its landing site (Bradbury Landing). A subsequent measurement performed on September 13th detected no discernible traces.
On December 16th, 2014, NASA reported that the Curiosity rover had detected a “tenfold spike”, likely localized, in the amount of methane in the Martian atmosphere. Samples measurements taken between late 2013 and early 2014 showed an increase of 7 ppb; whereas before and after that, readings averaged around one-tenth that level.
Regions where methane appears notably localized in Northern Summer and their relationship to mineralogical and geo-morphological domains. Credit: Mumma, et al., 2009.
Ammonia was also tentatively detected on Mars by the Mars Express satellite, but with a relatively short lifetime. It is not clear what produced it, but volcanic activity has been suggested as a possible source.
Historical Observations:
Earth astronomers have a long history of observing the “Red Planet”, both with the naked eye and with instrumentation. The first recorded mentions of Mars as a wandering object in the night sky were made by Ancient Egyptian astronomers, who by 1534 BCE were familiar with the planet’s “retrograde motion”. In essence, they deduced that the planet, though it appeared to be a bright star, moved differently than the other stars, and that it would occasionally slow down and reverse course before returning to its original course.
By the time of the Neo-Babylonian Empire (626 BCE – 539 BCE), astronomers were making regular records of the position of the planets, systematic observations of their behavior and even arithmetic methods for predicted the positions of the planets. For Mars, this included detailed accounts of its orbital period and its passage through the zodiac.
By classical antiquity, the Greeks were making additional observations on Mars’ behavior that helped them to understand its position in the Solar System. In the 4th century BCE, Aristotle noted that Mars disappeared behind the Moon during an occultation, which indicated it was farther away than the Moon.
Illustration of the Ptolemaic geocentric conception of the Universe by Portuguese cosmographer and cartographer Bartolomeu Velho from his work Cosmographia (1568). Credit: Bibilotèque nationale de France, Paris.
Ptolemy, a Greek-Egyptian astronomer of Alexandria (90 CE – ca. 168 CE), constructed a model of the universe in which he attempted to resolve the problems of the orbital motion of Mars and other bodies. In his multi-volume collection Almagest, he proposed that the motions of heavenly bodies were governed by “wheels within wheels”, which attempted to explain retrograde motion. This became the authoritative treatise on Western astronomy for the next fourteen centuries.
Literature from ancient China confirms that Mars was known by Chinese astronomers by at least the fourth century BCE. In the fifth century CE, the Indian astronomical text Surya Siddhanta estimated the diameter of Mars. In the East Asian cultures, Mars is traditionally referred to as the “fire star”, based on the Five elements.
Modern Observations:
The Ptolemaic model of the Solar System remained canon for western astronomers until the Scientific Revolution (16th to 18th century CE). Thanks to Copernicus’ heliocentric model, and Galileo’s use of the telescope, Mars proper position relative to Earth and the Sun began to become known. The invention of the telescope also allowed astronomers to measure the diurnal parallax of Mars and determine its distance.
This was first performed by Giovanni Domenico Cassini in 1672, but his measurements were hampered by the low quality of his instruments. During the 17th century, Tycho Brahe also employed the diurnal parallax method, and his observations were measured later by Johannes Kepler. During this time, Dutch astronomer Christiaan Huygens also drew the first map of Mars which included terrain features.
Map of Mars by Giovanni Schiaparelli (1877) that shows the famous Martian “canals”. Credit: Wikipedia Commons
By the 19th century, the resolution of telescopes improved to the point that surface features on Mars could be identified. This led Italian astronomer Giovanni Schiaparelli to produce the first detailed map of Mars after viewing it at opposition on September 5th, 1877. These maps notably contained features he called canali – a series of long, straight lines on the surface of Mars – which he named after famous rivers on Earth. These were later revealed to be an optical illusion, but not before spawning a wave of interest in Mars’ “canals”.
In 1894, Percival Lowell – inspired by Schiaparelli’s map – founded an observatory which boasted two of the largest telescopes of the time – 30 and 45 cm (12 and 18 inch). Lowell published several books on Mars and life on the planet, which had a great influence on the public, and the canals were also observed by other astronomers, like Henri Joseph Perrotin and Louis Thollon of Nice.
Seasonal changes like the diminishing of the polar caps and the dark areas formed during Martian summer, in combination with the canals, led to speculation about life on Mars. The term “Martian” became synonymous with extra-terrestrial for quite some time, though telescopes never reached the resolution needed to provide any proof. Even in the 1960s, articles were published on Martian biology, putting aside explanations other than life for the seasonal changes on Mars.
Exploration of Mars:
With the advent of the space age, probes and landers began to be sent to Mars by the late 20th century. These have yielded a wealth of information on the geology, natural history, and even the habitability of the planet, and increased our knowledge of the planet immensely. And while modern missions to Mars have dispelled the notions of there being a Martian civilization, they have indicated that life may have existed there at one time.
Efforts to explore Mars began in earnest in the 1960s. Between 1960 and 1969, the Soviets launched nine unmanned spacecraft towards Mars, but all failed to reach the planet. In 1964, NASA began launching Mariner probes towards Mars. This began with Mariner 3 and Mariner 4, two unmanned probes that were designed to carry out the first flybys of Mars. The Mariner 3 mission failed during deployment, but Mariner 4 – which launched three weeks later – successfully made the 7½-month long voyage to Mars.
Mariner 4 captured the first close-up photographs of another planet (showing impact craters) and provided accurate data about the surface atmospheric pressure, and noted the absence of a Martian magnetic field and radiation belt. NASA continued the Mariner program with another pair of flyby probes – Mariner 6 and 7 – which reached the planet in 1969.
During the 1970s, the Soviets and the US competed to see who could place the first artificial satellite in orbit of Mars. The Soviet program (M-71) involved three spacecraft – Cosmos 419 (Mars 1971C), Mars 2 and Mars 3. The first, a heavy orbiter, failed during launch. The subsequent missions, Mars 2 and Mars 3, were combinations of an orbiter and a lander, and would be the first rovers to land on a body other than the Moon.
They were successfully launched in mid-May 1971 and reached Mars about seven months later. On November 27th, 1971, the lander of Mars 2 crash-landed due to an on-board computer malfunction and became the first man-made object to reach the surface of Mars. In December 2nd, 1971, the Mars 3 lander became the first spacecraft to achieve a soft landing, but its transmission was interrupted after 14.5 seconds.
