Which part of the landing sequence was most worrisome?
The thing that we felt, mathematically, was the single lowest reliability element was probably the parachute, and that’s just [because of] the intrinsic uncertainties associated with parachutes. We throw more than 10,000 troops out of airplanes each year, and largely it’s into a very controlled environment, and we have parachutes designed exactly for those conditions. But we still give them a second parachute because even with all those controls, the odds just aren’t good enough. That’s not the case when you’re moving supersonically in an uncertain atmosphere 10 kilometers above the surface of Mars. So when you do the numbers, you end up convincing yourself that the single highest device risk is the parachute.
But that’s not the thing I was worried about. I was worried we could have missed something on the Sky Crane. It was so new and different. It was the unknown unknowns I was most concerned about, some unappreciated feature of this new landing system that we did not absorb and was waiting to bite us. So on landing night, as the data clicked by, I became more and more anxious. I said ‘Oh my God, is it really going to happen just this easily?’ I was pretty wound up for those last 20 or 40 seconds.
And your reaction afterward?
Tremendous relief, tremendous exhilaration, and, frankly, a slight sense of surrealness. To work on something for the better part of a decade, and then to have it done. Regardless of the outcome. It’s awesome that it was done successfully, but, I mean, all of a sudden it’s over. It was such a build-up, so much of my life was invested, and then it’s now, well, it just happened, now we move on.
The Sky Crane will also be used to land a rover similar to Curiosity in 2020. Are you studying any improvements to the system?
We knew going in that we had some points where the design was not all that we would have hoped it could have been. We were left holding some concentrations of risk that we would have preferred not to have. For example, we were measuring only two components of our velocity in the Sky Crane maneuver with our radar. We wanted to measure three, but because of late antenna development challenges, we could not get the right view angles to do that. So we said, ‘Well, we understand the local gravity of Mars fairly well at the landing site, so we’ll just estimate the third component.’ As it turned out, we didn’t understand the gravity well enough. There was a gravity anomaly at Gale Crater, which is probably not surprising because there’s a huge crater there. And that meant that we had an error in our estimates and we landed much more slowly than we’d anticipated. If that error had been flipped around, we might have landed faster than anticipated and we might have hurt the rover.
That’s an example of something the team that is going to be flying 2020 is going to look long and hard at, whether to put on a third antenna. For the lay person, the Sky Crane will look identical, but there will be some subtle changes the team will make to strengthen its reliability.