Elizabeth Howell is the senior writer at Universe Today. She also works for Space.com, Space Exploration Network, the NASA Lunar Science Institute, NASA Astrobiology Magazine and LiveScience, among others. Career highlights include watching three shuttle launches, and going on a two-week simulated Mars expedition in rural Utah. You can follow her on Twitter @howellspace or contact her at her website.
This artist’s impression of a supernova shows the layers of gas ejected prior to the final deathly explosion of a massive star. Credit: NASA/Swift/Skyworks Digital/Dana Berry
How can an exploding star appear far brighter than expected? This question vexed astronomers since the discovery of PS1-10afx, supernova that was about 30 times more luminous than other Type 1A supernovas. Astronomers have just confirmed in Science that it was likely due to well-known illusion in space.
The mirage is called a gravitational lens that happens when a huge object in the foreground (like a galaxy) bends the light of an object in the background. Astronomers use this trick all the time to spy on galaxies and even to map dark matter, the mysterious substance believed to make up most of the universe.
Check out some spectacular images below of the phenomenon in action.
Canada-France-Hawaii-Telescope (CFHT) image of the field before the supernova PS1-10afx. (Credit: Kavli IPMU / CFHT)Dark matter in the Bullet Cluster. Otherwise invisible to telescopic views, the dark matter was mapped by observations of gravitational lensing of background galaxies. Credit: X-ray: NASA/CXC/CfA/ M.Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/ D.Clowe et al. Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.;Hubble Space Telescope image shows Einstein ring of one of the SLACS gravitational lenses, with the lensed background galaxy enhanced in blue. A. Bolton (UH/IfA) for SLACS and NASA/ESA.The image is made from HST data and shows the four lensed images of the dusty red quasar, connected by a gravitational arc of the quasar host galaxy. The lensing galaxy is seen in the centre, between the four lensed images. Credit: John McKean/HST Archive dataThe HST WFPC2 image of gravitational lensing in the galaxy cluster Abell 2218, indicating the presence of large amount of dark matter (credit Andrew Fruchter at STScI).A picture of the object J1000+0221, which demonstrates the most distant gravitational lens ever discovered. This Hubble picture shows a normal galaxy’s center region (the glow in the picture), but the object is also aligned with a younger, star-creating galaxy that is in behind. The object in the foreground pulls light from the background galaxy with gravity — making rings of pictures. Credit: NASA/ESA/A. van der Wel
A view of "Pongsats" containing student experiments in a high-altitude balloon that goes to about 100,000 feet. Credit: John Powell / JP Aerospace / Kickstarter
Spring is a time of treasures in eggs — think about the Easter weekend that just passed, for example, or the number of chicks hatching in farms across the world. That’s also true of “near-space” exploration. A project called PongSats has sent thousands of tiny experiments into space, and is ready to send up another batch this coming September.
The concept is simple for the students participating — slice open a ball, put something inside you want to test at high experiments, then repackage it and decorate it for the big trip up. The balloons will soar to about 19 miles (30 kilometers), which is well below the Karman Line of 62 miles or 100 kilometers that marks the edge of space. Don’t discount that, however — you will still see black skies and the curvature of the Earth from that altitude.
Anyway, about the PongSats. A Kickstarter campaign (closing in five days) is asking for money to shoot these balls into the atmosphere, for science. While it’s aimed at young students, anybody can get an experiment on that balloon, the founder says.
Close-up view of the “PongSats” bound for high altitudes and carrying student experiments. Credit: John Powell / JP Aerospace / Kickstarter
“My favorite is the marshmallow. You put a marshmallow inside the ping pong ball. At 100,000 feet the marshmallow puffs up completely filling the ball. Then it freeze dries. The student gets to hold in her hand the direct results of traveling [to] the top of the atmosphere,” wrote John Powell, the founder of the project.
PongSat has already been through one successful Kickstarter round, when in 2012 the concept received $12,466 — a 138% increase over its $9,000 goal. That money was slated to send 1,000 student experiments into space. To date, the company has sent over 14,000 experiments aloft with only three losses — a 0.02% failure rate.
“However, the risk for a complete vehicle loss does exist,” Powell acknowledged, but said that after 164 flights, they “have gotten pretty good at it.” Any PongSats lost in flight will be flown again, no questions asked.
Powell’s project is part of a larger trend of sending items high in the sky — sometime for scientific purposes, and sometimes for other reasons. A Lego man and teddy bears are among those that made the journey.
Toronto Teens Launch Lego Main In Space to the Stratosphere – Jan 2012. Stunning space imagery was captured by Canadian teenagers Mathew Ho and Asad Muhammad when they lofted a tiny ‘Lego Man in Space’ astronaut to an altitude of 16 miles (25 kilometers) using on a helium filled weather balloon. Credit: Mathew Ho and Asad Muhammad. Watch the YouTube below
This 2012 project was called “Lego Man In Space” (although of course, balloons fly high in the atmosphere and well below the Karman Line at 62 miles or 100 kilometers above the surface.) Two teenagers from Toronto, Canada — Mathew Ho and Asad Muhammad — launched the weather balloon from a local field and captured some stunning video and pictures along the way.
“Upon launch we were very relieved. But we had a lot of anxiety on launch day because there were high winds when we were going up after all the hard work,” said Ho in a studio interview at the time on Canadian news channel CTV.
“We were also scared because now we would have to retrieve it back after it came down,” Asad said.
“We had no idea it would capture photos like that and would be so good,” said Ho. “We were blown away when we saw them back home.”
Two teddy bears sent high in the atmosphere by students in Cambridge, England in 2008. Credit: CU Spaceflight
It’s a teddy bear party in the sky! A group of English 11-13-year-olds designed the spacesuits on these stuffed animals, which were sent aloft to 30,085 meters (101,066 feet) in 2008. While at first glance the purpose looks esoteric, the goal was to test which spacesuit materials best insulated against the -53 degrees Celsius (-63 Fahrenheit) temperatures the teddies endured.
Student-run Cambridge University Spaceflight helmed the project along with a science club and community college.
“We want to offer young people the opportunity to get involved in the space industry whilst still at school and show that real-life science is something that is open to everybody,” stated Iain Waugh, then chief aeronautical engineer of CU Spaceflight.
