Whirlpool Galaxy M51 (NGC 5194). Credit: Hubble Heritage Team (STScI/AURA)
N. Scoville (Caltech)
On a clear night, you can make out the band of the Milky Way in the night sky. For millennia, astronomers looked upon it in awe, slowly coming to the realization that our Sun was merely one of billions of stars in the galaxy. Over time, as our instruments and methods improved, we came to realize that the Milky Way itself was merely one of billions of galaxies that make up the Universe.
Thanks to the discovery of Relativity and the speed of light, we have also come to understand that when we look through space, we are also looking back in time. By seeing an object 1 billion light-years away, we are also seeing how that object looked 1 billion years ago. This “time machine” effect has allowed astronomers to study how galaxies came to be (i.e. galactic evolution).
The process in which galaxies form and evolve is characterized by steady growth over time, which began shortly after the Big Bang. This process, and the eventual fate of galaxies, remain the subject of intense fascination, and is still fraught with its share of mysteries.
Illustration of the depth by which Hubble imaged galaxies in prior Deep Field initiatives, in units of the Age of the Universe. Credit: NASA and A. Feild (STScI)
Galaxy Formation:
The current scientific consensus is that all matter in the Universe was created roughly 13.8 billion years ago during an event known as the Big Bang. At this time, all matter was compacted into a very small ball with infinite density and intense heat called a Singularity. Suddenly, the Singularity began expanding, and the Universe as we know it began.
After rapidly expanding and cooling, all matter was almost uniform in distribution. Over the course of the several billion years that followed, the slightly denser regions of the Universe began to become gravitationally attracted to each other. They therefore grew even denser, forming gas clouds and large clumps of matter.
These clumps became primordial galaxies, as the clouds of hydrogen gas within the proto-galaxies underwent gravitational collapse to become the first stars. Some of these early objects were small, and became tiny dwarf galaxies, while others were much larger and became the familiar spiral shapes, like our own Milky Way.
Galactic Mergers:
Once formed, these galaxies evolved together in larger galactic structures called groups, clusters and superclusters. Over time, galaxies were attracted to one another by the force of their gravity, and collided together in a series of mergers. The outcome of these mergers depends on the mass of the galaxies in the collision.
Small galaxies are torn apart by larger galaxies and added to the mass of larger galaxies. Our own Milky Way recently devoured a few dwarf galaxies, turning them into streams of stars that orbit the galactic core. But when large galaxies of similar size come together, they become giant elliptical galaxies.
When this happens, the delicate spiral structure is lost, and the merged galaxies become large and elliptical. Elliptical galaxies are some of the largest galaxies ever observed. Another consequence of these mergers is that the supermassive black holes (SMBH) at their centers become even larger.
Not all mergers will result in elliptical galaxies, mind you. But all mergers result in a change in the structure of the merged galaxies. For example, it is believed that the Milky Way is experiencing a minor merger event with the nearby Magellanic Clouds; and in recent years, it has been determined that the Canis Major dwarf galaxy has merged with our own.
While mergers are seen as violent events, actual collisions are not expected to happen between star systems, given the vast distances between stars. However, mergers can result in gravitational shock waves, which are capable of triggering the formation of new stars. This is what is predicted to happen when our own Milky Way galaxy merges with the Andromeda galaxy in about 4 billion years time.
Galactic Death:
Ultimately, galaxies cease forming stars once they deplete their supply of cold gas and dust. As the supply runs out, star forming slows over the course of billions of years until it ceases entirely. However, ongoing mergers will ensure that fresh stars, gas and dust are deposited in older galaxies, thus prolonging their lives.
At present, it is believed that our galaxy has used up most of its hydrogen, and star formation will slow down until the supply is depleted. Stars like our Sun can only last for 10 billion years or so; but the smallest, coolest red dwarfs can last for a few trillion years. However, thanks to the presence of dwarf galaxies and our impending merger with Andromeda, our galaxy could exist even longer.
However, all galaxies in this vicinity of the Universe will eventually become gravitationally bound to each other and merge into a giant elliptical galaxy. Astronomers have seen examples of these sorts of “fossil galaxies”, a good of which is Messier 49 – a supermassive elliptical galaxy.
These galaxies have used up all their reserves of star forming gas, and all that’s left are the longer lasting stars. Eventually, over vast lengths of time, those stars will wink out one after the other, until the whole thing is the background temperature of the Universe.
After our galaxy merges with Andromeda, and goes on to merge with all other nearby galaxies in the local group, we can expect that it too will undergo a similar fate. And so, galaxy evolution has been occurring over billions of years, and it will continue to happen for the foreseeable future.
Astronauts need spacesuits to survive the temperature of the Moon. Image credit: NASA
Have you ever heard that it’s possible to buy property on the Moon? Perhaps someone has told you that, thanks to certain loopholes in the legal code, it is possible to purchase your very own parcel of lunar land. And in truth, many celebrities have reportedly bought into this scheme, hoping to snatch up their share of land before private companies or nations do.
Despite the fact that there may be several companies willing to oblige you, the reality is that international treaties say that no nation owns the Moon. These treaties also establish that the Moon is there for the good of all humans, and so it’s impossible for any state to own any lunar land. But does that mean private ownership is impossible too? The short answer is yes.
The long answer is, it’s complicated. At present, there are multiple nations hoping to build outposts and settlements on the Moon in the coming decades. The ESA hopes to build a “international village” between 2020 and 2030 and NASA has plans for its own for a Moon base.
The ESA recently elaborated its plan to create a Moon base by the 2030s. Credit: ESA/Foster + Partners
Because of this, a lot of attention has been focused lately on the existing legal framework for the Moon and other celestial bodies. Let’s take a look at the history of “space law”, shall we?
Outer Space Treaty:
On Jan. 27th, 1967, the United States, United Kingdom, and the Soviet Union sat down together to work out a treaty on the exploration and use of outer space. With the Soviets and Americans locked in the Space Race, there was fear on all sides that any power that managed to put resources into orbit, or get to the Moon first, might have an edge on the others – and use these resources for evil!
Astronaut Charles M. Duke Jr. shown collecting samples on the lunar surface with the Lunar Roving Vehicle during the Apollo 16 mission. Credit: NASA
The treaty is overseen the United Nations Office for Outer Space Affairs (UNOOSA). It’s a big document, with lots of articles, subsections, and legalese. But the most relevant clause is Article II of the treaty, where it states:
“Outer space, including the moon and other celestial bodies, is not subject to national appropriation by claim of sovereignty, by means of use or occupation, or by any other means.”
“Loophole” in the Treaty:
Despite clearly saying that Outer Space is the property of all humanity, and can only be used for the good of all, the language is specific to national ownership. As a result, there is no legal consensus on whether or not the treaty’s prohibition are also valid as far as private appropriation is concerned.
However, Article II addresses only the issue of national ownership, and contains no specific language about the rights of private individuals or bodies in owning anything in outer space. Because of this, there are some who have argued that property rights should be recognized on the basis of jurisdiction rather than territorial sovereignty.
About 20 minutes after the first step, Aldrin joined Armstrong on the surface and became the second human to set foot on the Moon. Credit: NASA
Looking to Article VI though, it states that governments are responsible for the actions of any party therein. So it is clear that the spirit of the treaty is meant to apply to all entities, be they public or private. As it states:
“States Parties to the Treaty shall bear international responsibility for national activities in outer space, including the moon and other celestial bodies, whether such activities are carried on by governmental agencies or by non-governmental entities, and for assuring that national activities are carried out in conformity with the provisions set forth in the present Treaty. The activities of non-governmental entities in outer space, including the moon and other celestial bodies, shall require authorization and continuing supervision by the appropriate State Party to the Treaty.”
