The University Record, February 20, 1996

NASA shuttle mission will duplicate Ben Franklin's experiment

By Sally Pobojewski
News and Information Services

U-M scientist hopes to repeat Ben Franklin's famous experiment with electricity next week. Instead of a kite, some string and a key, Gilchrist will use a 1,142-pound Italian satellite, a 13-mile-long cable and the space shuttle Columbia.

Gilchrist's experiment will take place during NASA's next space shuttle mission, scheduled for launch at 3:18 p.m. on Thursday (Feb. 22) from Kennedy Space Center. During the 13-day mission, NASA astronauts will test a new type of technology called the Tethered Satellite System (TSS), in which research satellites and the space shuttle orbit together, linked by very long, thin cables.

Gilchrist plans to explore TSS technology's ability to generate electricity from Earth's upper atmosphere and detect low-frequency radio signals from Earth. The TSS mission originally was scheduled for July 1992, but a mechanical malfunction caused the tether's reel mechanism to jam shortly after satellite deployment began.

"This mission will be much more than a reflight of the original experiment," says Gilchrist, assistant professor of electrical engineering and computer science and assistant professor of atmospheric, oceanic and space sciences in the College of Engineering. "During the past three years, we have enhanced our objectives for the mission considerably.

"One of NASA's most important objectives is to learn how to maneuver a space shuttle and a large satellite linked together by a 13-mile-long, copper-and-nylon tether," Gilchrist says. "With the tether fully deployed, this will be the largest man-made structure ever flown in space. It gives us an ideal opportunity to test our ability to fly multiple spacecraft in formation, like pearls on a string."

The ability to "fly" several spacecraft in formation will be important in future scientific efforts to obtain multi-point measurements of naturally occurring structures in the Earth's upper atmosphere, according to Gilchrist. It also could be important in space station operations.

The TSS mission's primary goal is to learn more about the electrodynamics of the ionospherea portion of Earth's upper atmosphere where interactions with solar radiation produce electrically charged gases called plasma, and very strong magnetic and electric fields. As the satellite orbits through the ionosphere at high speeds, charged electrons will collect on the surface of the satellite and flow down the conducting tether to the space shuttle where they will be safely discharged back into the ionosphere.

"We expect to generate between 4,000 and 5,000 volts from one end of the tether to the other," Gilchrist says. "Just as we can use the ground connection in our household power outlets to close an electrical circuit and conduct current on Earth, we will use the ionosphere to close the circuit and conduct current through the tether. Electrical currents occur naturally in space. We see the atmosphere's response to these currents in the aurora borealis or northern lights. Our experiments will allow us to study these ionospheric currents in a controlled manner."

Gilchrist adds that astronauts inside the space shuttle crew compartment are isolated from the electrical voltage and will not be in danger during the experiment.

The electrical response of the system will be measured and analyzed with an instrument package in the shuttle's payload bay called the Shuttle Electrodynamic Tether System (SETS), which was prepared by Gilchrist and colleagues at the Space Physics Research Laboratory, Utah State University and Stanford University. Other scientific instruments also will measure electrical response in the shuttle payload bay and at the satellite.

NASA engineers hope these experiments will help scientists learn how to harness electrical energy from the orbital motion of a spacecraft for use as a power source on future long-distance missions.

An additional objective of the TSS mission is to use the 13-mile-long tether as "the world's longest antenna for very low-frequency signals," Gilchrist says. "We will attempt to detect two types of signalsnatural background noise from the electrically active ionosphere and manmade radio navigation signals used by ships at sea to track their position. Astronauts also will use the tether to send low-frequency signals to special receiving stations on Earth."

During the 1992 TSS flight, NASA astronauts were able to unreel the tether for about 1,000 feet before the mechanism jammed. "While we were disappointed that we could not complete the experiment in 1992, it was encouraging to discover that the tethered satellite was much more stable in orbit than we thought it would be," Gilchrist says. "Now that the reel mechanism problem has been resolved, we are very optimistic we will be able to deploy the tether to its maximum length this time."