“High altitude balloon flights are a fantastic way of encouraging interest in science. They are easy to understand, and produce amazing results,” added Daniel Strange, treasurer of CU Spaceflight.
Sometimes the aim is more artistic, such as this German project that created a beautiful video showing the views from more than 100,000 feet (30,480 meters). You can see in the video above the careful preparations that go into launch, plus some of the side benefits — such as getting to make funny voices using helium! But it was the engineering challenges that attracted these students, they wrote Universe Today in 2011.
“Our challenge was to survive ambient air pressures as low as 1/100th of an atmosphere, temperatures as low as -60°C and finally to locate and recover the camera,” Tobias Lohf wrote . “We had a HD-Cam, GPS tracker and a heating pad on board, and all the construction had a total weight of about 1kg.”
The students emphasized that it doesn’t take a big budget or a lot of engineering to get that high. “All you need need is a camera, weather balloon and duct tape,” they said.
A collage of Saturn (bottom left) and some of its moons: Titan, Enceladus, Dione, Rhea and Helene. Credit: NASA/JPL/Space Science Institute
Saturn is well known for being a gas giant, and for its impressive ring system. But would it surprise you to know that this planet also has the second-most moons in the Solar System, second only to Jupiter? Yes, Saturn has at least 150 moons and moonlets in total, though only 62 have confirmed orbits and only 53 have been given official names.
Most of these moons are small, icy bodies that are little more than parts of its impressive ring system. In fact, 34 of the moons that have been named are less than 10 km in diameter while another 14 are 10 to 50 km in diameter. However, some of its inner and outer moons are among the largest and most dramatic in the Solar System, measuring between 250 and 5000 km in diameter and housing some of the greatest mysteries in the Solar System.
Saturn’s moons have such a variety of environments between them that you’d be forgiven for wanting to spend an entire mission just looking at its satellites. From the orange and hazy Titan to the icy plumes emanating from Enceladus, studying Saturn’s system gives us plenty of things to think about. Not only that, the moon discoveries keep on coming. As of April 2014, there are 62 known satellites of Saturn (excluding its spectacular rings, of course). Fifty-three of those worlds are named.
The Cassini spacecraft observes three of Saturn’s moons set against the darkened night side of the planet. Credit: NASA/JPL/Space Science Institute
Discovery and Naming
Prior to the invention of telescopic photography, eight of Saturn’s moons were observed using simple telescopes. The first to be discovered was Titan, Saturn’s largest moon, which was observed by Christiaan Huygens in 1655 using a telescope of his own design. Between 1671 and 1684, Giovanni Domenico Cassini discovered the moons of Tethys, Dione, Rhea, and Iapetus – which he collectively named the “Sider Lodoicea” (Latin for “Louisian Stars”, after King Louis XIV of France).
n 1789, William Herschel discovered Mimas and Enceladus, while father-and-son astronomers W.C Bond and G.P. Bond discovered Hyperion in 1848 – which was independently discovered by William Lassell that same year. By the end of the 19th century, the invention of long-exposure photographic plates allowed for the discovery of more moons – the first of which Phoebe, observed in 1899 by W.H. Pickering.
In 1966, the tenth satellite of Saturn was discovered by French astronomer Audouin Dollfus, which was later named Janus. A few years later, it was realized that his observations could only be explained if another satellite had been present with an orbit similar to that of Janus. This eleventh moon was later named Epimetheus, which shares the same orbit as Janus and is the only known co-orbital in the Solar System.
Collage of Saturn and its largest moons. Credit: NASA/JPL/SSI
By 1980, three additional moons were discovered and later confirmed by the Voyager probes. They were the trojan moons (see below) of Helene (which orbits Dione) as well as Telesto and Calypso (which orbit Tethys).
The study of the outer planets has since been revolutionized by the use of unmanned space probes. This began with the arrival of the Voyager spacecraft to the Cronian system in 1980-81, which resulted in the discovery of three additional moons – Atlas, Prometheus, and Pandora – bringing the total to 17. By 1990, archived images also revealed the existence of Pan.
This was followed by the Cassini-Huygens mission, which arrived at Saturn in the summer of 2004. Initially, Cassini discovered three small inner moons, including Methone and Pallene between Mimas and Enceladus, as well as the second Lagrangian moon of Dione – Polydeuces. In November of 2004,Cassini scientists announced that several more moons must be orbiting within Saturn’s rings. From this data, multiple moonlets and the moons of Daphnis and Anthe have been confirmed.
The study of Saturn’s moons has also been aided by the introduction of digital charge-coupled devices, which replaced photographic plates by the end of the 20th century. Because of this, ground-based telescopes have begun to discover several new irregular moons around Saturn. In 2000, three medium-sized telescopes found thirteen new moons with eccentric orbits that were of considerable distance from the planet.
The moons of Saturn, from left to right: Mimas, Enceladus, Tethys, Dione, Rhea; Titan in the background; Iapetus (top) and Hyperion (bottom). Credit: NASA/JPL/Space Science Institute
In 2005, astronomers using the Mauna Kea Observatory announced the discovery of twelve more small outer moons. In 2006, astronomers using Japan’s Subaru Telescope at Mauna Kea reported the discovery of nine more irregular moons. In April of 2007, Tarqeq (S/2007 S 1) was announced, and in May of that same year, S/2007 S 2 and S/2007 S 3 were reported.
The modern names of Saturn’s moons were suggested by John Herschel (William Herschel’s son) in 1847. In keeping with the nomenclature of the other planets, he proposed they be named after mythological figures associated with the Roman god of agriculture and harvest – Saturn, the equivalent of the Greek Cronus. In particular, the seven known satellites were named after Titans, Titanesses and Giants – the brothers and sisters of Cronus.
In 1848, Lassell proposed that the eighth satellite of Saturn be named Hyperion after another Titan. When in the 20th century, the names of Titans were exhausted, the moons were named after different characters of the Greco-Roman mythology, or giants from other mythologies. All the irregular moons (except Phoebe) are named after Inuit and Gallic gods and Norse ice giants.