Mariner 9 view of the Noctis Labyrinthus “labyrinth” at the western end of Valles Marineris. Credit: NASA
Meanwhile, NASA continued with the Mariner program, and scheduled Mariner 8 and 9 for launch in 1971. Mariner 8 also suffered a technical failure during launch and crashed into the Atlantic Ocean. But the Mariner 9 mission managed to not only make it to Mars, but became the first spacecraft to successfully establish orbit around it. Along with Mars 2 and Mars 3, the mission coincided with a planet-wide dust storm. During this time, the Mariner 9 probe managed to rendezvous and take some photos of Phobos.
When the storm cleared sufficiently, Mariner 9 took photos that were the first to offer more detailed evidence that liquid water might have flowed on the surface at one time. Nix Olympica, which was one of only a few features that could be seen during the planetary duststorm, was also determined to be the highest mountain on any planet in the entire Solar System, leading to its reclassification as Olympus Mons.
In 1973, the Soviet Union sent four more probes to Mars: the Mars 4 and Mars 5 orbiters and the Mars 6 and Mars 7 fly-by/lander combinations. All missions except Mars 7 sent back data, with Mars 5 being most successful. Mars 5 transmitted 60 images before a loss of pressurization in the transmitter housing ended the mission.
By 1975, NASA launched Viking1 and 2 to Mars, which consisted of two orbiters and two landers. The primary scientific objectives of the lander mission were to search for biosignatures and observe the meteorologic, seismic and magnetic properties of Mars. The results of the biological experiments on board the Viking landers were inconclusive, but a reanalysis of the Viking data published in 2012 suggested signs of microbial life on Mars.
Image from Mars taken by the Viking 2 lander. Credit: NASA
The Viking orbiters revealed further data that water once existed on Mars, indicating that large floods carved deep valleys, eroded grooves into bedrock, and traveled thousands of kilometers. In addition, areas of branched streams in the southern hemisphere, suggest that surface once experienced rainfall.
Mars was not explored again until the 1990’s, at which time, NASA commenced the Mars Pathfinder mission – which consisted of a spacecraft that landed a base station with a roving probe (Sojourner) on the surface. The mission landed on Mars on July 4th, 1997, and provided a proof-of-concept for various technologies that was would be used by later missions, such as an airbag landing system and automated obstacle avoidance.
This was followed by the Mars Global Surveyor (MGS), a mapping satellite that reached Mars on September 12th, 1997, and began its mission on March 1999. From a low-altitude, nearly polar orbit, it observed Mars over the course of one complete Martian year (nearly two Earth years) and studied the entire Martian surface, atmosphere, and interior, returning more data about the planet than all previous Mars missions combined.
Among key scientific findings, the MGS took pictures of gullies and debris flows that suggest there may be current sources of liquid water, similar to an aquifer, at or near the surface of the planet. Magnetometer readings showed that the planet’s magnetic field is not globally generated in the planet’s core, but is localized in particular areas of the crust.
The spacecraft’s laser altimeter also gave scientists their first 3-D views of Mars’ north polar ice cap. On November 5th, 2006, MGS lost contact with Earth, and all efforts by NASA to restore communication ceased by January 28th, 2007.
The North Polar Basin is the large blue low-lying area at the northern end of this topographical map of Mars. Its elliptical shape is partially obscured by volcanic eruptions (red, center left). Credit: NASA/JPL/USGS
In 2001, NASA’s Mars Odyssey orbiter arrived at Mars. Its mission was to use spectrometers and imagers to hunt for evidence of past or present water and volcanic activity on Mars. In 2002, it was announced that the probe had detected large amounts of hydrogen, indicating that there are vast deposits of water ice in the upper three meters of Mars’ soil within 60° latitude of the south pole.
On June 2, 2003, the European Space Agency (ESA) launched the Mars Express spacecraft, which consisted of the Mars Express Orbiter and the lander Beagle 2. The orbiter entered Martian orbit on December 25th, 2003, and Beagle 2 entered Mars’ atmosphere on the same day. Before the ESA lost contact with the probe, the Mars Express Orbiter confirmed the presence of water ice and carbon dioxide ice at the planet’s south pole, while NASA had previously confirmed their presence at the north pole of Mars.
In 2003, NASA also commenced the Mars Exploration Rover Mission (MER), an ongoing robotic space mission involving two rovers – Spirit and Opportunity – exploring the planet Mars. The mission’s scientific objective was to search for and characterize a wide range of rocks and soils that hold clues to past water activity on Mars.
TheMars Reconnaissance Orbiter (MRO) is a multipurpose spacecraft designed to conduct reconnaissance and exploration of Mars from orbit. The MRO launched on August 12th, 2005, and attained Martian orbit on March 10th, 2006. The MRO contains a host of scientific instruments designed to detect water, ice, and minerals on and below the surface.
Mosaic self-portrait of Curiosity at the John Klein outcrop on Feb. 3, 2013. Credit: NASA/JPL-Caltech/MSSS
Additionally, the MRO is paving the way for upcoming generations of spacecraft through daily monitoring of Martian weather and surface conditions, searching for future landing sites, and testing a new telecommunications system that will speed up communications between Earth and Mars.
The NASA Mars Science Laboratory (MSL) mission and its Curiosityrover landed on Mars in the Gale Crater (at a landing site named “Bradbury Landing”) on August 6th, 2012. The rover carries instruments designed to look for past or present conditions relevant to the habitability of Mars, and has made numerous discoveries about atmospheric and surface conditions on Mars, as well as the detection of organic particles.
NASA’s Mars Atmosphere and Volatile EvolutioN Mission (MAVEN) orbiter was launched on November 18th, 2013, and reached Mars on September 22nd, 2014. The purpose of the mission is to study the atmosphere of Mars and also serve as a communications relay satellite for robotic landers and rovers on the surface.
Most recently, the Indian Space Research Organisation (ISRO) launched the Mars Orbiter Mission (MOM, also called Mangalyaan) on November 5th, 2013. The orbiter successfully reached Mars on September 24th, 2014, and was the first spacecraft to achieve orbit on the first try. A technology demonstrator, who’s secondary purpose is to study the Martian atmosphere, MOM is India’s first mission to Mars, and has made the ISRO the fourth space agency to reach the planet.
