Curiosity validates a new way to land on planets
By the end of August, the nuclear-powered rover Curiosity will take its first, tentative steps on the surface of Mars, rolling a meter or two forward on its six aluminum wheels across the flat floor of Gale Crater.
By then, controllers at thehere probably will have finished checking out Curiosity's complex systems and calibrating its suite of 10 sophisticated instruments. With plans to spend at least two years exploring the crater and the 18,000-ft. mountain in its center, they are in no hurry.
Even in the unlikely event Curiosity suffers the interplanetary equivalent of a freeway crash that shuts the mission down suddenly and for good, it has already validated another way to put payloads on extraterrestrial surfaces. At a deeper level, it has also given the engineers who invented the now-famous “sky crane” technique more confidence that they know what it will take to do the same thing with humans.
“From an engineering perspective, this was really the linchpin we needed to prove that we can put a metric ton on the surface,” says Doug McQuistion,'s Mars exploration director. “That opens the door to all kinds of science instrumentation [and] science missions, whether they're rovers or fixed landers or palletized instrument sets or whatever they might be. All of the studies we've done over the years, we believe that pretty much anything we need to do scientifically or robotically can be done within a metric ton.”
Withreplanning its robotic and human exploration plans to accommodate lower budgets in the decades ahead, the success of the Mars Science Laboratory (MSL) landing is sure to add some flexibility to the deliberations. Rather than trying to figure out how to recover from a devastating failure, the Mars program planners will be able to continue on a track to develop options for missions to come.
All of those options will be supported by the skills built up here—and at NASA's Ames, Johnson and Langley field centers, as well as deep-space industrial houses likeSpace Systems and Pioneer Aerospace—that will advance the technologies needed to take the next steps beyond Curiosity.
“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-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.
The next critical phase of the landing occurred seconds later, just 6 min. after atmospheric entry and 85 sec. after heat shield separation, when the back shell separated at an altitude of around 1 mi. and a speed of 180 mph. Free-falling clear of the back shell and its parachute, the descent stage and rover briefly plummeted toward the surface. The parachute and back shell drifted away to land just over 2,000 ft. away from Curiosity's touchdown spot.
At 10:31 p.m. PDT, the sky crane's eight redundant Mars Landing Engines (MLE) were ignited and throttled up to begin the heart-stopping powered-descent for the final approach to the ground. “During the initial separation, all throttled up to between 50% and 70% thrust,” says Aerojet MLE chief engineer Matt Dawson. Immediately maneuvering laterally to ensure safe separation from the parachute and back shell, the engines throttled back and “knocked out the horizontal velocity,” says Dawson.
When the descent slowed to 1.7 mph, four of the engines throttled back to only 1%. Nozzles of the primary engines were splayed wider for reduced ground blast effect. At this sedate pace, the lander descended to just 66 ft. and entered sky crane mode to begin deploying the rover. “This is when it needs the finest control in terms of velocity change, and the four engines are at near 50% thrust,” says Dawson.
Derived from the engines used in the Viking lander, the MLE version has been “hugely upgraded” for the deep-throttling required by the Curiosity mission, says Roger Myers, general manager of Aerojet's operations at Redmond, Wash. “We were continually challenged by JPL to expand the capability of the engine, which originally had a thrust range of 85 lb. up to 635 lb. JPL asked us to go to 700 lb., and then also develop the capability to do less than 86 lb.,” says Myers. The eventual range from 7 lb. to more than 700 lb., represents a 100:1 throttling ratio. “The final challenge was they needed even more thrust and we did it by increasing the feed pressure.” Aerojet ultimately demonstrated more than 800 lb. thrust, “which was there if they needed it,” he adds.
With 12 sec. to go to touchdown, the sky crane hardware lowered the rover down on a bridle made of three 60-ft.-long nylon tethers attached to a bridle umbilical and descent rate limiter (BUD). Controlled by a descent brake, the tethers unspooled from the BUD until the rover touched down. At that moment, 10:32 p.m. PDT (10:17 Mars time), sensors activated devices to sever the tethers and the umbilical. The descent stage then performed a “fly-away” maneuver, ending up crashing 2,100 ft. to the north of the rover.
“We've shown we can land a ton of material on Mars using a new technique, and can do it precisely,” says Myers. “From here we have to see how we go to the next generation. We might update that architecture by using the same kind of flight dynamics with other propulsion systems to produce a two-to-five-ton lander.”
Aerojet's business development manager, Olwen Morgan, says more immediate options could involve a “'me-too' mission that goes to another location. “We'd just take the MSL to another place. The key is to exploit what's already been developed,” she adds.
The overall success of the sky crane concept means that Aerojet “hopes to play a role in any future missions,” says Myers. “We continue to develop the sky crane, but it is one piece of the exploration puzzle and there are other technologies which could play a part.” These include new, more powerful bi-propellant rockets as well as improved in-space propulsion technology that, combined with lighter-weight, inflatable reentry vehicle systems, could provide more capable sky cranes. “However, there are limits to the sky crane beyond a 10-ton-plus lander. At this point we don't know the answers yet,” says Myers.
On the surface, Curiosity will be a surrogate for a science team of some 400 scientists as it pokes around the alluvial fan where it landed before skirting a field of active dark-colored sand dunes to reach the base of Mt. Sharp, named by scientists here for the late Caltech Mars expert Robert P. Sharp. From there, it will climb a gully across what appear from orbit to be sedimentary layers, taking the time to analyze the composition of the material for clues to the history and habitability of the planet (AW&ST Aug. 1, 2011, p. 38).
