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.

A sudden loss of signal unexpectedly took place just before the closest approach, but this now appears to have been a safety measure taken by the onboard fault-management system, and did not result in a loss of important data, as was originally feared. It happened during an eighteen minute period when the probe was being eclipsed by the shadow of Mercury. With solar power temporarily cut off, the probe was supposed to operate on its own internal batteries during this time, but ten minutes after entering eclipse and four minutes before the closest approach point, the signal from the probe stopped.

According to Eric Finnegan of the Johns Hopkins Applied Physics Laboratory, the spacecraft had autonomously switched itself to a safe operating mode due to an unexpected configuration of the power system during the eclipse. While in this safe mode, the probe preserved all data on its solid-state recorder. When the probe came out of eclipse, it returned to operational mode and began transmitting the stored data to Earth. All the data have now been received, and are being analyzed to confirm the full sequence of events.

It now appears that the main purpose of the flyby, the gravity assist to alter the probe’s orbit, was completely successful. In addition, all of the observational data from the approach have been retrieved, including pictures of previously unseen terrain.

This was MESSENGER’s final flyby of Mercury. For the next 18 months, the probe and the planet will move in their respective orbits, and when they approach each other again, MESSENGER will not flyby, but instead will go into orbit around the planet. From that vantage point, it will be able to observe the planet in unprecedented detail.

It will be the culmination of a mission that has been a model of success from the start. MESSENGER was launched on August 3, 2004, the first probe to be sent to Mercury since the Mariner 10 flyby 33 years earlier. The planned course would be an ambitious voyage through the inner solar system, with course modifications provided by one flyby of Earth, two flybys of Venus and three flybys of Mercury.

The first of the Mercury encounters happened on January 14, 2008. In it, MESSENGER passed 200 kilometers from the planet, imaging portions of the surface that had never been seen before and collecting a wide array of scientific observations, including magnetometer readings and spectrographic data. The second flyby took place on October 6, 2008. Again the probe passed about 200 kilometers from the surface, and took advantage of the opportunity to make further observations.

The third flyby was the closest, bringing the probe within about 140 miles of Mercury’s surface. On this one, Mercury still had a few surprises. One of these was a double-ringed impact basin 180 miles across. The shape of the feature is remarkably well preserved, indicating that it is probably relatively young, perhaps only a billion years old. That’s unusual, since most impact craters date to the early days of the solar system, some four billion years ago. The inner floor of this basin is of a different color from its surroundings, and appears to be even younger than the rest of the feature. This may be the youngest volcanic material on Mercury.

Spectrographic observations performed on the third flyby also revealed that the surface material on Mercury is rusty. It contains a high percentage of iron oxide, which provides the answer to an earlier puzzle. Previous observations had indicated that there was a low amount of iron on the surface, which was a surprise for a small, rocky planet. Now we know that the iron is there after all; it’s just locked up in those oxides.

This flyby also provided the chance for further observations of Mercury’s magnetic field and its exosphere (extremely thin atmosphere) and the first detailed scans of the north and south poles. These polar regions will be a great source of interest when MESSENGER settles into its final orbit around the planet, since there is at least a slight possibility that they could contain water. This amazing possibility is due to a paradox: since Mercury keeps the same face toward the sun at all times, its dark side- and particularly the polar regions- may have some of the coldest real estate in the solar system. As incredible as it seems, Mercury may have ice that has been frozen since the planet’s birth.

That’s just one of the many questions that will be addressed when MESSENGER and Mercury come back together and the probe goes into its permanent orbit. When that happens, we will start to really get acquainted with the sun’s closest planet, until recently the least explored body in the solar system.