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 4 of 5)
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.
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.