In other words, any person, organization or company operating in space is answerable to their respective government. But since no specific mention is made of private ownership, there are those who claim that this represents a “loophole” in the treaty which allows them to claim and sell land on the Moon at this time. Because of this ambiguity, there have been attempts to augment the Outer Space Treaty.
The Moon Treaty:
On Dec. 18th, 1979, members of the United Nations presented an agreement which was meant to be a follow-up to the Outer Space Treaty and close its supposed loopholes. Known as the “Agreement Governing the Activities of States on the Moon and Other Celestial Bodies” – aka. “The Moon Treaty” or “Moon Agreement” – this treaty intended to establish a legal framework for the use of the Moon and other celestial bodies.
Buzz Aldrin’s panorama of the Apollo 11 landing site is the only good picture of mission commander Neil Armstrong on the lunar surface. Credit: NASA
Much like the Outer Space Treaty, the agreement established that the Moon should be used for the benefit of all humanity and not for the sake of any individual state. The treaty banned weapons testing, declared that any scientific research must be open and shared with the international community, and that nations and individuals and organizations could not claim anything.
In practice, the treaty failed because it has not been ratified by any state that engages in crewed space exploration or has domestic launch capability. This includes the United States, the larger members of the ESA, Russia, China, Japan and India. Though it expressly forbids both national and private ownership of land on the Moon, or the use thereof for non-scientific, non-universal purposes, the treaty effectively has no teeth.
Bottom line, there is nothing that expressly forbids companies from owning land on the Moon. However, with no way to claim that land, anyone attempting to sell land to prospective buyers is basically selling snake oil. Any documentation that claims you own land on the Moon is unenforceable, and no nation on the planet that has signed either the Outer Space Treaty or the Moon Treaty will recognize it.
The annotated features on the lunar nearside. You’ll notice, not one of them says “Land for Sale!” Credit: Wikimedia Commons/ Peter Freiman(Cmglee). Background photograph by Gregory H. Revera.
Then again, if you were able to fly up to the Moon and build a settlement there, it would be pretty difficult for anyone to stop you. But don’t expect that to the be the last word on the issue. With multiple space agencies looking to create “international villages” and companies hoping to create a tourist industry, you could expect some serious legal battles down the road!
But of course, this is all academic. With no atmosphere to speak of, temperatures reaching incredible highs and lows – ranging from 100 °C (212 °F) to -173 °C (-279.4 °F) – its low gravity (16.5 % that of Earth), and all that harsh Moon dust, nobody outside of trained astronauts (or the clinically insane) should want to spend a significant amount of time there!
A picture of Earth taken by Apollo 11 astronauts. Credit: NASA
The moment that the Apollo-11 mission touched down on the Moon, followed by Neil Armstrong‘s famous words – “That’s one small step for [a] man, one giant leap for mankind” – is one of the most iconic moments in history. The culmination of years of hard work and sacrifice, it was an achievement that forever established humanity as a space-faring species.
And in the year’s that followed, several more spacecraft and astronauts landed on the Moon. But before, during and after these missions, a number of other “lunar landings” were accomplished as well. Aside from astronauts, a number of robotic missions were mounted which were milestones in themselves. So exactly what were the earliest lunar landings?
Robotic Missions:
The first missions to the Moon consisted of probes and landers, the purpose of which was to study the lunar surface and determine where crewed missions might land. This took place during the 1950s where both the Soviet Space program and NASA sent landers to the Moon as part of their Luna and Pioneer programs.
The Soviet Luna 2 probe, the first man-made object to land on the Moon. Credit: NASA
After several attempts on both sides, the Soviets managed to achieve a successful lunar landing on Sept. 14th, 1959 with their Luna-2 spacecraft. After flying directly to the Moon for 36 hours, the spacecraft achieved a hard landing (i.e. crashed) on the surface west of the Mare Serenitatis – near the craters Aristides, Archimedes, and Autolycus.
The primary objective of the probe was to help confirm the discovery of the solar wind, turned up by the Luna-1mission. However, with this crash landing, it became the first man-made object to touch down on the Moon. Upon impact, it scattered a series of Soviet emblems and ribbons that had been assembled into spheres, and which broke apart upon hitting the surface.
The next craft to make a lunar landing was the Soviet Luna-3 probe, almost a month after Luna-2 did. However, unlike its predecessor, the Luna-3 probe was equipped with a camera and managed to send back the first images of the far side of the Moon.
The first US spacecraft to impact the Moon was the Ranger-7 probe, which crashed into the Moon on July 31st, 1964. This came after a string of failures with previous spacecraft in the Pioneer and Ranger line of robotic spacecraft. Prior to impact, it too transmitted back photographs of the Lunar surface.
The Ranger 7 lander, which became the first US spacecraft to land on the Moon. Credit: NASA
This was followed by the Ranger-8 lander, which impacted the surface of the Moon on Feb. 20th, 1965. The spacecraft took 7,000 high-resolution images of the Moon before crashing onto the surface, just 24 km from the Sea of Tranquility, which NASA had been surveying for the sake of their future Apollo missions. These images, which yielded details about the local terrain, helped to pave the way for crewed missions.
The first spacecraft to make a soft landing on the Moon was the Soviet Luna-9 mission, on February 3rd, 1966. This was accomplished through the use of an airbag system that allowed the probe to survive hitting the surface at a speed of 50 km/hour. It also became the first spacecraft to transmit photographic data back to Earth from the surface of another celestial body.
The first truly soft landing was made by the US with the Surveyor-1 spacecraft, which touched down on the surface of the Moon on June 2nd, 1966. After landing in the Ocean of Storms, the probe transmitted data back to Earth that would also prove useful for the eventual Apollo missions.
Several more Surveyor missions and one more Luna mission landed on the Moon before crewed mission began, as part of NASA’s Apollo program.
Launch of Apollo 11 mission aboard a Saturn V rocket on July 16th, 1969. Credit: NASA
Crewed Missions:
The first crewed landing on the Moon was none other than the historic Apollo-11 mission, which touched down on the lunar surface on July 20th, 1969. After achieving orbit around the Moon in their Command Module (aka. the Columbia module), Neil Armstrong and Buzz Aldrin rode the Lunar Excursion (Eagle) Module down to the surface of the Moon.
Once they had landed, Armstrong radioed to Mission Control and announced their arrival by saying: “Houston, Tranquility Base here. The Eagle has landed.” Once the crew had gone through their checklist and depressurized the cabin, the Eagles’ hatch was opened and Armstrong began walking down the ladder to the Lunar surface first.
When he reached the bottom of the ladder, Armstrong said: “I’m going to step off the LEM now” (referring to the Lunar Excursion Module). He then turned and set his left boot on the surface of the Moon at 2:56 UTC July 21st, 1969, and spoke the famous words “That’s one small step for [a] man, one giant leap for mankind.”
About 20 minutes after the first step, Aldrin joined Armstrong on the surface and became the second human to set foot on the Moon. The two then unveiled a plaque commemorating their flight, set up the Early Apollo Scientific Experiment Package, and planted the flag of the United States before blasting off in the Lunar Module.
