The University Record, November 21, 1995

Scientists play key role in Galilieo Investigation of Jupiter

By Sally Pobojewski
News and Information Services

If all goes as planned, U-M scientist Sushil Atreya will return to his Ann Arbor laboratory late in December with a 3.25-inch computer diskette tucked inside his jacket pocket. Encoded on the diskette will be answers to questions that have puzzled scientists since the Italian astronomer Galileo first observed Jupiter through a handmade telescope in 1610.

The information on Atreya's diskette will have traveled 550 million miles from NASA's Galileo spacecraft---launched in October 1989 on a mission to unlock the secrets of the mysterious giant planet.

At 2:07 p.m. PST on Dec. 7, a 750-pound Jupiter probe will make radio contact with the Galileo spacecraft orbiting 125,000 miles overhead, and then begin a 75-minute plunge into Jupiter's atmosphere. The probe will give scientists their first and perhaps only opportunity to remotely "sniff" the thick, multi-layered atmosphere of this gaseous planet.

George Carignan, associate dean for research in the College of Engineering, and Atreya, professor of atmospheric and space science, led a team of U-M scientists and research engineers who spent four years designing and building the key instrument on the probe called the Galileo Probe Mass Spectrometer or GPMS.

"GPMS is one of six sensing instruments on the probe," Atreya says. "While other instruments record temperature, pressure and density, and measure the amount of sunlight, cloud particles and lightning in Jupiter's atmosphere, GPMS will analyze its chemical composition. Currently, we have identified two dozen molecules in Jupiter's upper atmosphere, but we suspect there are a whole slew of chemical constituents below the topmost ammonia clouds. We have no idea what we might find.& quot;

Researchers in the Space Physics Research Laboratory, working with scientists at the Goddard Space Flight Center, designed and built the probe's mass spectrometer in the mid-1980s. (Because of the Challenger accident, the 1986 scheduled launch of the Galileo spacecraft from the space shuttle was delayed until 1989.) A key NASA requirement for the probe's mass spectrometer was that it be able to identify certain classes of molecules at the parts per trillion level---a level of sensitivity greater than any previous space sensing instrument.

"We really had to stretch the technology of that time to the limit to package such a sophisticated instrument in the small space available," Carignan says. "It was an intense project, the toughest I've ever been involved in."

According to Atreya, the probe will enter Jupiter's atmosphere 250 miles above the topmost cloud layer just north of the planet's equator, traveling at a speed of more than 100,000 miles per hour. A thick carbon phenolic heat shield will protect the probe's instruments from the 27,000 degrees Fahrenheit temperature produced from friction between the probe and the atmosphere.

After two minutes, when the probe has slowed to 1,000 miles per hour, the remains of the heat shield will be dropped and two parachutes will open. The probe will then float 125 miles down through layers of ammonia ice, ammonium hydrosulfide, water ice and perhaps an ammonia-water solution scientists suspect may be present beneath the upper cloud layers. During this descent, atmospheric pressures will increase from one-tenth to about 25 times the pressure at the Earth's surface.

"We have a 75-minute window to collect data and transmit it up to the orbiting Galileo spacecraft before it passes over Jupiter's horizon and can no longer receive radio signals from the probe," Atreya explained. "By that time, the probe will most likely have been crushed and scorched by Jupiter's extreme atmospheric pressure and 400 degree Fahrenheit temperature."

Originally, NASA scientists planned to use a tape recorder on board Galileo to record the probe data and then transmit it to Earth via a high-speed radio transmission antenna. Since the mission began, however, mechanical problems have disabled the antenna and reduced the tape storage capacity of the recorder.

"The current plan is to store probe data in the spacecraft's computer memory, with the tape recorder used as a backup storage device," Atreya says. Data will be transmitted back to Earth over the spacecraft's low-gain antenna, which can only send data at the agonizingly slow speed of 40 bits per second---thousands of times slower than an ordinary fax machine.

Even though their first direct look at Jupiter's atmosphere will arrive in a trickle of data, instead of a flood, Atreya and his colleagues are still enthusiastic. "We've waited 20 years," he says. "We can wait a few more weeks."

Other members of the U-M Galileo science and experiment team include Thomas M. Donahue, the Edward H. White II Distinguished University Professor Emeritus of Planetary Science, and Alan Macnee, professor emeritus of electrical engineering and computer science. Engineering support was provided by Bruce Block, John Maurer, John Caldwell, Bud Campbell, John Eder, Mima Carson, Plymouth Freed, Mark Huetteman and Lyle Slider.

Hasso Niemann, a scientist at the Goddard Space Flight Center and principal investigator for the GPMS project, also has a U-M connection. He received his Ph.D. in electrical engineering from U-M under the direction of William G. Dow, professor emeritus ofelectrical engineering and computer science.