Saturn’s Inner Large Moons
Saturn’s moons are grouped based on their size, orbits, and proximity to Saturn. The innermost moons and regular moons all have small orbital inclinations and eccentricities and prograde orbits. Meanwhile, the irregular moons in the outermost regions have orbital radii of millions of kilometers, orbital periods lasting several years, and move in retrograde orbits.
Saturn’s moon of Enceladus. Credit: NASA/JPL/Space Science Institute
Saturn’s Inner Large Moons, which orbit within the E Ring (see below), include the larger satellites Mimas, Enceladus, Tethys, and Dione. These moons are all composed primarily of water ice and are believed to be differentiated into a rocky core and an icy mantle and crust. With a diameter of 396 km and a mass of 0.4×1020 kg, Mimas is the smallest and least massive of these moons. It is ovoid in shape and orbits Saturn at a distance of 185,539 km with an orbital period of 0.9 days.
Some people jokingly call Mimas the “Death Star” moon because of the crater on its surface that resembles the machine from the Star Wars universe. The 140 km (88 mi) Herschel Crater is about a third the diameter of the moon itself and could have created fractures (chasmata) on the moon’s opposing side. There are in fact craters throughout the moon’s small surface, making it among the most pockmarked in the Solar System.
Enceladus, meanwhile, has a diameter of 504 km, a mass of 1.1×1020 kg, and is spherical in shape. It orbits Saturn at a distance of 237,948 km and takes 1.4 days to complete a single orbit. Though it is one of the smaller spherical moons, it is the only Cronian moon that is endogenously active – and one of the smallest known bodies in the Solar System that is geologically active. This results in features like the famous “tiger stripes” – a series of continuous, ridged, slightly curved, and roughly parallel faults within the moon’s southern polar latitudes.
Large geysers have also been observed in the southern polar region that periodically releases plumes of water ice, gas, and dust which replenish Saturn’s E ring. These jets are one of several indications that Enceladus has liquid water beneath its icy crust, where geothermal processes release enough heat to maintain a warm water ocean closer to its core.
Dione’s trailing hemisphere, showing the patches of “whispy terrain”. Credit: NASA/JPL
The moon has at least five different kinds of terrain, a “young” geological surface of less than 100 million years. With a geometrical albedo of more than 140%, which is due to it being composed largely of water ice, Enceladus is one of the brightest known objects in the Solar System.
At 1066 km in diameter, Tethys is the second-largest of Saturn’s inner moons and the 16th-largest moon in the Solar System. The majority of its surface is made up of heavily cratered and hilly terrain and a smaller and smoother plains region. Its most prominent features are the large impact crater of Odysseus, which measures 400 km in diameter, and a vast canyon system named Ithaca Chasma – which is concentric with Odysseus and measures 100 km wide, 3 to 5 km deep, and 2,000 km long.
With a diameter and mass of 1,123 km and 11×1020 kg, Dione is the largest inner moon of Saturn. The majority of Dione’s surface is heavily cratered old terrain, with craters that measure up to 250 km in diameter. However, the moon is also covered with an extensive network of troughs and lineaments which indicate that in the past it had global tectonic activity.
It’s covered in canyons, crackings, craters, and is coated from dust in the E-ring that originally came from Enceladus. The location of this dust has led astronomers to theorize that the moon was spun about 180 degrees from its original disposition in the past, perhaps due to a large impact.
Saturn’s Large Outer Moons:
The Large Outer Moons, which orbit outside of the Saturn’s E Ring, are similar in composition to the Inner Moons – i.e. composed primarily of water ice, and rock. Of these, Rhea is the second-largest – measuring 1,527 km in diameter and 23×1020 kg in mass – and the ninth-largest moon in the Solar System. With an orbital radius of 527,108 km, it is the fifth-most distant of the larger moons and takes 4.5 days to complete an orbit.
Views of Saturn’s moon Rhea. Credit: NASA/JPL/Space Science Institute
Like other Cronian satellites, Rhea has a rather heavily cratered surface and a few large fractures on its trailing hemisphere. Rhea also has two very large impact basins on its anti-Saturnian hemisphere – the Tirawa crater (similar to Odysseus on Tethys) and the Inktomi crater – which measure about 400 and 50 km across, respectively.
Rhea has at least two major sections, the first being bright craters with craters larger than 40 km (25 miles), and a second section with smaller craters. The difference in these features is believed to be evidence of a major resurfacing event at some time in Rhea’s past.
At 5150 km in diameter and 1,350×1020 kg in mass, Titan is Saturn’s largest moon and comprises more than 96% of the mass in orbit around the planet. Titan is also the only large moon to have its own atmosphere, which is cold, dense, and composed primarily of nitrogen with a small fraction of methane. Scientists have also noted the presence of polycyclic aromatic hydrocarbons in the upper atmosphere, as well as methane ice crystals.
The surface of Titan, which is difficult to observe due to persistent atmospheric haze, shows only a few impact craters, evidence of cryovolcanoes, and longitudinal dune fields that were apparently shaped by tidal winds. Titan is also the only body in the Solar System besides Earth with bodies of liquid on its surface, in the form of methane–ethane lakes in Titan’s north and south polar regions.
Image of Titan’s taken by the Cassini spacecraft, showing light passing through the periphery of the moon’s atmosphere. Credit: NASA/JPL-Caltech/Space Science Institute
Titan is also distinguished for being the only Cronian moon that has ever had a probe land on it. This was the Huygens lander, which was carried to the hazy world by the Cassini spacecraft. Titan’s “Earth-like processes” and thick atmosphere are among the things that make this world stand out to scientists, which include its ethane and methane rains from the atmosphere and flows on the surface.
With an orbital distance of 1,221,870 km, it is the second-farthest large moon from Saturn and completes a single orbit every 16 days. Like Europa and Ganymede, it is believed that Titan has a subsurface ocean made of water mixed with ammonia, which can erupt to the surface of the moon and lead to cryovolcanism.
Hyperion is Titan’s immediate neighbor. At an average diameter of about 270 km, it is smaller and lighter than Mimas. It is also irregularly shaped and quite odd in composition. Essentially, the moon is an ovoid, tan-colored body with an extremely porous surface (which resembles a sponge). The surface of Hyperion is covered with numerous impact craters, most of which are 2 to 10 km in diameter. It also has a highly unpredictable rotation, with no well-defined poles or equator.