Buzz Aldrin on the Moon during the Apollo 11 mission, with the reflection of Neil Armstrong visible in his face plate. Credit: NASA
Several more Apollo missions followed which expanded on the accomplishments of the Apollo-11 crew. The US and NASA would remain the only nation and space agency to successfully land astronauts on the Moon, an accomplishment that has not been matched to this day.
Today, multiple space agencies (and even private companies) are contemplating returning to the Moon. Between NASA, the European Space Agency (ESA), the Russian Space Agency (Roscosmos), and the Chinese National Space Administration (CNSA), there are several plans for crewed missions, and even the construction of permanent bases on the Moon.
Hubble image of variable star RS Puppis (NASA, ESA, and the Hubble Heritage Team)
The Universe is a really, really big place. We’re talking… imperceptibly big! In fact, based on decades worth of observations, astronomers now believe that the observable Universe measures about 46 billion light years across. The key word there is observable, because when you take into account that which we cannot see, scientists think it’s actually more like 92 billion light years across.
The hardest part in all of this is making accurate measurements of the distances involved. But since the birth of modern astronomy, increasingly accurate methods have evolved. Aside from redshift and examining the light coming from distant stars and galaxies, astronomers also rely on a class of stars known as Cepheid Variables (CVs) to determine the distance of objects within and beyond our Galaxy.
Definition:
Variable stars are essentially stars that experience fluctuations in their brightness (aka. absolute luminosity). Cepheids Variables are special type of variable star in that they are hot and massive – five to twenty times as much mass as our Sun – and are known for their tendency to pulsate radially and vary in both diameter and temperature.
What’s more, these pulsations are directly related to their absolute luminosity, which occurs within well-defined and predictable time periods (ranging from 1 to 100 days). When plotted as a magnitude vs. period relationship, the shape of the Cephiad luminosity curve resembles that of a “shark fin” – do its sudden rise and peak, followed by a steadier decline.
The name is derived from Delta Cephei, a variable star in the Cepheus constellation that was the first CV to be identified. Analysis of this star’s spectrum suggests that CVs also undergo changes in terms of temperature (between 5500 – 66oo K) and diameter (~15%) during a pulsation period.
Use in Astronomy:
The relationship between the period of variability and the luminosity of CV stars makes them very useful in determining the distance of objects in our Universe. Once the period is measured, the luminosity can be determined, thus yielding accurate estimates of the star’s distance using distance modulus equation.
This equation states that: m – M = 5 log d – 5 – where m is the apparent magnitude of the object, M is the absolute magnitude of the object, and d is the distance to the object in parsecs. Cepheid variables can be seen and measured to a distance of about 20 million light years, compared to a maximum distance of about 65 light years for Earth-based parallax measurements and just over 326 light years for the ESA’s Hipparcos mission.
Calibrated Period-luminosity Relationship for Cepheids. Credit: NASA
Because they are bright, and can be clearly seen million of light years away, they can be easily distinguished from other bright stars in their vicinity. Combined with the relationship between their variability and luminosity, this makes them highly useful tools in deducing the size and scale of our Universe.
Classes:
Cepheid variables are divided into two subclasses – Classical Cepheids and Type II Cepheids – based on differences in their masses, ages, and evolutionary histories. Classical Cepheids are Population I (metal-rich) variable stars that are 4-20 times more massive than the Sun and up to 100,000 times more luminous. They undergo pulsations with very regular periods on the order of days to months.
These Cepheids are typically yellow bright giants and supergiants (spectral class F6 – K2) and they experience radius changes in the millions of kilometers during a pulsation cycle. Classical Cepheids are used to determine distances to galaxies within the Local Group and beyond, and are a means by which the Hubble Constant can be established (see below).
Type II Cepheids are Population II (metal-poor) variable stars which pulsate with periods of typically between 1 and 50 days. Type II Cepheids are also older stars (~10 billion years) that have around half the mass of our Sun.
Type II Cepheids are also subdivided based on their period into the BL Her, W Virginis, and RV Tauri subclasses (named after specific examples) – which have periods of 1-4 days, 10-20 days, and more than 20 days, respectively. Type II Cepheids are used to establish the distance to the Galactic Center, globular clusters, and neighboring galaxies.
There are also those that do not fit into either category, which are known as Anomalous Cepheids. These variables have periods of less than 2 days (similar to RR Lyrae) but have higher luminosities. They also have higher masses than Type II Cepheids, and have unknown ages.
A small proportion of Cepheid variables have also been observed which pulsate in two modes at the same time, hence the name Double-mode Cepheids. A very small number pulsate in three modes, or an unusual combination of modes.
History of Observation:
The first Cepheid variable to be discovered was Eta Aquilae, which was observed on September 10th, 1784, by English astronomer Edward Pigott. Delta Cephei, for which this class of star is named, was discovered a few months later by amateur English astronomer John Goodricke.
Hubble image of variable star RS Puppis, one of the brightest known Cepheid variable stars in the Milky Way galaxy. Credit: NASA/ ESA/Hubble Heritage Team
In 1908, during an investigation of variable stars in the Magellanic Clouds, American astronomer Henrietta Swan Leavitt discovered the relationship between the period and luminosity of Classical Cepheids. After recording the periods of 25 different variables stars, she published her findings in 1912.
In the following years, several more astronomers would conduct research on Cepheids. By 1925, Edwin Hubble was able to establish the distance between the Milky Way and the Andromeda Galaxy based on Cepheid variables within the latter. These findings were pivotal, in that they settled the Great Debate, where astronomers sought to established whether or not the Milky Way was unique, or one of many galaxies in the Universe.
By gauging the distance between the Milky Way and several other galaxies, and combining it with Vesto Slipher’s measurements of their redshift, Hubble and Milton L. Humason were able to formulate Hubble’s Law. In short, they were able to prove that the Universe is in a state of expansion, something that had been suggested years prior.
Further developments during the 20th century included dividing Cepheids into different classes, which helped resolve issues in determining astronomical distances. This was done largely by Walter Baade, who in the 1940s recognized the difference between Classical and Type II Cepheids based on their size, age and luminosities.
Limitations:
Despite their value in determining astronomical distances, there are some limitations with this method. Chief among them is the fact that with Type II Cepheids, the relationship between period and luminosity can be effected by their lower metallicity, photometric contamination, and the changing and unknown effect that gas and dust have on the light they emit (stellar extinction).
These unresolved issues have resulted in different values being cited for Hubble’s Constant – which range between 60 km/s per 1 million parsecs (Mpc) and 80 km/s/Mpc. Resolving this discrepancy is one of the largest problems in modern cosmology, since the true size and rate of expansion of the Universe are linked.
However, improvements in instrumentation and methodology are increasing the accuracy with which Cepheid Variables are observed. In time, it is hoped that observations of these curious and unique stars will yield truly accurate values, thus removing a key source of doubt about our understanding of the Universe.
Io and Europa cast simultaneous shadows on Jupiter on March 22nd, 2016. Image credit and copyright: Andrew Symes.
Missing Jove? The largest planet in our solar system is currently on the far side of the Sun and just passed solar conjunction on September 26th, 2016. October now sees Jupiter slowly return to the dawn sky. Follow that gas giant, as an interesting set of double shadow transits transpires in late October leading in to early November.
This particular cycle of double shadow transits involves the large Jovian moons of Europa and Ganymede.
The scene on October 24th at 23:55 UT. Image credit: Created using Starry Night Education software.
