As often happens in the field of space exploration, the idea was way ahead of the reality.  The mission’s unusual name is a salute to Italian astrophysicist Guiseppe “Bepi” Colombo, who originally figured out the course that such a flight should take.  In 1975, Colombo proposed a plan to NASA by which a spacecraft could swing close to Mercury three times by making gravity-assist maneuvers around Venus.  The plan would have worked, but was too ambitious for NASA at the time; it got shelved and forgotten.  Now the years have passed, and things that were once unattainable seem within reach.

Unfortunately, Bepi Colombo died in 1984.  Another sad fact about space exploration is that those who think up the great ideas often don’t live long enough to see them come of age.  The mission that he proposed years ago is going to happen at last, and it will bear his name.

BepiColombo will be launched aboard a Soyuz-Fregat rocket from Kourou, French Guiana.   The date seems to be uncertain; the JAXA website says it’s 2013, but the ESA site says 2014.  (It’s possible that the mission plans were changed in the early stages, as often happens, and that these two postings reflect different versions of the plan.)  BepiColombo’s objectives are to make precise observations of the planet’s magnetic field, magnetosphere, interior and surface.

One of the main questions that it will try to answer is why, of all the planets in the solar system, only Earth and Mercury have active magnetic fields.   Actually, we know why Earth has one, but knowing that just makes Mercury more perplexing.   Earth has an active magnetic field because it has a molten layer surrounding its solid inner core.  The innermost core is compressed into a solid by gravity, but above that, there is a viscous, molten layer called the outer mantle.  The fluid contains iron, and as it moves around the core, the entire planet becomes a giant electrical generator.  The principal is exactly the same as generators here on Earth, in which a rotor moves around a stator, creating an electrical field.  In this case, we see the basic concept expanded to a planetary scale, and the field that results is so strong, it forms lines of magnetic force that stretch around the whole planet.  The important point here is that without the molten outer layer and the solid inner layer moving past each other, you wouldn’t get the magnetic field.  It’s being constantly generated and replenished by that spinning motion inside the planet.

If Earth’s molten outer core were to cool down and become solid- which will happen eventually, in our planet’s old age- the magnetic field would no longer be generated.  Of course, this does not mean that the earth’s magnetic field would suddenly disappear.  We all know that when you place a piece of iron in a magnetic field for a while, it becomes magnetized, too.  If you cut off the magnetism, the iron will still retain some of that energy, but it will be weaker than the original source, and it will fade with time.  If the iron is all mixed up with a bunch of rocks and other stuff, and the mixture is of a very uneven consistency, then the magnetism will fade at an uneven rate.  If there’s more iron in one spot than another, then of course, that spot will hold the magnetism longer.  Eventually you will end up with a glob of iron and rock and stuff which has a very weak, uneven magnetic field around it.  If you had the instruments to measure it, it would be immediately obvious that this was an old, fading field rather than an active one that is being constantly replenished from within.

Let’s leave Mercury for a moment and look at Mars.  This planet has a weak, spotty magnetic field, which tells us that it once had a molten outer core like Earth’s, but that core has now become solid and is no longer generating magnetism.  This is not surprising, because Mars is smaller than Earth, and must have lost the initial heat of its birth faster than Earth has.  Mars is said to have a “fossil” magnetic field.

So here we have the two extremes: Earth with its active field, and poor old Mars with its weak, faded one.  In the old days, when we were looking at Mercury from afar, the popular thought was that it would probably have no magnetic field at all, or if it did, the field would only be a fossil field.   The reasoning was that since Mercury is considerably smaller than either Earth or Mars, it should have long since lost all of the original heat from its formation.  And despite the fact that Mercury is the closest planet to the sun, it still isn’t hot enough to melt rock.  After all, the planet’s surface is still quite solid.  Since the inner region of the planet should have lost all of its original heat, and is insulated by all the rock that lies above it, it must be cooler than the sun-baked surface.  So, if the surface is solid, the core should be, too.  Looking at it logically, you get a picture of a small planet with a relatively cool and very solid interior.