An artist’s conception of the MAVEN orbiter arriving at Mars. Credit: NASA/Goddard Space Flight Center
Future missions to Mars include NASA’s Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSIGHT) lander. This mission, which is planned for launch in 2016, involves placing a stationary lander equipped with a seismometer and heat transfer probe on the surface of Mars. The probe will then deploy these instruments into the ground to study the planets interior and get a better understanding of its early geological evolution.
The ESA and Roscosmos are also collaborating on a large mission to search for biosignatures of Martian life, known as Exobiology on Mars (or ExoMars). Consisting of an orbiter that will be launched in 2016, and a lander that will be deployed to the surface by 2018, the purpose of this mission will be to map the sources of methane and other gases on Mars that would indicate the presence of life, past and present.
The United Arab Emirates also has a plan to send an orbiter to Mars by 2020. Known as Mars Hope, the robotic space probe will be deployed in orbit around Mars for the sake of studying its atmosphere and climate. This spacecraft will be the first to be deployed by an Arab state in orbit of another planet, and is expected to involve collaboration from the University of Colorado, the University of California, Berkeley and Arizona State University, as well the French space agency (CNES).
Crewed Missions:
Numerous federal space agencies and private companies have plans to send astronauts to Mars within the not-too-distant future. For instance, NASA has confirmed that it plans to conduct a manned mission to Mars by 2030. In 2004, human exploration of Mars was identified as a long-term goal in the Vision for Space Exploration – a public document released by the Bush administration.
Concept for NASA Design Reference Mission Architecture 5.0 (2009). Credit: NASA
In 2010, President Barack Obama announced his administration’s space policy, which included increasing NASA funding by $6 billion over five years and completing the design of a new heavy-lift launch vehicle by 2015. He also predicted a U.S.-crewed orbital Mars mission by the mid-2030s, preceded by an asteroid mission by 2025.
The ESA also has plans to land humans on Mars between 2030 and 2035. This will be preceded by successively larger probes, starting with the launch of the ExoMars probe and a planned joint NASA-ESA Mars sample return mission.
In Robert Zubrin (founder of the Mars Society) proposed a low-cost human mission to NASA known as Mars Direct, which he expanded on in his 1996 book The Case for Mars. According to Zubrin, the plan calls for a new class of heavy-lift rockets (similar to the Saturn V) to send human explorers and payloads to the Red Planet. A modified proposal, known as “Mars to Stay”, involves a possible one-way trip, where the astronauts would become Mars’ first colonists.
Similarly, MarsOne, a Netherlands-based non-profit organization, hopes to establish a permanent colony on the planet beginning in 2027. The original concept included launching a robotic lander and orbiter as early as 2016 to be followed by a human crew of four in 2022. Subsequent crews of four will be sent every few years, and funding is expected to be provided in part by a reality TV program that will document the journey.
Artist’s concept of a Martian astronaut standing outside the Mars One habitat. Credit: Bryan Versteeg/Mars One
SpaceX and Tesla CEO Elon Musk has also announced plans to establish a colony on Mars. Intrinsic to this plan is the development of the Mars Colonial Transporter (MCT), a spaceflight system that would rely of reusable rocket engines, launch vehicles and space capsules to transport humans to Mars and return to Earth.
As of 2014, SpaceX has begun development of the large Raptor rocket engine for the Mars Colonial Transporter, and a successful test was announced in September of 2016. In January 2015, Musk said that he hoped to release details of the “completely new architecture” for the Mars transport system in late 2015.
In June 2016, Musk stated in the first unmanned flight of the MCT spacecraft would take place in 2022, followed by the first manned MCT Mars flight departing in 2024. In September 2016, during the 2016 International Astronautical Congress, Musk revealed further details of his plan, which included the design for an Interplanetary Transport System (ITS) – an upgraded version of the MCT.
Mars is the most studied planet in the Solar System after Earth. As of the penning of this article, there are 3 landers and rovers on the surface of Mars (Phoenix, Opportunity and Curiosity), and 5 functional spacecraft in orbit (Mars Odyssey, Mars Express, MRO, MOM, and MAVEN). And more spacecraft will be on their way soon.
These spacecraft have sent back incredibly detailed images of the surface of Mars, and helped discover that there was once liquid water in Mars’ ancient history. In addition, they have confirmed that Mars and Earth share many of the same characteristics – such as polar icecaps, seasonal variations, an atmosphere, and the presence of flowing water. They have also shown that organic life can and most likely did live on Mars at one time.
In short, humanity’s obsession with the Red Planet has not waned, and our efforts to explore its surface and understand its history are far from over. In the coming decades, we are likely to be sending additional robotic explorers, and human ones as well. And given time, the right scientific know-how, and whole lot of resources, Mars may even be suitable for habitation someday.
The strangest feature on Iapetus is the equatorial ridge. What could possibly create a feature like this?
To paraphrase the British geneticist J.B.S Haldane, “in my suspicion, the Universe is not only stranger than we suppose, it’s stranger than we can suppose.” The context was life and evolution, but he might as well been talking about Saturn’s moons. Those teeny worlds are some of the strangest places we’ve ever seen.
Titan is a massive moon with an atmosphere thicker than Earth’s. If it wasn’t for the bone crippling cold and unbreathable atmosphere, you could wear a pair of wings and fly around in the Titanic skies.
There’s Enceladus, an icy moon that blasts water out into space through geysers at its southern pole. But the Saturnian moon that fascinates me the most has got to be Iapetus, also known as Saturn’s yin-yang moon.
Here’s a photo captured by Cassini. Check out the bizarre surface features, where half of the moon is icy white and the other is brownish red. Astronomers believe this strange coloration comes from the ice on the warmer side sublimating away, leaving this darker material beneath.
Sure that’s a bit odd, but the strangest feature on Iapetus is the equatorial ridge. This feature measures 1,300 km long and it makes the moon look like a space walnut. Because of the heavy cratering on the ridge, astronomers know that it’s ancient, nearly as old as the moon itself. At 13 kms high, it’s tall enough to keep out the most persnickety white walker or wildling mammoth & giant battalion.
