The air up there
BU planetary scientist tracks Spirit's trip to Mars
David J. Craig
As NASA's space rover Spirit pokes around the surface of Mars collecting and analyzing samples of sediment for the next three months, scientists will wait eagerly for signs of water and life on the red planet.
But for Paul Withers, a CAS research associate at the Center for Space Physics, the most exciting part of Spirit's trip is over. An expert in the upper regions of the Martian atmosphere, Withers is interested in the aerodynamic measurements Spirit took as it approached Mars, so he can better understand how the chemistry and dynamics of its atmosphere compare to those of Earth's.
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 last month, the 28-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.
The B.U. Bridge spoke to Withers recently about his role in the Spirit mission and about what scientists hope to learn by analyzing atmospheric data gathered during the spacecraft's descent.
B.U. Bridge: How would you characterize the Martian atmosphere?
Withers: It's cold and there isn't much of it. It's made largely of carbon dioxide and it weighs about one percent of what Earth's nitrogen-oxygen atmosphere weighs. So atmospheric pressure on Mars is much lower than on Earth. If you blew up a balloon on Earth and released it on Mars, it would explode immediately because there would be hardly any atmosphere pushing back against it. The temperature near the surface of Mars is around minus 100 degrees Fahrenheit. Near the top of the atmosphere, say above an altitude of 100 kilometers, the temperature is about minus 210 degrees Fahrenheit.
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 amount of gas in the atmosphere can actually change by as much as 25 percent between summer and winter. That has no parallel on Earth.
B.U. Bridge: What kind of information did you help analyze for NASA? Withers: Instruments on board Spirit measured the aerodynamics the spacecraft experienced when it passed through the Martian atmosphere. 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' atmosphere, from its outer regions all the way down to where the spacecraft hit the dirt.
B.U. Bridge: How does that help NASA?
Withers: NASA engineers need to know if the spacecraft functioned as designed — for instance, if the parachute opened at the right time and if the speed with which its airbags hit the surface was what was expected. Spirit performed very well, which is especially important because an identical spacecraft, Opportunity, will try to land on Mars at the end of the month. The engineers also need to know that they're using the most accurate atmospheric models to study the spacecraft's performance.
B.U. Bridge: What basic questions about the atmosphere on Mars might the spacecraft's measurements help answer?
Withers: The major scientific finding that will come out of these missions regarding the atmosphere is a detailed understanding of how the temperature changes from the top to the bottom of the Martian atmosphere. The spacecrafts' measurements may also help us understand how areas of heat move around in the Martian atmosphere. We know this happens because measurements showed that as Spirit descended, the atmospheric temperature didn't increase smoothly, but showed fluctuations of as much as 10 to 20 percent.
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.
B.U. Bridge: Have scientists had previous opportunities to gather information about the Martian atmosphere?
Withers: There have been only three other chances to gather a top-to-bottom profile of the Martian atmosphere: NASA's two Viking missions to Mars in the late 1970s, and its Pathfinder mission in 1997. Other spacecraft have orbited Mars, but have never descended below about 100 kilometers from the planet's surface.
B.U. Bridge: How might new knowledge about the Martian atmosphere apply to Earth's atmosphere?
Withers: The laws of physics that govern Earth's atmosphere — involving, for instance, how the sun's heat and light are absorbed in different regions of the atmosphere, 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.
BU Bridge: Why did Spirit survive the trip through the Martian atmosphere, while England's Beagle 2 failed just a few weeks ago?
Withers: Beagle 2 weighed only about a 10th as much as Spirit, which means it had less space for safety features that would have helped it survive entry into the Martian atmosphere. For example, Spirit and Opportunity have rockets that help them compensate for unwanted motion as they approach the surface. Beagle 2 didn't have features like that because the Beagle program has always been a low-cost and relatively high-risk attempt to land on Mars.
Beagle 2 and Spirit had similarities, though. To slow their entry through the Martian atmosphere, both were designed to use aerobraking, which means using the atmosphere to direct your trajectory and to slow down without the use of rockets. If Spirit had descended straight down through the Martian atmosphere, there would not have been enough atmosphere to absorb its speed. So when it entered the upper atmosphere, traveling 20 times faster than the speed of sound, or about 5 kilometers per second, it traveled more horizontally than vertically, at an angle of only about 15 or 20 degrees. After a couple of minutes in the atmosphere, it slowed to about one kilometer per second, and during the last 10 to 20 kilometers of its descent lost the rest of its speed, until a parachute opened up at an altitude of between 5 and 10 kilometers. Eventually, stabilizing rockets brought it to almost a dead stop about 10 meters above the ground, its cocoon of airbags inflated, and it dropped to the ground and bounced several times.
One of the more likely explanations for Beagle 2's failure, I think, is that the speed at which it reached Mars' surface was too much for its airbags to handle.