At 1,470 km in diameter and 18×1020 kg in mass, Iapetus is the third-largest of Saturn’s large moons. And at a distance of 3,560,820 km from Saturn, it is the most distant of the large moons and takes 79 days to complete a single orbit. Due to its unusual color and composition – its leading hemisphere is dark and black whereas its trailing hemisphere is much brighter – it is often called the “yin and yang” of Saturn’s moons.
The two sides of Iapetus, Saturn’s “yin-yang moon”. Credit: NASA/JPL
Saturn’s Irregular Moons:
Beyond these larger moons are Saturn’s Irregular Moons. These satellites are small, have large radii, are inclined, have mostly retrograde orbits, and are believed to have been acquired by Saturn’s gravity. These moons are made up of three basic groups – the Inuit Group, the Gallic Group, and the Norse Group.
The Inuit Group consists of five irregular moons that are all named from Inuit mythology – Ijiraq, Kiviuq, Paaliaq, Siarnaq, and Tarqeq. All have prograde orbits that range from 11.1 to 17.9 million km, and from 7 to 40 km in diameter. They are all similar in appearance (reddish in hue) and have orbital inclinations of between 45 and 50°.
The Gallic group consists of four prograde outer moons that are named after characters in Gallic mythology – Albiorix, Bebhionn, Erriapus, and Tarvos. Here too, the moons are similar in appearance and have orbits that range from 16 to 19 million km. Their inclinations are in the 35°-40° range, their eccentricities around 0.53, and they range in size from 6 to 32 km.
Last, there is the Norse group, which consists of 29 retrograde outer moons that take their names from Norse mythology. These satellites range in size from 6 to 18 km, their distances from 12 and 24 million km, their inclinations between 136° and 175°, and their eccentricities between 0.13 and 0.77. This group is also sometimes referred to as the Phoebe group, due to the presence of a single larger moon in the group – which measures 240 km in diameter. The second-largest, Ymir, measures 18 km across.
Saturn’s rings and moons Credit: NASA
Within the Inner and Outer Large Moons, there are also those belonging to the Alkyonide group. These moons – Methone, Anthe, and Pallene – are named after the Alkyonides of Greek mythology, are located between the orbits of Mimas and Enceladus, and are among the smallest moons around Saturn. Some of the larger moons even have moons of their own, which are known as Trojan moons. For instance, Tethys has two trojans – Telesto and Calypso, while Dione has Helene and Polydeuces.
Moon Formation:
It is thought that Saturn’s moon of Titan, its mid-sized moons and rings developed in a way that is closer to the Galilean moons of Jupiter. In short, this would mean that the regular moons formed from a circumplanetary disc, a ring of accreting gas, and solid debris similar to a protoplanetary disc. Meanwhile, the outer, irregular moons are believed to have been objects that were captured by Saturn’s gravity and remained in distant orbits.
However, there are some variations to this theory. In one alternative scenario, two Titan-sized moons were formed from an accretion disc around Saturn; the second one eventually broke up to produce the rings and inner mid-sized moons. In another, two large moons fused together to form Titan, and the collision scattered icy debris that formed to create the mid-sized moons.
However, the mechanics of how the moon formed remains a mystery for the time being. With additional missions mounted to study the atmospheres, compositions, and surfaces of these moons, we may begin to understand where they truly came from.
Much like Jupiter, and all the other gas giants, Saturn’s system of satellites is extensive as it is impressive. In addition to the larger moons that are believed to have formed from a massive debris field that once orbited it, it also has countless smaller satellites that were captured by its gravitational field over the course of billions of years. One can only imagine how many more remain to be found orbiting the ringed giant.
Steve Swanson, commander of Expedition 40, during a spacewalk on 2007 shuttle mission STS-117. Credit: NASA
UPDATE, 11:42 a.m. EDT: Rick Mastracchio and Steve Swanson finished their spacewalk in just 1 hour and 36 minutes, nearly an hour faster than what NASA budgeted for. Early tests show the replacement computer is working well, providing backup once again for the robotics, solar arrays and other systems on station.
Can two astronauts fix a broken computer quickly on the International Space Station, preventing possible problems with the solar arrays and robotics? Watch live (above) to find out.
The NASA spacewalk involving Rick Mastracchio and Steve Swanson is scheduled to start today (April 23) at 9:20 a.m. EDT (1:20 p.m. UTC), with coverage starting around 8:30 a.m. EDT (12:30 p.m. UTC). The spacewalk is scheduled to last 2.5 hours. Bear in mind that the times could change as circumstances arise.
The computer, also called a multiplexer/demultiplexer (MDM), failed for unknown reasons a couple of weeks ago. While the primary computer is working perfectly and the crew is in no danger, things get more risky if the primary computer also breaks. That’s why NASA worked to get the spacewalkers outside as quickly as possible. You can see a full briefing of the rationale here.
As a note, all non-urgent spacewalks have been suspended because NASA is still working on addressing the recommendations given after a life-threatening water leak took place in a NASA spacesuit last summer. Urgent spacewalks can still go ahead because the agency has implemented safety measures such as snorkels and helmet absorption pads in case of another leak.
That said, in the months since NASA has traced the problem to contamination in a filter in the fan pump separator. After replacing the separator, the leaky spacesuit was used during two contingency spacewalks in December with no water problems at all.
In our Solar System, astronomers often divide the planets into two groups — the inner planets and the outer planets. The inner planets are closer to the Sun and are smaller and rockier. The outer planets are further away, larger and made up mostly of gas.
The inner planets (in order of distance from the sun, closest to furthest) are Mercury, Venus, Earth and Mars. After an asteroid belt comes the outer planets, Jupiter, Saturn, Uranus and Neptune. The interesting thing is, in some other planetary systems discovered, the gas giants are actually quite close to the sun.
This makes predicting how our Solar System formed an interesting exercise for astronomers. Conventional wisdom is that the young Sun blew the gases into the outer fringes of the Solar System and that is why there are such large gas giants there. However, some extrasolar systems have “hot Jupiters” that orbit close to their Sun.
