Courtesy NASA
11/6/96
CONTACT: David F. Salisbury, News Service (415) 725-1944
STANFORD -- Len Tyler and his colleagues have been waiting 10 years to start their daily weather reports from Mars. Some time next year, when they fire up the instruments scheduled for launch today on the spacecraft Mars Global Surveyor (MGS), they'll get their chance.
Tyler, professor of electrical engineering, leads the international radio science team for the MGS satellite, which is designed to orbit Mars and send back data for at least one Martian year (687 Earth days). The team expects to obtain the most precise measurements ever made of the pressures and temperatures on the Red Planet. Using radio occultation a technique invented and developed at Stanford they will acquire weather data about Mars more accurate than any currently available about some parts of the atmosphere of Earth.
Courtesy NASA
Engineers place the Mars Global Surveyor spacecraft on the test stand at Cape Canaveral, Fla., for final adjustments before loading onto the Delta Rocket.
The radio scientists will measure pressure changes thought to kick up dust storms that sweep across the surface of Mars and sometimes engulf the whole planet. They will measure the vertical structure of the largely carbon dioxide atmosphere, to track both seasonal changes and local weather conditions. They may be able to spot changes in Martian polar ice caps as they absorb and then evaporate carbon dioxide. "[All of] this will tell us a great deal about the physics of the [Martian] atmosphere," Tyler said.
The team also will map the gravity field of Mars with unprecedented accuracy. And once the weather reports start streaming in from the Mars Global Surveyor, they'll set up a study program on the World Wide Web, so that eighth-grade students can track atmospheric changes and compare the very different weather patterns on Mars and on Earth.
Other Stanford members of the MGS radio science team are senior research scientists David Hinson and Richard Simpson, and software engineer Joseph Twicken. They are working with scientists from the Centre National d'Etudes Spatiales in Toulouse, France, and the National Aeronautics and Space Administration. Two Stanford electrical engineering graduate students, Bilal Ahmad and Tuna Karayel, have made major contributions to the project.
The radio scientists originally developed the experiment for the ill-fated Mars Observer satellite that disappeared when NASA lost all communications with it in August 1993, just three days before it reached Mars. NASA approved the Mars Global Surveyor as a low-cost "recovery" program to make up for this loss.
Radio occultation technique
The radio occultation technique was first suggested in the early 1960s by Von R. Eshleman, professor emeritus of electrical engineering, and his student, Gunnar Fjeldbo. It had its first test on Mars with the Mariner 4 spacecraft in July 1965.
Tyler, Hinson and Eshleman further developed the theory and it was perfected in the early 1980s, during the dramatic Voyager fly-bys past Jupiter, Saturn, Neptune and Uranus. By now, every planet in the solar system except Pluto has had its atmosphere measured using radio occultation, and scientists have begun applying the technique to provide better data about Earth as well.
Courtesy NASA
A Delta II launch vehicle, being readied for takeoff at NASA/Kennedy Space Center at Cape Canaveral, Fla., was scheduled to take off Nov. 6 and carry Global Surveyor space-
craft to Mars.
"It was a good thing that we started with Mars," Tyler said. The Red Planet turned out to be a good "laboratory rat," with the simplest atmosphere in the solar system. Tyler said that what they learned on Mars allowed them to overcome some "very hairy problems" as they moved to analyze more complex atmospheres.
Now the Mars Global Surveyor will carry the most sophisticated radio occultation equipment to date, so accurate that the orbiting satellite will be able to get substantially more accurate reports of atmospheric conditions than the Mars landers of the 1970s were able to obtain as they sat on the Martian surface
Here is how radio occultation works: the Mars Global Surveyor will send back signals to Earth as it orbits the Red Planet. The spacecraft's orbit will carry it regularly out of sight behind the planet, a process scientists call occultation. Because the satellite will be placed in a two-hour polar orbit, it will disappear every other hour behind the planet's north pole and emerge a hour later over the south pole.
In the moments just before the satellite disappears and just after it reappears, Tyler and his team will carefully monitor its radio signals. At these moments only, the radio signals pass through the Martian atmosphere, enabling the scientists to use the radiowaves as a precision instrument to measure the temperatures and pressures in the distant envelope of gases.
As the radio waves pass through the atmosphere, the gases act to slow down the radio waves. This has the effect of lengthening the radio path and slightly altering their frequency and wavelength. By measuring these changes the scientists can determine the temperature and pressure of the atmosphere that the radio signal traveled through with extreme precision.