Europa and Ganymede double shadow transit season begins later this month, as both cast shadows on the Jovian cloud tops. This series of simultaneous shadow transits runs from October 17th to November 8th, and includes four weekly events.
The inner three large moons Io, Europa and Ganymede are in a 4:2:1 resonance. Europa orbits Jove once every 3.6 days and makes two circuits for Ganymede’s one. This means there’s a double shadow transit once every week in the current season:
The double shadow transit season of 2016. Created by author.
When can you first spy Jupiter, post solar conjunction? Catching this particular series of double shadow transits is challenging this time around, owing to the planet’s position low in the dawn twilight. The first event on October 17th starts with Jupiter just 16 degrees west of the Sun, and the cycle ends with Jove 38 degrees west of the Sun on November 8th.
Keep in mind, it is possible to track Jupiter up in to the daytime sky, post sunrise. To do this, you’ll need a ‘scope with a solid equatorial mount and good sidereal tracking. The trick is to lock on to Jupiter before sunrise and track it up in to the dawn sky. Be sure to physically block that dazzling rising Sun out of view behind a hill or building, and NEVER aim your telescope at the Sun!
Using this method opens up the possibility of nabbing a given double shadow event to longitudes due east of the quoted locales above.
The waning crescent Moon also passes 1.4 degrees NNE of Jupiter on October 28th, offering another chance to spy the gas giant in the dawn sky, using the nearby crescent Moon as a guide.
The Moon and Jupiter in the daytime skies on October 28th. Image credit: Stellarium.
And another interesting pairing is coming right up on the morning of Tuesday, October 11th, when Mercury passes just 0.8 degrees (48′) NNE of Jupiter. Both are only 12 degrees west of the Sun at closest approach, which occurs around 10:00 UT. Still, both will appear as an interesting pseudo-double star, with Mercury shining at magnitude -1.1 and Jupiter only half a magnitude fainter at -1.6.
You can even see Jupiter coming off of solar conjunction and headed toward dawn skies courtesy of SOHO’s LASCO C3 camera:
Jupiter (arrowed) exiting the 15 degree wide field of view of SOHO’s LASCO C3 camera on October 5th. Image credit: NASA/ESA/SOHO.
Callisto, the outermost large moon of Jupiter, ceased casting its shadow on Jupiter earlier this year on September 1st 2016. Callisto is the only large moon that can ‘miss’ the gas giant’s cloud tops. Callisto must be involved for a triple shadow transit to occur, and the moon resumes regularly casting its shadow on Jove on December 4th, 2019.
Callisto can also experience total solar eclipses similar to those seen from the Earth during the mutual eclipse season for Jupiter’s moons, albeit shorter in duration:
And don ‘t forget: we’ve got a spacecraft currently exploring Jupiter for the next year and a half: NASA’s very own Juno.
Be sure to check out the Jovian action over the next month, gracing a dawn sky near you.
NASA's MSL Curiosity. NASA is the only agency to successfully place a lander on Mars. This self portrait shows Curiosity doing its thing on Mars. Image: NASA/JPL-Caltech/MSSS
We may be living in the Golden Age of Mars Exploration. With multiple orbiters around Mars and two functioning rovers on the surface of the red planet, our knowledge of Mars is growing at an unprecedented rate. But it hasn’t always been this way. Getting a lander to Mars and safely onto the surface is a difficult challenge, and many landers sent to Mars have failed.
The joint ESA/Roscosmos Mars Express mission, and its Chiaparelli lander, is due at Mars in only 15 days. Now’s a good time to look at the challenges in getting a lander to Mars, and also to look back at the many failed attempts.
A model of the Schiaparelli lander. The lander is part of the ExoMars mission. By Pline – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=26837226
For now, NASA has the bragging rights as the only organization to successfully land probes on Mars. And they’ve done it several times. But they weren’t the first ones to try. The Soviet Union tried first.
The USSR sent several probes to Mars starting back in the 1960s. They made their first attempt in 1962, but that mission failed to launch. That failure illustrates the first challenge in getting a craft to land on Mars: rocketry. We’re a lot better at rocketry than we were back in the 1960’s, but mishaps still happen.
Then in 1971, the Soviets sent a pair of probes to Mars called Mars 2 and Mars 3. They were both orbiters with detachable landers destined for the Martian surface. The fate of Mars 2 and Mars 3 provides other illustrative examples of the challenges in getting to Mars.
Mars 2 separated from its orbiter successfully, but crashed into the surface and was destroyed. The crash was likely caused by its angle of descent, which was too steep. This interrupted the descent sequence, which meant the parachute failed to deploy. So Mars 2 has the dubious distinction of being the first man-made object to reach Mars.
Mars 3 was exactly the same as Mars 2. The Soviets liked to do missions in pairs back then, for redundancy. Mars 3 separated from its orbiter and headed for the Martian surface, and through a combination of aerodynamic breaking, rockets, and parachutes, it became the first craft to make a soft landing on Mars. So it was a success, sort of.
A model of the Mars 3 lander with its petals open after landing. By NASA – http://nssdc.gsfc.nasa.gov/image/spacecraft/mars3_lander_vsm.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=14634254
But after only 14.5 seconds of data transmission, it went quiet and was never heard from again. The cause was likely an intense dust storm. In an odd turn of events, NASA’s Mariner 9 orbiter reached Mars only days before Mars 2 and 3, becoming the first spacecraft to orbit another planet. It captured images of the planet-concealing dust storms, above which only the volcanic Olympus Mons could be seen. These images provided an explanation for the failure of Mars 3.
This image from the Mariner 9 orbiter shows Olympus Mons above the dust storms that concealed much of the planet when it arrived at Mars in 1971. Image: NASA
In 1973, the Soviets tried again. They sent four craft to Mars, two of which were landers, named Mars 6 and Mars 7. Mars 6 failed on impact, but Mars 7’s fate was perhaps a little more tragic. It missed Mars completely, by about 1300 km, and is in a helicentric orbit to this day. In our day and age, we just assume that our spacecraft will go where we want them to, but Mars 7 shows us that it can all go wrong. After all, Mars is a moving target.
In the 1970s, NASA was fresh off the success of their Apollo Program, and were setting their sites on Mars. They developed the Viking program which saw 2 landers, Viking 1 and Viking 2, sent to Mars. Both of them were probe/lander configurations, and both landers landed successfully on the surface of Mars. The Vikings sent back beautiful pictures of Mars that caused excitement around the world.
The Viking 2 lander captured this image of itself on the Martian surface. By NASA – NASA website; description,[1] high resolution image.[2], Public Domain, https://commons.wikimedia.org/w/index.php?curid=17624
In 1997, NASA’s Martian Pathfinder made it to Mars and landed successfully. Pathfinder itself was stationary, but it brought a little rover called Sojourner with it. Sojourner explored the immediate landing area around Pathfinder. Sojourner became the first rover to operate on another planet.
Pathfinder was able to send back over 16,000 images of Mars, along with its scientific data. It was also a proof of concept mission for technologies such as automated obstacle avoidance and airbag mediated touchdown. Pathfinder helped lay the groundwork for the Mars Exploration Rover Mission. That means Spirit and Opportunity.