The Inner Planets:
The four inner planets are called terrestrial planets because their surfaces are solid (and, as the name implies, somewhat similar to Earth — although the term can be misleading because each of the four has vastly different environments). They’re made up mostly of heavy metals such as iron and nickel, and have either no moons or few moons. Below are brief descriptions of each of these planets based on this information from NASA.
Mercury: Mercury is the smallest planet in our Solar System and also the closest. It rotates slowly (59 Earth days) relative to the time it takes to rotate around the sun (88 days). The planet has no moons, but has a tenuous atmosphere (exosphere) containing oxygen, sodium, hydrogen, helium and potassium. The NASA MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft is currently orbiting the planet.
The terrestrial planets of our Solar System at approximately relative sizes. From left, Mercury, Venus, Earth and Mars. Credit: Lunar and Planetary Institute
Venus: Venus was once considered a twin planet to Earth, until astronomers discovered its surface is at a lead-melting temperature of 900 degrees Fahrenheit (480 degrees Celsius). The planet is also a slow rotator, with a 243-day long Venusian day and an orbit around the sun at 225 days. Its atmosphere is thick and contains carbon dioxide and nitrogen. The planet has no rings or moons and is currently being visited by the European Space Agency’s Venus Express spacecraft.
Earth: Earth is the only planet with life as we know it, but astronomers have found some nearly Earth-sized planets outside of our solar system in what could be habitable regions of their respective stars. It contains an atmosphere of nitrogen and oxygen, and has one moon and no rings. Many spacecraft circle our planet to provide telecommunications, weather information and other services.
Mars: Mars is a planet under intense study because it shows signs of liquid water flowing on its surface in the ancient past. Today, however, its atmosphere is a wispy mix of carbon dioxide, nitrogen and argon. It has two tiny moons (Phobos and Deimos) and no rings. A Mars day is slightly longer than 24 Earth hours and it takes the planet about 687 Earth days to circle the Sun. There’s a small fleet of orbiters and rovers at Mars right now, including the large NASA Curiosity rover that landed in 2012.
The outer planets of our Solar System at approximately relative sizes. From left, Jupiter, Saturn, Uranus and Neptune. Credit: Lunar and Planetary Institute
The Outer Planets:
The outer planets (sometimes called Jovian planets or gas giants) are huge planets swaddled in gas. They all have rings and all of plenty of moons each. Despite their size, only two of them are visible without telescopes: Jupiter and Saturn. Uranus and Neptune were the first planets discovered since antiquity, and showed astronomers the solar system was bigger than previously thought. Below are brief descriptions of each of these planets based on this information from NASA.
Jupiter: Jupiter is the largest planet in our Solar System and spins very rapidly (10 Earth hours) relative to its orbit of the sun (12 Earth years). Its thick atmosphere is mostly made up of hydrogen and helium, perhaps surrounding a terrestrial core that is about Earth’s size. The planet has dozens of moons, some faint rings and a Great Red Spot — a raging storm happening for the past 400 years at least (since we were able to view it through telescopes). NASA’s Juno spacecraft is en route and will visit there in 2016.
Saturn: Saturn is best known for its prominent ring system — seven known rings with well-defined divisions and gaps between them. How the rings got there is one subject under investigation. It also has dozens of moons. Its atmosphere is mostly hydrogen and helium, and it also rotates quickly (10.7 Earth hours) relative to its time to circle the Sun (29 Earth years). Saturn is currently being visited by the Cassini spacecraft, which will fly closer to the planet’s rings in the coming years.
Near-infrared views of Uranus reveal its otherwise faint ring system, highlighting the extent to which it is tilted. Credit: Lawrence Sromovsky, (Univ. Wisconsin-Madison), Keck Observatory.
Uranus: Uranus was first discovered by William Herschel in 1781. The planet’s day takes about 17 Earth hours and one orbit around the Sun takes 84 Earth years. Its mass contains water, methane, ammonia, hydrogen and helium surrounding a rocky core. It has dozens of moons and a faint ring system. There are no spacecraft slated to visit Uranus right now; the last visitor was Voyager 2 in 1986.
Neptune: Neptune is a distant planet that contains water, ammmonia, methane, hydrogen and helium and a possible Earth-sized core. It has more than a dozen moons and six rings. The only spacecraft to ever visit it was NASA’s Voyager 2 in 1989.
To learn more about the planets and missions, check out these links:
A radar view of Venus taken by the Magellan spacecraft, with some gaps filled in by the Pioneer Venus orbiter. Credit: NASA/JPL
There are dozens upon dozens of moons in the Solar System, ranging from airless worlds like Earth’s Moon to those with an atmosphere (most notably, Saturn’s Titan). Jupiter and Saturn have many moons each, and even Mars has a couple of small asteroid-like ones. But what about Venus, the planet that for a while, astronomers thought about as Earth’s twin?
The answer is no moons at all. That’s right, Venus (and the planet Mercury) are the only two planets that don’t have a single natural moon orbiting them. Figuring out why is one question keeping astronomers busy as they study the Solar System.
Astronomers have three explanations about how planets get a moon or moons. Perhaps the moon was “captured” as it drifted by the planet, which is what some scientists think happened to Phobos and Deimos (near Mars). Maybe an object smashed into the planet and the fragments eventually coalesced into a moon, which is the leading theory for how Earth’s Moon came together. Or maybe moons arose from general accretion of matter as the solar system was formed, similar to how planets came together.
The International Space Station captured as it passed in front of the Moon on Dec. 6, 2013, as seen from Puerto Rico. Credit and copyright: Juan Gonzalez-Alicea.
Considering the amount of stuff flying around the Solar System early in its history, it’s quite surprising to some astronomers that Venus does not have a moon today. Perhaps, though, it had one in the distant past. In 2006, California Institute of Technology researchers Alex Alemi and David Stevenson presented at the American Astronomical Society’s division of planetary sciences meeting and said Venus could have been smacked by a large rock at least twice. (You can read the abstract here.)
