The U.S. government criticized the unannounced move at the time, and many experts have expressed concerns about the field of debris left in low-Earth orbit.
But the latest "satellite kill," U.S. officials argue, is not a weapons test and is necessary to prevent the release of the largely unspent rocket fuel on board the craft.
The big question is just how toxic the satellite's fuel might be.
According to the defense department's press release, the fuel tank contains about 1,000 pounds (453 kilograms) of hydrazine, a hazardous chemical related to ammonia.
Hydrazine's effect on people depends on the dose, duration, and manner of exposure, according to a 1997 fact sheet released by the U.S. Agency for Toxic Substances and Disease Registry.
(National Geographic News's phone calls to the agency were not returned by press time.)
Various forms of the chemical are used safely in medicines, farm chemicals, and plastic foams and are naturally present in foods such as mushrooms.
(Related news: "Bacteria Eat Human Sewage, Produce Rocket Fuel" [November 9, 2005].)
Pure hydrazine is unstable in air and quickly decomposes, generally within minutes to hours.
But high doses of the gas can damage skin, burn lung tissue, and affect the nervous system, leading to seizures.
And at least three agencies—the International Agency for Research on Cancer, the U.S. Environmental Protection Agency, and the California Office of Environmental Health Hazard Assessment—have declared hydrazine to be a carcinogen.
Still, toxicologists generally view cancer as a result of long-term exposures, not one-shot accidents.
Cloud of Toxic Gas
So, critics have been saying, how big is the risk?
Unless the missile destroys the fuel tank, it is quite likely that the tank and its cargo wouldn't burn up in the atmosphere, defense officials said.
"We had a similar one on [the space shuttle] Columbia that survived reentry" after the shuttle broke apart in February 2003, Marine General James E. Cartwright, vice chair of the Joint Chiefs of Staff, said in a statement.
The tank would probably rupture when it hit the ground, spreading toxic gas over an area Cartwright estimated to be roughly the size of two football fields.
"If you inhale a lot of it, it could be deadly," Cartwright said.
It's likely that the tank holds enough hydrazine to be dangerous, said Kenneth Ramos, a professor at the University of Louisville School of Medicine and incoming president of the Society of Toxicology.
In fact, Ramos implied, the defense department's estimate for how far the gas would spread might be conservative.
"If they do not destroy it and the intact tank ruptures, it is likely that the volume would be sufficient to cause harm," he said by email.
"After all, a thousand pounds of hydrazine is a large enough amount to spread over a considerable distance," he said.
"[W]eather conditions would likely be a major determinant of dispersion potential," he added.
The threat also would depend on where the tank hits.
The equivalent of two football fields is a large impact zone in a big city. But in a remote desert or in the ocean, few people would be affected.
Currently the Navy cruiser U.S.S. Lake Erie waits in the Pacific Ocean west of Hawaii with two SM-3 heat-seeking missiles to target the satellite and hopefully bring down the debris over unpopulated waters.
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