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In 2001, a titanium motor casing from a Delta II ended up in Saudi Arabia. (NASA)

The Things That Fell to Earth

How NASA can predict when space junk will fall in your back yard.

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(Continued from page 1)

While out walking her dog with friends before sunrise on January 22, 1997, a woman in Tulsa, Oklahoma, was hit by a man-made object that fell out of the sky. Half an hour before, Lottie Williams had watched an impressive fireball streaking through the sky from north to south. “I noticed in the sky there was this big bright light, like a fire,” she told a reporter from the Tulsa World newspaper. “I turned to my friends to say look, and when I turned back it was coming towards us.” Then two sparks shot from the fireball and disappeared over a building.

Later, when a slowly falling piece of charred woven material brushed Williams’ left shoulder and hit the ground “with a metallic sound,” she concluded that there was a connection between the two events, especially since the next day’s news was full of stories about “space junk” found on the ground in Texas. A large stainless steel fuel tank, which bore evidence of surface melting, had landed in the front yard of a farmer near Georgetown, Texas, partially collapsing on impact. And outside the town of Seguin, a titanium pressurant sphere, undamaged except for some discoloration, had embedded itself halfway into a field.

Nicholas Johnson, chief scientist of the Orbital Debris Program Office at Johnson Space Center, soon got word of the discovered space debris. A few days later, he made the drive from Houston to Georgetown, where he identified the tank as having come from the one-ton second stage of a Delta II rocket booster. The U.S. Space Command in Colorado Springs, Colorado, had been tracking the Delta II for several days. Nine months earlier, it had launched a U.S. Department of Defense payload. After the rocket stage’s orbit finally decayed, it had reentered the atmosphere around 3:30 a.m. over the south-central part of the country. The reentry was seen by observers in Texas, Kansas, Missouri, and Arkansas.

The collected orbital debris was shipped to the Johnson Space Center. The fuel tank and the pressurant sphere found in central Texas were obviously from the fireball. But investigators initially doubted that the piece of metal mesh that had fallen on Lottie Williams was from the rocket, since it had been recovered so far upstream of the bulk of the Delta II’s debris.

At the Orbital Debris Program Office, Johnson is in charge of NASA’s efforts to predict what sorts of space junk can be expected to reach the ground after the natural decay of objects’ orbits. Despite the fact that hundreds of fragments of space objects have been found around the world and sent to various government agencies, conventional wisdom is that entering objects burn up. “Some launch companies until recently claimed in commercial launch license applications that spent stages totally burn up in the atmosphere,” says Johnson. When the Russians remotely command the supply vehicles that service the space station to reenter the atmosphere, they claim that the vehicles “cease to exist,” yet they choose to dump them over the far southern Pacific rather than over Russia—just in case.

According to William Ailor, director of the Center for Orbital and Reentry Debris Studies, between 100 and 200 large (bigger than a breadbox) man-made objects reenter each year. In 1999, Ailor has estimated, for example, that 212 tons of hardware hit the atmosphere, and a quarter of it, about 42 tons, probably reached the surface. And during the first 40 years of the Space Age, Ailor estimates that as much as 1,400 tons of man-made material has reached the surface of Earth. He says, though, that worldwide, only about 250 discoveries of authentic spacecraft pieces have been reported because most pieces have landed in water.

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.

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