Slowing down while falling at a star

It takes a long time to slow down when you go to Mercury. The closest planet to the Sun, Mercury is a small rocky world that has only been visited by one probe before, over thirty years ago. Back in 1974/75 Mariner 10 flew by Mercury three times, imaging over half of the planet. Mariner 10 was the first space craft to use a gravity assist, using the mass of Venus to alter is speed and direction in order to get in line with Mercury. Gravity assists have been used in many missions since then, allowing craft with limited fuel to go further than they would otherwise be able to. In March of 1975 the mission ended and Mariner 10 continued to swing around the Sun in silence.

Since then probes have visited all the other planets, but, until now, none have gone back to Mercury. Flybys like Mariner 10’s are fine as they go, but for a much better analysis of a planet you really want to get an orbiter. NASA are sending just such a probe to Mercury outfitted with the latest sensors that should greatly increase our understanding of this half-mapped planet. But in order to get into an orbit around Mercury, the probe MESSENGER will need to be traveling very slowly alongside the small planet. Unfortunately the closeness of the Sun makes it difficult to slow down, it’s immense gravity pulling on the small probe which can’t hold enough fuel to slow down on it’s own.

In order to change it’s velocity, MESSENGER also uses gravity assists, sling shotting around various planets. Launched back in 2004 MESSENGER has already swung past Earth once, then on to Venus twice, and finally it will swing past Mercury three times before it powers itself into an orbit around the rocky planet.

Although it will be another three years before the main part of it’s mission can start, MESSENGER will be doing as much observation as it can on it’s upcoming flyby next week. The results will take a little while to come back though, since all the transmission capacity will be taken up with making sure that the sunshade is in the correct position so that the instruments don’t fry while doing their work. Once it has flown by the planet the data will come back and give us new views of this planet that hasn’t been seen close up in over three decades. Then in 2011 MESSENGER will finally have slowed down enough to do it’s main mission without plunging into the Sun.