In conducting these flybys, the ESA is working in cooperation with ROSCOSMOS, the Russian Federal Space Agency.  Russia is planning to place an unmanned lander on Phobos by 2012 to collect soil samples, and images taken on these flybys will be used to select the landing site.

It is hoped that all of this scrutiny will unlock the secret of this moon’s origin, which is a point of much speculation now.  There are three conflicting theories about this, which we will examine in more detail in a moment, and while the final verdict won’t come out until the actual return of soil samples, these flybys are already filling in some of the blanks in our understanding of this body.

We talked about the Mars Express probe in a previous article (still posted at this site).  This spacecraft was launched by the ESA on June 2, 2003, and arrived at Mars in December of that year.  Since then, it has returned huge amounts of data on the red planet, including many beautiful and striking images of Martian landforms.  One of the accomplishments of this mission has been the detection of methane in some areas of Mars, which may be an indication of some sort of biological activity.  Mars Express has also contributed to the growing body of evidence for frozen water just beneath the Martian surface.

This is not the first time Mars Express has encountered Phobos.  In fact, it’s a regular event for this probe, whose orbit periodically brings it close to the moon.  The folks at ESA call it “Phobos flyby season,” and typically use this time to study the body.  But this time is the closest approach to the moon yet, and will present a chance for more exact measurements than ever before.  The observations will include very precise radiometric readings to determine exactly what the gravity of Phobos is, and how mass is distributed within the body.  This information will be useful to the Russians in planning their lander expedition, and will also address a key fact which has come to light about Phobos: it seems to be hollow.

Well, maybe not completely hollow.  Scientists are estimating that between 25 and 35 percent of Phobos’ interior is empty space.  They have arrived at this conclusion because Phobos simply doesn’t have as much gravity as it should.  The dimensions of this moon are quite well established, so it’s possible to get a rough estimate of how much gravity there should be if the whole body is solid rock.  While the flybys that are happening now will give us our most precise measurement of the moon’s real gravity to date, less precise measurements have already revealed that the gravity of Phobos is much less than it should be.  The conclusion is inescapable: this body doesn’t have as much matter as it seems to have.

This is not as big a mystery as it seems.  In fact, Phobos appears to be a type of body that was predicted before any were actually found, and which now seems to be quite common in the solar system.  Scientists call them “rubble piles,” and that’s exactly what they are.  You see, there are an awful lot of rocks flying around out there, especially in the asteroid belt, which is between the orbits of Mars and Jupiter.  In the early solar system, there were even more of them than there are today.  (Nowadays, the majority of these rocks have already crashed into some larger body, and the asteroids that are left are just the small percentage that have managed to avoid this fate.)

Since all matter exerts some gravitational attraction, every one of those rocks has its own gravity, and when rocks come close to each other, they tend to attract.  If two of these space rocks drift together and stick to each other because of their mutual gravitation, then of course, they’re exerting a stronger gravitational attraction than either one did separately, so they tend to attract still more rocks.  Eventually, if more and more of them come together and stick to each other, you end up with a big mass of rocks loosely held together by gravity.  Of course, they don’t fit together very well, and while they are touching each other in some places, there will also be a lot of gaps.  What you’ve got is a classic rubble pile.

Over millions or billions of years, more rocks will keep hitting this pile, smashing up the outer surface and spraying a lot of asteroid dust around.  Eventually, our rubble pile acquires a coating of this dust which fills in the cracks on the outside of the body, giving the illusion of a solid, unbroken surface.  To an outside observer, there is no obvious sign that this body is not solid- but deep inside, all those gaps are still there.  You wouldn’t suspect a thing unless you measured the body’s gravity, at which point it would become evident that there was an awful lot of empty space inside it.

So, that’s the giveaway: if you find a body that has a much lower gravity pull than it logically should have, you know you’ve got a rubble pile.  We have already found a few of these bodies out there.  For example, it is strongly suspected that Jupiter’s moon, Amalthea, is a rubble pile.

This brings us back to a point mentioned earlier: the fact that there are conflicting theories about the origins of the Martian moons.  One of them is the scenario that we’re looking at here: Mars, being so close to the asteroid belt, has attracted some far-roaming rocks into orbit around itself.  The fact that Phobos- and possibly the other moon, Deimos- is a rubble pile instead of a solid chunk doesn’t really effect this scenario; the rocks may have formed into a pile in the asteroid belt, or after they were captured by Mars.

