Reverse Robotic Origami

Categories: Feature Stories Mars

"We asked the rover engineers to fit a square peg in a round hole," said the Spirit rover mission manager, Jennifer Trosper of NASA JPL. "On a very big day today, the engineering team unfolded that square rover from its triangular landing petal."

The rover has about half a meter to go before putting its six wheels into martian soil.

Gusev crater, panoramic camera. Depressions thought to be secondary fallout from crater impact, hill in distance. A mixture of angular and round rocks, with three percent rock coverage of the crater floor at the current rover position, compared to twenty percent at the three previsou sites, Viking I and II, and Pathfinder.
Credit: NASA/JPL

The Spirit rover successfully passed through its four part ‘stand-up’ routine–a staged series of maneuvers that transform its scientific laboratory on wheels "from a lander to a rover", said Chris Voorhees, one of the JPL mechanical engineers tasked with the stand-up. The wheels and arms of the rover have remained nested underneath its main solar panels for seven months, since it launched from Florida seven months ago.

"The tetrahedral landing petal is not a shape that a rover wants to be in," continued Voorhees, "so we have to stand-up in stages. They happen like reverse robotic origami."

This unfolding prepares the rover for driving off its base, which will likely become metal debris after next Tuesday and for the rest of the rover’s 90-day mission. From Tuesday night onwards, "the Spirit will be supported on six wheels in the soil", said Voohees, "on her own."

"This is one of the most complex deployments ever done", said Voorhees, to get the rover "standing on all sixes. It involves twelve pyrotechnics, nine motors, six structural latches, and two hazard avoidance cameras."

There are two main steps which will wait until the rover is closer to its final egress, which are moving the robotic arm into a ‘ready-to-do science position’, according to Trosper, followed by releasing the last restraint, called ‘cutting cable number three’.

The rover follows a plan to turn in place from south, to northwest facing the large martian volcano, Apollinaris Patera, and then driving off a deck height of forty centimeters to the soil. This turning maneuver is preferred because overnight simulations have raised the small chance that going south will brush the rover’s rear left solar panel against a deflated airbag on the way off its base.

Trosper summarized the overnight science data return, as ten times [or 200 megabits] more than the 1997 Pathfinder mission could deliver.

Dr. Joy Crisp, Project Scientist, reported that the science group is "thrilled with the match" between their predicted models for the martian atmosphere and what Spirit actually went through during its descent last Saturday. Thos same models will be used when the second rover, Opportunity, tries to land in two weeks.

"That atmospheric reconstruction is complete," Crisp said, "and only for the fourth time in history do we have a temperature versus height plot for Mars. This is a measurement taken thousands of times every day on Earth, with atmospheric sounding balloons, but it gives us confidence for the next rover’s landing" to know the models match well with what Spirit recorded. Since these models for the martian atmosphere panned out, there are currently no plans to change Opportunity’s entry.

Prime Site

Dr. Matt Golombek of the mission science team, described how the Gusev crater site has stacked up so far against their initial predictions. "There are two consideration in selecting a site: safety, safety, safety…" said Golumbek, who was also the project scientist for the 1997 Mars Pathfinder mission. "We can’t do science, with a safe rover. This is the first time we’ve been able to certify sites as ‘safe’ using two remote sensing orbiters" –the Mars Global Surveyor and Mars Odyssey. Using the three meter per pixel resolution of these orbital cameras, "these sites are by far the best imaged sites ever studied on Mars."

The science team predicted that Gusev would be safe for both landing and driving, with fewer rocks expected as obstacles to driving than the three previous Mars landing sites. Those predictions have been borne out, based on the images returned from Spirit’s panoramic cameras. The site is judged using radar and topographical maps, then rated on two key factors: the albedo (dustiness, or surface brightness) and thermal inertia (change of temperature during a day).

Both these factors –albedo and thermal inertia– depend on the dust quantity and quality. As Spirit’s principal investigator, Dr. Steve Squyres, described these thermal effects, "if you go to a beach during the heat of the day, the fine sand is hot to your feet, but any denser concrete nearby is cool. At night, that same beach will have cooler sand, while the concrete will remain warm."

Dust or albedo changes in the atmosphere, showing two different times of day in dark and light illumination. Gusev is about thirty percent more dusty than initially predicted from orbit, but much less dusty than Viking during storms. The second rover site, Meridiani, is predicted to have nearly zero dust comparatively. Golombek said that 93% of a ‘mission success panorama’ has been photographed, including now nearly 1200 ‘marvelous images’ in either full or down-sampled photographs.
Credit: NASA/JPL/Spirit

Squyre’s beach analogy is not far from describing Mars ‘thermal inertia’, or how scientists are measuring the how fast the martian soil heats up during the day, and how slow the bigger, denser rocks cool during the night.

The precise temperatures are being evaluated using a kind of infrared eye (called mini-TES) on the Gusev landscape. Unlike a sunny beach however, the seasonal change in temperatures at Gusev varies up to 100 degrees, between about 0 to -100 degrees Fahrenheit.

In anticipation of driving around with their new roving geologist, the mission team is pleased with the smoother, flatter plain they have to drive across. "The three previous landings showed twenty percent rock coverage," said Golombek, "while Gusev shows three percent coverage."

Dust Devils: The Movie?

In addition to understanding what Dr. Charles Elachi, JPL Center Director, described as Gusev’s flatness,–or the first interplanetary, ‘interstate highway’– the mission science team is also beginning to quantify what happens above the soil, in the hills and farther into the hazy distance. Because the martian equator is clearing from a seasonal dust storm, the visibility varies considerably, with best conditions probably around ’20 kilometers’, said mission science team member, Dr. Mark Lemmon, of Texas A&M University.

From orbit, Gusev was predicted to be about 30% less hazy than it appears now, but the site remains much clearer than the previous Viking sites that had periodically weathered large dust storms over many martian seasons on the surface.

The crater is like "a smoggy day in a big city", said Lemmon, "but not the worst smog you’ve ever seen. …We are interested in how dust heats the atmosphere and how this affects power supplies from solar panels and future explorers."

Gusev is predicted to witness about ten times more dust devils or tiny tornadoes, than any previous landing sites. How many Spirit might photograph depends on the afternoon heating of the surface and atmosphere.

What’s Next

In two weeks, the second Opportunity rover will attempt to touchdown on the other side of Mars, near the equator at a site called Terra Meridiani. Meridiani is likely to have a more grey, volcanic look to it than Gusev’s flat plain, but with intriguing possibilities for also having a water-formed history and an aqueous mineral called hematite. The rock density at Meridiani is also hoped to be less than twenty percent of the landscape, which will make driving longer distances possible.

Golombek previewed what those initial panoramas from Meridiani might appear like: "It will look completely different. Meridiani has virtually no dust (low albedo). You will see a dark-grey basaltic plain, with much more of the rolling landscape" seen by Pathfinder.