Mars Polar Lander’s Liftoff Will Set the Stage for a Crash-Landing
An unusually risky ground-pounding experiment is hitching a ride on NASA’s next spacecraft to Mars.
Aboard the Mars Polar Lander, due to lift off today and arrive at the Red Planet in December, are two probes designed to slam into the martian surface between 400 mph and 500 mph and, if they survive, burrow 3 feet deep.
It is the first attempt by NASA to crash-land a spacecraft on another planet with the intent of penetrating its surface and sending back radio signals.
The trick is getting the probes to withstand the impact, estimated at up to 60,000 times the force of Earth’s gravity.
To come up with a durable design, project manager Sarah Gavit and her team at the Jet Propulsion Laboratory in Pasadena hurled sample probes from planes and shot them out of a cannon into the ground.
They settled on a probe the size of a cantaloupe, enclosed in a protective shell the size of a basketball. Total weight: eight pounds.
The silicon carbide aeroshell is meant to shatter on impact, with the probe inside separating into two parts connected by a cable.
One part, made of aluminum and magnesium, is supposed to remain on the surface of Mars to serve as a radio-relay station. The other, a 4-inch by 1 1/2-inch tube, is intended to penetrate up to 3 feet.
This stainless steel and tungsten tube contains a miniature drill that will be propelled out the side to collect bits of soil. The dirt will fall into a cup that will be sealed and then heated, turning any water into vapor that would flow into a chamber holding a laser and mirrors.
The laser will emit a beam of light. Scientists will know whether water vapor is present if any of that light is absorbed.
At the same time, temperature sensors will measure the rate at which each penetrating tube cools after plunging into the ground, allowing scientists to determine how fast heat dissipates in the soil.
If all goes well--a big if--the probes could collect data for one to three days, the expected lifetime of their tiny lithium batteries.
Almost everything in the probes was developed from scratch. “Either there was nothing out there small enough, or nothing that was packaged to survive the kind of forces we’re going into,” Gavit said.
In 1996, the Russians used different designs and technologies for a much larger pair of soil-penetrating probes that were to have drilled 20 feet beneath the martian surface. But the spacecraft on which they were launched, unfortunately, came crashing down through the atmosphere shortly after liftoff.
Even with the best design, which engineers believe they have after three years of testing, luck will play a large role when the probes divebomb into Mars near the South Pole on Dec. 3, 1999.
They are supposed to peel away from the Mars Polar Lander five minutes before the lander enters the planet’s upper atmosphere. They should hit the martian surface a minute before the three-legged lander plops down some 60 miles away, where it will shovel up dirt for analysis.
If the probes hit a small rock or the side of a rock, that’s OK as long as they penetrate. “But if we go splat right in the middle of a big one, we’re history,” Gavit said.
The terrain near the planet’s unexplored South Pole shouldn’t be as rocky as the spot where NASA’s Mars Pathfinder bounced down on July 4, 1997.
Just as bad would be landing in soft, fluffy layers of dust. “The whole thing could be buried,” Gavit said, preventing radio signals from getting out.
Another problem is if a big gust of wind tilts the probes the wrong way just before they hit. If the angle at which they descend is too great, the probes will skip along the surface like pebbles on a pond.
If Gavit and her team get even one radio signal back, they’ll be thrilled. That will mean at least part of their package survived and most of the technologies were demonstrated.
Those in charge of the program put the chance of success at better than 50-50--maybe. So two probes are flying to double the odds.
That’s also why the $29-million experiment is part of NASA’s New Millennium program to test revolutionary, high-risk technologies.
The first New Millennium project is Deep Space 1, launched last October. It is en route to an asteroid, its ion propulsion engine working well after a slow start.
The two penetrating probes on the Mars Polar Lander constitute Deep Space 2.
“It’s meant to be pushing the boundaries of what we think is possible,” said Carl Pilcher, NASA’s science director for solar system exploration.
The science goal--hunting for water in the martian soil--is secondary to proving that the technology works, notes Richard Zurek, project scientist. “The idea is to demonstrate just how small you can get these devices,” he said. “If they’re small enough, then you can send many of them.”
Imagine a string of mini weather stations on Mars, or a chain of seismographs to register Marsquakes. “There’s a whole area of science . . . that you could do if you could put down a network of instruments,” Zurek said.
