“What we're doing here, in the test methodologies and the modeling methodologies, are directly applicable and portable to getting those technologies to high [technology readiness levels],” says Rob Manning, chief engineer on the MSL project.
It was that testing and modeling that gave MSL mission managers confidence the untried entry, descent and landing (EDL) technique would work, even though there was no way to flight-test the whole thing on Earth. Instead, engineers tested what they could of the hardware, and then used data from those tests to shape sophisticated supercomputer simulations of the entire EDL sequence. Those simulations were run over and over until the engineering teams were confident the systems could handle whatever Mars threw at them.
“Those models become our truth,” says Manning. “We say, 'this is the EDL we intend to fly. Now let's build it so it matches that model.'”
Manning, who was also chief engineer on the 1996-98 Mars Pathfinder mission, says engineers at NASA and its industrial partners “discovered” the integrated system-level Monte Carlo simulation capability as they built Pathfinder, the two Mars Exploration Rovers, the Mars Phoenix lander and the other deep-space machines that send planetary-science data streaming back through NASA's Deep Space Network (DSN) ground stations. It will be invaluable as engineers modify the sky crane approach to land future robotic Mars missions, and it will help shape the systems that will be needed to land the much heavier human-spaceflight missions being planned by NASA.
Aside from the fact that Curiosity went straight into “surface nominal mode” at the end of the “7 minutes of terror” that comprised EDL, evidence that the modeling techniques Manning describes really work is scattered across the floor of Gale crater. Mission managers say imagery collected from orbit by the Mars Reconnaissance Orbiter (MRO) show the heat shield, back shell and parachute, and descent stage all landed in the expected locations around the rover, as did the six 55-lb. tungsten weights jettisoned to restore central balance to the descending vehicle before the 51-ft.-dia. supersonic parachute deployed.
Those “entry balance mass devices” were the final step in a sequence that allowed the MSL entry vehicle to fly itself to a landing in a target ellipse measuring only 20 X 7 km (12 X 4 mi.). Before atmospheric entry, two heavier tungsten weights were jettisoned to unbalance the vehicle, giving it lift and the ability to steer itself into the Gale Crater landing site.
The lander propulsion system incorporated eight 68-lb.-thrust MR-107U engines and eight MR-80B descent thrusters. All the Aerojet-developed engines for the lander were fueled by N2H4 Hydrazine, eliminating any chance of carbon remnants confusing the scientific search for Martian carbon. The MR-107U upper thrusters were used for attitude control during the hypersonic phase of atmospheric entry, which (not counting the 13.8-min. communications delay from Mars) began around 10:24 p.m. PDT, or some 10 min. after separation from the cruise stage. The thrusters fired through fixed nozzles in the back shell during the descent that, for the first time on a Mars landing, was controlled by a real-time autonomous maneuvering system using closed-loop flight-control data.
Four minutes after atmospheric entry at around 13,200 mph, hypersonic aero-maneuvering and deceleration slowed the vehicle to Mach 2.4. The final set of six tungsten ballast weights was ejected, shifting Curiosity's center of gravity back to the middle of the vehicle, and correcting its attitude to allow the parachute to deploy at around 900 mph and an altitude of 7 mi. Subsequent images from the MRO indicated the ballast impacted an area 7.5 mi. downrange from the rover's final landing spot.
Following parachute deployment at 10:29 p.m., the heat shield was jettisoned as the craft slowed to around 280 mph at an altitude of 5 mi. The heat shield landed 4,900 ft. away from Curiosity's landing area. The exposure of the lower aeroshell activated the landing radar, as well as the Mars Decent Imager that began to capture high-resolution images at the rate of up to four per second.