14 November 2014

A rocket from *The Adventures of Tintin*. Hergé
A rocket from The Adventures of Tintin.

Of all the inner planets, only Mercury hasn’t had a probe land on it. It likely won’t for quite some time. The reason isn’t because of lack of desire, or worthy science to be done, but because of a simple thing known as delta-v.

Delta-v, or change in velocity, is the amount of speed a spacecraft need to gain or lose in order to reach a destination. If you launch a spacecraft from Earth, it is basically moving at the speed of Earth in its orbit around the Sun. To get to another planet you have to either increase your speed to reach Mars and the outer planets, or lose speed to reach Venus or Mercury.

There’s two ways to get delta-v. One is by simply using fuel. Fire rockets in the right direction and you can speed up or slow down as necessary. The only downside is that the more you need delta-v, the more you need fuel, and that adds mass and cost to your spacecraft. Another way is to make a close fly-by of a planet. Basically, if you approach a planet in the direction of its orbit (coming up from behind, if you will), then the gravity between the planet and your spacecraft will cause the spacecraft to speed up at the cost of slowing down the planet by a tiny, tiny amount. Making a flyby in the opposite direction can cause your spacecraft to slow down. This costs you nothing in terms of fuel, but takes time because you need to orbit the Sun in just the right way.

Messenger's complex orbit to reach Mercury. Wikipedia
Messenger’s complex orbit to reach Mercury.

So what does this have to do with Mercury? Since Mercury is close to the Sun, you have to lose a lot of delta-v to reach it. In fact, even with a very efficient orbit, the delta-v to reach Mercury is about the same as that needed to reach Jupiter. Of course we’ve reached Jupiter several times, put a probes in orbit, and even dropped a probe into Jupiter’s atmosphere. We only put an orbiter around Mercury in 2011 with the Messenger spacecraft. Before that, there was only the Mariner 10 flyby of Mercury, so it’s been a while since we’ve visited the planet. It also took several gravitational assists with Earth and Venus just to get Messenger into Mercury orbit.

Still, Messenger is just an orbiter. There’s no lander component to the mission. That again has to do with delta-v. The reason we’ve been able to put so many landers and rovers on Mars is that it has a mild atmosphere. We can use that atmosphere to “air-brake” a spacecraft, slowing it enough to make a landing without crashing. Mercury has no atmosphere, so any lander would have to be slowed by fuel alone, which is a hefty challenge.

There are currently no solid plans to put a lander on Mercury. Delta-v is a harsh mistress.