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NASA X-ray astronomy probe slated for rocket launch next week

NASA's first X-ray astronomy spacecraft mission since 1980-the X-ray Timing Explorer (XTE)-is scheduled to be placed in orbit December 10 carrying a trio of instruments that will probe the Milky Way and the more remote areas of space.

One of the instruments, designed by a team at MIT's Center for Space Research (CSR), will make sure the other two don't miss anything exciting. It is called the All Sky Monitor (ASM).

Professor Hale Bradt of the Department of Physics is principal investigator for the CSR team. Alan M. Levine, a CSR principal research scientist, is the project scientist.

The mission has several unique aspects, including the fact that all of the observing time is available to the international community of scientists. On many missions, blocks of observing time are reserved for the principal scientists.

Also unique is the XTE's ability to detect and record X-ray events in sub-millisecond time, its rapid response ability-thanks to the ASM-and its sensitivity, which ranges from 2-200 keV (kilo-electron volts).

Another contribution to the mission by the MIT team was the design, fabrication and testing of a new, powerful flight data system, the Experiment Data System (EDS), which preprocesses X-ray data from the ASM and from one of the other two instruments on board. Data are then transmitted continuously for almost the entire orbit-1.5 hours-via NASA's orbiting Tracking Data Relay Satellite System. The EDS contains eight independent event analyzers that work in parallel, two processing data from one instrument and six processing data from the other, making it possible to do several analyses simultaneously.

The primary objective of the XTE mission, sponsored by NASA's Office of Space Science and Applications, is the time and spectral study of compact objects, which include white dwarfs, neutron stars and black holes. The spacecraft has been designed to remain in low-earth circular orbit for at least two years with a goal of five years.

The other instruments on board, designed by teams at Goddard Space Flight Center and the University of California at San Diego, will work together as a powerful "telescope," examining a single source in a one-degree field of view for long periods of time. Their energy range spans from 2-200 keV (kilo-electron volts) and they can study time scales from microseconds to hours or days.

Meanwhile, the ASM will act as the "look out," sweeping 80 percent of the sky every 90 minutes as it monitors the intensities and spectra of the brightest sources-about 75 in number.

This constant vigilance is necessary because some X-ray sources are only occasionally active. For example, most of the time matter from the normal star in a binary system-two stars orbiting each other, shackled by their mutual gravity-will not flow onto its companion neutron star. But occasionally this flow, called accretion, can suddenly become very strong and will appear in the sky as a brilliant X-ray flare before disappearing from sight as accretion slows or stops. If the MIT All Sky Monitor spots an event like this, it will alert scientists on the ground to reorient the spacecraft so that the sensitive instruments point in the right direction.

"The flaring source can be acquired within several hours of its initial detection," Professor Bradt said. "This relatively rapid response to unpredictable temporal phenomena is one of the major new features of XTE."

The ASM has three wide-angle scanning shadow cameras, each consisting of a position-sensitive proportional counter-a relative of the Geiger tube-behind a random slit mask. The mask casts an X-ray shadow on the proportional counter. The total collecting area is 90 square centimeters. A pinhole camera would be a very low-tech analogy.

X-ray astronomy has led to many exciting discoveries, including accreting neutron-star binaries, hot gases in clusters of galaxies and black-hole candidates.

Among X-ray astronomers, black-hole "sightings" are high on the list of hoped-for events the mission will see. Black holes are celestial objects so dense, it is theorized, that nothing, not even light, can escape them. They have not been observed directly, but their presence is inferred from the behavior of rapidly moving matter, such as a companion star in a binary system.

The 6,700-pound spacecraft, about 6 by 6 by 19 feet long, will be launched aboard an expendable Delta II rocket from Cape Canaveral. Several of the CSR team will be on hand for the launch.

Over the years, many people have played important roles at MIT in the project.

CSR research scientist Edward Morgan has led the EDS work, collaborating with digital engineer Dorothy Gordon, software engineer James Francis, packaging engineer Fred Kasparian, thermal engineer Ellen Sen and several technicians, including Joanne Vining, John Hughes and Mariano Hellwig, provided important assistance.

Other members of the science team include CSR research scientist Ronald Remillard, CSR staff scientist Wei Cui, and J. Garrett Jernigan of the University of California at Berkeley, formerly with the CSR, as well as project manager Dr. William Mayer, project engineer Robert Goeke, and quality assurance manager Brian Klatt.

Mechanical engineer John Tappan, analog electronic engineer Hans Govaert, electronic engineer Mike Doucette, senior technician James O'Connor and mechanical technician Myron E. Mac Innis also had a hand in the ASM. Technician Mary Briggs did quality assurance inspections of circuit boards and other components for both the ASM and the EDS.

Others involved include Stephen P. Berczuk, Robert J. Blozie, Royce E. Buehler, James R. Cook, Ann M. Davis, Michael Enright, William W. Forbes, Deborah A. Gage, Daniel R. Hanlon, Bruce Jones, Robert A. Laliberte, Mitch Lyons, Rita B. Somigliana and William J. Ward.

A version of this article appeared in MIT Tech Talk on December 6, 1995.

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