Another theory about the formation of the Martian moons is that some large body slammed into Mars in the remote past, and Phobos and Deimos are fragments from the collision, thrown off the planet with enough speed to achieve orbit.  If this is the case, it is probable that we won’t know for certain until soil samples can be collected from Mars and both moons, so they can be compared.  If Mars and its moons were originally part of the same body, they should have similar compositions.

However, even if this is the case, it still will not be conclusive, and further research will have to be done to get a definitive answer.  If Mars and its moons are made of similar stuff, there is another possible explanation for it, and this gets to the third theory about the formation of these moons.  This theory holds that the planet and both moons were all formed at the same time, from the same primordial accretion disc that gave birth to all the other planets.  In other words, they may be first generation objects (formed in the birth of the solar system) rather than second generation (formed later from smaller fragments coming together).

So far, it’s all a big mystery, but we are fast closing in on the answer.   The information gained from this series of flybys has already taught us a few things about Phobos, and further analysis of it will reveal more.  Radiometric data from these flybys are being studied right now, and should be precise enough to tell us where the gaps are within the moon.  As said earlier, the gravity analysis from Mars Express will be used to help ROSCOSMOS select the spot to set down its lander in 2012, and the samples from that encounter will answer more questions.  In time, we will pry all of the secrets out of Phobos.

One final note about the possible future of Phobos: it may be a ready-made home for settlers.  What you’ve got is a big rock with a lot of holes in it, and some of those holes can probably be smoothed out and modified to make living spaces.  Since it now appears that at least a quarter of Phobos’ area is empty, that amounts to a lot of space.  If we should ever want a convenient space station orbiting Mars (and we will, eventually) Phobos might be it.  Using a body that’s already there would be a lot easier and cheaper than building something from scratch.  In some far-future time, this little rock might be riddled with underground colonies.

As you can see, the work is just starting on Phobos- and we haven’t even looked at Deimos yet.  The data from the recent encounters will be studied for years to come, and new findings will undoubtedly come to light.  Stick with us; we’ll keep you posted.

Even before they were launched, these three missions were successes. Mars Express, Venus Express and Rosetta were all built using the same design and many of the same data-gathering instruments. The same facilities were used to assemble the probes, and even many of the same people worked on all three projects. By using this strategy, the ESA was able to greatly reduce the amount of time and expense required to launch the missions.

But at first, Rosetta had a troubled childhood. It was originally approved in November 1993, and the destination was to be the comet 46 P/Wirtanen- but the project ran into delays and was not able to make that destination. Preparations continued with a new goal: comet 67 P/Churyumov-Gerasimenko. The new itinerary was an ambitious 10-year journey that would include two asteroid flybys: 2867 Steins in 2008 and 21 Lutetia in 2010.

This was the mission that finally made it into space on March 2, 2004, launched on an Ariane-5G rocket from Kourou, French Guiana. After that, the probe spent almost four years modifying its orbit by making two passes near Earth, and one near Mars. This put it on course for its first destination, asteroid 2867 Steins.

This was our first chance to get a close-up look at a rare kind of asteroid, the E-type. These bodies are thought to be fragments of larger asteroids that fragmented. They are highly reflective, with a featureless flat spectrum, and are thought to be the source of a type of meteorite called Aubrites. They are greatly outnumbered by other types of asteroids, mainly the M-types. While other probes had obtained pictures of eight asteroids, none of them was an E-type. As Rosetta approached 2867 Steins, the planetary scientists were keenly anticipating their first peek at one of these rare bodies.

The flyby took place flawlessly on September 5, 2008. The closest approach was 800 kilometers, and the relative velocity was 8.62 km/sec. Rosetta immediately confirmed the calculations of ground-based astronomers by affirming that 2867 Steins rotates once every 6.05 hours. The probe took some beautiful, clear pictures of the asteroid, imaging about 60 per cent of its surface. While the pictures were in color, 2867 Steins still looked gray; there was no significant color variation over its surface. Rosetta measured the asteroid’s albedo (reflectivity) and found it to be .4, meaning that it reflected about 40 per cent of the sunlight hitting it. (This is about four times as reflective as Earth’s moon.)