“Most likely, Venus was slammed early on and gained a moon from the resulting debris. The satellite slowly spiraled away from the planet, due to tidal interactions, much the way our Moon is still slowly creeping away from Earth,” Sky and Telescope wrote of the research.
“However, after only about 10 million years Venus suffered another tremendous blow, according to the models. The second impact was opposite from the first in that it ‘reversed the planet’s spin,’ says Alemi. Venus’s new direction of rotation caused the body of the planet to absorb the moon’s orbital energy via tides, rather than adding to the moon’s orbital energy as before. So the moon spiraled inward until it collided and merged with Venus in a dramatic, fatal encounter.”
Venus as photographed by the Pioneer spacecraft in 1978. Some exoplanets may suffer the same fate as this scorched world. Credit: NASA/JPL/Caltech
There could be other explanations as well, however, which is part of why astronomers are so interested in revisiting this world. Figuring out the answer could teach us more about the solar system’s formation.
Artist's impression of the planets in our solar system, along with the Sun (at bottom). Credit: NASA
The eight planets in our solar system each occupy their own orbits around the Sun. They orbit the star in ellipses, which means their distance to the sun varies depending on where they are in their orbits. When they get closest to the Sun, it’s called perihelion, and when it’s farthest away, it’s called aphelion.
So to talk about how far the planets are from the sun is a difficult question, not only because their distances constantly change, but also because the spans are so immense — making it hard for a human to grasp. For this reason, astronomers often use a term called astronomical unit, representing the distance from the Earth to the Sun.
The table below (first created by Universe Today founder Fraser Cain in 2008) shows all the planets and their distance to the Sun, as well as how close these planets get to Earth.
Mercury:
Closest: 46 million km / 29 million miles (.307 AU)
Farthest: 70 million km / 43 million miles (.466 AU)
Average: 57 million km / 35 million miles (.387 AU)
Closest to Mercury from Earth: 77.3 million km / 48 million miles
Venus:
Closest: 107 million km / 66 million miles (.718 AU)
Farthest: 109 million km / 68 million miles (.728 AU)
Average: 108 million km / 67 million miles (.722 AU)
Closest to Venus from Earth: 40 million km / 25 million miles
The planet Venus, as imaged by the Magellan 10 mission. Credit: NASA/JPL
Earth:
Closest: 147 million km / 91 million miles (.98 AU)
Farthest: 152 million km / 94 million miles (1.01 AU)
Average: 150 million km / 93 million miles (1 AU)
Mars:
Closest: 205 million km / 127 million miles (1.38 AU)
Farthest: 249 million km / 155 million miles (1.66 AU)
Average: 228 million km / 142 million miles (1.52 AU)
Closest to Mars from Earth: 55 million km / 34 million miles
Jupiter:
Closest: 741 million km /460 million miles (4.95 AU)
Farthest: 817 million km / 508 million miles (5.46 AU)
Average: 779 million km / 484 million miles (5.20 AU)
Closest to Jupiter from Earth: 588 million km / 346 million miles
Artist’s impression of Jupiter and Io. Credit: NASA/JPL
Saturn:
Closest: 1.35 billion km / 839 million miles (9.05 AU)
Farthest: 1.51 billion km / 938 million miles (10.12 AU)
Average: 1.43 billion km / 889 million miles (9.58 AU)
Closest to Saturn from Earth: 1.2 billion km /746 million miles
Uranus:
Closest: 2.75 billion km / 1.71 billion miles (18.4 AU)
Farthest: 3.00 billion km / 1.86 billion miles (20.1 AU)
Average: 2.88 billion km / 1.79 billion miles (19.2 AU)
Closest to Uranus from Earth: 2.57 billion km / 1.6 billion miles
Neptune:
Closest: 4.45 billion km /2.77 billion miles (29.8 AU)
Farthest: 4.55 billion km / 2.83 billion miles (30.4 AU)
Average: 4.50 billion km / 2.8 billion miles (30.1 AU)
Closest to Neptune from Earth: 4.3 billion km / 2.7 billion miles
As a special bonus, we’ll include Pluto too, even though Pluto is not a planet anymore.
Uranus and Neptune, the Solar System’s ice giant planets. Credit: Wikipedia Commons
Pluto:
Closest: 4.44 billion km / 2.76 billion miles (29.7 AU)
Farthest: 7.38 billion km / 4.59 billion miles (49.3 AU)
Average: 5.91 billion km / 3.67 billion miles (39.5 AU)
Closest to Pluto from Earth: 4.28 billion km / 2.66 billion miles
To learn more:
Online resources demonstrating the scale of the Solar System:
Planets in our Solar system size comparison.
Largest to smallest are pictured left to right, top to bottom: Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars, Mercury. Via Wikimedia Commons.
If you’re interested in planets, the good news is there’s plenty of variety to choose from in our own Solar System. From the ringed beauty of Saturn, to the massive hulk of Jupiter, to the lead-melting temperatures on Venus, each planet in our solar system is unique — with its own environment and own story to tell about the history of our Solar System.
What also is amazing is the sheer size difference of planets. While humans think of Earth as a large planet, in reality it is dwarfed by the massive gas giants lurking at the outer edges of our Solar System. This article explores the planets in order of size, with a bit of context as to how they got that way.
A Short History of the Solar System:
No human was around 4.5 billion years ago when the Solar System was formed, so what we know about its birth comes from several sources: examining rocks on Earth and other places, looking at other solar systems in formation and doing computer models, among other methods. As more information comes in, some of our theories of the Solar System must change to suit the new evidence.
Today, scientists believe the Solar System began with a spinning gas and dust cloud. Gravitational attraction at its center eventually collapsed to form the Sun. Some theories say that the young Sun’s energy began pushing the lighter particles of gas away, while larger, more solid particles such as dust remained closer in.
Artist’s conception of a solar system in formation. Credit: NASA/FUSE/Lynette Cook
Over millions and millions of years, the gas and dust particles became attracted to each other by their mutual gravities and began to combine or crash. As larger balls of matter formed, they swept the smaller particles away and eventually cleared their orbits. That led to the birth of Earth and the other eight planets in our Solar System. Since much of the gas ended up in the outer parts of the system, this may explain why there are gas giants — although this presumption may not be true for other solar systems discovered in the universe.
