Above the atmosphere bits and pieces of debris zip around the Earth at tens of thousands of miles an hour. Some of these objects are natural cosmic dust, produced by comets, meteoroid impacts or other natural processes, while others are debris resulting from human activity in space.
These objects have caused varying levels of damage to space shuttles, satellites and the Hubble Space Telescope. And although detection systems currently track the largest pieces of man-made debris, many more particles are too small to track, ranging in size from pebbles or sand grains down to particles that can only be seen with a microscope.
"Many of these particles are produced by collisions between larger debris objects, and so information about these particles is important for understanding the whole debris population in Earth orbit," said Bruce McKibben, Senior Scientist at the University of Chicago's Laboratory for Astrophysics and Space Research.
A Chicago instrument designed to provide data to help reach that understanding will be launched Jan. 15 on the Advanced Research and Global Observation Satellite (ARGOS) from Vandenberg Air Force Base in southern California. ARGOS's scientific payload will include the University's space dust experiment (SPADUS), which will measure the mass, speed and trajectory of dust particles in low-Earth orbit, and will allow scientists to determine whether they are particles left in the wake of comets or man-made orbital debris.
"This is the first active experiment where you can separate these two phenomena," said John Simpson, Chicago's Arthur Holly Compton Distinguished Service Professor Emeritus. "We will be able to tell whether the debris is uniformly distributed or in clouds around the Earth, and even whether there's a ring around the Earth, like Saturn's, but very weak. This is one of the discovery possibilities."
ARGOS, an unclassified U.S. Air Force satellite, will circle the Earth for three years in a polar orbit at an altitude of 516 miles. This altitude is near a region heavily used by commercial, scientific and government spacecraft, where ground tracking of the larger objects indicates a concentration of man-made debris. In addition to the Chicago experiment, ARGOS will conduct high-temperature superconductivity experiments, upper atmospheric imaging and environmental studies, and test electric propulsion methods.
Much of the current data regarding the quantity of debris particles in low-Earth orbit was collected by NASA's Long Duration Exposure Facility (LDEF) from 1984 to 1990. But LDEF could not distinguish between natural and man-made debris and could not determine where or when in its orbit an impact occurred. SPADUS will be able to do this by measuring the time of impact and the particle trajectory and velocity of debris with enough sensitivity to detect particles smaller than the particles contained in a puff of cigarette smoke.
Man-made debris in a circular orbit races about the Earth at speeds of nearly 17,000 miles an hour. "This debris consists of everything from rocket casings and dead satellites on down to the very small dust particles that can result from the grinding down of these large objects as they collide with each other and with dust particles already in orbit," McKibben said.
Cometary debris, on the other hand, travels more than 25,000 miles an hour. There is also a remote possibility that SPADUS will be able to detect dust particles entering the solar system from interstellar space.
"They'll have even higher velocities," McKibben said. "Very small ones have been detected by spacecraft in the outer solar system right now. We might see them, but I wouldn't count on it."
SPADUS will be used to study the Leonid meteor shower next Nov. 17. The Leonid meteor stream consists of the boiled off remains of Comet Tempel-Tuttle and is usually fairly mild. But the shower was expected to be far more intense in 1998 and possibly 1999, because last February, the comet made its closest approach to the sun, which happens once every 33 years.
Last year, satellite controllers changed the orientation of their satellites to reduce surface area exposed to the cometary stream. "The meteor shower was not as strong as some predictions in 1998, so maybe this year we'll get the whole works," Simpson said.
Earth's orbital path will take it across other cometary streams as well, said Anthony Tuzzolino, Senior Scientist at Chicago's Laboratory for Astrophysics and Space Research. "There are 15 or 16 streams that are possible candidates for detection based on how close we'll come to them," he said. "There are a lot of things to look for."
Helping in the data analysis will be Herbert Gursky and his associates at the Naval Research Laboratory in Washington, D.C. The NRL contributed the instrument's mechanical design and construction of the experiment housing. Similarly, important contributions were made by Lockheed Martin, which provided the digital electronics box, including the microchips that make the sophisticated SPADUS measurements possible in a package small enough for space flight.
H.N. Voss, then of Lockheed Martin and now a physics professor at Taylor University in Upland, Ind., also led the effort to provide a small radiation sensor system, known as the ADS as part of the digital box. The ADS will monitor the radiation environment of the spacecraft, and data analysis for will take place at Taylor University.
Instruments related to SPADUS are components on NASA's current Cassini mission to Saturn and the Stardust mission to Comet Wild 2 (pronounced "Vilt" 2). Similar Chicago-built dust instruments also flew aboard the Russian Vega 1 and 2 spacecraft that visited Comet Halley in 1986.
Simpson and Tuzzolino built Cassini's High Rate Detector (HRD), part of a larger instrument, the Cosmic Dust Analyzer from Germany, which will collect and analyze dust particles found in interplanetary space and those that form the major components of Saturn's rings.
On Dec. 31, 1998, the HRD was turned on for the first time since Cassini's launch on Oct. 15, 1997, for three weeks of testing. The HRD, which is working fine, will be turned on again in June to collect several months of data as Cassini flies by Earth to gain gravity- assisted momentum toward its destination.
Cassini will pass Earth at an altitude of 620 miles this summer. "That's where the maximum of orbital debris is expected to be," Tuzzolino said. "The flyby will be a very exciting period."
Tuzzolino and Simpson, along with McKibben, also are providing the Dust Flux Monitor instrument for Stardust, which is scheduled for launch Feb. 6.
The $2 million SPADUS instrument is funded by NASA, the Office of Naval Research, the Naval Research Laboratory and Lockheed Martin.
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