Leftover Instruments Will Pave Way For New Propulsion Test

March 22, 1999

March 22, 1999: Instruments that flew on a total of four Space Shuttle missions are being prepared for one last trip into space, piggyback on a university satellite, to help validate a new method of studying electrified gases in space.

"We want to prove this new technique, the Deflection Plate Analyzer, or DPA, which characterizes ions and electrons in a plasma, and tell us what direction they are coming from" explained Fred Berry, PEST Project Manager in the Space Sciences Laboratory at NASA's Marshall Space Flight Center.

"Our first tests of the new detector have been successful," Berry said on March 18. "We have it functioning in the plasma chamber and have detected our first plasma with it."

The DPA will be flown as part of the Plasma Experiment Satellite Test (PEST) on the Joint Air Force-Weber State College Satellite, or JAWSAT, scheduled for launch in August. Assembly of the PEST payload will start at NASA/Marshall in April.

"This will really benefit ProSEDS, the propulsive tether project NASA/Marshall is developing for launch in 2000," said Dr. Nobie Stone, the principal investigator for the DPA project. "It will give us a chance to get an in-flight diagnostic test of one of the ProSEDS instruments."

ProSEDS, the Propulsive Small Expendable Deployer System, will use a wire tether to connect with the plasmas in low Earth orbit and form an "electric motor" that will slowly put the brakes on a spent Delta rocket stage. This will make it re-enter the Earth's atmosphere much faster than natural decay alone (less than two weeks rather than the normal 5 to 6 months). The motor effect employs the Earth's magnetic field and the electrically conducting ionosphere. Space is not a true vacuum, but is thinly populated by plasmas-gasses made up of atoms that have been stripped of one or more electrons, and the free electrons themselves.

Plasmas are generated by the ionizing effect of solar ultra violet (UV) radiation on the upper regions of the earth's atmosphere. This region of ionized gas, or plasma, is called the ionosphere and is heavily involved in space weather effects that are important to anyone who builds satellites. Some of the plasma effects can be caused by spacecraft themselves, so NASA/Marshall has placed plasma instruments aboard several Space Shuttle missions to measure the craft's own effects.

Because plasmas vary widely in space, ProSEDS will have to carry instruments that measure the ambient ionosphere in order to determine the conditions under which the experiment was conducted and the tether system's performance. It's analogous to determining the performance of an airplane where, in addition to its ground speed, it is necessary to know the speed of the wind in which the plane is moving.

The DPA uses a new analysis technique that can measure multiple ion streams and determine the intensity, flow direction, and energy and mass distributions for each stream. But it can't be fully tested on Earth because space simulation chambers can't fully mimic the complex conditions of space.

The space test requires comparing the DPA with instruments that have been successfully used in space many times. So, PEST will combine the old and the new, giving the DPA its first flight test while instruments from previous space missions look over its shoulder. Specifically, a Retarding Potential Analyzer (RPA) was part of the instrument suite on the Plasma Diagnostic Package flown on the third Space Shuttle mission (STS-3, 1982) and Spacelab 2 (STS 51-F, 1985).

That's our standard candle," Berry said of the RPA. Stone noted that the RPA design has been used for more than 30 years and will provide a good point of comparison. But where the RPA just measures along a narrow line of sight and under equilibrium, or "quiet" plasma conditions, the DPA will "see" ions arriving from different directions and should make reliable measurements even under highly turbulent conditions.

The other instrument making a return trip to space is the Soft Particle Energy Spectrometer (SPES), which flew aboard the Tethered Satellite System carried by the Shuttle (TSS-1 on STS-46 in 1992 and TSS-1R on STS-75 in 1996). Chasing down an oddity noted during the Tethered Satellite System missions is another objective of the PEST mission.

"We noticed something strange in the characteristics of the RM400 conducting thermal coating used on the tethered satellite," Stone explained. "The data suggested tremendous emissions of secondary electrons due to particle bombardment or solar ultraviolet or both. We had no reason to suppose it would do that before the TSS mission launched."

To find out what happened, PEST will include a test panel coated with stripes of gold - which has well known characteristics - and stripes of RM400. Stone hopes that data from the DPA and SPES instruments will help him and other scientists sort through the effects caused by the RM400 coating on the tethered satellite experiment.

The Tethered Satellite System stands atop its deployment boom during the STS-46 Shuttle mission in 1992.

Full size image available through contact

In addition to testing a new science instrument, PEST will also help scientists improve their knowledge of the space environment at 700 km (420 mi) altitude where JAWSAT will orbit from pole to pole.

"We think that flying a new generation of instruments - at this altitude over the Earth's poles - will provide a good data set that can be used to improve our ability to model that part of the magnetosphere," Stone said. "It's an important area because the polar regions are active, including the aurora borealis, energetic particle streams, and outflowing plasma and gas escaping from the earth."

JAWSAT will be launched by a surplus U.S. Air Force Minuteman ballistic missile with a Pegasus fourth stage added. The primary payload will be the U.S. Air Force Academy's Falconsat. Also piggybacking are Stanford University's OPAL satellite and the Arizona State University Satellite (ASUSat).

Total launch cost is $3 million. NASA/Marshall's expenses will be $232,000: $40,000 to piggyback, and $192,000 to develop the DPA instrument suite, the RM400 test panel, and the power and data handling electronics. The entire process, from the offer of a ride, to delivery of the flight hardware in June, to launch in August, will have taken only 11 months.

Berry said that PEST will operate for at least two months, and that the data can be collected by amatuer radio operators.

"We are really interested in hams being able to use these data," Berry said. Details on the frequencies that PEST will use, and how the data are formatted, will be published this summer, he added.

"To launch an experiment of this complexity - an array of three instruments, a materials test panel, and the supporting power and data systems - for $230,000 in only 11 months is unprecedented," Stone said. "It holds great promise for future small satellites. It's very encouraging to know we can do this."

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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