What could possibly create a feature like this?
Astronomers are of a few camps. The first think it formed through convective activity early on in the moon’s history. Saturn pulls Iapetus with its tremendous gravity, and the moon undergoes massive tidal forces. This generates heat in the moon’s interior, and it might have caused it to blob out at the equator.
A ridge that follows the equator of Saturn’s moon Iapetus gives it the appearance of a giant walnut. This image was taken by the Cassini spacecraft. Credit: NASA/JPL/SSI
A second idea is that Iapetus consumed one of Saturn’s rings, billions of years ago. The moon might have slowly wandered through the ring plane, and accreted all the ring material, like snow piling up in front of a plow.
A third is that Iapetus was smashed into by a massive asteroid billions of years ago. This impact caused the moon to fly apart, but then mutual gravity pulled it back together. The force of this recombination squeezed out material at the equator, which then solidified in place.
Alternately, it might be a walnut from a Galactus family Christmas stocking. So which is it?
It turns out that Saturn has two more moons in its system with similar equatorial ridges. Its moon Atlas is just 15 km across, but it’s dominated by an equatorial ridge. It looks like a UFO, and Pan has a similar feature.
Saturn’s relatively thin main rings are about 250,000 km (156,000 miles) in diameter. (Image: NASA/JPL-Caltech/SSI/J. Major)
Astronomers know that both of these created their ridges by pulling material out of the rings and piling it up on their surface. This is the mechanism that seems to match what’s going on with Iapetus.
One mystery, is how distantly Iapetus orbits Saturn. There’s no ring that far out, so where did it get the material to consume? Is it possible that Iapetus drifted outward, or had a ring system of its own?
You want puzzles? Iapetus is one of the strangest places in the Solar System, and it would be my candidate for a future orbiter or lander. Let’s explore it closer.
What’s your favorite bizarre object in the Solar System? Tell us in the comments below.
Pluto was re-classified as a dwarf planet based on our growing understanding of its nature. Will Schlaufman's new study help us more accurately classify gas giants and brown dwarfs? NASA's New Horizons spacecraft captured this high-resolution enhanced color view of Pluto on July 14, 2015. Credit: NASA/JHUAPL/SwRI
After being officially discovered by Clyde Tombaugh in 1930, Pluto spent close to a century being thought of as the ninth planet of our Solar System. In 2006, it was reclassified as a “dwarf planet” due to the discovery of other Trans-Neptunian Objects (TNOs) of comparable size. However, that does not change its significance one bit. In addition to being the largest TNO, it is the largest and second-most massive dwarf planet in our Solar System.
As a result, a great deal of time and study has been devoted to this former planet. And with the successful flyby of the New Horizons mission this month, we finally have a clear picture of what it looks like. As scientists pour over the voluminous amounts of data being sent back, our understanding of this world at the edge of our Solar System has grown by leaps and bounds.
Discovery:
The existence of Pluto was predicted before it was observed. In the 1840s, French mathematician Ubrain Le Verrier used Newtonian mechanics to predict the position of Neptune (which had not yet been discovered) based on the perturbation of Uranus. By the late 19th century, subsequent observations of Neptune led astronomers to believe that a planet was perturbing its orbit as well.
In 1906, Percival Lowell – an American mathematician and astronomer who founded the Lowell Observatory in Flagstaff, Arizona, in 1894 – initiated a project to locate “Planet X”, the possible ninth planet of the Solar System. Unfortunately, Lowell died in 1916 before a confirmed discovery was made. But unbeknownst to him, his surveys had captured two faint images of Pluto (March 19th and April 7th, 1915), which were not recognized for what they were.
The discovery photographs of Pluto, dated January 23rd and 29th , 1930. Credit: Lowell Observatory Archives
After Lowell’s death, the search did not resume until 1929, at which point the director of the Lowell Observatory (Vesto Melvin Slipher) entrusted the job of locating Planet X to Clyde Tombaugh. A 23 year-old astronomer from Kansas, Tombaugh spent the next year photographing sections of the night sky and then analyzing the photographs to determine if any objects had shifted position.
On February 18th, 1930, Tombaugh discovered a possible moving object on photographic plates taken in January of that year. After the observatory obtained further photographs to confirm the existence of the object, news of the discovery was telegraphed to the Harvard College Observatory on March 13th, 1930. The mysterious Planet X had finally been discovered.
Naming:
After the discovery was announced, the Lowell Observatory was flooded with suggestions for names. The name Pluto, based on the Roman god of the underworld, was proposed by Venetia Burney (1918–2009), a then eleven-year-old schoolgirl in Oxford, England. She suggested it in a conversation with her grandfather who passed the name on to astronomy professor Herbert Hall Turner, who cabled it to colleagues in the United States.
Pluto’s surface as viewed from the Hubble Space Telescope in several pictures taken in 2002 and 2003. Credit: NASA/Hubble
The object was officially named on March 24th, 1930, and it came down to a vote between three possibilities – Minerva, Cronus, and Pluto. Every member of the Lowell Observatory voted for Pluto, and the name was announced on May 1st, 1930. The choice was based on part on the fact that the first two letters of Pluto – P and L – corresponded to the initials of Percival Lowell.
The name quickly caught on with the general public. In 1930, Walt Disney was apparently inspired by it when he introduced a canine companion for Mickey Mouse named Pluto. In 1941, Glenn T. Seaborg named the newly created element plutonium after Pluto. This was in keeping with the tradition of naming elements after newly discovered planets – such as uranium, which was named after Uranus, and neptunium, which was named after Neptune.
Size, Mass, and Orbit:
With a mass of 1.305±0.007 x 1o²² kg – which is the equivalent of 0.00218 Earths and 0.178 Moons – Pluto is the second most-massive dwarf planet and the tenth-most-massive known object directly orbiting the Sun. It has a surface area of 1.765×107 km, and a volume of 6.97×109 km3.
Map of Pluto’s surface features, with (informal) names for some of the largest surface features. Credit: NASA/JHUAPL
Pluto has a moderately eccentric and inclined orbit, which ranges from 29.657 AU (4.4 billion km) at perihelion to 48.871 AU (7.3 billion km) at aphelion. This means that Pluto periodically comes closer to the Sun than Neptune, but a stable orbital resonance with Neptune prevents them from colliding.
