By now everyone is familiar with the Curiosity rover’s system for entry, descent and landing, the preposterously complicated Rube Goldberg sequence that a Los Angeles Times reporter said is like “something Wile E. Coyote devised to catch the Road Runner”. It’s crazy to believe that it will get the rover to the surface of Mars in one piece, right? Wrong. Curiosity is the least-risky landing that NASA has ever attempted.
NASA doesn’t like to publicize internal estimates of failure and success rates. At a press briefing on Saturday at the Jet Propulsion Laboratory (JPL) in Pasadena, California, Doug McCuistion, director of the Mars Exploration Program, said: “I don’t think there’s a single number that we can put on this.” And it’s true. there are so many complicating factors — some well known and quantified and others not even known — that any attempt at quantifying risk is a bit reductive. But the Curiosity team still tries. Earlier, mission-team members told me that the risk of a landing failure, according to millions of simulations, was 1.7% — with 1% of the risks wrapped up in parachute problems and the remaining 0.7% owing to terrain hazards (see ‘7 minutes of terror‘). Those numbers depend on a key assumption: that rover hardware functions perfectly to specifications. But that’s not a bad assumption, given that the JPL has been scrupulously testing much of this landing system since 2005.
And a success rate of 98.3% is much better than the calculated rates for the Mars Exploration Rovers (MER) Spirit and Opportunity. Allen Chen, the JPL operations lead for entry, descent and landing, told me that Spirit, which landed at Gusev Crater, had a success rate in the “high 80s”, and that Opportunity, which landed at the slightly more benign Meridiani Planum, was in the “mid-90s”.
What about the Viking landers? For that, you’d have to talk to the guy who, on Thursday, was lurking in the back of the briefing room wearing one of his trademark Los Angeles Dodgers baseball caps: Gentry Lee. Now JPL chief engineer for Solar System exploration, Lee has had his hand in just about every one of NASA’s planetary missions. At the time of the Viking landings in 1976, Lee was director of science analysis and mission planning.
Back then, he says, computer simulations were crude, the ability to test hardware was less robust and knowledge about Mars — and its atmosphere in particular — was thin. Now, he says, “the computational ability to analyse the bejeezus out of everything is absolutely staggering.” And so, when Lee told Viking project manager Jim Martin that the probability of landing success for at least one of the two landers was 90%, it really was a bit of a guess. Engineers working on Curiosity have every right to be more confident in their landing, he says. Asked whether he was more confident in Curiosity than he was in the Viking landings, Lee says, “Goodness, yes. And I’m breathing easier now than I was for MER.”