Observatory on a suicide mission

In the interest of safety, NASA has decided to plunge a spacecraft into the Pacific Ocean.

Late last year, one of the three gyroscopes on the Compton Gamma Ray Observatory (GRO) failed, leaving the satellite with the minimum number necessary to steer the craft. The space agency wants to direct the craft back through Earth’s atmosphere in a controlled manner, while the two other gyros still function.

So on May 26, after 9 years of mapping the gamma-ray sky, GRO will close up shop. Engineers then will fire the craft’s thrusters, slowing the satellite and causing it to lose altitude. If all goes according to plan, GRO will reenter the atmosphere in early June.

Most of the 17-ton satellite will vaporize as it crashes through Earth’s atmosphere, but some 30 to 40 titanium pieces, a few as heavy as tens of kilograms, will survive. Debris will be scattered over an area about 25 kilometers wide and 1,500 km long in a remote region of the eastern Pacific about 4,000 km southeast of Hawaii.

“Enough will survive to present a small but still unacceptable risk to populated areas if Compton were allowed to reenter in an uncontrolled manner,” says Preston Burch, deputy program manager for space science operations at NASA’s Goddard Space Flight Center in Greenbelt, Md.

“We actively pursued the option that provided the lowest risk to human lives,” notes Edward J. Weiler, NASA’s associate administrator for the Office of Space Science in Washington, D.C.

Scientists working on GRO, however, maintain that the craft could operate reliably even if another gyro fails. The satellite could rely on its magnetometers and sun sensors for navigation, and setting GRO slowly spinning would allow for a controlled reentry, Goddard engineers have proposed. Although NASA’s plan would minimize the threat to human life, the proposed alternatives could have safely extended the mission for several months to years, maintains GRO investigator James M. Ryan of the University of New Hampshire in Durham.

Among its landmark findings, the satellite revealed that the mysterious flashes of light known as gamma-ray bursts are spread uniformly across the sky and come from outside the galaxy. GRO’s studies of high-energy emissions from the sun have taken on added importance, GRO scientists note, with the delay in launch of the High Energy Solar Spectroscopic Imager, which suffered damage during recent testing (SN: 4/1/00, p. 215: Reviewers see red over recent Mars programs). Ending the GRO mission now, even though it has long surpassed its estimated 5-year lifetime, “is a terrible loss,” says Hugh S. Hudson of the Solar Physics Research Corp. in Tucson.

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