Until the 1990s, scientists only knew of planets in our own Solar System and at that point accepted there were nine planets. As telescope technology improved, however, two things happened. Scientists discovered exoplanets, or planets that are outside of our solar system. This began with finding massive planets many times larger than Jupiter, and then eventually finding planets that are rocky — even a few that are close to Earth’s size itself.
The other change was finding worlds similar to Pluto, then considered the Solar System’s furthest planet, far out in our own Solar System. At first astronomers began treating these new worlds like planets, but as more information came in, the International Astronomical Union held a meeting to better figure out the definition.
Hubble image of Pluto and some of its moons, Charon, Nix and Hydra. Image Credit: NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team
The result was redefining Pluto and worlds like it as a dwarf planet. This is the current IAU planet definition:
“A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.”
Jupiter (69,911 km / 43,441 miles) – 1,120% the size of Earth
Saturn (58,232 km / 36,184 miles) – 945% the size of Earth
Uranus (25,362 km / 15,759 miles) – 400% the size of Earth
Neptune (24,622 km / 15,299 miles) – 388% the size of Earth
Earth (6,371 km / 3,959 miles)
Venus (6,052 km / 3,761 miles) – 95% the size of Earth
Mars (3,390 km / 2,460 miles) – 53% the size of Earth
Mercury (2,440 km / 1,516 miles) – 38% the size of Earth
Eight planets and a dwarf planet in our Solar System, approximately to scale. Pluto is a dwarf planet at far right. At far left is the Sun. The planets are, from left, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Credit: Lunar and Planetary Institute
Jupiter is the behemoth of the Solar System and is believed to be responsible for influencing the path of smaller objects that drift by its massive bulk. Sometimes it will send comets or asteroids into the inner solar system, and sometimes it will divert those away.
Saturn, most famous for its rings, also hosts dozens of moons — including Titan, which has its own atmosphere. Joining it in the outer solar system are Uranus and Neptune, which both have atmospheres of hydrogen, helium and methane. Uranus also rotates opposite to other planets in the solar system.
The inner planets include Venus (once considered Earth’s twin, at least until its hot surface was discovered); Mars (a planet where liquid water could have flowed in the past); Mercury (which despite being close to the sun, has ice at its poles) and Earth, the only planet known so far to have life.
To learn more about the Solar System, check out these resources:
NASA astronaut Steve Swanson does a spacesuit fit check prior to the launch of Expedition 39 in March 2014. Credit: NASA
It’s a good thing that next week’s urgent spacewalk is pegged as a short one, because the coming days will be hectic for the Expedition 39 crew.
Finding a spot for even a 2.5-hour excursion on the International Space Station was extremely challenging, NASA officials said in a news conference today (April 18), because crew time also is needed for two cargo spacecraft: the SpaceX Dragon launch scheduled for today and subsequent Progress undocking/redocking on station.
Here’s a rundown of some things NASA was juggling as it moves hastily to replace a failed backup computer on the outside of the station. Rick Mastracchio and Steve Swanson are expected to go “outside” on Wednesday (April 23), but if today’s SpaceX launch is delayed the spacewalk will be moved up to Sunday (April 20).
Why it’s urgent
The U.S. portion of the station has 46 computers, with 24 of them external. The multiplexer/demultiplexer or MDM (one of two) controls 12 of these external computers and is responsible for everything for how the solar arrays are pointed to how some robotics operate. It should be noted here that the primary MDM is working just fine, but if it fails with no backup, there will be problems. NASA will lose telemetry or data from the external ammonia cooling systems operating on station (although the systems themselves will work automatically). Some redundant equipment can’t be turned on, either. The agency also won’t be able to point the solar arrays to get power or to move them aside when spacecraft come in, to protect the arrays from thruster plumes (although further below you can see some backups they have for the array problems.)
NASA astronaut Mike Hopkins during a contingency spacewalk in December 2013 to replace a faulty ammonia pump. Hopkins was part of Expedition 37/38. Credit: NASA
Fixing the spacesuits
Since last summer’s life-threatening water leak, NASA has been moving quickly to fix the spacesuits it has. All non-urgent spacewalks are off the table until at least this summer while NASA addresses a panel’s recommendations to fix the problem. A faulty fan pump separator was swapped out on the bad suit (Suit 3011) last December, but two spacesuits still needed to be fixed on station. The crew spent much of the past week changing out a fan pump separator on Suit 3005 (which will also be used in the spacewalk) and flushing out the cooling lines in the suit and on station, since contamination is believed to have led to the failure. (More parts will arrive on Dragon, but they won’t be used this time, NASA has determined.)
Spacewalk preps on the ground
Also today, NASA astronaut Chris Cassidy was in “the pool” (at NASA’s Neutral Buoyancy Laboratory) simulating the spacewalk. He’s part of a team working to see what could go wrong on the spacewalk and come up with procedures dealing with that. “As best we can we have all those answers in our hip pockets so as they get thrown out on the game day, we can give the crew a quick answer,” he said in an interview Wednesday (April 16) on NASA TV.
Preparing the new computer
A spare MDM is inside the station, but it was an older model that needed to be reconfigured. Astronauts changed out a processing card and did other hardware/software changes to prepare the MDM to sit outside of the station. They also thoroughly tested it to make sure it’s working before mounting it outside. As a point of interest, no one yet knows why the backup MDM failed, but astronauts will inspect the site for damage (and take pictures). It’s expected that once they bring the broken MDM inside, any failed cards will be swapped out and sent to the ground sometime for analysis. The MDM itself will stay on station to be used again, as needs arise.
SpaceX’s Dragon spacecraft berthed to the International Space Station during Expedition 33 in October 2012. Credit: NASA
Grappling Dragon
SpaceX’s Dragon is a cargo spacecraft controlled by the ground, but the astronauts need to be ready to nab it with the robotic Canadarm2 once it arrives (now scheduled for Sunday, April 20). The crew has their normal amount of training and preparation for the procedures, then the time it takes to capture the spacecraft, and then the time to unload the vehicle (which is somewhat urgent as there are certain research experiments that need to come off fairly quickly, NASA said.)
