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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 2)

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.”

The ORSAT model can accurately predict entry heat loads on falling objects because it factors in the actual processes at work. In contrast, the idea that air friction is the cause of reentry heat is persistent but misleading.

Atmospheric entry heating of man-made objects was first noted in 1944, when Nazi Germany’s V-2 rocket warheads hit the atmosphere over London at about 6,000 mph. As they reentered, compression-induced shock waves heated the air ahead of the plunging warheads enough to prematurely detonate the explosives inside. German engineers solved the problem by lining the warheads with plywood to serve as a heat shield.

But after the Bell X-1 rocketplane made its sound-barrier-breaking flight on October 14, 1947, confusion began to set in about atmospheric heating. The X-1 and other high-speed aircraft, such as the North American X-15, which first flew in 1959, faced severe thermal environments. Supersonic air rubbing across the aircraft’s outer skins created frictional heating, which had to be endured or actively cooled. And from then on, the notion of atmospheric heating was indelibly linked with air friction in media explanations and thus in the public mind.

But air friction has little to do with the process that heats objects entering Earth’s atmosphere. The key source of the heating is compression: Air molecules in front of an incoming object can’t move out of the way fast enough, so they pile up, or compress, which makes them very hot. The air molecules get “aggravated,” as the late Max Faget liked to say when he explained how he invented the heat shield for the spacecraft of NASA’s Mercury manned space program. Or as space engineer Jim Davis says: “This is due to the spacecraft performing work on the atmosphere like a piston in a cylinder.”

The air molecules caught up in the shock wave created by the incoming object can heat up to 11,000 degrees Fahrenheit, as hot as the surface of the sun. This heat reaches the reentering object mainly by conduction, as the superheated air molecules repeatedly strike its surface. At higher reentry speeds—say, when the U.S. Apollo manned space capsules returned from the moon at 25,000 mph—the compression-induced shock wave becomes so hot that it transfers much of its heat into the reentering object through direct thermal radiation. And at the speeds at which meteors hit Earth’s atmosphere, up to 150,000 mph, nearly all of the heat transfer is through radiation.

Understanding how objects break up and scatter in the atmosphere is a relatively new science for NASA, but one with a wide range of applications. During space shuttle launches, for example, the external fuel tank, which weighs 44 tons empty, hits the atmosphere an hour or so after launch and breaks apart, with metal fragments scattering along a footprint in the Atlantic Ocean. Mission planners must place the entire footprint in a region that sees little commercial sea and air traffic, and for some launches, planners had difficulty finding a big enough dumping ground. But then a Lockheed Martin computer analysis showed that the external tank was breaking up at a substantially lower altitude than first estimated and the pieces were scattering over a correspondingly smaller area. Once the computer prediction was confirmed by direct observation, shuttle mission planners had more leeway in calculating where the remnants of the tank could plunge.

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