An artist’s conception of Spirit/Opportunity working on Mars. By NASA/JPL/Cornell University, Maas Digital LLC – http://photojournal.jpl.nasa.gov/catalog/PIA04413 (image link), Public Domain, https://commons.wikimedia.org/w/index.php?curid=565283
But after Pathfinder, and before Spirit and Opportunity, came a time of failure for Martian landing attempts. Everybody took part in the failure, it seems, with Russia, Japan, the USA, and the European Space Agency all experiencing bitter failure. Rocket failures, engineering errors, and other terminal errors all contributed to the failure.
Japan’s Nozomi orbiter ran out of fuel before ever reaching Mars. NASA’s Mars Polar Lander failed its landing attempt. NASA’s Deep Space 2, part of the Polar Lander mission, failed its parachute-less landing and was never heard from. The ESA’s Beagle 2 lander made it to the surface, but two of its solar panels failed to deploy, ending its mission. Russian joined in the failure again, with its Phobos-Grunt mission, which was actually headed for the Martian moon Phobos, to retrieve a sample and send it back to Earth.
In one infamous failure, engineers mixed up the use of English units with Metric units, causing NASA’s Mars Climate Orbiter to burn up on entry. These failures show us that failure is not rare. It’s difficult and challenging to get to the surface of Mars.
After this period of failure, NASA’s Spirit and Opportunity rovers were both unprecedented successes. They landed on the Martian surface in January 2004. Both exceeded their planned mission length of three months, and Opportunity is still going strong now.
So where does that leave us now? NASA is the only one to have successfully landed a rover on Mars and have the rover complete its mission. But the ESA and Russia are determined to get there.
The Schiaparelli lander, as part of the ExoMars mission, is primarily a proof of technology mission. In fact, its full name is the Schiaparelli EDM lander, meaning Entry, Descent, and Landing Demonstrator Module.
It will have some small science capacity, but is really designed to demonstrate the ability to enter the Martian atmosphere, descend safely, and finally, to land on the surface. In fact, it has no solar panels or other power source, and will only carry enough battery power to survive for 2-8 days.
Schiaparelli faces the same challenges as other craft destined for Mars. Once launched successfully, which it was, it had to navigate its way to Mars. That took about 6 months, and since ExoMars is only 15 days away from arrival at Mars, it looks like it has successfully made its way their. But perhaps the trickiest part comes next: atmospheric entry.
Schiaparelli is like most Martian craft. It will make a ballistic entry into the Martian atmosphere, and this has to be done right. There is no room for error. The angle of entry is the key here. If the angle is too steep, Schiaparelli may overheat and burn up on entry. On the other hand, if the angle is too shallow, it could hit the atmosphere and bounce right back into space. There’ll be no second chance.
The entry and descent sequence is all pre-programmed. It will either work or it won’t. It would take way too long to send any commands to Schiaparelli when it is entering and descending to Mars.
If the entry is successful, the landing comes next. The exact landing location is imprecise, because of wind speed, turbulence, and other factors. Like other craft sent to Mars, Schiaparelli’s landing site is defined as an ellipse.
Schiaparelli will land somewhere in this defined ellipse on the surface of Mars. Image: IRSPS/TAS-I
The lander will be travelling at over 21,000 km/h when it reaches Mars, and will have only 6 or 7 minutes to descend. At that speed, Schiaparelli will have to withstand extreme heating for 2 or 3 minutes. It’s heat shield will protect it, and will reach temperatures of several thousand degrees Celsius.
It will decelerate rapidly, and at about 10km altitude, it will have slowed to approximately 1700 km/h. At that point, a parachute will deploy, which will further slow the craft. After the parachute slows its descent, the heat shield will be jettisoned.
Schiaparelli’s Descent and Landing Sequence. Image: ESA/ATG medialab. Click here for larger image.
On Earth, a parachute would be enough to slow a descending craft. But with Mars’ less dense atmosphere, rockets are needed for the final descent. An onboard radar will monitor Schiaparelli’s altitude as it approaches the surface, and rockets will fire to slow it to a few meters per second in preparation for landing.
In the final moments, the rockets will stop firing, and a short free-fall will signal Schiaparelli’s arrival on Mars. If all goes according to plan, of course.
We won’t have much longer to wait. Soon we’ll know if the ESA and Russia will join NASA as the only agencies to successfully land a craft on Mars. Or, if they’ll add to the long list of failed attempts.
The sign of a truly great scientific theory is by the outcomes it predicts when you run experiments or perform observations. And one of the greatest theories ever proposed was the concept of Relativity, described by Albert Einstein in the beginning of the 20th century.
In addition to helping us understand that light is the ultimate speed limit of the Universe, Einstein described gravity itself as a warping of spacetime.
He did more than just provide a bunch of elaborate new explanations for the Universe, he proposed a series of tests that could be done to find out if his theories were correct.
One test, for example, completely explained why Mercury’s orbit didn’t match the predictions made by Newton. Other predictions could be tested with the scientific instruments of the day, like measuring time dilation with fast moving clocks.
Since gravity is actually a distortion of spacetime, Einstein predicted that massive objects moving through spacetime should generate ripples, like waves moving through the ocean.
The more massive the object, the more it distorts spacetime. Credit: LIGO/T. Pyle
Just by walking around, you leave a wake of gravitational waves that compress and expand space around you. However, these waves are incredibly tiny. Only the most energetic events in the entire Universe can produce waves we can detect.
It took over 100 years to finally be proven true, the direct detection of gravitational waves. In February, 2016, physicists with the Laser Interferometer Gravitational Wave Observatory, or LIGO announced the collision of two massive black holes more than a billion light-years away.
Any size of black hole can collide. Plain old stellar mass black holes or supermassive black holes. Same process, just on a completely different scale.
Colliding black holes. Credit: LIGO/A. Simonnet
Let’s start with the stellar mass black holes. These, of course, form when a star with many times the mass of our Sun dies in a supernova. Just like regular stars, these massive stars can be in binary systems.
Imagine a stellar nebula where a pair of binary stars form. But unlike the Sun, each of these are monsters with many times the mass of the Sun, putting out thousands of times as much energy. The two stars will orbit one another for just a few million years, and then one will detonate as a supernova. Now you’ll have a massive star orbiting a black hole. And then the second star explodes, and now you have two black holes orbiting around each other.
As the black holes zip around one another, they radiate gravitational waves which causes their orbit to decay. This is kind of mind-bending, actually. The black holes convert their momentum into gravitational waves.
As their angular momentum decreases, they spiral inward until they actually collide. What should be one of the most energetic explosions in the known Universe is completely dark and silent, because nothing can escape a black hole. No radiation, no light, no particles, no screams, nothing. And if you mash two black holes together, you just get a more massive black hole.
The gravitational waves ripple out from this momentous collision like waves through the ocean, and it’s detectable across more than a billion light-years.
Arial view of LIGO Livingston. Credit: The LIGO Scientific Collaboration
This is exactly what happened earlier this year with the announcement from LIGO. This sensitive instrument detected the gravitational waves generated when two black holes with 30 solar masses collided about 1.3 billion light-years away.
This wasn’t a one-time event either, they detected another collision with two other stellar mass black holes.
Regular stellar mass black holes aren’t the only ones that can collide. Supermassive black holes can collide too.
From what we can tell, there’s a supermassive black hole at the heart of pretty much every galaxy in the Universe. The one in the Milky Way is more than 4.1 million times the mass of the Sun, and the one at the heart of Andromeda is thought to be 110 to 230 million times the mass of the Sun.
In a few billion years, the Milky Way and Andromeda are going to collide, and begin the process of merging together. Unless the Milky Way’s black hole gets kicked off into deep space, the two black holes are going to end up orbiting one another.