Or so we thought, back in the olden days.  When Mariner 10 made its flyby of Mercury in March of 1974, it revealed what nobody had expected to find: an active magnetic field.  Amazingly, this little planet seems to have a molten portion in its interior, just as Earth does.  Either that, or the current theory about how planetary magnetic fields are generated is completely wrong.  If that’s the case, it means we’ll have to throw away all the textbooks and start over at square one- and since this would be a scandalous waste of paper, we can only hope that it won’t be necessary.  A less revolutionary explanation is that Mercury may have large amounts of radioactive material in its interior, which generates enough heat to melt iron and form a molten mantle- but at the moment, this is pure speculation.

In any case, the planetary scientists certainly want to have a closer look.  The part of BepiColombo’s instrumentation which was contributed by JAXA is the Mercury Magnetospheric Orbiter (MMO) which will observe the planet’s magnetic field with unprecedented accuracy in an effort to better understand its configuration and how it is being created.  It is hoped that the probe can also reveal how Mercury’s magnetosphere differs from Earth’s.  Since these are the only bodies known to have magnetospheres, it is possible that comparing the two will help us understand  something about both of them.

Another job of the MMO is to observe the tenuous exosphere of Mercury.  Rather than a gaseous atmosphere like Earth’s, Mercury is surrounded by a very thin layer of atoms, mainly sodium, that have been blasted off the surface by the fierce solar radiation and the impacts of micrometeorites.  These atoms quickly heat up and escape into space, so Mercury never builds up as much pressure as Earth’s atmosphere has.  This exosphere has never been studied in detail before, and the MMO will observe its structure and the process of formation and escape into space.

The other component of the BepiColombo mission is the Mercury Planetary Orbiter (MPO), contributed by the European Space Agency.  The function of this probe is to study Mercury as a planet: its form, interior, structure, geology, composition and craters.  This will result in a more accurate and detailed map of Mercury than we have ever had before, as well as a better understanding of just what the planet is made of.

One specific question that will be addressed by the MPO is whether there is frozen water at Mercury’s poles.  In the flyby of Mariner 10 back in 1974, the satellite received radar reflections from the polar regions, so we know that there’s something highly reflective there.  Nature doesn’t produce many things that are that reflective; frozen water is the most common.  And it makes sense, because there are areas at Mercury’s poles that are permanently shaded, and probably have been been since the planet was formed.  As strange as it seems, the closest planet to the sun has places that have never felt sunlight, and may be the coldest piece of ground in the solar system.  The MPO will subject these areas to closer scrutiny, looking for ice.

Like some other places in the solar system, Mercury is interesting because it can tell us something about the system’s early times.  On Earth, any substances that we find have been crushed, melted, reformed, eroded and then reformed again, possibly many times over.  There really isn’t much pristine matter from the solar system’s birth for us to study here.   On some of the other bodies in the system, there may be more unchanged matter that can give us a better picture of the original accretion disc which gave birth to all of this.  In particular, if there is ice at the Mercurian poles, it may be a sample of the water vapor that was present in that disc around the sun, which froze in those early times and has remained frozen ever since.  As you can imagine, the planetary scientists are itching to get a peek at it.

The questions are there, just waiting to be answered- and BepiColombo will bring us some of those answers.  By the time it arrives in 2020, we should already have a lot of new information on Mercury from NASA’s MESSENGER probe, which will have been in orbit around the planet for nine years.  BepiColombo will fill in more of the gaps in our knowledge, providing us with our most complete picture yet of the sun’s first planet.    The projected mission is only supposed to last for one Earth year, but there is the possibility of an extension beyond that.

BepiColombo is a big, ambitious mission with huge potential.  It is a remarkable technical achievement, and in the coming years it will bring us some fascinating science about one of the last unexplored bodies in our system.

The fun is just beginning!  As the news comes in, you get always get it here.

Sources:
Mercury Exploration Mission “BepiColombo” at website of Japan Aerospace Exploration Agency:  jaxa.jp/projects/sat/bepi/index_e.html

BepiColombo page at website of the European Space Agency:  sci.esa.int/science-e/www/area/index.cfm?fareaid=30

BepiColombo Overview on the Science and Technology page at website of the European Space Agency:  esa.int/esaSC/120391_index_0_m.html

World Book at NASA- Mercury:  nasa.gov/worldbook/mercury_worldbook.html

Mercury: Closest Planet to the Sun at website of National Geographic:  science.nationalgeographic.com/science/space/solar-system/mercury-article.html

BepiColombo: Mercury Interior at website of the European Space Agency:  sci.esa.int/science-e/www/object/index.cfm?fobjectid=31272

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.