Steins is shaped like a child’s top: rounded on “top,” pointed on the “bottom.” The rounded end is dominated by a huge impact crater, 2.1 kilometers in diameter. Running down the side of the asteroid is a row of seven circular indentations. While these look like impact craters, their similar size and shape, and the fact that they are in a row, would seem to indicate that there is a fracture along that line, and the indentations are actually collapsed pits where dust has settled into the crack. There is also a similar groove along the other side of the asteroid, which may be the same fracture running completely through the body.

The surface features of Steins can be used to make inferences about its history. Its surface is not saturated with impact craters; in other words, there is some empty space visible between them. But we know that the early solar system was a violent place, with many impacts occurring frequently, so we would expect that an asteroid would be completely covered with impact craters. If we find an asteroid that doesn’t look like that, we must assume that something happened to erase the older craters. When looking for a likely cause of this phenomenon on Steins, our attention is immediately drawn to that 2.1-kilometer crater on its top. This is a puzzle, because the impact was so great, it should have shattered a solid body. Normally, if you take a solid rock that’s about 6 km wide and hit it with another rock that’s also of considerable size, they should both split into lots of little fragments.

The conclusion is inescapable: 2867 Steins was never a solid body. It’s a “rock pile,” a collection of fragments held loosely together by their mutual gravitation. When the big impact happened, these fragments were shifted around; some of them probably even flew away and then came back and re-collided with the larger body. Since the asteroid itself was the greatest source of gravity nearby, all the pieces eventually settled back together. The crater from the big impact was so huge, it was still recognizable after the shift- but any smaller craters were completely obliterated. Except for that big crater, Steins got a completely fresh surface, and any smaller craters that we see on it now have occurred since then.

So now we have another portrait of an asteroid, and we know something about its past. Before now, this type of body was an abstraction that existed only on the pages of astronomy textbooks. Now it is a real object that we can see and study. ESA has posted a video of the approach to Steins, and the sight of this little jewel-like object spinning out of the darkness is truly inspiring.

And Rosetta hasn’t even reached its destination yet! This is a really ambitious project, and the hits just keep on coming. In November of 2009, Rosetta modified its course by flying past Earth one more time, which put it on course for the asteroid 21 Lutetia. That encounter will happen in July of this year, and we will gain another asteroid for our photo gallery. These encounters always give us some interesting science, and this one will certainly be no exception.

After this, the probe will go into a period of hibernation. When it wakes up, it will be at it final destination, the comet 67P/Churyumov-Gerasimenko. If all goes well, the probe will go into orbit around the comet and put down a lander, which will anchor itself by shooting harpoons into the comet’s surface. The lander and the probe will stay with the comet through its entire approach to the sun, observing the changes that occur as it heats up and begins to give off the gasses that form the comet’s “tail.” If this is successful, we will finally know in detail exactly what happens to a comet as it approaches the sun.

The show is just starting! Watch for updates at this website.

Sources:

Rosetta homepage at the website of the european Space Agency: sci.esa.int/science-e/www/area/index.cfm?fareaid=13

Space Topics: Rosetta at the website of the Planetary Society: planetary.org/explore/topics/rosetta/

Rosetta: the United States’ Contribution at the NASA website: search.nasa.gov/search/search.jsp?nasaInclude=rosetta+comet+mission

Mars Express was launched from Baikonur, Kazakhstan on June 2, 2003 on board a Russian Soyuz/Fregate launcher. The launcher provided all of the thrust needed to put the spacecraft on an orbit that would take it to Mars. The craft would only use its onboard propulsion system for minor course corrections, and to slow down for orbital insertion upon arrival at Mars.

Mars Express took about six months to complete its journey, arriving at Mars in December of 2003. Immediately before insertion into orbit around Mars, the probe was supposed to deploy a small rover which would land on the surface and perform close-up observations of surface features. Unfortunately, this was the only disappointment of the mission. The rover was supposed to send a signal to mission control on Earth confirming successful deployment, but the signal never came. After unsuccessful attempts to re-establish contact, the ESA finally had to face the reality: their rover was lost.