Just with the stellar mass black holes, they’re going to radiate away angular momentum in the form of gravitational waves, and spiral closer and closer together. Some point, in the distant future, the two black holes will merge into an even more supermassive black hole.
View of Milkdromeda from Earth “shortly” after the merger, around 3.85-3.9 billion years from now. Credit: NASA, ESA, Z. Levay and R. van der Marel (STScI), T. Hallas, and A. Mellinger
The Milky Way and Andromeda will merge into Milkdromeda, and over the future billions of years, will continue to gather up new galaxies, extract their black holes and mashing them into the collective.
Black holes can absolutely collide. Einstein predicted the gravitational waves this would generate, and now LIGO has observed them for the first time. As better tools are developed, we should learn more and more about these extreme events.
NASA astronauts exploring Mars on future missions starting perhaps in the 2030’s will require protection from long term exposure to the cancer causing space radiation environment. Credit: NASA.
History was made on July 20th, 1969, when Apollo 11 astronauts Neil Armstrong and Buzz Aldrin set foot on the surface of the Moon. The moment was the culmination of decades of hard work, research, development and sacrifice. And since that time, human beings have been waiting and wondering when we might achieve the next great astronomical milestone.
So really, when will we see a man or woman set foot on Mars? The prospect has been talked about for decades, back when NASA and the Soviets were still planning on setting foot on the Moon. It is the next logical step, after all. And at present, several plans are in development that could be coming to fruition in just a few decades time.
Original Proposals:
Werner Von Braun, the (in)famous former Nazi rocket scientist – and the man who helped spearhead NASA’s Project Mercury – was actually the first to develop a concept for a crewed mission to Mars. Titled The Mars Project (1952), his proposal called for ten spacecraft (7 passenger, 3 cargo) that would transport a crew of 70 astronauts to Mars.
In between launching V-2s in New Mexico and developing rockets at Redstone Arsenal, Von Braun had time to write Mars Projekt (1952). Credit: Mars Project, Von Braun
His proposal was based in part on the large Antarctic expedition known as Operation Highjump (1946–1947), a US Navy program which took place a few years before he started penning his treatise. The plan called for the construction of the interplanetary spacecraft in around the Earth using a series of reusable space shuttles.
He also believed that, given the current pace of space exploration, such a mission could be mounted by 1965 (later revised to 1980) and would spend the next three years making the round trip mission. Once in Mars orbit, the crew would use telescopes to find a suitable site for their base camp near the equator.
A landing crew would then descend using a series of detachable winged aircraft (with ski landing struts) and glide down to land on the polar ice caps. A skeleton crew would remain with the ships in orbit as the surface crew would then travel 6,500 km overland using crawlers to the identified base camp site.
They would then build a landing strip which would allow the rest of the crew to descend from orbit in wheeled gliders. After spending a total of 443 days on Mars conducting surveys and research, the crew would use these same gliders as ascent craft to return to the mother ships.
Astronaut Eugene A. Cernan during the Apollo 17 mission, December 11th, 1972, shown conducting a checkout of the Lunar Roving Vehicle (LRV). Credit: NASA
Von Braun not only calculated the size and weight of each ship, but also how much fuel each would require for the round trip. He also computed the rocket burns necessary to perform the required maneuvers. Because of the detailed nature, calculations and planning in his proposal, The Mars Project remains one of the most influential books on human missions to the Red Planet.
Obviously, such a mission didn’t happen by 1965 (or 1980 for that matter). In fact, humans didn’t even return to the Moon after Eugene Cernan climbed out of the Apollo 17 capsule in 1972. With the winding down of the Space Race and the costs of sending astronauts to the Moon, plans to explore Mars were placed on the backburner until the last decade of the 20th century.
In 1990, a proposal called Mars Direct was developed by Robert Zubrin, founder of the Mars Society and fellow aerospace engineer David Baker. This plan envisioned a series of cost-effective mission to Mars using current technology, with the ultimate goal of colonization.
The initial missions would involve crews landing on the surface and leaving behind hab-structures, thus making subsequent missions easier to undertake. In time, the surface habs would give way to subsurface pressurized habitats built from locally-produced Martian brick. This would represent a first step in the development of in-situ resource utilization, and eventual human settlement.
Artist’s rendering Manned Habitat Units and Mars vehicles, part of the Mars Design Reference Mission 3.0. Credit: NASA
During and after this initial phase of habitat construction, hard-plastic radiation- and abrasion-resistant geodesic domes would be deployed to the surface for eventual habitation and crop growth. Local industries would begin to grow using indigenous resources, which would center around the manufacture of plastics, ceramics and glass out of Martian soil, sand and hydrocarbons.
While Zubrin acknowledged that Martian colonists would be partially Earth-dependent for centuries, he also stated that a Mars colony would also be able to create a viable economy. For one, Mars has large concentrations of precious metals that have not been subjected to millennia of human extracting. Second, the concentration of deuterium – a possible source for rocket fuel and nuclear fusion – is five times greater on Mars.
In 1993, NASA adopted a version of this plan for their “Mars Design Reference” mission, which went through five iterations between 1993 and 2009. And while it involved a great deal of thinking and planning, it failed to come up with any specific hardware or projects.
Current Proposals:
Things changed in the 21st century after two presidential administrations made fateful decisions regarding NASA. The first came in 2004 when President George W. Bush announced the “Vision for Space Exploration“. This involved retiring the Space Shuttle and developing a new class of launchers that could take humans back to the Moon by 2020 – known as the Constellation Program.
Then, in February of 2010, the Obama administration announced that it was cancelling the Constellation Program and passed the Authorization Act of 2010. Intrinsic to this plan was a Mars Direct mission concept, which called for the development of the necessary equipment and systems to mount a crewed mission to Mars by the 2030s.
In 2015, NASA’s Human Exploration and Operations Mission Directorate (HEOMD) presented the “Evolvable Mars Campaign”, which outlined their plans for their “Journey to Mars’ by the 2030s. Intrinsic to this plan was the use of the new Orion Multi-Purpose Crew Vehicle (MPCV) and the Space Launch System (SLS).
The proposed journey would involve Three Phases, which would involve a total of 32 SLS launches between 2018 and the 2030s. These missions would send all the necessary components to cis-lunar space and then onto near-Mars space before making crewed landings onto the surface.
Phase One (the “Earth Reliant Phase”) calls for long-term studies aboard the ISS until 2024, as well as testing the SLS and Orion Crew capsule. Currently, this involves the planned launch of Exploration Mission 1 (EM-1) in Sept. of 2018, which will be the first flight of the SLS and the second uncrewed test flight of the Orion spacecraft.
NASA’s Journey to Mars. NASA is developing the capabilities needed to send humans to an asteroid by 2025 and Mars in the 2030s. Credit: NASA/JPL
NASA also plans to capture a near=Earth asteroid and bring it into lunar orbit, as a means of testing the capabilities and equipment for a Mars mission. Known as the Asteroid Redirect Mission, this mission is scheduled to take place in the 2020s and would primarily involve a robotic mission towing the asteroid and returning samples.
Exploration Mission 2 (EM-2), the first crewed flight using the Orion capsule, would conduct a flyby around the Moon and this asteroid between 2021 and 2023. At this point, NASA would be moving into Phase Two (“Proving Ground”) of the Journey to Mars, where the focus would move away from Earth and into cis-lunar space.
