The Phoenix Mars Lander is set to scope out Mars' frozen pole for life-supporting conditions next May, but researchers are predicting a tricky soil-sampling mission.
If the spacecraft's rockets don't blast the topsoil away, the exhaust fumes could contaminate it with fuel. Or, Martian surface winds might simply blow away the samples before they can be dumped into the lander.
To combat the looming technical troubles, a team led by engineer and atmospheric scientist Nilton Renno at the University of Michigan has developed several unique tests to prepare the three-legged spacecraft's operators for success.
Nutty soil conditions
The lander's first hurdle will be to avoid blasting the icy soil away when it touches down, Renno said.
"Phoenix will be traveling around 10 meters per second when it lands, so it's going to use thrusters to soften the landing," Renno said. The price for this relatively soft descent is that the thrusters will be ejecting exhaust at more than twice the speed of sound, he explained.
To measure the effects of the 12 thrusters on Mars' surface, where gravity is just 38 percent as strong as on Earth, Renno and his colleagues created a glorified shower stall armed with a downward-pointing air cannon. At the stall's base, they placed sawdust and crushed walnut shells to mimic the Martian soil and blasted it with air for several seconds to simulate the thrusters' blasts.
"The walnuts help us simulate not only the weight, but also the density of the soil," Renno said. "It's going to be pretty darn close," he said, when the laboratory performs the experiment again under vacuum conditions at NASA's Ames Research Center in California.
So far, Renno explained, the results show that if the lander travels horizontal to the ground while landing, it should leave an intact "wake" of soil for chemical, conductivity and microscope analyses. Another benefit is that thruster exhaust would spread over a larger area and cut down onany soil contamination that would be more concentrated in a vertical landing.
So long, sample
Assuming a top-notch landing, Renno said Phoenix's next challenge will be for its 8-foot robotic arm to collect and fully deliver the samples of icy soil. Mission engineers had planned to dump the scooped-up samples into the lander's mini-laboratory from a height of about 4 inches, but Martian winds of up to 11 mph are expected at the landing site.
"The finer material will blow away at that height, leaving only the heavier, denser soil to sample," Renno said. "To make the trip worthwhile, we need the whole sample." The more clearance the arm has from the craft, however, the less likely the arm is to snag on or damage the spacecraft
Renno thinks shrinking that distance to about 1.2 inches from the drop-zone should address the competing concerns, but the team is planning tests in a wind tunnel to be certain. In the upcoming experiment, they'll drop bits of ebony, bamboo and balsa to represent Martian ice, soil and dust in the wind and determine the optimum drop distance using high-speed camera images.
Renno said addressing such problems may seem simple, but the number of issues to fix add up quickly. "With missions like these, there are so many things to worry about," Renno said. "But you solve them one by one and get it right, until it's perfect."
The team's experiments were funded by NASA as well as Lockheed Martin, the spacecraft's manufacturer.