But there have been few space missions that have not had some minor malfunction, or some part that did not perform as planned. These failures are disappointing, but the overall merit of the mission should be judged by its successes, not its failures. For Mars Express, the successes have been stunning.

Mars Express (so called because it was made and launched with record speed, using parts and facilities from previous missions) isn’t much to look at. It’s a plain box, 1.5 x 1.8 x 1.4 meters large, with a small dish antenna on one side and two solar panels, which were deployed immediately after its detachment from the launcher. Though Mars is farther from the sun than Earth, and the solar radiation is correspondingly weaker, these solar panels are still capable of delivering 500 watts of electricity, which is enough to run the probe. When Mars is between the probe and the sun, a lithium-ion battery, charged when the probe was in the sunlight, takes over the power requirements. Mars Express contains seven scientific instruments contributed by Sweden, Germany, Italy, England and France. Besides the ESA member nations, the U.S. and Poland are also involved in the project.

When Mars Express was launched in 2003, everyone involved undoubtedly had a moment of deja vu remembering the ESA’s earlier attempt to launch a Mars probe. In November of 1997, the Mars 96 probe was launched, only to end up at the bottom of the Pacific Ocean when its second booster stage failed to propel it into a stable orbit. Mars Express was ESA’s attempt at a “faster, better, cheaper” mission, using nearly 80% of the design and components from the earlier Rosetta spacecraft and elements of the solar array taken from the Globalstar satellite series. Several of the scientific instruments on Mars Express are spares or modified designs of experiments lost on the ill-fated Mars 96 mission.

Part of the mission’s objective was to compile a series of photos which would make up the first 3D image of the Martian surface. In pursuit of this goal, Mars Express has taken pictures of landforms all over the planet, which, in addition to the scientific information that they can yield, are also stunningly beautiful. Analysis of these images will go on for years to come, and will undoubtedly reveal much about the planet and how its various features formed.

This mission has already yielded at least two pieces of information which, if confirmed by future investigation, have enormous implications in our understanding of the red planet. One is the possible presence of large amounts of frozen water beneath the Martian surface, and the other is the presence of methane gas, which could have been created by living organisms.

Let’s be clear about this. We’re talking about water, and maybe even life, which are present on Mars now, not in the distant past. For some years, there has been growing evidence for large amounts of water in the early days of planet Mars. Landforms show unequivocal signs of erosion from flowing water, including extensive river systems, deltas, etc. But all of that was long ago, when the atmosphere was denser and liquid water could form. At present, the atmosphere of Mars is so thin, water would sublimate directly from ice into water vapor without passing through a liquid phase. It was assumed that as the air got thinner, the surface water had turned into vapor and eventually been lost into space.

Maybe not. Mars Express has raised the possibility that a significant percentage of this ancient water is still there, beneath the Martian surface. In 2005, the probe found evidence for a large mass of pack ice just under the ground in an area close to the Martian equator. This ice, scientists theorize, may have collected in the geologically recent past, perhaps two to five million years ago, when flooding brought it down from the nearby Elysium region. The intervening land clearly shows the signs of this flooding. Planetary scientists theorize that the ice may be prevented from sublimating by a layer of volcanic ash which covers it.

The hugeness of this possibility cannot be overstated. Without native water, permanent colonies on Mars will be impossible. With it, our descendants can become Martians. That’s how big it is.

But there may already be Martians- at least small ones. Mars Express has found spectrographic evidence of methane in the Martian atmosphere above the same region where the water was detected. Methane can be made by inorganic processes- but on Earth, more than 90% of the methane was made by the activity of living organisms. And since methane breaks down when exposed to solar radiation, it only has a lifetime of 300 to 600 years.

It should be stressed that the methane findings are controversial, and are not accepted by all planetary scientists. But if the methane is really there, it must have been made in the last few centuries- and the most likely source is life. If it is confirmed by future observations, this could be our first incontrovertible evidence for life off the Earth.

This may be only the beginning for Mars Express. Its mission was supposed to end in 2005, but has now been extended through December of 2012. Considering what it has told us already, what else might Mars Express have in store for us?

As more information comes in, this site will feature updates. Watch for future articles.