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Compton Comes Down
May 30, 2000
This map, created by CGRO, combines two years (91-92) of gamma ray observations. image: NASA
NASA plans to end the nine-year mission of the Compton Gamma Ray Observatory this Sunday, when it will crash the enormous telescope into the Pacific Ocean. However, some researchers question the reasoning behind the National Aeronautics and Space Administration’s decision to bring down one of the agency’s greatest successes.
Over its life, Compton has provided reams of new data to scientists eager to ferret out the mysteries of the universe’s beginning. Why does our nation’s space agency want to drown $600 million in astrophysical instruments?
Hubble’s quiet sister creates a big stir in science circles
Dissention in the ranks: James Ryan voices his displeasure over NASAs decision to kill Compton.
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The Compton Gamma Ray Observatory (CGRO), named for physics pioneer Arthur Holly Compton, has operated since 1991. One of NASA’s "great observatories," Compton’s contribution to the field of gamma-ray astronomy is legendary. More than 400 new gamma-ray sources and 2,500 bursts have been discovered by Compton’s four instrument packages.
CGRO’s data have also led researchers to a greater understanding of fundamental physical processes. According to James Ryan, a professor of physics at the University of New Hampshire who has worked with the Compton project for almost twenty years, "We’re learning a great deal about the high-energy physics that take place on the sun." Officials at NASA’s Goddard Space Flight Center, which oversees the orbiting observatory’s operations, note that a scientific paper based on Compton data appears about every other day.
But CGRO’s flashy sister, the Hubble Space Telescope has garnered the lion’s share of the public’s attention. The reason? Hubble’s breathtaking photographs of our universe. Ryan notes that "Hubble...produces more things that the public can relate to easily...Our pictures are not quite so pretty."
Now, scientists are alarmed that a prime resource for studying the origins and evolution of our universe faces the executioner.
Compton nears the end of her journey
Where Compton might land. image: NASA
At more than 35,000 pounds and about the size of a school bus, Compton was the largest payload to ride in the space shuttle at the time of its April 1991 launch. After some initial problems at launch, CGRO has reliably performed its mission of detecting and locating previously mysterious gamma ray bursts and sources.
In December 1999, one of Compton’s three gyroscopes seized up. The gyros are essential to keep the space craft in a stable orbit. Since the craft was built with particularly dense materials (like titanium, as opposed to lighter materials like aluminum that are used in some spacecraft), NASA worries that an uncontrolled descent would injure or even kill large numbers of people. Guenther Riegler, the director of NASAs research program management division, states that if Compton falls uncontrolled the chances of a human being struck by a stray piece could be 1 in 4,000—an unacceptable safety risk. "[CGRO] will not burn up in the atmosphere," says Riegler. "There will be chunks that can be as heavy as tens, maybe tens of thousands of pounds in weight."
Gamma Rays: What Are They?
A gamma ray is a photon with the highest energy level, and shortest wavelength of any electromagnetic radiation. Gamma rays are said to exhibit characteristics of both particles and waves. Gamma rays in the cosmos are produced by:
a high-energy particle-particle collision
a particle-anti particle (electron-positron) collision and annihilation
radioactive decay of an element
acceleration of a charged particle
from NASAs "Imagine the Universe!"
Although Riegler admits that Compton "recently discovered a new class of very high energy gamma ray sources... and the first detailed observations of solar events in the current solar maximum" (the peak period for solar flares), the verdict is in: Compton must come down. A shuttle mission to replace the defective gyro is out of the question, says Riegler; it would take two years to design and execute such a mission.
Beginning on Wednesday, NASA plans to fire Compton’s thrusters to initiate a descent into Earth’s atmosphere, culminating on Sunday with a splashdown in the Pacific Ocean at least 400 miles from any known island.
Some ask: Is this necessary?
Ryan, among others, questions NASA’s decision. "In my opinion, Compton does not have to come down," he says. "It has a new back-up mode [developed by NASA] for bringing it in in case something else fails; and even if everything fails, the space craft, by its own design and nature, will just settle into a nice steady position that could eventually be retrieved by the space shuttle.... The probability of anyone getting hurt by Compton falling into the ocean by way of the zero gyro reentry procedure are about the same as me winning the state lottery."
Ryan also notes that NASA has never attempted to perform "any servicing of the observatory.... There’s never been a need to do any." Ryan contends that a shuttle mission could be successful: "Compton is made to be serviced and recovered in orbit."
The Burst And Transient Source Experiment (BATSE), a series of eight gamma ray detectors, locates and records sources and brightness variations of strong transient gamma-ray bursts and other gamma-ray sources over the entire sky; BATSE detects energy ranges of 30 keV to 1.9 MeV.
The Imaging Compton Telescope (COMPTEL) uses the Compton effect, the "scattering of high-energy photons by electrons," to create pictures of gamma ray sources across a wide field with a few degrees of resolution. COMPTEL operates over the energy ranges 1 to 30 MeV.
The Energetic Gamma Ray Experiment Telescope (EGRET) measures the high energy transfers in neutron stars and other intra- and extra-galactic objects, gamma ray diffuse radiation, energetic solar phenomena (such as solar flares), cosmic rays and supernovae, and high-energy gamma-ray bursts from unknown origins. EGRET evaluates the highest energy phenomena, from 20 MeV to 30 GeV.
The Oriented Scintillation Spectrometer Experiment (OSSE) alternates gamma-ray source observations with those of nearby background regions for accurate measurement of the energy spectra of nuclear lines in solar flares, radioactive decay of nuclei in remnants of supernovae, and (most exciting) the spectral signature of matter-antimatter (electron-positron) interactions in the center of our Milky Way Galaxy. OSSE works in the 0.05 to 10 MeV range.
The timing of Compton’s end couldn’t be worse, Ryan says. "We had plans to operate the space craft through 2005. We’d taken steps several years ago to boost the space craft so that it lasted several more years, through the maximum of the sunspot cycle." The next gamma-ray observatory, the Gamma Ray Large Area Space Telescope won’t be launched until 2005.
An independent panel of space scientists should be consulted before Compton comes down, says Ryan. "We’d like to postpone this plan to have it come down.... If it does come down, it’d be a crying shame in my book."
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