Pluto has an orbital period of 247.68 Earth years, meaning it takes almost 250 years to complete a single orbit of the Sun. Meanwhile, its rotation period (a single day) is equal to 6.39 Earth days. Like Uranus, Pluto rotates on its side, with an axial tilt of 120° relative to its orbital plane, which results in extreme seasonal variations. At its solstices, one-fourth of its surface is in continuous daylight, whereas another fourth is in continuous darkness.
Composition and Atmosphere:
With a mean density of 1.87 g/cm3, Pluto’s composition is differentiated between an icy mantle and a rocky core. The surface is composed of more than 98% nitrogen ice, with traces of methane and carbon monoxide. The surface is very varied, with large differences in both brightness and color. A notable feature is a large, pale area nicknamed the “Heart”.
The theoretical structure of Pluto, consisting of 1. Frozen nitrogen 2. Water ice 3. Rock. Credit: NASA/Pat Rawlings
Scientists also suspect that Pluto’s internal structure is differentiated, with the rocky material having settled into a dense core surrounded by a mantle of water ice. The diameter of the core is believed to be approximately 1700 km, 70% of Pluto’s diameter. Thanks to the decay of radioactive elements, it is possible that Pluto contains a subsurface ocean layer that is 100 to 180 km thick at the core–mantle boundary.
Pluto has a thin atmosphere consisting of nitrogen (N2), methane (CH4), and carbon monoxide (CO), which are in equilibrium with their ices on Pluto’s surface. However, the planet is so cold that during part of its orbit, the atmosphere congeals and falls to the surface. The average surface temperature is 44 K (-229 °C), ranging from 33 K (-240 °C) at aphelion to 55 K (-218 °C) at perihelion.
Satellites:
Pluto has five known satellites. The largest, and closest in orbit to Pluto, is Charon. This moon was first identified in 1978 by astronomer James Christy using photographic plates from the United States Naval Observatory (USNO) in Washington, D.C. Beyond Charon lies the four other circumbinary moons – Styx, Nix, Kerberos, and Hydra, respectively.
Nix and Hydra were discovered simultaneously in 2005 by the Pluto Companion Search Team using the Hubble Space Telescope. The same team discovered Kerberos in 2011. The fifth and final satellite, Styx, was discovered by the New Horizons spacecraft in 2012 while capturing images of Pluto and Charon.
Artist’s concept comparing the scale and brightness of the moons of Pluto. Credit: NASA/ESA/M. Showalter
Charon, Styx and Kerberos are all massive enough to have collapsed into a spheroid shape under their own gravity. Nix and Hydra, meanwhile, are oblong in shape. The Pluto-Charon system is unusual, since it is one of the few systems in the Solar System whose barycenter lies above the primary’s surface. In short, Pluto and Charon orbit each other, causing some scientists to claim that it is a “double-dwarf system” instead of a dwarf planet and an orbiting moon.
In addition, it is unusual in that each body is tidally locked to the other. Charon and Pluto always present the same face to each other; and from any position on either body, the other is always at the same position in the sky, or always obscured. This also means that the rotation period of each is equal to the time it takes the entire system to rotate around its common center of gravity.
In 2007, observations by the Gemini Observatory of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryo-geysers. This would seem indicate that Pluto does have a subsurface ocean that is warm in temperature, and that the core is geologically active. Pluto’s moons are believed to have been formed by a collision between Pluto and a similar-sized body early in the history of the Solar System. The collision released material that consolidated into the moons around Pluto.
Classification:
From 1992 onward, many bodies were discovered orbiting in the same area as Pluto, showing that Pluto is part of a population of objects called the Kuiper Belt. This placed its official status as a planet in question, with many asking whether Pluto should be considered separately or as part of its surrounding population – much as Ceres, Pallas, Juno and Vesta, which lost their planet status after the discovery of the Asteroid Belt.
On July 29h, 2005, the discovery of a new Trans-Neptunian Object (TNO), Eris, was announced, which was thought to be substantially larger than Pluto. Initially referred to the as the Solar System’s “tenth planet”, there was no consensus on whether or not Eris constituted the planet. What’s more, others in the astronomic community considered its discovery the strongest argument for reclassifying Pluto as a minor planet.
The debate came to a head on August 24th, 2006 with an IAU resolution that created an official definition for the term “planet”. According to the XXVI General Assembly of the International Astronomical Union, a planet must meet three criteria: it needs to be in orbit around the Sun, it needs to have enough gravity to pull itself into a spherical shape, and it needs to have cleared its orbit of other objects.
Pluto fails to meet the third condition, because its mass is only 0.07 times that of the mass of the other objects in its orbit. The IAU further decided that bodies that do not meet criterion 3 would be called dwarf planets. On September 13th, 2006, the IAU included Pluto, and Eris and its moon Dysnomia, in their Minor Planet Catalog.
The IAUs decision was met with mixed reactions, especially from within the scientific community. For instance, Alan Stern, the principal investigator with NASA’s New Horizons mission to Pluto, and Marc W. Buie – an astronomer with the Lowell Observatory – have both openly voiced dissatisfaction with the reclassification. Others, such as Mike Brown – the astronomer who discovered Eris – have voiced their support.
Our evolving understanding of Pluto, represented by images taken by Hubble in 2002-3 (left), and images taken by New Horizons in 2015 (right). Credit: theguardian.com
On August 14th – 16th, 2008, in what came to be known as “The Great Planet Debate“, researchers on both sides of the issue gathered at Johns Hopkins University Applied Physics Laboratory. Unfortunately, no scientific consensus was reached; but on June 11th 2008, the IAU announced in a press release that the term “plutoid” would henceforth be used to refer to Pluto and other similar objects.
Exploration:
Pluto presents significant challenges for spacecraft because of its small mass and great distance from Earth. In 1980, NASA began to contemplate sending the Voyager 1 spacecraft on a flyby of Pluto. However, the controllers opted instead for a close flyby of Saturn’s moon Titan, resulting in a trajectory incompatible with a Pluto flyby.