Multiple SLS launches would deliver the mission components during this time – including a habitat that would eventually be transported to Martian orbit, landing craft, and exploration vehicles for the surface of Mars. This phase also calls for the testing of key technologies, like Solar Electric Propulsion (aka. the ion engine).
By the early 2030s, Phase Three (“Earth Independent”) would begin. This calls for testing the entry, descent and landing techniques needed to get to the Martian surface, and the development of in-situ resource utilization. It also calls for the transferring of all mission components (and an exploration crew) to Martian orbit, from which the crews would eventually mount missions to designated “Exploration Zones” on the surface.
The European Space Agency (ESA) has long-term plans to send humans to Mars, though they have yet to build a manned spacecraft. As part of the Aurora Program, this would involve a crewed mission to Mars in the 2030s using an Ariane M rocket. Other key points along that timeline include the ExoMars rover (2016-2020), a crewed mission to the Moon in 2024, and an automated mission to Mars in 2026.
Roscosmos, the Russian Federal Space Agency, is also planning a crewed mission to Mars, but doesn’t envision it happening until between 2040 and 2060. In the meantime, they have conducted simulations (called Mars-500), which wrapped up in Russia back in 2011. The Chinese space agency similarly has plans to mount a crewed mission to Mars between 2040 and 2060, but only after crewed missions to Mars take place.
In 2012, a group of Dutch entrepreneurs revealed plans for a crowdfunded campaign to establish a human Mars base, beginning in 2023. Known as MarsOne, the plan calls for a series of one-way missions to establish a permanent and expanding colony on Mars, which would be financed with the help of media participation.
Other details of the MarsOne plan include sending a telecom orbiter by 2018, a rover in 2020, and the base components and its settlers by 2023. The base would be powered by 3,000 square meters of solar panels and the SpaceX Falcon 9 Heavy rocket would be used to launch the hardware. The first crew of 4 astronauts would land on Mars in 2025; then, every two years, a new crew of 4 astronauts would arrive.
SpaceX and Tesla CEO Elon Musk has also announced plans to establish a colony on Mars in the coming decades. 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.
According to Musk’s estimates, the ITS would cost $10 billion to develop and would be ready to ferry the first passengers to Mars as early as 2024. Each of the SpaceX vehicles would accommodate 100 passengers, with trips being made every 26 months (when Earth and Mars are closest). Musk also estimated that tickets would cost $500,000 per person, but would later drop to a third of that.
And while some people might have a hard time thinking of MarsOne’s volunteers or SpaceX’s passengers as astronauts, they would nevertheless be human beings setting foot on the Red Planet. And if they should make it there before any crewed missions by a federal space agency, are we really going to split hairs?
So the question remains, when will see people sent to Mars? The answer is, assuming all goes well, sometime in the next two decades. And while there are plenty who doubt the legitimacy of recent proposals, or the timetables they include, the fact that we are speaking about going to Mars a very real possibility shows just how far we’ve come since the Apollo era.
And does anyone need to be reminded that there were plenty of doubts during the “Race to the Moon” as well? At the time, there were plenty of people claiming the resources could be better spent elsewhere and those who doubted it could even be done. Once again, it seems that the late and great John F. Kennedy should have the last word on that:
“We choose to go to the Moon! … We choose to go to the Moon in this decade and do the other things, not because they are easy, but because they are hard; because that goal will serve to organize and measure the best of our energies and skills, because that challenge is one that we are willing to accept, one we are unwilling to postpone, and one we intend to win.”
If you’d like more information about humans traveling to Mars, check out the Mars Society’s homepage. And here’s a link to MarsDrive, and another group looking to send people to Mars.
M24 (the Small Sagittarius Cloud) and nearby Messier Objects. Credit: Wikisky
Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Messier 24 star cloud. Enjoy!
Back in the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of these objects so that other astronomers wouldn’t make the same mistake. Consisting of 100 objects, the Messier Catalog has come to be viewed as a major milestone in the study of Deep Space Objects.
One such object is Messier 24, otherwise known as the Sagittarius Star Cloud (or Delle Caustiche). Located in the Sagittarius constellation, located approximately 100,000 light years from Earth, this cluster of the Milky Way is one of the densest concentration of individual stars in the night sky.
Description:
Messier 24 is one of the most curious of the catalog entries because it really isn’t a star cluster – simply an oddity. What we are looking at is thousands of stars that belong to the Sagittarius arm of the Milky Way galaxy seen through a chance hole in the gas and dust… a clear “window” in space.
Messier 24 (Sagittarius Star Cloud, Delle Caustiche), also showing like the dark nebula Barnard 92, the dark nebula Barnard 93, and the open cluster NGC 6603. Credit: Wikipedia Commons/Tomasmazon
And speaking of space, M24 fills a space of significant volume, to a depth of 10,000 to 16,000 light-years. This makes it the most dense concentration of individual stars visible using binoculars, with around 1,000 stars visible within a single field of view!
Still, it is sometimes referred to as the Small Sagittarius Star Cloud in order to differentiate it from the Great Sagittarius Star Cloud located north of Gamma Sagittarii and Delta Sagittarii. When viewing this awesome area, take into account how many different objects you can spot just within this region – like dim open cluster, NGC 6603.
E.E. Barnard has cataloged two dark nebulae in the northern region as objects 92 and 93. How about lesser known clusters like Collinder 469 and Markarian 38? Along the southern edge you’ll find emission nebula IC 1283-1284, with two adjacent reflection nebulae, NGC 6589 and NGC 6590.
Their fueling source is the notable little open cluster NGC 6595. Take a tour on the western edge of M24 and see if you can spot 12th-magnitude planetary nebula NGC 6567. Need more? Then how about Delta Cephei variable WZ Sagittarii in the southern area. Its a pulsating giant star that varies in brightness between magnitude 7.5 and 8.5 in slightly less than 22 days!
The Sagittarius constellation. Credit: iau.org
History of Observation:
As bright as the Sagittarius Star Cloud is, we know that Messier probably wasn’t the first to see it – but he was the first to catalog it. As he wrote about it in his notes:
“In the same night, June 20 to 21, 1764, I have discovered on the same parallel as the star cluster I have just been talking about and near the extremity of the bow of Sagittarius, in the milky way, a considerable nebulosity, of about one degree and a half extension: in that nebulosity there are several stars of different magnitudes; the light which is between these stars is divided in several parts. I have determined approximately the position of the middle of this cloud of light; its right ascension is 270d 26′, and its declination 18d 26′, south.”
While other historic astronomers would also look at Messier’s “discovery”, they realized they were looking at a portion of the Milky Way and were somewhat less than enthusiastic. The Sagittarius Star Cloud was named “Delle Caustiche” by Fr. Secchi, “from the peculiar arrangement of its stars in rays, arches, caustic curves, and intertwined spirals.”
As is often the case with Messier Objects, it was the late Admiral Smyth who described it with flowering prose. As he wrote of the large star cloud in July of 1835:
“A beautiful field of stars, below the sinister base of the Polish shield, and in a richly clustering portion of the Milky Way. This object was discovered by Messier in 1764, and described as a mass of stars — a great nebulosity of which the light is divided in several parts. This was probably owing to want of power in the instruments used, as the whole is fairly resolvable, though there is a gathering spot with much star dust [This is NGC 6603!].”
