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Holy Hunks of Junk, It's Raining Boosters!
By Leonard David
Senior Space Writer
posted: 04:01 pm ET
10 May 2000
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raining_boosters_000510 WASHINGTON -- The remains of rockets are raining from space, hitting Earth with disturbing frequency. While no one yet has been killed or seriously hurt, scientists say the risk of injury is greater, as more and more pieces reach the ground. Through the years, the space business has seen some spectacular falls from grace. Whole chunks of Skylab fell over Western Australia in July 1979. And NASA is preparing to bring the school-bus sized Compton Gamma-Ray Observatory into a "controlled" reentry into the eastern Pacific Ocean early next month.But scientists are most worried about space junk that hits land. Late last month, the skies over Cape Town, South Africa, thundered with sonic booms as white-hot pieces of metal slammed to the ground around several farms. Eyewitnesses to the late-afternoon plunge from space on April 27 heard and watched the fragments come down. Not surprisingly, the incident touched off rumors of an out-of-control UFO or a fallen aircraft. U.S. space-debris experts have now identified the more than 700 pounds (318 kilograms) of space junk that fell that day as the remains of a Delta 2 second stage. 
A Delta 2 lifts off from the launch pad. What fell were parts of a large, egg-shaped main fuel tank, a rocket motor combustion chamber and a small pressurization sphere. The fragments belonged to a Delta upper stage that was used to orbit a Global Positioning System (GPS) satellite back in March 1996. U.S. Space Command in Colorado Springs tracks about 9,000 objects -- most of them space junk -- that are in orbit around Earth. When a dead satellite or space litter tumbles from orbit, the pieces cut through thin to thicker regions of Earth's atmosphere. They are often incinerated or vaporized by atmospheric friction. Or if they survived at all, they typically would fall into the ocean. Breaking up is hard to do But as the case in South Africa proved, sometimes the ground has been on the receiving end of a Delta rocket upper stage. It's not likely to be the last time, either. In January 1997, a 580-pound (264-kilogram) stainless-steel tank -- identical to the one recovered in South Africa -- plowed into the ground in Georgetown, Texas. It missed a house by a scant 50 yards (45 meters) and major highway by 150 yards (135 meters). Other areas reported hits from the same rocket breakup. In Seguin, Texas a titanium sphere, which holds gas that forces propellant into the stage's engine, survived the fiery fall from orbit and was recovered. And in Tulsa, Oklahoma, a small piece of charred metal mesh hit a woman in the shoulder but did not injure her. 
The 500-pound stainless steel tank pictured above, once part of a satellite injection stage, landed 50 yards from a Texas farmer's home. The image below is the side view of a Delta II upper stage rocket, similar to the one that landed in Texas. 
Space debris experts now concede that the conventional wisdom that such objects either burn up, fall unnoticed in the ocean or careen into remote areas of Earth doesn't quite ring true. "We now see that's not the case all the time," said William Ailor, director of the Center for Orbital and Reentry Debris Studies at The Aerospace Corporation in El Segundo, California. Ailor and his colleagues want to get their hands on the South African pieces as they did with the rocket hardware from the Texas event. "When you're in the research mode like we are, the more opportunity to look at any material recovered the better," he said. "Finding something on the ground is a bit unusual, but in 40 years we've never had anybody actually get hurt by anything, as far as we know," Ailor said. Last year, perhaps nearly a dozen stages like those that fell in South Africa and Texas came back to Earth. Those surely wound up in the ocean or other out-of-view locales, he said. "We've got a little ways to go before we understand the breakup process. Ultimately, we'll have sufficient information to be able to allow a spacecraft and a rocket-stage builder to design features into the hardware to basically guarantee it will pose as small a hazard as possible," Ailor said. Experts who have studied the large Texas tank have some idea how it survived the plunge. They found that a hole had formed in the tank during its fall that acted as an aerodynamic scoop that slowed it down, reducing the tank's heat. That kept it cool enough to stay intact all the way to the surface. Earth: A hard-hat area? The fact that both the Texas and South Africa incidents yielded identical pieces, though they happened three years apart, is not surprising. "The bottom line is that every time a Delta comes back, these components are going to survive," said Nicholas Johnson, chief scientist and program manager in NASA's Orbital Debris Program Office at the Johnson Space Center in Houston. Computer models of the Delta 2 upper stage show very clearly that the main tank and four titanium spheres should survive reentry every single time, Johnson said. In fact, the survivability of the Delta second stage exceeds a NASA safety criterion, where the chance of a human casualty is one in 10,000. "On average, the risk from the Delta second stage exceeds that. But because of a cost-effectiveness clause, we've never required a redesign of a Delta second stage for a NASA mission. We accept that," Johnson said. Now slipping through space above Earth are nearly 50 Delta rocket upper stages, primarily the result of military-launched GPS spacecraft. But don't rush out and buy that hard hat just yet. Most of the rocket stages are in high orbits and are expected to remain in space for tens to hundreds of years. "But I wouldnt be surprised to have a few more reenter later this year," Johnson said.
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