When Russian mission controllers bring the 135-ton orbital complex down later this month, they will be aiming for a 3,700-mile- (6,000-kilometer-) long stretch of the south Pacific. Will they succeed? Or will pieces strike populated areas, causing injury and damaging property? That's a question that has been faced before, not only by the Russians, but also by NASA.
The Last Big Deorbit
Mir"s finale was somewhat previewed by last year"s reentry of the Compton Gamma Ray Observatory (CGRO). In two spectacular videos obtained by SPACE.com, you can see the satellite just before it left orbit, and its fiery trail across the night sky.
Back in the summer of 1979, the space agency sweated out
the reentry of its Skylab space station, which had been lofted into orbit six years before. Solar activity, which can heat and expand Earth's upper atmosphere, had intensified faster than expected, hastening the decay of the station's orbit. Planners hoped to be able to save Skylab from falling out of orbit by sending a propulsion module to the outpost on one of the early flights of the space shuttle. But when the shuttles maiden voyage fell behind schedule, NASA had no choice but to let Skylab reenter. Knowing that large pieces of the 83-ton (75,000-kilogram) station would likely survive reentry, space watchers both inside NASA and in the public anxiously waited to see where the debris would come down.
Aerospace engineer Bill Ailor remembers the tension of those times; he and his colleagues at the Aerospace Corporation in El Segundo, California helped NASA to plan for Skylabs plunge. NASA, Ailor remembers, was particularly concerned about the so-called breakup altitude, where the station would be torn apart by friction with the atmosphere because that would affect
the size of the impact area, or "footprint," of the debris. "The higher it comes apart, of course, the bigger the footprint," Ailor explained.
NASAs calculations, Ailor says, suggested the station would break apart at around 69 miles (111 kilometers), "which would have meant a very large footprint." But research by Ailor and his colleagues, using data from
other satellite reentries, pointed to a much lower height, only 48 miles (77 kilometers). In fact, Ailor said, "thats sort of a magic number for this sort of thing" no matter what kind of spacecraft is reentering. "We helped them to realize the altitudes were lower."
Still, because Skylab had no retrorocket, NASA had no sure way of controlling where the station would reenter, and where the resulting debris footprint would be. They could only change Skylab's orientation to offer more or less resistance to the atmosphere, to help adjust its reentry point. As it turned out, Skylab's "footprint" fell mostly in the Indian Ocean,
but pieces also struck Australia. "I heard stories of debris raining on peoples roofs," said Ailor, who adds that the majority of the fragments landed in the unpopulated Australian Outback, "and most of it has never been found.There was debris recovered, but nobody was hurt. So it worked out." But Skylab served as a warning on the dangers of uncontrolled spacecraft reentries.
That was a lesson the Soviet Union had already learned the hard way. In 1978, a nuclear-powered surveillance satellite designated Cosmos 954 reentered, raining radioactive debris on northwest Canada. Although no subsequent health problems were reported, the event was a political lightning rod for the Soviets, who subsequently abandoned the use of nuclear-powered satellites in low Earth orbit.
By 1991 the Soviets were facing another potential problem: Just as it had with Skylab, solar activity was hastening the demise of
their Salyut 7 space station. As NASA had done, Soviet controllers tried turning the station to adjust its interaction with the atmosphere. But this time the technique didn't work as well. "Its iffy," Ailor said. "The atmospheric density is not the worlds most predictable thing at those altitudes. It dances all over the place. If you hit it you hit it; if you dont, you dont. Its not easy to call." Fragments of the 20-ton station struck a village about 250 miles (400 kilometers) from Argentina. As with Skylab, there were no reports of injuries. You might say the Soviets, like NASA, had been lucky. But is "luck" really the right word?
~
Not according to Nicholas Johnson, who heads
the Orbital Debris office at NASAs Johnson Space Center. "Its not a matter of luck," Johnson said. "Its a matter of statistics. Because the world is a big place, and even 6.1 billion people go a long way; in terms of [area], its not very densely populated." In fact, there is only one known case of anyone being hurt by debris from space. In 1969 pieces assumed to be from a Soviet rocket struck a Japanese ship and injured several sailors. And yet, said Johnson, "Theres been an uncontrolled reentry, on average, one a day for the last 40 years."
"Its an odds game," said Johnson, "But anyone whos been to Las Vegas knows the odds can win. Everybody knows that its improbable that someone will be hit, but improbable is not sufficient. You want to make those odds vanishingly small."
With Mir, the Russians plan to minimize risk by controlling the reentry with
an attached Progress freighter, which will provide the retrorocket that was unavailable for Skylab and Salyut 7. Whether they will succeed, however, is something Johnson says is too difficult for analysts to predict.
When it comes to the reentry of ordinary satellites and rocket bodies, Johnson said, "Were reasonably good at being able to predict the kinds of components that will survive," and where those pieces come down. About four years ago, when the second stage of a Delta rocket reentered the atmosphere, two fuel tanks came down in Texas. After obtaining details of the event, Johnsons team put their computer models to work on recreating it in detail.
"Our model not only predicted those two objects would survive," Johnson said, "but if I remember the numbers correctly, it predicted they were going to land about 145 kilometers (90 miles) apart, and they really landed about 136 kilometers (84 miles) or something like that." This astounding accuracy is far from easy to achieve, explains Johnson. "Its still more of an art than a science, and theres only a few people in the world who can really do it well."
And such mastery, Johnson says, is next to impossible when it comes to Mir. Unlike the simple geometry of a rocket stage or a small satellite, the 135-ton station is composed of modules of different sizes and shapes, arranged at all kinds of angles. "When you get
something as complex as Mir," Johnson said, "the best models in the world just cant do it. Theres too many variables."
"Youre going to have these modules coming off at unknown times and unknown velocities," said Johnson, adding that each module could tumble unpredictably. Any residual fuel left in Mirs tanks could become hot enough to ignite, or at least to cause the tanks to explode. Air inside pressurized modules could also burst through the cabin walls. All these things, says Johnson, could affect the path of the debris as it speeds through the atmosphere.
But Johnson is optimistic. He notes that the Russians have successfully brought down five of their Salyut space stations, and many more Progress vehicles. "In terms of being able to do controlled deorbits, I'm not sure anybody has more experience, or more success, than the Russians," Johnson said. "They've dropped about 80 Progress vehicles in the south Pacific already. And to the best of my knowledge, they've never failed with an attempt." When it comes to Mir, Johnson said, "I think there's every reason to believe that the Russians will be able to do what they've set out to do."
When Mir does come down,
Johnson and his NASA colleagues will be watching especially closely, because they have a space station of their own to worry about, one that will eventually be several times bigger than Mir. "We have every intention, and its in our plans, to deorbit the International Space Station when its time comes," Johnson said. "Obviously we hope thats 30 years down the road or more."
advertisement
