The Things That Fell to Earth
How NASA can predict when space junk will fall in your back yard.
- By James E. Oberg
- Air & Space magazine, January 2005
(Page 3 of 5)
For years, researchers had no reliable numerical models to predict which pieces of a space vehicle would survive entry and reach the ground intact. Estimates were made “by guess and by golly,” says Johnson. But in the 1990s ITT Systems in Alexandria, Virginia, developed a numerical model that took all known thermal processes into account in order to predict the fate of entering objects. Around the same time, a team of engineers from NASA and Lockheed Martin worked jointly to create a numerical model called Object Reentry Survival Analysis Tool (ORSAT).
Johnson and his team at NASA have found the ORSAT program particularly helpful in understanding what happens to an object after the stress of deceleration causes it to disintegrate. Once a satellite breaks up, for example, and its individual components—often in the form of spheres, cylinders, and plates—are streaking in on their own, the reentries of the basic shapes are much easier to predict than those of the irregular shapes common to most intact satellites. “Tumbling titanium spheres survive reentry totally intact,” says Johnson. Further, components with protuberances are affected by aerodynamic drag differently than smooth components, with the protruding parts forming a “tail,” so that the front end of the object gets really roasted (some of NASA’s reports contain photographs of recovered spheres with burn holes opposite the protuberances).
Johnson’s group has applied the ORSAT model to known entry events, including the reentry and breakup of the nuclear-powered Russian satellite Kosmos 954, which rained radioactive debris over Canada on January 24, 1978. As the ORSAT model predicted, Kosmos 954’s beryllium fuel rods became very hot during reentry. But the rods survived because they were made of beryllium. “This is because of the extremely high heat of fusion of beryllium,” says Johnson. Steel and metals such as titanium and nickel share beryllium’s ability to handle the heat, while aluminum and copper objects usually vaporize soon after breakup.
Johnson is particularly proud of the ORSAT model’s results for debris from the Delta II rocket stage that reentered over Texas in January 1997. Using data such as size, weight, and composition for the fuel tank, pressurant sphere, and rocket nozzle, the ORSAT model indicated that all three pieces would survive reentry, which they did. Additionally, the ORSAT program’s prediction of the landing sites for all three pieces matched well with the actual locations.
Unlike the fuel tank and the pressurant sphere, the Delta II’s rocket nozzle is made of the metal columbium, which is mechanically weak but can withstand high temperatures. The ORSAT model showed the rocket nozzle being heated quickly, then cooling quickly and eventually falling to the ground at a speed of about 33 feet a second (compared to the impact speed of the heavier tanks, 260 feet a second). As the nozzle approached the ground, it was already at air temperature. “Our research has shown that the material does survive reentry,” wrote Johnson in a NASA report, “and that it ‘floats’ down, landing approximately 30 minutes after the steel tank impact and 500–600 kilometers uprange.”
What about the piece of mesh that hit Lottie Williams: Had it also been shed from a Delta II? Williams has never loaned the object to NASA, but she did send a fragment to the Center for Orbital and Reentry Debris Studies, which concluded that its composition is consistent with Delta II insulation. Because the mesh has no identifying marks or numbers, though, it cannot be proven to have come from a particular rocket. Still, the “circumstantial evidence is highly convincing,” says Johnson, who points out that the mesh’s location and time of landing are consistent with the 1997 Delta II reentry.
When an object reenters the atmosphere and breaks up, the debris is scattered along a field, or footprint, with lighter fragments landing near the “heel” of the footprint and heavier objects traveling farther downrange toward the “toe”; this explains why Williams’ mesh floated down in Oklahoma, far uprange of the heavier pieces that plowed into Texas. The ballistics characteristics of the heavy pieces also ensure that they’ll travel at a higher velocity—and reach the ground sooner—than the lighter pieces.
Lottie Williams wasn’t happy with these results, however. “I was thinking I had something celestial,” she told the Tulsa World reporter. “And here I got something man-made.”