Voyager 2 never had a plausible trajectory for reaching Pluto, but it’s flyby Neptune and Triton in 1989 led scientists to once again begin contemplating a mission that would take a spacecraft to Pluto for the sake of studying the Kuiper Belt and Kuiper Belt Objects (KBOs). This led to the formation of the Pluto Kuiper Express mission proposal, and NASA instructing the JPL to being planning for a Pluto, Kuiper Belt flyby.
By 2000, the program had been scrapped due to apparent budget concerns. After much pressure had been brought to bear by the scientific community, a revised mission to Pluto, dubbed New Horizons, was finally granted funding from the US government in 2003. New Horizons was launched successfully on January 19th, 2006.
From September 21st-24th, 2006, New Horizons managed to capture its first images of Pluto while testing the LORRI instruments. These images, which were taken from a distance of approximately 4,200,000,000 km (2.6×109 mi) or 28.07 AU and released on November 28th, confirmed the spacecraft’s ability to track distant targets.
Distant-encounter operations at Pluto began on January 4th, 2015. Between January 25th to 31st, the approaching probe took several images of Pluto, which were released by NASA on February 12th. These photos, which were taken at a distance of more than 203,000,000 km (126,000,000 mi) showed Pluto and its largest moon, Charon.
Pluto and Charon, captured by the New Horizons spacecraft from January 25th to 31st. Credit: NASA
The New Horizons spacecraft made its closest approach to Pluto at 07:49:57 EDT (11:49:57 UTC) on July 14th, 2015, and then Charon at 08:03:50 EDT (12:03:50 UTC). Telemetries confirming a successful flyby and the health of the spacecraft reached Earth on 20:52:37 EDT (00:52:37 UTC).
During the flyby, the probe captured the clearest pictures of Pluto to date, and full analyses of the data obtained is expected to take years to process. The spacecraft is currently traveling at a speed of 14.52 km/s (9.02 mi/s) relative to the Sun and at 13.77 km/s (8.56 mi/s) relative to Pluto.
Though the New Horizons mission has shown us much about Pluto – and will continue to do so as scientists pour over all the data collected by the probe’s instruments – we still have much to learn about this distant and mysterious world. In time, and with more missions to the outer Solar System, we may eventually be able to unlock some of its deeper mysteries.
Artist’s impression of the New Horizons spacecraft in orbit around Pluto (Charon is seen in the background). Credit: NASA/JPL
Until then, we offer all information that is currently available on Pluto. We hope that you find what you are looking for in the links below and, as always, enjoy your research!
What would it take to destroy our moon, and eliminate the enemy of stellar astronomy for all time?
In the immortal words of Mr. Burns, “ever since the beginning of time, man has wished to destroy the Sun.” Your days are numbered, Sun.
But supervillains, being the practical folks they are, know that a more worthy goal would be to destroy the Moon, or at least deface it horribly. Nothing wrecks a beautiful night sky like that hideous pockmarked spotlight. What would it take to destroy it and eliminate the enemy of stellar astronomy for all time?
Crack out your Acme brand blueprint paper and white pencils, it’s Wile E. Coyote time.
The energy it takes to dismantle a gravitationally held object is known as its binding energy, we talked about it in a Death Star episode and inventive ways to overcome it.
For example, the binding energy of the Earth is 2.2 x 10^32 joules. It’s a lot. The binding energy of a smaller object, like our Moon is a tidy little 1.2 x 10^29 joules. It takes about 1800 times more energy to destroy the Earth than it takes to destroy the Moon.
It’s 1800 times easier. That’s downright doable, isn’t it? That’s almost 2000 times easier. Which, on the scale of easy to less easy, is definitely closer to easy.
Take the event that created the Caloris Basin on Mercury. It’s a crater, 1,500 km across. Astronomers think that a big fat asteroid, a fatsteroid(?) around 100 km in diameter crashed into Mercury billions of years ago. This event released 1.3 x 10^26 joules of energy, carving out this giant pit. It’s a thousandth of the binding energy of the Moon. We’ll need something more.
Our Sun produces 3.8 x 10^26 joules of energy every second, the equivalent of about a billion hydrogen bombs. If you directed the full power of the Sun at the Moon for 15 minutes, it’d tear apart.
That’s quite a superweapon you’ve got there, perhaps you’ll want to mount that on a space station and take it for a cruise through a galaxy far far away?
If that scene took that long, we’d have fallen asleep. It’s as if millions of voices gradually became a little hoarse from crying out for a quarter of an hour. There’s another way you could tear the Moon apart that doesn’t require an astral gate accident: gravity.
Astronomers use the Roche Limit to calculate how close an object – like a moon – can orbit another object – like a planet.
This is the point where the difference between the tidal forces on the “front” and “backside” are large enough that the object is torn apart, and if this sounds familiar you might want to look up “spaghettification”.
Spaghettification. Credit: Streeter
This is all based on the radius of the planet and the density of the planet and moon. If the Moon got close enough to the Earth, around 18,000 km, it would pull apart and be shredded into a beautiful ring.
And then the objects in the ring would enter the Earth’s atmosphere and rain down beautiful destruction for thousands of years.
Fortunately or unfortunately, depending your position in this “Die Moon, Die” discussion, the Moon is drifting away from the Earth. It’ll never be closer than it is right now, at almost 400,000 km, without a little nudge.
Phobos, the largest moon orbiting Mars is slowly approaching the planet, and astronomers think it’ll reach the Roche Limit in the next few million years.
It turns out that if we really want to destroy the Moon, we’ll need to destroy all life on Earth as well.
Now we know your new supervillain project, what’s your supervillain name? Tell us your handle in the comments below.
Artist's impression of the New Horizons spacecraft in orbit around Pluto (Charon is seen in the background). Credit: NASA/JPL
Humans have been sending spacecraft to other planets, as well as asteroid and comets, for decades. But rarely have any of these ventured into the outer reaches of our Solar System. In fact, the last time a probe reached beyond the orbit of Saturn to explore the worlds of Neptune, Uranus, Pluto and beyond was with the Voyager 2 mission, which concluded back in 1989.
But with the New Horizons mission, humanity is once again peering into the outer Solar System and learning much about its planets, dwarf planets, planetoids, moons and assorted objects. And as of July 14th, 2015, it made its historic rendezvous with Pluto, a world that has continued to surprise and mystify astronomers since it was first discovered.
