BU Researchers Help Rovers Explore Atmosphere, Surface of Mars

by Brian Fitzgerald and David J. Craig

The robotic arms on both the Spirit and Opportunity Mars exploration rovers have instruments that can grind away rock layers, take microscopic images, and analyze the elemental composition of rock and soil. Illustration courtesy of NASA

Last spring College of Engineering graduate student Matt Heverly waved farewell to identical robotic arms he helped design for Spirit and Opportunity, two exploration rovers that NASA was sending to Mars. Then he prayed they would make it to the red planet without being damaged. They did, surviving a rough ride into Mars's atmosphere at 12,000 miles per hour, and successfully landing on January 3 and January 24, respectively. For Paul Withers, a CAS research associate at the Center for Space Physics, the most exciting part of the trip was over as soon as the rovers touched down. An expert in the upper regions of Mars's atmosphere, Withers has been analyzing the rovers' fiery descent to better understand the unusual Martian atmosphere. Both Heverly (ENG'05) and Withers had an integral role in NASA's historic mission to explore the atmosphere and surface of Earth's nearest neighbor.

Heverly had further cause for celebration in early February, when he saw his devices for the first time in nine months. "The most stressful time during the Spirit and Opportunity webcasts was watching the small pyrotechnic devices fire and release the arms from the cables that are holding them," says Heverly. "Up to that point, the mission had been a success - the rovers came off the landers fine and started driving around. But it really wasn't a total success until the arms deployed. So many things could have gone wrong. I was asking myself, did I check all those calculations right? Is dirt going to get into the motor? We had only one shot to get it right. I was so relieved when the arms worked."

Not only are the rovers' panoramic cameras sending back the highest resolution pictures ever taken of Mars, their arms are also busy examining the planet's soil, analyzing its minerals and chemistry. The ultimate goal is to gather enough information about rock and soil structure to determine if water once evaporated from the planet, which will give researchers a better indication of whether Mars might have been suitable for sustaining life in the past.

	The capsule containing NASA's space rover Spirit descended through the upper regions of the Martian atmosphere at twenty times the speed of sound, cutting a largely horizontal path so that the atmosphere could absorb its speed. Illustration courtesy of NASA/JPL-Caltech

Heverly was a mechanical engineer for Alliance Spacesystems, Inc. (ASI), in Pasadena, California, before coming to BU last fall to further study robotics and controls. At ASI, he worked with three other engineers for almost two years on the robotic arms, formally known as instrument deployment devices.

Each arm has roughly the size and motion capabilities of a human arm, allowing it to position its four instruments in contact with rocks and soil. "My duties included portions of the structural design and analysis of the arm," says Heverly, who also made most of the mechanical drawings of the arm and was responsible for design portions of the actuators and contact sensors. "It was amazing to be part of a space hardware project," he says, "and it certainly is fun to turn on my computer and look at the footage of the rovers, and see the parts that I designed. There they are, right on Mars."

A microscope on Opportunity's arm has examined a patch of soil, revealing structures as thin as a human hair, and a Moessbauer spectrometer has collected information to identify minerals. The arm has also examined soil with another instrument, the alpha particle X-ray spectrometer, which reveals chemical elements. On the other side of Mars, Spirit's arm dusted off a rock with an abrasion tool, and used a microscope and two spectrometers to examine it. The rovers have found evidence of water and will continue studying the pebbly Martian soil.

Graduate student Matt Heverly (ENG'05) (right), who came to BU to further study robotics and controls, and Pierre Dupont, an ENG associate professor of aerospace and mechanical engineering, who has recruited Heverly to a team developing technology for fetal cardiac surgery. Photograph by Vernon Doucette

The Air Up There

As a member of NASA's Spirit Atmospheric Advisory Team, Withers spoke nightly with NASA officials by telephone during the final stages of Spirit's seven-month, 300-million-mile voyage to Mars and for several days after it landed on January 3, analyzing data that helped gauge the spacecraft's performance. And as a consultant to Britain's failed Beagle 2 Mars mission, the twenty-eight-year-old native of England wrote computer programs that, had the spacecraft survived, would have helped scientists and engineers reconstruct its trajectory through the Martian atmosphere.

Spirit and Opportunity slowed their entries into the Martian atmosphere by aerobraking, using the atmosphere to direct their trajectories and slow down without rockets. "If Spirit had descended straight down through the Martian atmosphere, there would not have been enough air to absorb its speed," Withers says. "So when it entered the upper atmosphere at twenty times the speed of sound, or about five kilometers per second, it traveled more horizontally than vertically, at an angle of only about fifteen or twenty degrees. During the last ten to twenty kilometers of its descent, it lost the rest of its speed, until a parachute opened up at an altitude of between five and ten kilometers. Eventually, stabilizing rockets brought it to almost a dead stop about ten meters above the ground, its cocoon of airbags inflated, and it dropped to the ground and bounced several times."

Spirit's fiery descent is helping scientists learn more about the cold, thin Martian atmosphere. "Instruments on board Spirit measured the aerodynamics the spacecraft experienced when it passed through the atmosphere," Withers explains. "With that information, and with our knowledge of Spirit's shape and mass, other engineers and I were able to determine the density, pressure, and temperature of Mars's atmosphere, from its outer regions all the way down to where the spacecraft hit the dirt."

	Paul Withers, CAS research associate. Photograph by Vernon Doucette

Martian air contains mostly carbon dioxide and weighs only about one percent of Earth's nitrogen-oxygen atmosphere. "Atmospheric pressure on Mars is much lower than on Earth," Withers says. "If you blew up a balloon on Earth and released it on Mars, it would explode immediately. Another distinguishing feature of the Martian atmosphere, and one that poses interesting scientific questions, is that in winter the gas that makes up the atmosphere freezes to the surface near the poles, so that the volume of the atmosphere can actually change by as much as 25 percent between summer and winter. That has no parallel on Earth."

The major scientific finding emerging from these missions, Withers says, will be a detailed understanding of how the temperature changes from the top to the bottom of Mars's atmosphere. "The spacecrafts' measurements may also help us understand how areas of heat move around in the Martian atmosphere," he says. "In addition, by looking at atmospheric temperatures, we can determine the structure of clouds that the spacecraft might have passed through, such as if they're made of carbon dioxide, water, or some other component of the atmosphere."

These lessons may help scientists better understand Earth's thin shell of air. "The laws of physics that govern Earth's atmosphere," Withers says, "such as how the sun's heat and light are absorbed in different regions and how air moves around between the equator and the poles, should apply in the Martian atmosphere. So by looking at a system that is very different from our own, we can test our theories about how atmospheres work in general."