09/23/92
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STANFORD - The exploration of Mars resumes Friday after more than 15 years, and a team of Stanford engineers will be making the most precise measurements of the Martian atmosphere and gravity ever taken.
The unmanned expedition, Mars Observer, is scheduled for launch on a Titan III rocket from Cape Canaveral Sept. 25, after a two-week mechanical delay. The voyage will be the first American exploration of the red planet since th e landing of the Viking automated spacecraft in 1976.
Researchers for Stanford's electrical engineering department and Space Telecommunications and Radioscience Lab (STAR Lab) will be using radio occultation techniques perfected during explorations of Saturn, Neptune and Jupiter t o take the temperature, measure the pressure and help plot the gravitation of the planet.
They will be joined in the radioscience experiment by one French scientist, and researchers from the Goddard Space Flight Center in Maryland and the Jet Propulsion Laboratory of the California Institute of Technology in Pasaden a, said Stanford Professor G. Leonard Tyler, who serves as team leader for the mission's radioscience investigation.
While the Stanford team is gathering radioscience data, other experimenters will be surveying other phenomena on this mysterious planet, a world with the solar system's highest volcanoes, frigid deserts, wispy air, wild dust st orms and canyons that look suspiciously as though rivers once ran through them.
The Viking spacecraft found no evidence of life on Mars, even microscopic, but the possibility that some primitive form exists in or just underneath the surface remains. Mars Observer, however, is not looking for life.
The radio scienceequipment is among seven scientific instruments aboard the 5,470-pound (2,487-kilogram) spacecraft. Data from the spacecraft will match or even better data now being obtained of Earth by weather and scientific satellites, one researcher at Caltech said, including pictures taken with equipment similar to that once launched in spy satellites.
The spacecraft cost more than $510 million. With launch costs, the price of the mission will exceed $800 million.
The journey to Mars, which has a mean distance from Earth of 141.6 million miles (227.9 million kilometers), will take 11 months. Initially, the spacecraft will enter an elliptical polar orbit and then will be shifted to a lowe r, circular mapping orbit with an average altitude of about 235 miles (378 km).
The orbits will take 118 minutes each, and the orbiter will fly over nearly the same area every seven Martian days, which are 5 percent longer than an Earth day. Every 26 Martian days, the Observer will have completely mapped t he surface once.
Because the orbit will be synchronized with the sun, it always will be 2 p.m.. Martian time on the red plains and mountains below the satellite. Tyler said this was designed so that the sun and shadows are constant during the o ne-Martian-year (687 Earth days) mission. The scientists can better observe seasonal and weather changes that way.
Researchers, of course, are hoping the equipment lasts much longer than one year, adding to the data, Tyler said.
Unlike previous expeditions, the data from Mars Observer will be distributed electronically so researchers won't all have to gather in Pasadena. The Stanford contingent will be based at STAR Lab in the Durand building on campus . In some cases, a "quick and dirty" reading will be no more than a half hour old, including the 11 minutes it takes for the radio signal to get from Mars to Earth. It might take several weeks to refine the measurements.
Another unique feature is that every instrument works without the satellite changing altitude or attitude, which means one experiment will not have to wait while another one is proceeding, although there are some power limits. Richard Simpson, senior research associate at Stanford's Electrical Engineering Research Administration, said the radio data, which eventually will consist of as much as six gigabytes collected on 10 CD-ROMs, will be analyzed on Sun wo rkstations here.
David Hinson, a Stanford senior research scientist, said the radioscience experiment can measure the atmosphere and gravity of Mars with very high accuracy.
Martian gravity will be measured by determining the perturbations in the orbit of the spacecraft changes in the gravitational field caused by anomalies below the surface or huge rocks. Precise measurements of the surface, neede d for the study, will be made by a laser onboard Observer.
The researchers hope to discern the effects of the solar tide on the planet - that is, how much the orb of Mars is changed by the gravitational force of the sun. They also should be able to spot seasonal changes as carbon dioxi de is absorbed and then evaporated by the polar ice caps. The caps are mostly frozen carbon dioxide, probably with a little water ice.
The second goal of the radioscience experiment is to measure the temperature, circulation and pressure of the Martian atmosphere to a precision never before possible, making use of the radio signals sent from the Observer back to Earth as the spacecraft passes behind the planet.
Every time Mars Observer sinks below the Martian horizon, the radio signal is occulted - that is, blocked. By watching the signal blink out, the researchers can measure the density, temperature and pressure of the largely carbo n dioxide atmosphere and characteristics of the ionosphere, mostly above the polar regions. The same thing happens as the satellite reappears on the other side.
The information obtained from the lower atmosphere is similar to what is learned from weather balloons on Earth. Overall, it's as if the radio team were able to launch a weather balloon every hour on Mars.
Tyler, Hinson and the Stanford team will be particularly interested in watching how the polar regions cool down and warm up as periodic worldwide dust storms tear across Mars. (The dust storms are not expected to interfere with the experiment.) They also will be looking at places where the carbon dioxide appears to condense, which might explain the so-called polar hoods - gossamer clouds that have been seen hanging over the poles.
They will be looking at the vertical structure of atmosphere waves or weather fronts, measuring both seasonal changes and the local weather, all of which will lead to a better understanding of the circulation of the Martian atm osphere. They also should be able to pick up temperature changes near the surface as day changes to night.
The researchers also will be acquiring a Mars year's worth of data on the ionosphere and its relationship to the solar wind, the atmosphere and Mars' magnetic field, if it has one.
And while they are at it, they will be able to measure the surface radius of Mars.
All this depends on the equipment working after a trip of 100 million miles.
The solar panels (23 by 12 feet, or 7 by 3.7 meters in size) that provide power to Mars Observer are only partially unfolded during the flight, and get fully extended just before the spacecraft reaches Mars.
"We don't test them until we're close to orbit," Simpson said.
The radioscience experiment consists largely of two redundant radio transponders. They receive at 7.2 GHz and transmit at 8.4 GHz, in what is called the X band of the radio spectrum.
At least two graduate students will be involved in analyzing the data, and there probably is room for three, perhaps four more in the future, Simpson said.
The expedition itself is unusually international in scope, with teams from France, Germany and Britain, as well as a dozen Russians, participating. The Russians will launch their expedition to Mars in two years, with considerab le American assistance.
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