This satellite is the first to carry a high-precision crystal clock to Mars. Having such a clock on board allows the scientists to measure extremely small changes in the received radio frequencies. The measurements are so precise that they correspond to differences of only two tenths of the millimeter in the 100-million-plus mile path that the radio signal follows. They allow the scientists to measure changes in pressure of less than one-tenth of a percent and changes in temperature of less than one-tenth of a degree Celsius.
The scientists also can analyze the signal to create vertical temperature and pressure profiles of sections of the Mars atmosphere, with measurements stacked at intervals of 200 meters (600 feet).
"If the efforts of one of our graduate students is successful, we could reduce that to intervals of 10 meters (33 feet)," Tyler said.
On Earth, similar profiles of the atmosphere at different altitudes are constructed using data from weather balloons. "[This is] like sending up weather balloons at alternate poles at alternate hours," Tyler said. The team hopes to get more than 8,000 occultations during the course of the experiment.
The Mars Global Surveyor radio team has a second, related goal: to map the planet's gravity field with unprecedented accuracy. Many of the experiments on board the spacecraft depend on pinpointing the location of the spacecraft with extreme precision. To accomplish that, the radio scientists will monitor shifts in the radio frequencies that they receive from MGS. Those shifts are caused as the spacecraft orbits the planet and its position responds to slight changes in the planet's gravitational force.
When combined with the data from MGS's laser altimeter, for example, the improved knowledge of the planet's gravity will help scientists pin down major uncertainties regarding the shape of Mars. Surface elevations in many places are not well known.
Aerobraking
After losing years of work when the Mars Observer mission failed, Tyler and his colleagues are watching the Mars Global Surveyor with more than a little concern. Tyler said that the mission's first challenge will be to achieve the correct orbit, swinging around Mars from pole to pole every hour.
Previous planetary orbiters used onboard thrusters to achieve their desired positions, but that approach adds substantially to the spacecraft's weight and cost. The use of "aerobraking" on the global surveyor helped slash the mass of the payload from more than two tons to slightly more than 2,200 pounds, which reduced launch costs from $350-400 million down to about $60 million, Tyler said.
In the aerobraking method, NASA puts the satellite into an initial elliptical course around the planet that forces it to dip into the upper reaches of the atmosphere with each orbit. The aerodynamic drag gradually pulls the spacecraft into a circular orbit, if it is targeted precisely enough.
"One kilometer too low and it will burn up. One kilometer too high and the effect isn't strong enough," Tyler said. While it is relatively easy to achieve the targeting accuracy needed, he said, the tricky part will be monitoring expansion and contraction of the Martian atmosphere and adjusting the satellite's position accordingly. The aerobraking approach was successfully tested at Venus by the Magellan spacecraft in 1993, but the Venusian atmosphere is not as variable as the envelope of gases around Mars, Tyler said.
Martian weather reports for science education
Once the Mars Global Surveyor is orbiting Mars and sending back data, software engineer Joseph Twicken will set up a Web-based study program for eighth graders. The students will be able to track the changes in the Martian weather over its long year, which lasts 687 Earth days. Educators who are interested in participating can contact Twicken at the e-mail address: joe@nova.stanford.edu.
In a note to K-12 educators posted on the Web, Twicken writes, "Planetary exploration
is . . . one of the rare subjects which interests almost anyone. It clearly demonstrates to young students that science and mathematics are not inherently dry and uninteresting. Furthermore, it demonstrates that there are jobs for people who study whose subjects which are both rewarding and fun. Members of the MGS Radio Science Team have participated in almost every U.S. planetary mission since the dawn of the space age. Most of them would not do anything else."
As part of the project, the radio occultation data from the Mars orbiter will be made available to students in participating classes. The researchers also will provide the tools required so that the students can convert the data into weather information.
In addition, the scientists intend to provide the students with comparable data from Antarctica, which is the most Mars-like place on Earth.
The students will collect local weather data and then make comparisons.
"After they do this they will realize that Mars is a very different place from Earth," Tyler said.
One thing the students may notice is that they can get better data about Mars than atmospheric scientists have collected about some parts of Earth's atmosphere. "On Earth, [radio occultation] could do better in the upper atmosphere, above the tropopause, than is possible with other methods," said Tyler.
Currently, several groups are attempting to apply radio occultation to get better information about Earth. They hope to use radio signals from the 20-plus satellites in the Global Positioning System to get improved pressure, temperature and humidity data. It would require a second set of small, relatively inexpensive satellites. Tyler and Hinson are working with scientists at NASA's Jet Propulsion Laboratory and elsewhere on the preliminary design of such a system.
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