Mars Landing Hinges on the Precise and Unpredictable
“Here Be Dragons.”
As ancient mapmakers groped for words to describe the terra incognita beyond the edge of the world, they settled on these none-too-encouraging words; it was all mariners of the time had to go on.
The modern mariners at NASA’s Jet Propulsion Laboratory in Pasadena are ready to meet their own dragons this evening as their robot, Spirit, lands on the alien world of Mars at 8:35 p.m. PST.
If Spirit survives, it will be the most ambitious explorer ever to roam an alien world, its mission no less than a search for signs of water that would have once allowed life to bloom on Mars.
But like their ancient counterparts, the scientists at JPL are worried about the devils they don’t -- and can’t -- know.
“You just hope your imagination is wide enough to think of all the possibilities,” said Rob Manning, who heads the Mars rover entry, descent and landing team. “It’s not inconceivable that Mars could throw something at us significantly outside our expectations.”
Mars, after all, is no place like home. The sky is pink, the sunset blue. The temperature can be 50 degrees colder at your head than at your feet -- meaning you could walk around barefoot yet have to wear a wool hat.
There’s practically no atmosphere, and most of the landing site -- a 4-billion-year-old crater the size of Connecticut -- “has no analogue with anything on Earth,” said JPL geologist Matthew Golombek.
Two NASA orbiters have been madly snapping bird’s-eye pictures like so many space tourists, taking the planet’s temperature, mapping its topology and reading its chemistry. Their efforts have made Gusev Crater the most studied place on Mars.
Even so, the satellites can’t make out features smaller than about 15 feet, meaning that very large green monsters indeed could be waiting below -- or scarier still, very sharp rocks.
“Now we sit and wait and don’t sleep much and think of all the what if’s,” said Jaime Dyk, the engineer in charge of testing the craft’s landing systems.
It’s like approaching the top of a very steep roller coaster, she said. “There’s nothing you can do. You’re strapped in. You just hope it all works.”
A long string of much-talked-about Mars failures (including JPL’s Mars Climate Orbiter and Mars Polar Lander) has added to the pressure to set Spirit down safely; but so, too, has success.
Six years ago, Pathfinder bounced onto Mars for a picture-perfect landing; its toy-like rover, Sojourner, wobbled around the surface for months, winning the hearts of the world.
You almost wish it hadn’t been quite so perfect, said project system engineer Jennifer Trosper. “People would have lower expectations. Pathfinder didn’t have such a ‘this-has-to-work’ mentality.”
Spirit and its twin, Opportunity, (due to land Jan. 24) are far more ambitious, expensive and heavier -- 384 pounds each, more than 15 times Sojourner’s weight.
“It’s a chance to redeem ourselves,” Trosper said. “We want to prove to the world that we can do complicated stuff and do it right.”
The team of in-house Martians at JPL are feeling like nervous parents waiting to hear if their child has safely landed after a first solo flight in the family car.
Except that landing on Mars is nothing like a trip around the block.
Fifteen minutes before touchdown, explosive bolts will free the lander from the cruise stage -- the life-support system that sustained the craft for the seven-month trip to Mars. The craft will skid into that strange pink sky at a precise 11.5 degree angle. (Any more than .2 degrees off, and it bounces into space, or crashes and burns.)
Then it brakes from 12,000 mph to zero in the space of six minutes.
A heat shield will glow like the sun as it sheds most of Spirit’s speed; a parachute will get shot out with a cannon while Spirit is still moving at nearly 1,000 mph -- and slow it to about 200 mph.
A 66-foot tether as thin a shoelace will lower the craft away from the landing module, and a 17-foot-high complex of protective airbags will inflate in half a second around it.
Just before hitting the surface, retro rockets strong enough to lift trucks will bring the spacecraft to a dead stop, and a pyrotechnic guillotine will cut the tether, dropping the lander to the ground like a steamer trunk full of bricks.
There are dragons every step of the way.
“There’s too many ways to count what could go wrong,” said Golombek. “Seventy or 80 different pyrotechnic devices must work absolutely perfectly for this thing to have a chance. One doesn’t work? Toast. One solar flare resets your computer just before you arrive? It’s over.”
The system has been tested many times over, so engineers at JPL say they are confident it will work.
But ultimately, each pyrotechnic bolt, each airbag, each rocket, is a single-use affair, a one-shot event.
“You haven’t tested these rockets,” said project manager Peter Theisinger. “You’ve only tested their brothers.”
As the lander got heavier to accommodate the collection of complex instruments, engineers learned the hard way that the Pathfinder airbag system couldn’t carry the load.
In practical tests, they failed.The bags were improved with more outer layers and a double inner bladder.
But like everything else, they haven’t been tested on Mars.
Even a system that performs perfectly won’t help much if the planet doesn’t cooperate. And that, virtually everyone agrees, is the scariest part.
“You can’t tell Mars not to have a dust storm,” Dyk said. “You can’t place the hills where you want them and the cliffs and the rocks and the shapes of the rocks. If they’re all stalagmites, it’s a bad day.”
Second to rocks, the major concern is wind. The lander’s airbag cocoon is designed to bounce 50 feet high, for as long as several minutes, traveling more than a mile. But a strong sideways gust could drag the airbags over the ground, shredding all six protective layers.
A steep slope could fool the radar, making the lander drop at the wrong time.
With so many potential dragons, it would be comforting, at least, if the lander would phone home to tell the folks on Earth that it arrived safely. But that’s unlikely to happen right away.
Several dozen tones will signal various stages during the descent. But the transmitter is very weak. “It’s like a candle,” Manning said. “And you’re doing all sorts of nasty things to that poor little radio, the antenna is bouncing all over the place, the candle is moving, blurring the signal.”
As Spirit drops, there’ll be another brief chance to make contact. Mars Global Surveyor has been programmed to fly overhead at just the right time and could relay an “all’s well” signal to Earth. “It’s been tuned with incredible precision,” Manning said. “It’s like the Blue Angels flying overhead at the beginning of a parade.”
Three hours later at about 11 p.m. PST, there’s another opportunity when a second orbiting satellite, Mars Odyssey, flies overhead. But the lander has much to do on the ground before that connection can be made.
The rectangular rover is stuffed inside a pyramid-shaped lander that unpeels like an artichoke. The petals have to open just so and set the craft upright (should it land on its side). The mass of deflated airbags needs to be neatly tucked out of harm’s way -- lest the rover trip over its own placental sac.
Retracting the airbags is what worries Dyk most. “As mechanical engineers, we understand metallics.” But fabrics pull and stretch in unpredictable ways, she said.
“There’s a lot more to this than there was with Pathfinder,” Trosper said. “It’s a rectangular peg in a tetrahedronal box.”
If there’s time before the sun sets on Mars, Spirit will poke up its 5-foot-tall head, look around, find the sun, perhaps even send pictures back to Earth tonight.
“If you don’t hear anything, you’re going to get pretty antsy,” Golombek said. “That doesn’t mean anything has gone wrong. It just means you have to wait another day. But it will be tough to sleep that night.”
If Spirit doesn’t survive, the JPL team will learn as much as it can from the disaster; it has three weeks to reprogram Opportunity to avoid the same fate.
If all goes well, however, Spirit will spend the next week or more standing up, stretching its limbs, trying out its chemical senses, testing its environs -- one tentative toe at a time.
Eventually, it will roll down onto the cotton-candy sands of Mars like a strange bird with a 5-foot neck and 14-square-foot solar-panel wings.
“It’s funny,” Manning said, “but as human beings we think of how scary it would be traveling through space, hitting the atmosphere, being in a parachute. But our robots are happiest in that environment. We designed them to be there.”