Background:
In 1980, after Voyager 1‘s flyby of Saturn, NASA scientists began to consider the possibility of using Saturn to slingshot the probe towards Pluto to conduct a flyby by 1986. This would not be the case, as NASA decided instead to conduct a flyby of Saturn’s moon of Titan – which they considered to be a more scientific objective – thus making a slingshot towards Pluto impossible.
Because no mission to Pluto was planned by any space agency at the time, it would be years before any missions to Pluto could be contemplated. However, after Voyager 2′s flyby of Neptune and Triton in 1989, scientists once again began contemplating a mission that would take a spacecraft to Pluto for the sake of studying the Kuiper Belt and Kuiper Belt Objects (KBOs).
Artist’s impression of the Voyager spacecraft in flight. Credit: NASA/JPL
In May 1989, a group of scientists, including Alan Stern and Fran Bagenal, formed an alliance called the “Pluto Underground”. Committed to the idea of mounting an exploratory mission to Pluto and the Kuiper Belt, this group began lobbying NASA and the US government to make it this plan a reality. Combined with pressure from the scientific community at large, NASA began looking into mission concepts by 1990.
During the course of the late 1990s, a number of Trans-Neptunian Objects (TNOs) were discovered, confirming the existence of the Kuiper Belt and spurring interest in a mission to the region. This led NASA to instruct the JPL to re-purpose the mission as a Pluto and KBO flyby. However, the mission was scrapped by 2000, owing to budget constraints.
Backlash over the cancellation led NASA’s Science Mission Directorate to create the New Frontiers program which began accepting mission proposals. Stamatios “Tom” Krimigis, head of the Applied Physics Laboratory’s (APL) space division, came together with Alan Stern to form the New Horizons team. Their proposal was selected from a number of submissions, and officially selected for funding by the New Frontiers program in Nov. 2001.
Despite additional squabbles over funding with the Bush administration, renewed pressure from the scientific community allowed the New Horizons team managed to secure their funding by the summer of 2002. With a commitment of $650 million for the next fourteen years, Stern’s team was finally able to start building the spacecraft and its instruments.
Engineers working on the New Horizons spacecraft’s Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument. Credit: NASA
Mission Profile:
New Horizons was planned as a voyage to the only unexplored planet in the Solar System, and was originally slated for launch in January 2006 and arrival at Pluto in 2015. Alan Stern was selected as the mission’s principal investigator, and construction of the spacecraft was handled primarily by the Southwest Research Institute (SwRI) and the Johns Hopkins Applied Physics Laboratory, with various contractor facilities involved in the navigation of the spacecraft.
Meanwhile, the US Naval Observatory (USNO) Flagstaff Station – in conjunction with NASA and JPL – was responsible for performing navigational position data and related celestial frames. Coincidentally, the UNSO Flagstaff station was where the photographic plates that led to the discovery of Pluto’s moon Charon came from.
In addition to its compliment of scientific instruments (listed below), there are several cultural artifacts traveling aboard the spacecraft. These include a collection of 434,738 names stored on a compact disc, a piece of Scaled Composites’s SpaceShipOne, and a flag of the USA, along with other mementos. In addition, about 30 g (1 oz) of Clyde Tombaugh’s ashes are aboard the spacecraft, to commemorate his discovery of Pluto in 1930.
The New Horizons spacecraft takes off on Jan. 19, 2006 from the Kennedy Space Center for its planned close encounter with Pluto. Credit: NIKON/Scott Andrews/NASA
Instrumentation:
The New Horizons science payload consists of seven instruments. They are (in alphabetically order):
Alice: An ultraviolet imaging spectrometer responsible for analyzing composition and structure of Pluto’s atmosphere and looks for atmospheres around Charon and Kuiper Belt Objects (KBOs).
LORRI: (Long Range Reconnaissance Imager) a telescopic camera that obtains encounter data at long distances, maps Pluto’s farside and provides high resolution geologic data.
PEPSSI: (Pluto Energetic Particle Spectrometer Science Investigation) an energetic particle spectrometer which measures the composition and density of plasma (ions) escaping from Pluto’s atmosphere.
Ralph: A visible and infrared imager/spectrometer that provides color, composition and thermal maps.
REX: (Radio Science EXperiment) a device that measures atmospheric composition and temperature; passive radiometer.
SDC: (Student Dust Counter) built and operated by students, this instrument measures the space dust peppering New Horizons during its voyage across the solar system.
SWAP: (Solar Wind Around Pluto) a solar wind and plasma spectrometer that measures atmospheric “escape rate” and observes Pluto’s interaction with solar wind.
The instruments New Horizons will use to characterize Pluto. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
Launch:
Due to a series of weather-related delays, the New Horizons mission launched on January 19th, 2006, two days later than originally scheduled. The spacecraft took off from Cape Canaveral Air Force Station, Florida, at 15:00 EST (19:00 UTC) atop an Atlas V 551 rocket. This was the first launch of this particular rocket configuration, which has a third stage added to increase the heliocentric (escape) speed.
The spacecraft left Earth faster than any spacecraft to date, achieving a launch velocity of 16.5 km/s. It took only nine hours to reach the Moon’s orbit, passing lunar orbit before midnight (EST) on the same day it was launched. It has not, however, broken Voyager 1‘s record – which is currently traveling at 17.145 km/s (61,720 km/h, 38,350 mph) relative to the Sun – for being the fastest spacecraft to leave the Solar System.
Inner Solar System:
Between January and March, 2006, mission controllers guided the probe through a series of trajectory-correction maneuvers (TCMs). During the week of February 20th, 2006, controllers conducted in-flight tests on three of the major on board science instruments. On April 7th, the spacecraft passed the orbit of Mars, moving at roughly 21 km/s (76,000 km/h; 47,000 mph) away from the Sun.
At this point in its journey, the spacecraft had reached a distance of 243 million kilometers from the Sun, and approximately 93.4 million km from Earth. On June 13th, 2006, the New Horizons spacecraft passed the tiny asteroid 132524 APL at a distance of 101,867 km (63,297 mi) when it was closest.
Using the Ralph instrument, New Horizons was able to capture images of the asteroid, estimating to be 2.5 km (1.6 mi) in diameter. The spacecraft also successfully tracked the asteroid from June 10th-12th, 2006, allowing the mission team to test the spacecraft’s ability to track rapidly moving objects.
