A Flash in the Night Sky

You can see it early on some evenings: a glowing dot streaking halfway up the twilight sky, with occasional flashes of light sparkling its path. Those flashes foretell the doom of satellite USA 193. It's tumbling in orbit as the outer wisps of the earth's atmosphere jar its path; as it falls, facets of the craft catch the dying sunlight. USA 193 is doomed. It will plunge to earth sometime in the next two weeks. And the United States has decided to mount a $74 million effort to fragment it before it lands. The first attempt to do this may come as early as Thursday.

Why? Conspiracy theorists have had a field day speculating on possible secret reasons behind the Bush administration's decision to launch up to three SM-3 Standard missiles from U.S. Navy warships somewhere west of Hawaii to hit the satellite.

The suspicions are understandable. USA 193, weighing around 5,000 pounds, is the size of a school bus. But the odds against the satellite's hitting a person are literally millions to one. Three-quarters of the earth's surface is water. Ninety-five percent is uninhabited. Suppose USA 193's debris were to cover a few square miles, which is a plausible estimate. The earth's surface is 197 million square miles—all but one-20th of which is uninhabited. As National Security Council spokesman Gordon Johndroe said on Jan. 20, announcing USA 193's impending demise, "The likely percentage of this satellite or any debris falling into a populated area is very small."

History validates this confidence. In the 50 years of satellite launches, some 17,000 objects have plunged back to earth, according to the Pentagon. The biggest were the 78-ton U.S. Skylab, which came down in 1979, and the 100-ton Soviet Mir space station, which fell in 2001. By one independent count there were, last year alone, some 42 "major re-entries"—which is to say, big things hurtling down. Nine of those were satellites; perhaps a dozen were the upper stages of rockets that had lofted satellites into orbit. There has never been a report of a human being struck by space debris.

The detailed rationale given by administration officials for the shoot-down makes little more sense. USA 193 carries on board a tank of hydrazine, the fuel U.S. satellites use to change orbit in space. This fuel is contained in a spherical steel tank about three feet in diameter. After the space shuttle Columbia was destroyed while re-entering the atmosphere in 2003, its hydrazine tank was found, breached but intact, in a wooded area of Texas. Hydrazine is moderately toxic, with effects akin to chlorine gas. The hydrazine cloud from USA 193's tank would, if released, diffuse over an area of roughly two football fields. The cloud would dissipate in minutes. Nevertheless, we are told, that is the risk that impelled President Bush to order the satellite's midair destruction. Arguing against this are the facts of physics. The roughly 1,000 pounds of hydrazine in the tank is pressurized to a frozen slush. When heated, hydrazine turns into a gas. When USA 193 re-enters, friction from the atmosphere will roast the satellite to 7,500 degrees centigrade, hotter than the surface of the sun. Those who remember Charles's Law from high school will recall that gas expands on heating. The overwhelming probability is that the hydrazine tank—which is designed to withstand the zero temperature of space, not the superheating of re-entry—will simply explode as the hydrazine expands. (Why didn't Columbia's tank burst? Most likely because, as Gen. James Cartwright, vice chairman of the Joint Chiefs of Staff, explained in the Pentagon's Feb. 14 briefing on the planned shoot-down, "they'd burned most of it" on their mission. "So it [had] almost no hydrazine left." In other words, the tank was almost empty.)