M24, located in the direction of the Sagittarius constellation, shares that region of the sky with many Deep Sky Objects. Credit: freestarcharts.com
Locating Messier 24:
From a dark sky location, M24 is easily located with the unaided eye. It will appear as a large hazy patch in northern portion of the constellation of Sagittarius, about a handspan above the teapot-shaped Sagittarius asterism. For those observing under urban skies, even the slightest optical aid will easily reveal this massive cloud of stars.
Spanning a degree and a half of sky means this huge object is going to cover anywhere from about 1/3 to 1/2 the field of view in most binoculars. It can easily be seen in all optical finderscopes and requires minimum magnification in all telescopes. Even then, you’ll only be able to study portions of the Sagittarius Star Cloud at a time. given its sheer size.
So go forth, and gather ye some star dust of your own. There’s plenty for everyone!
And here are the quick facts on the Sagittarius Stat Cloud to help you get started:
Object Name: Messier 24 Alternative Designations: M24, IC 4715, Sagittarius Star Cloud, Delle Caustiche Object Type: Star Cloud – contains Open Cluster NGC 6603 and NGC 6595, Barnard 92, Barnard 93, Collinder 469, IC 1283-1284, NGC 6589/90 and planetary nebula NGC 6567 Constellation: Sagittarius Right Ascension: 18 : 16.9 (h:m) Declination: -18 : 29 (deg:m) Distance: 10.0 (kly) Visual Brightness: 4.6 (mag) Apparent Dimension: 90 (arc min)
This illustration shows a stage in the predicted merger between our Milky Way galaxy and the neighboring Andromeda galaxy, as it will unfold over the next several billion years. In this image, representing Earth's night sky in 3.75 billion years, Andromeda (left) fills the field of view and begins to distort the Milky Way with tidal pull. (Credit: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger)
We don’t want to scare you, but our own Milky Way is on a collision course with Andromeda, the closest spiral galaxy to our own. At some point during the next few billion years, our galaxy and Andromeda – which also happen to be the two largest galaxies in the Local Group – are going to come together, and with catastrophic consequences.
Stars will be thrown out of the galaxy, others will be destroyed as they crash into the merging supermassive black holes. And the delicate spiral structure of both galaxies will be destroyed as they become a single, giant, elliptical galaxy. But as cataclysmic as this sounds, this sort of process is actually a natural part of galactic evolution.
Astronomers have know about this impending collision for some time. This is based on the direction and speed of our galaxy and Andromeda’s. But more importantly, when astronomers look out into the Universe, they see galaxy collisions happening on a regular basis.
The Antennae galaxies, a pair of interacting galaxies located 45 – 65 million light years from Earth. Credit: Hubble / ESA
Gravitational Collisions:
Galaxies are held together by mutual gravity and orbit around a common center. Interactions between galaxies is quite common, especially between giant and satellite galaxies. This is often the result of a galaxies drifting too close to one another, to the point where the gravity of the satellite galaxy will attract one of the giant galaxy’s primary spiral arms.
In other cases, the path of the satellite galaxy may cause it to intersect with the giant galaxy. Collisions may lead to mergers, assuming that neither galaxy has enough momentum to keep going after the collision has taken place. If one of the colliding galaxies is much larger than the other, it will remain largely intact and retain its shape, while the smaller galaxy will be stripped apart and become part of the larger galaxy.
Such collisions are relatively common, and Andromeda is believed to have collided with at least one other galaxy in the past. Several dwarf galaxies (such as the Sagittarius Dwarf Spheroidal Galaxy) are currently colliding with the Milky Way and merging with it.
However, the word collision is a bit of a misnomer, since the extremely tenuous distribution of matter in galaxies means that actual collisions between stars or planets is extremely unlikely.
Image obtained by the Hubble Space Telescope and the Keck-II telescope, showing a collision that took place billions of years ago. Credit: ESO/NASA/ESA/W. M. Keck Observatory
Andromeda–Milky Way Collision:
In 1929, Edwin Hubble revealed observational evidence which showed that distant galaxies were moving away from the Milky Way. This led him to create Hubble’s Law, which states that a galaxy’s distance and velocity can be determined by measuring its redshift – i.e. a phenomena where an object’s light is shifted toward the red end of the spectrum when it is moving away.
However, spectrographic measurements performed on the light coming from Andromeda showed that its light was shifted towards the blue end of the spectrum (aka. blueshift). This indicated that unlike most galaxies that have been observed since the early 20th century, Andromeda is moving towards us.
In 2012, researchers determined that a collision between the Milky Way and the Andromeda Galaxy was sure to happen, based on Hubble data that tracked the motions of Andromeda from 2002 to 2010. Based on measurements of its blueshift, it is estimated that Andromeda is approaching our galaxy at a rate of about 110 km/second (68 mi/s).
At this rate, it will likely collide with the Milky Way in around 4 billion years. These studies also suggest that M33, the Triangulum Galaxy – the third largest and brightest galaxy of the Local Group – will participate in this event as well. In all likelihood, it will end up in orbit around the Milky Way and Andromeda, then collide with the merger remnant at a later date.
Images from Hubble’s ACS in 2004 and 2005 show four examples of interacting galaxies (at various stages in the process) far away from Earth. Credit: NASA/ESA/J. Lotz, STScI/M. Davis, University of California, Berkeley/A. Koekemoer, STScI.
Consequences:
In a galaxy collision, large galaxies absorb smaller galaxies entirely, tearing them apart and incorporating their stars. But when the galaxies are similar in size – like the Milky Way and Andromeda – the close encounter destroys the spiral structure entirely. The two groups of stars eventually become a giant elliptical galaxy with no discernible spiral structure.
Such interactions can also trigger a small amount of star formation. When the galaxies collide, it causes vast clouds of hydrogen to collect and become compressed, which can trigger a series of gravitational collapses. A galaxy collision also causes a galaxy to age prematurely, since much of its gas is converted into stars.
After this period of rampant star formation, galaxies run out of fuel. The youngest hottest stars detonate as supernovae, and all that’s left are the older, cooler red stars with much longer lives. This is why giant elliptical galaxies, the results of galaxy collisions, have so many old red stars and very little active star formation.
Despite the Andromeda Galaxy containing about 1 trillion stars and the Milky Way containing about 300 billion, the chance of even two stars colliding is negligible because of the huge distances between them. However, both galaxies contain central supermassive black holes, which will converge near the center of the newly-formed galaxy.
Two galaxies colliding in the Corvus constellation. Credit: B. Whitmore (STScI), F. Schweizer (DTM),
This black hole merger will cause orbital energy to be transferred to stars, which will be moved to higher orbits over the course of millions of years. When the two black holes come within a light year of one another, they will emit gravitational waves that will radiate further orbital energy, until they merge completely.
Gas taken up by the combined black hole could create a luminous quasar or an active nucleus to form at the center of the galaxy. And last, the effects of a black hole merger could also kick stars out of the larger galaxy, resulting in hypervelocity rogue stars that could even carry their planets with them.
Today, it is understood that galactic collisions are a common feature in our Universe. Astronomy now frequently simulate them on computers, which realistically simulate the physics involved – including gravitational forces, gas dissipation phenomena, star formation, and feedback.
And be sure to check out this video of the impending galactic collision, courtesy of NASA:
![The Viking 2 lander captured this image of itself on the Martian surface. By NASA - NASA website; description,[1] high resolution image.[2], Public Domain, https://commons.wikimedia.org/w/index.php?curid=17624](https://www.universetoday.com/wp-content/uploads/2016/10/608px-Viking2lander1.jpg)