First images of Pluto taken by New Horizons in September 2006. Credit: NASA
From September 21st-24th, New Horizons managed to capture its first images of Pluto while testing the LORRI instruments. These images, which were taken from a distance of approximately 4,200,000,000 km (2.6×109 mi) or 28.07 AU and released on November 28th, confirmed the spacecraft’s ability to track distant targets.
Outer Solar System:
On September 4th, 2006, New Horizons took its first pictures of Jupiter at a distance of 291 million kilometers (181 million miles). The following January, it conducted more detailed surveys of the system, capturing an infrared image of the moon Callisto, and several black and white images of Jupiter itself.
By February 28th, 2007, at 23:17 EST (03:17, UTC) New Horizons made its closest approach to Europa, at a distance of 2,964,860 km (1,842,278 mi). At 01:53:40 EST (05:43:40 UTC), the spacecraft made its flyby of Jupiter, at a distance of 2.3 million km (1.4 million mi) and received a gravity assist.
The Jupiter flyby increased New Horizons‘ speed by 4 km/s (14,000 km/h; 9,000 mph), accelerating the probe to a velocity of 23 km/s (83,000 km/h; 51,000 mph) relative to the Sun and shortening its voyage to Pluto by three years.
The encounter with Jupiter not only provided NASA with the opportunity to photograph the planet using the latest equipment, it also served as a dress rehearsal for the spacecraft’s encounter with Pluto. As well as testing the imaging instruments, it also allowed the mission team to test the communications link and the spacecraft’s memory buffer.
Black and white image of Jupiter viewed by LORRI in January 2007. Credit: NASA/John Hopkins University Applied Physics Laboratory/Southwest Research Institute
One of the main goals during the Jupiter encounter was observing its atmospheric conditions and analyzing the structure and composition of its clouds. Heat-induced lightning strikes in the polar regions and evidence of violent storm activity were both observed. In addition, the Little Red Spot, was imaged from up close for the first time. The New Horizons spacecraft also took detailed images of Jupiter’s faint ring system. Traveling through Jupiter’s magnetosphere, the spacecraft also managed to collect valuable particle readings.
The flyby of the Jovian systems also gave scientists the opportunity to examine the structure and motion of Io’s famous lava plumes. New Horizons measured the plumes coming from the Tvashtar volcano, which reached an altitude of up to 330 km from the surface, while infrared signatures confirmed the presence of 36 more volcanoes on the moon.
Callisto’s surface was also analyzed with LEISA, revealing how lighting and viewing conditions affect infrared spectrum readings of its surface water ice. Data gathered on minor moons such as Amalthea also allowed NASA scientists to refine their orbit solutions.
After passing Jupiter, New Horizons spent most of its journey towards Pluto in hibernation mode. During this time, New Horizons crossed the orbit of Saturn (June 8, 2008) and Uranus on (March 18, 2011). In June 2014, the spacecraft emerged from hibernation and the team began conducting instrument calibrations and a course correction,. By August 24th, 2014, it crossed Neptune’s orbit on its way to Pluto.
New Horizons Long Range Reconnaissance Imager (LORRI) captured these two images of Jupiter’s outermost large moon, Callisto, during its flyby in February 2007. Credit: NASA/JPL
Rendezvous with Pluto:
Distant-encounter operations at Pluto began on January 4th, 2015. Between January 25th to 31st, the approaching probe took several images of Pluto, which were released by NASA on February 12th. These photos, which were taken at a distance of more than 203,000,000 km (126,000,000 mi) showed Pluto and its largest moon, Charon.
Investigators compiled a series of images of the moons Nix and Hydra taken from January 27th through February 8th, 2015, beginning at a range of 201,000,000 km (125,000,000 mi), while Kerberos and Styx were captured by photos taken on April 25.
On July 4th, 2015, NASA lost contact with New Horizons after it experienced a software anomaly and went into safe mode. On the following day, NASA announced that they had determined it to be the result of a timing flaw in a command sequence. By July 6th, the glitch had been fixed and the probe had exited safe mode and began making its approach.
The New Horizons spacecraft made its closest approach to Pluto at 07:49:57 EDT (11:49:57 UTC) on July 14th, 2015, and then Charon at 08:03:50 EDT (12:03:50 UTC). Telemetries confirming a successful flyby and a healthy spacecraft reached Earth on 20:52:37 EDT (00:52:37 UTC).
During the flyby, the probe captured the clearest pictures of Pluto to date, and full analyses of the data obtained is expected to take years to process. The spacecraft is currently traveling at a speed of 14.52 km/s (9.02 mi/s) relative to the Sun and at 13.77 km/s (8.56 mi/s) relative to Pluto.
Full trajectory of New Horizons space probe (sideview). Credit: pluto.jhuapl.edu
Future Objectives:
With its flyby of Pluto now complete, the New Horizons probe is now on its way towards the Kuiper Belt. The goal here is to study one or two other Kuiper Belt Objects, provided suitable KBOs are close to New Horizons‘ flight path.
Three objects have since been selected as potential targets, which were provisionally designated PT1 (“potential target 1”), PT2 and PT3 by the New Horizons team. These have since been re-designated as 2014 MU69 (PT1), 2014 OS393 (PT2), and 2014 PN70 (PT3).
All of these objects have an estimated diameter of 30–55 km, are too small to be seen by ground telescopes, and are 43–44 AU from the Sun, which would put the encounters in the 2018–2019 period. All are members of the “cold” (low-inclination, low-eccentricity) classical Kuiper Belt, and thus very different from Pluto.
Even though it was launched far faster than any outward probe before it, New Horizons will never overtake either Voyager 1 or Voyager 2 as the most distant human-made object from Earth. But then again, it doesn’t need to, given that what it was sent out to study all lies closer to home.
What’s more, the probe has provided astronomers with extensive and updated data on many of planets and moons in our Solar System – not the least of which are the Jovian and Plutonian systems. And last, but certainly not least, New Horizons is the first spacecraft to have it made it out to such a distance since the Voyager program.
And so we say so long and good luck to New Horizons, not to mention thanks for providing us with the best images of Pluto anyone has ever seen! We can only hope she fares well as she makes its way into the Kuiper Belt and advances our knowledge of the outer Solar System even farther.
