NASA’s Jet Propulsion Laboratory
Pasadena, CA – If you’ve ever played the claw machine at an arcade, you know how hard it can be to maneuver the metal “hand” to pick up a prize. Imagine trying to play that game when the claw is on Mars, the objects you’re trying to grasp are far more fragile than a stuffed bear and all you have is a stitched-together panorama of the environment you’re working in. Oh, and there might be a dust storm.
NASA’s InSight lander, slated to arrive on Mars November 26th, 2018, will be the first mission to use a robotic arm to grasp instruments from the spacecraft and release them into place on another planet. These instruments will help scientists study the deep interior of Mars for the first time.
“We have a lot riding on InSight’s robotic arm, so we’ve been practicing our version of the claw game dozens of times,” said Tom Hoffman, InSight’s project manager at NASA’s Jet Propulsion Laboratory in Pasadena, California. “The difference, of course, is that, unlike the claw machine designers, our robotic arm team works hard to allow us to win every time.”
InSight’s robotic arm (called the Instrument Deployment Arm) will pick up two sensitive science packages from the spacecraft deck and gently lower them to the ground: the Heat Flow and Physical Properties Package, which will assess Mars’ interior energy, and the Seismic Experiment for Interior Structure, which will study vibrations of the ground set off by marsquakes and meteorite impacts.
InSight also needs to place a Wind and Thermal Shield over the seismometer, like a cloche — or rounded dish cover — at a fancy dinner service.
“The robotic arm has to place everything perfectly,” said Ashitey Trebi-Ollennu, team lead for InSight’s instrument deployment system operations at JPL. “But we like a challenge.”
Luckily, engineers didn’t have to start from scratch. The JPL engineering team had in storage a leftover robotic arm — made for the Mars Surveyor 2001 lander mission that never flew. The arm wasn’t as beefy as ones built for missions like the Mars Curiosity Rover, which carries more weight at the end of its arm.
But the 2001 arm was designed for lifting, making it appropriate for InSight’s mission. And it was long (5.9 feet, or 1.8 meters, to be exact). InSight needs to put the seismometer and heat probe a significant distance away from itself for the sensitive instruments to function optimally.
The original grapple design had two stiff “toes” emanating from a central base, which Trebi-Ollennu likens to a crow’s foot. Each instrument was outfitted with a knob, or “grapple point,” that resembled a lollipop with a long stem for the stiff foot to grab. In tests on sloped surfaces, the lollipop often got stuck in the toes. Given the possibility of slight slopes at the InSight landing site, engineers didn’t want to take that chance.
The second proposed design was an idea familiar to those who have seen junkyard operators maneuver crushed cars. Engineers hung a magnet on an umbilical cord from the robotic arm and put steel plates on the instruments. Tests showed, however, that dust collected on both the magnet’s surface and the instruments’ steel plate, decreasing the ability of the two parts to stick together. Given that InSight’s landing date falls within the typical dust storm season on Mars, engineers decided against this magnet design.
The third idea was the charm: a clawlike grapple with five metal fingers about the length of human fingers (about 2.5 inches, or 63 millimeters, long) hanging off the end of an umbilical cord to compensate for any slopes. The grapple point on each instrument resembled the original spherical lollipop, but with the top half of the sphere cut off and a shorter stem.
An especially clever feature of this robotic hand, Trebi-Ollennu explained, is that melting of paraffin wax — a common constituent of candles and crayons — controls the opening of InSight’s fingers.
To begin the process, an actuator heats a very pure paraffin wax to 84°F (29°C), which takes about 15 minutes in the average ambient Mars temperature of about minus 60°F (minus 50°C). The wax expands as it melts and pushes out a rod that pushes on a spring that opens the fingers. When the fingers open, a microswitch turns off the heater, and the cooling, contracting wax allows the rod — and therefore the fingers — to retract. At rest, the fingers are closed so that if the hand happens to lose power, it won’t drop an instrument.
A few days after landing, InSight engineers will put the robotic arm into action. The arm will move so the camera attached to it can take images of the area around the lander site. Back on Earth, engineers will use those images to figure out where the instruments can be safely set down.
“We’re looking forward to the demanding work of getting InSight’s claw machine in motion,” said Bruce Banerdt, InSight’s principal investigator at JPL. “But the prize for the InSight team won’t be a fuzzy bear. It’ll be the stream of science data flowing in from precisely placed instruments — telling us what Mars is really like on the inside.”
JPL, a division of Caltech in Pasadena, California, manages InSight for NASA’s Science Mission Directorate in Washington. InSight is part of NASA’s Discovery Program, managed by the agency’s Marshall Space Flight Center in Huntsville, Alabama. The InSight spacecraft, including its cruise stage and lander, was built and tested by Lockheed Martin Space in Denver.
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