MESSENGER was a great example of how NASA’s reputation for solid engineering can extend the life of their spacecraft far beyond anyone’s expectations. Originally slated for a one year mission once it reached it’s destination (a 7 year long journey in itself) MESSENGER continued to operate around Mercury for another 3 years past its original mission date, providing all sorts of great data on the diminutive planet that hugs our sun. However after being in orbit for so long its fuel reserves ran empty leaving it unable to maintain its orbit. Then last week MESSENGER crash landed on Mercury’s surface putting an end to the 10 year long mission. However before that happened MESSENGER sent back some interesting data around Mercury’s past.
As MESSENGER’s orbit deteriorated it creeped ever closer to the surface of Mercury allowing it to take measurements that it couldn’t do previously due to concerns about the spacecraft not being able to recover from such a close approach. During this time, when MESSENGER was orbiting at a mere 15KMs (just a hair above the max flight ceiling of a modern jetliner) it was able to use its magnetometer to detect the magnetic field emanating from the rocks on Mercury’s surface. These fields showed that the magnetic field that surrounds Mercury is incredibly ancient, dating back almost 4 billion years (right around the creation of our solar system). This is interesting for a variety of reasons but most of all because of how similar Mercury’s magnetic field is to ours.
Of all the planets in our solar system only Earth and Mars have a sustained magnetic field that comes from an internal dynamo of undulating molten metals. Whilst the gas giants also generate magnetic fields they come from a far more exotic form of matter (metallic hydrogen) and our other rocky planets, Venus and Mars, have cores that have long since solidified, killing any significant field that might have once been present. Mercury’s field is much weaker than Earth’s, on the order of only 1% or so, but it’s still enough to produce a magnetosphere that deflects the solar wind. Knowing how Mercury’s field evolved and changed over time will give us insights not only into our own magnetic field but of those planets in our solar system who have long since lost theirs.
There’s likely a bunch more revelations to come from the data that MESSENGER gathered over all those years it spent orbiting our tiny celestial sister but discoveries like this, ones that could only be made in the mission’s death throes, feel like they have a special kind of significance. Whilst it might not be the stuff that makes headlines around the world it’s the kind of incremental discovery that gives us insight into the inner workings of planets and their creation, something we will most definitely need to understand as we venture further into space.
Mercury is a strange little beast of a planet. It’s the closest planet to our sun and manages to whip around it just under 88 days. Its “days” are 59 earth days long and whilst it’s not tidally locked to our parent star (like the moon is to us, always showing the same face to the earth) it is in a 3:2 spin-orbit resonance. This has led to some interesting phenomena when we’ve sent probes to image it as the only probe to ever visit it, Mariner 10, only managed to image 45% of the planet’s surface on it’s 2 encounter trip with the tortured little planet. That all changed a few years ago when MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) made its first approach to Mercury in January 2008 and sent back images of the as of yet unseen side of the planet. Ever since then MESSENGER has been on a long trajectory that will eventually bring it into orbit with Mercury and it will begin it’s year long mission of observations.
It just so happens that that day is today.
MESSENGER has been in space for an extremely long time, almost 7 years. You might be wondering why it has taken this craft so long to reach Mercury and the answer requires that you understand a little about orbital mechanics. You see as a heavenly body, in this case a satellite, moves closer to another body it will tend to speed up. This is known as the conservation of angular momentum and it’s the same principle that governs the increase in speed when you bring your arms in closer whilst you’re spinning. Thus for a satellite that’s launched from Earth to be able to orbit Mercury it has to shed all that extra speed so it can match up to it, otherwise it would just whiz right past it. Since doing this with a rocket is rather expensive (the fuel required would be phenomenal) NASA instead opts to shed velocity by a complicated set of maneuvers between planets, each of which removes a portion of the satellite’s velocity. This is cheap fuel wise but means the space craft will have to endure many years in space before it reaches its destination.
As I write this MESSENGER is making its final preparations to insert itself into an orbit around Mercury. MESSENGER hopes to demystify the diminutive planet by providing hi-resolution imaging of the planet (there’s still 5% we haven’t seen yet), doing chemical analysis to determine the planet’s makeup and attempting to figure out why Mercury has a magnetic field. Probably the most interesting part of MESSENGER will be the last part as our current theories on planet formation point to Mercury being much like our moon with a solid core and no magnetic field to speak of. The presence of one there suggests that part of Mercury’s core is still molten and raises a number of questions over how planets and natural satellites like our moon form. It will also be the first ever artificial satellite of Mercury, something that still eludes many of the other planets in our solar system.
This is the kind of science that NASA really excels at, the stuff that just hasn’t been done before. It’s really amazing to see NASA flex their engineering muscle, designing systems that survive in the most unforgiving environment we know for decades and still function as expected. The next year will be filled with all kinds of awesome discoveries about our tortured little cousin Mercury and I for one can’t wait to see how the analysis of its magnetic field changes the way we model planet formations in the future.