Today, the MESSENGER orbiter, which had been circling Mercury since 2011, executed a lithobraking manoeuvre.

What is lithobraking? It’s one of the ways of reducing the velocity of a spacecraft. Yes, it’s braking as in braking, that thing you do to slow down a car or a bike. The verb brake in this sense comes from the noun brake, ‘device for slowing or stopping a wheel’, which comes from one or both (by mutual influence) of the Dutch verb breken ‘break’ (in reference to crushing flax in this case, apparently) and the Old French brac ‘arm’ (in reference to a lever, which is used for applying the brake), which comes from Latin bracchium.

Spacecraft are gliding through space, so you can’t just slow down a wheel that’s in contact with a surface; you have to use some external force. One way of doing this is by aerobraking. A spacecraft that has come a long distance at high speeds needs to slow down enough to get into the right orbit around a planet, and if you dip into the atmosphere and back out, you can use the friction of the atmosphere to slow it down rather than needing to burn a lot of fuel firing rockets to the same effect. So: aero, from Latin for ‘air’. And braking.

The MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) probe didn’t use aerobraking at any point; Mercury doesn’t have enough atmosphere for that to work. When it arrived at Mercury in 2011, it had to use a complex and large set of manoeuvres to take it to the right speed and into orbit. But it settled into its orbit and commenced mapping Mercury. It was set for a one-year mission. But at the end of the year, it still had lots of juice, so they decided to keep it going as long as possible. And they started live-tweeting it: a person on Earth tweeting as the first-person voice of MESSENGER.

Orbit is not a pure and simple thing. Your straight-line speed has to be just right to pull you away from what you’re orbiting with the same energy as what you’re orbiting is exerting through gravity to pull you towards it. Orbits can decay: the speeds aren’t quite matched. MESSENGER needed to fire some rockets occasionally to fix decay of its orbit. And finally it no longer had the fuel to do so. Its orbit would inevitably give way to the pull of gravity. It was determined that the resolution of this would be lithobraking.

Do you recognize lith? It’s a Greek-derived root meaning ‘stone’; you see it in lithograph (a printmaking technique using etched stone) and monolith (speaking of things in space, cue 2001: A Space Odyssey). Lithobraking is using rock to reduce the speed of a spacecraft. It has been used with the Mars Pathfinder and Mars Exploration Rover, for instance.

Where do you get rock in space? You don’t. You get it at the surface of the planet.

The Mars landers inflated big balloons and bounced along the surface of the planet until friction with the stone finally stopped them.

MESSENGER did not have big balloons. It was not designed to be a lander.

Lithobraking is a technical term for a precise manoeuvre calculatedly using the rock of a planet’s surface to stop a lander. It is also a more sarcastically euphemistic term for crashing.

The MESSENGER ground crew on Earth knew that this was the inevitable ending. The orbit was decaying and gravity would win. They let this be known on April 16. @MESSENGER2011 tweeted:

Oh No!! I’m going to be creating a new crater on Mercury? Hum.. This should be interesting.

They made a manoeuvre on April 24 to set the trajectory. The probe would come into the planet at 3.9 km per second and would contact the planet’s surface at about 54˚ north on April 30, at 2:36:06 EST, Earth time. The probe tweeted more information, including this:

I’m only ~ 3m across, but I will create a crater about 16 meters across.

Yesterday (April 29), it tweeted:

Well my lithobraking will occur tomorrow @ 3:26pm EDT. More info here:

A Twitter user commented this morning on the use of lithobraking. @MESSENGER2011 responded:

It’s called I don’t want to think about crashing. Lithobraking sounds a lot better.

And at just the predicted time, in just the predicted way, MESSENGER lithobraked from 3.9 km/s  to 0 km/s instantaneously – on the other side of Mercury from the Earth, so there are no pictures of it as it happened. But there’s a new crater on Mercury now. You can find out more at @MESSENGER2011 and

And now so many more of us know this word lithobraking. The first half is smooth and classical, the liquid /l/ and soft /θ/, like flying through space though it refers to stone. The second half is hard, abrupt, an odd and uncertain form from Germanic and/or Latin. Collided together, they make quite the contrast. Of course, contrast is what lets you see craters…

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