Artist's concept of NASA's Compton Gamma Ray Observatory in space. Photo: NASA

NASA's $760 million Compton Gamma Ray Observatory - crippled by old age and a threat to people around the world when it eventually falls out of orbit - will be ordered to make a kamikaze plunge back to Earth on June 3, ending a successful nine-year mission to study the most explosive objects in the universe.

By ordering the 33,000-pound satellite to deorbit while ground controllers still have some level of control over it, NASA will be able to make sure the craft breaks up over the Pacific Ocean, hundreds of miles from any populated area. While 30 to 40 pieces of debris weighing 40 pounds or more are expected to survive the heat of entry and reach the surface, the odds anyone might get hit will be a remote 1-in-29 million.

"It's not a pleasant day when you have to make a decision to stop doing world-class research," said Ed Weiler, NASA's associate administrator for space science. "But the bottom line is there can be no tradeoff between lost science and even the slightest increased chance of loss of human life."

Built by TRW, the Hubble-class GRO was designed to operate at least two years with a design goal of eight years. Launched from the shuttle Atlantis on April 5, 1991, GRO has been in orbit for nine years, providing a steady stream of data about some of the most energetic objects in the cosmos.

But the spacecraft has suffered several equipment malfunctions in recent years and last December, one of its three stabilizing gyroscopes failed. To eventually drive GRO to a specific point in the atmosphere to ensure breakup over an ocean, flight controllers must be able to precisely control the satellite's orientation.

With one gyro out of action, NASA managers immediately began studying a variety of scenarios to determine whether GRO could be safely operated as is or whether it made more sense to go ahead and bring it down. A shuttle repair or retrieval mission was ruled out because of safety issues.

If NASA did nothing, Weiler said, the spacecraft would fall back to Earth on its own in three to 11 years, depending on the effects of solar activity on the atmosphere. And there is a 10 percent chance one of GRO's two remaining gyroscopes will fail in the next three years. If that happened, NASA would be unable to precisely target the satellite's impact point.

Gamma Ray Observatory is raised out shuttle Atlantis' payload bay. Photo: NASA

"GRO's one of the heaviest spacecraft ever launched by NASA," Weiler said. "Through in-depth analysis, we know many chunks would make it to the ground, some pieces as small as titanium bolts, some pieces as large as a ton, traveling 200 miles per hour. And this would be over a very large debris field.

"There is a 10 percent chance we could lose (a second) gyro over the next three years," he said. "If GRO re-enters on its own, without control, there is a 1-in-1,000 chance of a human fatality. That would be in a case where we didn't do anything and we let it come down by itself."

To an astronomer, Weiler said, 1-in-1,000 "sounds like pretty slim odds. But to many Americans who spend their dollars on lottery tickets ... 1-in-1,000 is a real chance when you're dealing with human life. And that 1-in-1,000 gets worse every day that passes after June 3."

GRO project scientist Neil Gehrels of NASA's Goddard Space Flight Center said he was disappointed with the decision to deorbit GRO. But he said he understood the reasoning behind it.

"Astronomers throughout the world will be quite disappointed with this decision and I'm personally, profoundly disappointed," he said. "But I do respect the importance of safety and instead of looking at this as a loss, I think we'll celebrate the great scientific success the mission has had over the last nine years."

Since its launch in 1991, the Gamma Ray Observatory has discovered 70 gamma ray quasars, 10 rare gamma ray pulsars and more than 2,000 gamma ray bursters, the most titanic explosions since the big bang. More than 1,000 scientific papers have been written based on data from the space observatory.

"We have 62 days 19 hours and 40 minutes left before (entry) and we're going to make the best use of every one of those minutes," Gehrels said.

GRO is prepared for release into space from Atlantis. Photo: NASA

GRO's orbit carries it over every point on the Earth's surface between 28.5 degrees north and 28.5 degrees south latitude. The current plan calls for a series of rocket firings beginning May 31 to carefully lower the perigee, or low point, of GRO's orbit. The first burn will put the craft in a 318-by-221-mile orbit. The second burn on June 1 will lower the perigee to 157 miles. On June 3, perigee will be lowered to 93 miles and on the next orbit, a fourth rocket firing will drive the craft to the desired re-entry point.

The 16-mile-wide debris impact "footprint" stretches 2,550 miles along GRO's flight path along a northwest-to-southeast track crossing the equator west of South America. The nearest land is 684 miles away. Flight controllers say entry will take place during darkness but it will be too far away from land for anyone to see any flaming debris.

Assuming GRO's two operational gyroscopes remain healthy, the odds of debris falling outside the planned target zone are believed to be around 1-in-29,000 with a probability of human casualty in the 1-in-29 million range. If no gyros are operational, the probability of debris falling outside the target zone is 1-in-4,100 with a probability of human casualty of 1-in-4.1 million.

"This was a difficult decision due to the loss of potential science from the GRO over the next few years," Weiler said. "But the safest option is clearly to bring it down in June."