Once the heat shield has done its job, the MSL spacecraft should have slowed to around 400m/s. With the rover still encased in the shell of the heat shield, the parachute deploys to slow the descent down even further. Parachutes have a proven track record on Mars – most recently with the Phoenix mission. But with MSL, the parachute is only the first stage in a much more complex landing process. This will be the first mission to use a ‘sky crane’.
“It looks scary but it actually eliminates a lot of the problems that previous designs had,” says Nasa’s Steve Sell, an engineer at the Jet Propulsion Laboratory in Pasadena who helped develop the concept.
On the face of it, the sky crane looks unnecessarily complicated. Once released from its parachute and protective shell, the lander uses engines to slow the descent. At this stage, the MSL resembles a flying bedstead with the rover slung underneath. As this combination nears touchdown, the rover is lowered towards the surface on cables and its wheels unfurl. On contact, the cables are severed and the skycrane (bedstead) flies away and crashes and the rover starts work.
What could possibly go wrong?
I ask Sell how they ended up with something that’s so complex. “When you’re designing rover style missions in particular, you quickly run into a problem where if you’re going to put the rover on top of a lander, with legs and rockets, you have to get it off there somehow when you’re going to touch down,” he explains.
“There’s a very complicated issue of designing a way to reliably, on all kinds of different circumstances – on slopes, on rocks, on sand dunes - drive something off the top of a platform. You also have the additional complexity that for Curiosity, this rover is over five times larger than any previous rover built to land on Mars.”
That means that if they put the rover on top of the lander, there’s every chance it would make the whole arrangement too top heavy, causing it to topple over. Likewise, if you attached the rover beneath the lander, it could get trapped if the legs sank into sandy ground or landed at an odd angle. So that’s out too.
Another option that’s been used successfully – most notably on the recent Spirit and Opportunity missions – is airbags. With this design, the rover bounces across the surface encased in a cocoon. When it comes to a stop, the airbags deflate, the cocoon unfolds and the rover trundles away. So why not use that system?
“When you start trying to make that [airbag system] bigger, so it could safely land a one tonne rover, you end up with an airbag system that is very, very large…it just doesn’t scale up.”
So, it was back to the drawing board. Eventually, by a process of elimination, they ended up with the sky crane.
“You can think of the rover wearing a jet pack on its back,” says Sell. “When we get 20m above the surface, the jet pack lowers the rover on three bridles and that whole system keeps moving down towards the surface with the rover held seven and a half metres below the jetpack. When it touches down, the jet pack stops, it cuts the bridles free, then the jetpack flies away and crashes...it’s a way of landing without ever having to land the spacecraft.”