When the Compton Gamma Ray Observatory begins to rip apart as it falls into Earth's atmosphere in the wee hours of Sunday, June 4, technicians aboard an Air Force intelligence plane will be tracking the fragments as they burn and tumble toward an unpopulated target zone in the Pacific Ocean.

The Cobra Ball's sensors should detect the disintegrating parts of the spacecraft as they heat, and then follow the debris until it either burns up in the atmosphere, cools down below a certain temperature or crashes -- smoldering -- into the sea. The plane will fly far enough away from the danger zone that it will not be threatened by falling telescope parts, and its instruments will have a good, wide-field view of the fall, Johnson said. A crew seated at computer consoles on board the plane will monitor and process the data the Cobra Ball's instruments gather.

"We will be trying to follow as many of the individual pieces as possible -- their initial trajectories -- and determine if our calculated footprint appears to be consistent with what we're observing. And, if possible, how many large pieces there are versus how many we expected." Johnson said.

Engineers working for NASA use a complicated set of codes called the Object Reentry Survival Analysis Tool, or ORSAT, to calculate how and where spacecraft parts will fall when they reenter Earth's atmosphere. They used this code to predict how much of the gamma-ray observatory would survive reentry, and where the surviving pieces would fall.

Taking into account the speed, trajectory, size, shape and composition of every major part of the spacecraft, the engineers calculated which parts of the spacecraft would melt completely, and which parts would survive. Then, using models of the aerodynamics of various shapes and how these pieces might be tumbling, they calculated where each piece would be likely to land. They came up with an area about 388 miles (626 kilometers) long and a few dozen miles (kilometers) wide where the debris would be scattered.

Various structural beams, batteries and propellant tanks are expected to survive reentry. The largest of the components that will land on the surface has a mass of about 1,300 pounds (590 kilograms). Although some of it will melt and burn away as it falls through the air, it is expected weigh more than 1,000 pounds (467 kilograms) and be speeding at 323 miles per hour (145 meters per second) when it slams into the water.

More than 60 square yards (50 square meters) within the landing zone is called "casualty area." That is, the area where a person would be hurt or killed if they were unfortunate enough to be in it.

So much of the satellite will hit the surface because Compton was built to be very heavy and very stiff in order to be a stable platform from which to observe space, said Joe Loftus, an orbital debris specialist at the Johnson Space Center.

If NASA is able to get good tracking data from the Air Force, the engineers will be able to determine where the ORSAT codes succeed and where they need to be improved. It will make for more accurate prediction tools in the future, Loftus said.

NASA and Air Force officials are confident that the Cobra Ball observations will yield valuable results.

"Once you get the data, interpreting the data is a challenge, but we have been successful in doing this in the past, so we are obviously optimistic that this is a worthwhile effort," Johnson said.

NASA and the Air Force have teamed up in the past to follow the fall of the space shuttle's external tank and its solid-fuel rocket boosters, as well as to track tanks and motors from the European Space Agency's Ariane 5 rocket.