Cav's Place

Saw this line in a comment someone posted when it was first announced that MESSENGER has been successfully inserted into orbit around Mercury and couldn’t help but use it as the title of this post. Indeed it seems like an achievement worthy of such a title, mainly because of the way in which it was achieved. You probably couldn’t make a trip from Earth to Mercury more complicated if you tried, so it’s that much more amazing to see that such an apparently insane plan worked out in the end.

But let’s take a moment to go through this mission’s timeline… After the planning, preparations and actual construction took over five years, on August 3, 2004, the MESSENGER probe was launching, with the aim of becoming the first to orbit around Mercury. It then flew by Earth on August 2, 2005, and by Venus on October 24, 2006 and June 5, 2007, before finally encountering Mercury on January 14, 2008. However, at that time it was traveling much too fast, so this was only a flyby meant to slow it down even more, just like the three before it and the two others, also of Mercury, that came after it, on October 6, 2008 and September 29, 2009. Only then was it slow enough for its thrusters to be able to complete the final braking maneuver, during the next encounter.

Prior to this mission, such a feat appeared prohibitively expensive at best, if not even highly unlikely to even be possible with our current technology, because of the huge amount of fuel necessary to slow down the probe enough to be caught in the gravitational field of such a small planet this close to the Sun. However, this utterly insane trajectory, apparently designed in 1985, proved to be the solution. Even so, nearly a third of the total amount of fuel was used for this final maneuver, which required firing the thrusters for 15 minutes in order to slow the probe down by just over 3100 km/h and insert it on a highly elliptical orbit around Mercury.
Fuel is necessary for the actual mission as well, as adjustment maneuvers are planned at least once per Mercury year, which lasts just about 88 Earth days. These maneuvers are necessary because otherwise the minimum distance from Mercury would increase past 500 km, which is the maximum permitted by the mission plan. However, each such maneuver, meant to reduce the minimum distance back to 200 km, has the undesired but unavoidable side effect of reducing the orbital period by about 15 minutes, which means that some more fuel needs to be burned soon after, at the other end of the orbit, in order to get the orbital period back to the planned 12 hours.

Though the distance between the semi-major axes of Earth and Mercury is just under 91.7 million kilometers, MESSENGER traveled nearly 7.9 billion kilometers over more than six and a half years in order to get and stay there, orbital insertion being achieved at around 1 AM GMT on March 18, 2011. It will now remain in history as the first man-made satellite of Mercury and only the second probe to directly observe the planet. And it already imaged the planet almost completely and shocked everyone by discovering significant amounts of water in Mercury’s exosphere during the previous flybys. As a result, anything it will discover during its primary mission, which is scheduled to properly start on April 4, can at this point be considered a bonus.