The Things That Fell to Earth
How NASA can predict when space junk will fall in your back yard.
In 2001, a titanium motor casing from a Delta II ended up in Saudi Arabia.
James E. Oberg
Knowing what kinds of materials and structures are likely to survive entry and reach Earth intact also enables NASA to calculate more reliable probabilities of property damage and personal injury. In 2001, such calculations ended the mission of the Compton Gamma Ray Observatory when it was shown that the satellite's heavy structural materials presented a greater than 1-in-10,000 chance of harming property and people. The satellite had a gimpy control system, so instead of waiting for it to fail and leave ground controllers with no means of directing the craft's reentry, mission control dumped the satellite into the far southern Pacific Ocean while it was still controllable.
Following the 2003 Columbia disaster, the Center for Orbital and Reentry Debris Studies became involved in assessing the scatter pattern of fragments from the shuttle. On March 17, coincidentally just a day before Columbia's flight data recorder was recovered, CORDS director William Ailor testified before a public hearing of the Columbia Accident Investigation Board in Houston. Because the Columbia accident investigators needed to know whether the damage they saw on recovered fragments resulted from events that happened earlier in the shuttle's flight (and that may have led to the disaster) or from the stresses endured during reentry, they were interested in learning how different materials react to entering Earth's atmosphere. The Columbia accident investigators also wanted a way to judge how thorough their search was by comparing the weight of recovered Columbia material to calculations of how much should have reached the ground.
"For unprotected space hardware, the heating and loads will gradually tear it apart," Ailor explained to the investigators during the hearing. "The kinds of things that we've seen that survive reentry are things that you would probably guess might - things like steel sometimes, glass, titanium, and then parts that are sheltered by other parts. One of the things about the reentry breakup process is that the heating is like, in a sense, cooking an onion. You basically start from the outside, and then as you heat the pieces up to a point where the materials will fail, that will expose some new materials. They'll go through the same process and the object can be broken apart. We do have objects that are melted and shedded away, things like aluminum [and] solar panels.
"For example, when an object comes off of a parent body, it now experiences the air stream that exists there, and it will respond based on its own characteristics. If you've got a very lightweight piece that comes off of a heavier object that's coming through the atmosphere, it's like throwing a piece of paper out of a car. That will decelerate very quickly, and the same things happen even at Mach 20."
Ailor went on to explain how timing and release conditions can affect survivability, both of particular significance in the hunt for Columbia debris. "If an object comes out late in the reentry, after being shielded for a portion of the reentry, that means a lot of the energy has been taken out of that trajectory prior to that object's release, and that object is more likely to survive," he said. This shielding effect explains why Columbia searchers found documents, videotape, cloth patches, and astronaut remains among the items that made it to the ground.
Ailor estimated that anywhere from 10 to 40 percent of a space object would actually survive reentry. The odds of its being found, however, are much lower, even with the models that CORDS has developed.
Careful study of the debris field from the Columbia accident helped improve the odds of success in that search. As truckload after truckload of debris was gathered, NASA engineers began to hope they could find particular pieces. "We did analyze some specific components," says Nicholas Johnson. "Most already had been found, and we were asked to go back and use our models to look at those locations. We had very close agreement."
Insights from such computer simulations and a reconstruction of the moments of Columbia's breakup enabled accident investigators to take the next step. With the Orbiter Experiment Recorder still missing, but with pieces known to have been installed near it found, the region where it ought to have fallen was carefully plotted, leading to its recovery and its use to solve the more heartbreaking mystery of what destroyed the space shuttle.
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