Mariner, Messenger, and Space Missions to Mercury

In December 2009, NASA scientists created the first-ever world atlas of the planet Mercury. This seemingly banal feat was hailed as a considerable accomplishment because Mercury is the closest planet to the Sun, and is therefore extremely difficult to visit with space probes. Just two space probes have flown past the "iron planet" in the history of human space flight, Mariner 10 and the still-active Messenger probe. In 2011, Messenger will enter orbit around Mercury -- the first space probe ever to do so -- and will almost certainly begin transmitting the sort of string of scientific data and discoveries we currently associate with Mars and Venus. This brief article will help you understand the history of our investigation of this mysterious little planet.

Small, Barren, and Fast

Mercury does not, to be fair, rank highest on the list of space travel priorities. It is very tiny; whereas Mars is reasonably Earth-like and might have once held life, Mercury is more like our moon than like any of the other planets. It does have an iron core, much like Earth's, but it has no moons of its own, its surface is cratered and pockmarked from billions of years of meteor impacts, and its atmosphere is extremely thin. Much of its original atmosphere, and most of its original mass as well, have been lost over time, either burned away by the Sun or blasted away by an impact with a giant planetesimal during the early period of the solar system.

If Mercury is seemingly a barren rock, there are certain things about it that make it interesting. It has some very strange geological features, probably the result of whatever cataclysmic event resulted in the loss of its surface and its atmosphere. It also has a very strange orbit: Mercury's year (the time it takes to orbit the Sun) lasts just 88 Earth days, but it spins extremely slowly. One day on Mercury, from sunup to sunup, lasts an incredible 176 days -- half a year here on Earth, and two years in Mercury time. Perhaps most interestingly, its craters are deep enough that the evidence suggests there are probably pockets of frozen waters at the north and south poles of the planet.

And the planet's magnetic field is extremely strong, at least in relation to the planet's size. It's theorized that this is because the original iron core is still intact, but seems disproportionately large following the loss of most of the planet's original surface.

Aside from being of less scientific interest than planets like Mars, Venus, Jupiter, and Saturn, Mercury's complex orbital dynamics have also scared away a lot of would-be researchers and space probe engineers. When travelling away from the Sun (for example, on a mission to Mars or Jupiter), space probes slow down slightly in transit because of the pull of the distant Sun's gravity. This is not actually a problem, because some deceleration is required in order to enter orbit around another planet anyways.

Moving inward into the solar system, however, reverses the situation and creates a serious problem. Space probes travelling inward from the Earth are speeding up as they go -- so a probe that orbits Mercury, as Messenger will attempt to do in 2011, must do even more braking of its own (and carry even more fuel in order to do so). What's more, once the spacecraft is in Mercury's orbit, it will require yet more fuel in order to correct its course regularly. If it doesn't, the Sun is far too close, and the probe will eventually be drawn out of Mercury's orbit and pulled straight into the sun.

Mariner 10 -- http://sse.jpl.gov/ -- Mariner 10 was not actually a Mercury probe, but the last in the Mariner series of probes, in its case intended to study Mercury and Venis. (The final two projects in the Mariner series were reorganized and became the more famous Voyager probes.) Mariner flew by Venus, then made three flybys of Mercury, before running out of fuel in 1975. It's still in a solar orbit, having been switched off by NASA. Most likely its instruments have been too damaged by radiation now to be switched back on.

During its flybys, Mariner mapped slightly less than half of Mercury's total surface area, taking almost 3000 photos. It's Mariner 10's readings that confirmed that Mercury had an atmosphere, albeit a thin one, as well as a dense iron core and a magnetic field. Because of its long day and proximity to the sun, the temperature range between day and night identified by the Mariner probe is truly shocking: a thermometer on the surface would reach 190 C (370 F), but during the long night temperatures plunge to -180 C (-300 F).

Messenger -- http://messenger.jhuapl.edu/ -- Messenger, launched in 2004, is NASA's ambitious attempt to enter Mercury's orbit for the first time and set up a long-range study of the planet from orbit. Its cameras are around 100 times more powerful than those on Mariner 10, and it is expected to spend one year in orbit. (As with all time estimates for NASA space missions, this is a deliberately lowball figure: this is the time it will take to meet the basic scientific objectives of the mission, but probes often survive for much longer periods of time.) Because it will be there for such a long time, it will be able to survey the entire surface in detail, lit by the Sun; Mariner, by contrast, was only able to see the single half of the planet which was experiencing daytime when the probe flew past.

Messenger's trip was a long one, to say the least: it left Earth in 2004 and has swince swung into and out of the inner solar system, swinging by Earth, Venus, and lastly Mercury, several times, on an extremely complicated course intended to get it into Mercury's orbit with the greatest fuel efficiency. The most recent activity on this flight was the close flyby which occurred in September 2009, when Messenger zoomed by just 230 km from the surface of Mercury. It will return again in 2011, to enter Mercury orbit.

In addition to detailed mapping of the surface, scientists expect to study the atmosphere as well as to examine whether Mercury is gradually shrinking (this is what is expected, since the outer regions of the very large core should be slowly cooling, freezing, and compressing).

BepiColombo -- http://sci.esa.int/ -- BepiColombo is a planned joint European-Japanese spacecraft currently expected to launch in 2014, spend six years on a meandering course into the inner solar system, and enter Mercurian orbit in 2020. Once there, the spacecraft will actually split into three pieces: the transfer module which handled navigation from Earth, and two satellites which will enter Mercury orbit carrying different research packages. Originally, the mission called for a third element, a lander which would reach Mercury's surface in the same way that we have already landed on Mars, Venus, and Titan. However, the lander was eliminated due to budget cuts.