The terrain where Curiosity rests looks remarkably like the Mojave Desert, a fact mission chief scientist John Grotzinger attributes to its probable formation from material brought down from the northern rim of the crater, much as running water paved the surface of the California desert.
“With Curiosity, we landed in something that actually looks very Mars-like, but also it looks Earth-like, with those mountains in the background,” Grotzinger says. “It's really cool, so it kind of makes you feel at home. I think the great experience there is, we're looking at a place that feels very comfortable, and what's going to be interesting is going to be to find out all the ways that it is different.”
Scientists already are studying a pair of half-meter-deep pits in the loose Martian soil north of the rover that apparently were dug by the plumes of the MSL descent-stage rockets. At the bottom of one of the holes is what appears to be bedrock, giving the science team an early baseline for soil depth as they explore across the alluvial fan.
With its fiscal 2013 budget request, NASA dropped its plans to work with the European Space Agency's (ESA) ExoMars program on a mutual campaign to return samples from Mars, starting with selection and caching of samples for future return to Earth. John Grunsfeld, NASA associate administrator for science, said here the night Curiosity landed that sample return continues to be the U.S.' next step on Mars, although hopefully at lower cost than the $2.5 billion MSL-class missions envisioned earlier.
Manning, who likes to point out that he was the chief engineer on both the cheapest and the costliest Mars-landing missions to date, says it should be possible to realize some cost-savings by reusing the MSL sky-crane technology to put payloads on Mars. One more sky crane can be built from engineering spares, and some of the development work would not need to be repeated for possible landing missions after that. But the MSL sky crane was custom-made for the current mission, a decision made to save money that may turn out to be more expensive in the long run. And, regardless of the specific robotic-mission options, the payloads still will need to be developed.
There will be more robotic missions to scout locations for human missions. “This isn't 'Star Trek;' we won't boldly go where no man has been before,” says NASA planetary-science chief Jim Green. While those future robotic missions probably will fit into the one-metric-ton landing capability demonstrated by the MSL sky crane, human missions will be much heavier—20-80 metric tons.
The technologies that will be needed to land those payloads safely—possibilities include hypersonic aerodynamic decelerators, supersonic inflatable decelerators and supersonic retropropulsion—are still being developed. But Manning and other engineers here believe there is wide appreciation that future exploration options must include the kind of expertise that allowed Curiosity to land.
“I do think people understand and would expect extensive simulation, characterization and verification,” says Matt Wallace, the MSL flight system manager, who has been working with NASA's Mars Program Planning Group.
Opportunities to launch missions to Mars come up every 26 months. Planetary scientists and the engineers who support them consider the 2018 opportunity as particularly favorable for transporting a sizeable payload to Mars. But Orlando Figueroa, NASA's “Mars czar” brought back from retirement to head the Mars planning group, has said the $700-800 million that will be available for Mars exploration in 2018 probably will not be enough to mount a landing mission (AW&ST May 14, p. 27).
If that turns out to be true, it will be worrisome for NASA managers trying to maintain the fragile skills here and elsewhere in the agency that enabled the MSL landing.
“This has got a lot of art to it as well as engineering skills,” says McQuistion. “It's one of the most important core competencies this agency has developed over the years. We need to maintain that . . . . If we can afford a lander in '18, I would be comfortable that we would be able to keep the team around, keep them engaged, keep them together, keep the skill set.”
|Mars Science Laboratory||Mars Exploration Rovers|
|Rovers||1 (Curiosity)||2 (Spirit and Opportunity)|
|Launch Vehicle||Atlas V||Delta II|
|Heat Shield Diameter||14.8 ft. (4.5 meters)||8.7 ft. (2.65 meters)|
|Design Mission Life on Mars||1 Mars year (98 weeks)||90 Mars sols (13 weeks)|
|Science Payload||10 instruments, 165 lb. (75 kg)||5 instruments, 11 lb. (5 kg)|
|Rover Mass||1,982 lb. (899 kg)||374 lb. (170 kg)|
|Rover Size (excluding arm)||Length 10 ft. (3 meters);||Length 5.2 ft. (1.6 meters);|
|Width 9 ft. (2.7 meters);||Width 7.5 ft. (2.3 meters);|
|Height 7 ft. (2.2 meters)||Height 4.9 ft. (1.5 meters)|
|Robotic Arm||7 ft. (2.1 meters) long, deploys two instruments, collects powdered samples from rocks, scoops soil, prepares and delivers samples for analytic instruments, brushes surfaces||2.5 ft. (0.8 meters) long, deploys three instruments, removes surfaces of rocks, brushes surfaces|
|Entry, Descent and Landing||Guided entry, sky crane||Ballistic entry, air bags|
|Landing Ellipse (99% confidence area)||12 mi. (20 km) long||50 mi. (80 km) long|
|Power Supply on Mars||Multi-mission radioisotope thermoelectric generator (about 2,700 watt hours per sol)||Solar photovoltaic panels (less than 1,000 watt hours per sol)|
|Computer||Redundant pair, 200 MHz, 250 MB of RAM, 2 GB of flash memory||Single, 20 MHz, 128 MB of RAM, 256 MB of flash memory|