Moving the solar array
NASA not only needs to have the solar arrays out of the way from thruster plumes from Dragon and Progress, but it also needs to keep power to the station and configure the arrays so that if the other MDM fails, the arrays will automatically be placed in a safe spot. The array would autotrack for 24 hours after the MDM fails, then go to a “preset angle” that NASA carefully chose. As for whether there would be power shortages on station, NASA says it depends on the sun’s angle and what needs to be done on station at a particular time.
Moving the Progress spacecraft
Russian cargo ship Progress 53 is supposed to undock from the Zvezda service module on Wednesday (April 23) to test an automated rendezvous system that controls approaches to station. Then it’s docking again on Friday (April 25).
Unless otherwise noted, information in this article is based on comments from the following officials in today’s NASA news conference: Mike Suffredini, International Space Station program manager; Brian Smith, International Space Station flight director and Glenda Brown, lead spacewalk officer.
Astronaut Drew Feustel reenters the space station after completing an 8-hour, 7-minute spacewalk at on Sunday, May 22, 2011. He and fellow spacewalker Mike Fincke conducted the second of the four EVAs during the STS-134 mission. Credit: NASA
Talk about a high-flying career! Being a government astronaut means you have the chance to go into space and take part in some neat projects — such as going on spacewalks, moving robotic arms and doing science that researches the nature of the human body.
Behind the glamor and the giddiness of flight, however, astronauts also need to pay their bills on Earth. How much you get paid as an astronaut depends on what agency you work for – as well as your experience, just like any other career.
The information below for NASA, the European Space Agency (ESA) and the Canadian Space Agency (CSA) is current as of April 2014, unless otherwise noted. Three agencies do not disclose salary scales online, at least in English pages: the Japan Aerospace Exploration Agency (JAXA), the Russian Federal Space Agency (Roscosmos) and the China National Space Administration (CNSA).
NASA
Astronaut Chris Cassidy training for a spacewalk in NASA’s Neutral Buoyancy Laboratory. Credit: Robert Markowitz
NASA has 43 active astronauts and eight astronauts-in-training who were selected in 2013. Until basic training is completed, which takes about two years, selectees are called “astronaut candidates”. (Astronauts from other agencies, such as ESA and CSA, often join NASA selectees for basic training.) Then even after they’re selected, it could be years more before they take a spaceflight.
Some astronauts are hired as civilian employees while others come over from the military. Civilian astronauts are paid according to a government scale that ranges from classifications GS-11 to GS-14.
In 2012, employees living in Houston (where astronaut training facilities are located) make a minimum of $64,724 for a GS-11 to a maximum of $141,715 for a GS-14. As employees pick up more qualifications, responsibility and experience, their salaries increase.
Military salaries were not disclosed, but NASA said those employees from the armed forces “remain in an active duty status for pay, benefits, leave, and other similar military matters.”
European Space Agency
Expedition 36/37’s Luca Parmitano, a European Space Agency astronaut, moments after landing Nov. 10, 2013. Credit: NASA/Carla Cioffi
ESA’s most recent astronaut class was selected in 2009. They have all either flown in space, or have been assigned to future missions aboard the International Space Station. Astronauts are paid between the A2 and A4 scales set by the Coordinated Organisations, a group of European intergovernmental groups.
“Upon entering the ESA Astronaut Corps, new recruits will generally be paid at the A2 level. Following the successful completion of the basic astronaut training, the recruit will be paid in accordance with the grade A3. The promotion to the grade A4 generally follows after the first spaceflight,” the European Space Agency stated.
While ESA’s website does not specify the salaries for astronauts beyond the grade, another Coordinated Organisation – called the North Atlantic Treaty Organisation – lists the annual A2 salary as 58,848 Euros ($81,404) and the A4 salary as 84,372 Euros ($116,619.)
Canadian Space Agency
David Saint-Jacques (left) with fellow Canadian astronaut trainee Jeremy Hansen. The two men were selected as astronauts in 2009. Credit: NASA
Canada has two active astronauts, neither of which have been assigned to a spaceflight yet. The CSA does not disclose on its website how much astronauts make, but some information is available on the website of the Privy Council Office – an advisory group to Canada’s prime minister and senior officials.
As of 2011, astronauts are paid a minimum of $89,100 Canadian ($80,897) in Grade 1 and a maximum of $174,000 Canadian ($158,470) in Grade 3. Newly minted astronaut candidates appear to move to Level 2 upon completing basic astronaut training, which takes two years, and then increase their salary with more experience.
Military astronauts are paid according to a separate scale that was not disclosed in PCO documents.
Becoming a government astronaut
The European Space Agency’s astronaut class of 2009 (left to right): Andreas Mogensen, Alexander Gerst, Samantha Cristoforetti, Thomas Pesquet, Luca Parmitano, Timothy Peake. Credit: European Space Agency/S. Corvaja
Generally, you must be the citizen of a particular country with a space program to apply as an astronaut. U.S. astronauts are U.S. citizens, European astronauts are citizens of European countries, and so forth.
Each space agency has periodic astronaut selections where they put out a call for candidates and then winnow down the list to a handful of people selected for astronaut training. The United States had its last selection in 2013, and ESA, CSA and JAXA did theirs in 2009.
While space agencies are careful not to specify certain kinds of degrees or universities for applicants, generally speaking astronauts have technical, medical or military backgrounds.
Astronauts are best known by the public for their time in space, but in reality they will spend most of their careers on the ground. International Space Station astronauts are expected to be proficient in station systems, science and spacewalks. They also must learn how to operate the Soyuz spacecraft that gets them into space, and to learn Russian since that country is a major partner of the International Space Station.
When astronauts aren’t training, they’re working to support other missions — sometimes in locations such as NASA’s Mission Control or in pools used for spacewalk training. They additionally spend hours of time doing outreach for schools and other audiences, and travelling all over the world to the various training centers used to get people ready for spaceflight.
It’s a tough career, but those who make the trek into space say the view is totally worth it.
Astronaut Edward White, the first American to walk in space. Image Credit: NASA
Want to learn more?
The following pages give you more information on becoming an astronaut, and what to expect once you get selected.
