The complexity of the landing sequence that the JPL engineers envisioned was unprecedented. First, the space capsule carrying Curiosity would eject its heat shield and deploy a supersonic parachute, which would slow its descent to 200 mph. Then an array of bolts would explode, releasing the chute and dropping the rover—attached to its landing gear—into freefall for a few seconds before firing rocket thrusters. The landing gear would hover at an altitude of 60 feet, while a crane lowered the rover to the surface using cables. After the rover landed, cable cutters would sever the link, allowing the crane to hurl itself away before crashing into the dusty Martian soil. Small wonder the code name for the EDL sequence was “Audacity.”
NASA had briefly considered a similar sky crane system (dubbed “rover on a rope”) for the 1997 Mars Pathfinder mission, but had shelved the idea because a tethered vehicle would have to contend with pendulum forces and wind shear on top of all the other problems. But when Curiosity’s EDL team did its analysis of the revamped design, “To our surprise the pendulum behaved,” says Miguel San Martin, chief engineer for guidance, navigation and control.
Still, there were other challenges. Given its larger size, the rover needed a soft touchdown, and this required a precise radar system to scan and map the terrain during descent. The EDL team tested the radar by mounting it on a helicopter—which, like the proposed lander, was capable of a slow descent and then hovering above the surface—in the middle of California’s Mojave Desert. That’s how they discovered that sand dunes could pose a problem for the delicate sensors in the radar system: The helicopter rotor whipped up grains of sand, much like the rover’s rocket boosters might do on Mars, creating a large error in the measurements. There was little they could do to change the design of the radar by then, but they were able to account for this effect in their calibrations.
Despite these precautions, it was impossible to test the entire landing sequence in advance. The only complete live experiment was the mission itself, monitored in the JPL control room from 352 million miles away.
First, Curiosity had to eject the final section of the rocket (the “cruise stage”) that had propelled it to Mars. At that point it needed to enter the planet’s atmosphere at just the right angle to avoid burning up. There was a harrowing nine-minute delay after the cruise stage separation before the first signal came back: Curiosity had arrived at the outskirts of the Martian atmosphere and was beginning its descent. Initially, the news was not good: “Beta out of bounds catastrophic.” (Translation: “Curiosity is tilting too much to the side.”)
After another agonizing four minutes, the next signal came in, indicating that all was normal. Curiosity had made it through the atmosphere.
Now the descent and landing sequence began. The parachute deployed, the heat shield separated and the radar system scanned the ground. Flight Dynamics and Operations Lead Allen Chen, who was broadcasting the play-by-play, announced the start of the sky crane sequence. “I am like, really?” Steltzner recalls. “Nine years and it’s just going to happen.”
Three crucial pieces of data needed to come in. First, the rover would send a message telling its creators back on Earth that it had landed safely. Next would be to confirm that Curiosity hadn’t landed on a crater wall or was being dragged along the surface by the still-connected descent stage. Finally, the descent stage had to fly off as planned, rather than landing on top of the rover and crushing its UHF antenna.
One by one, the messages came in.