Richard A. Lovett, National Geographic News 20 Feb 08;
The U.S. Navy is gearing up to use modified anti-missile rockets to shoot down an ailing spy satellite late tonight or tomorrow, depending on weather conditions at the North Pacific launch site.
As the countdown approaches, observers are still split on whether the primary motivation for the move is to protect public safety or to test weapons technology under the watchful eyes of foreign powers such as China.
The satellite, known as USA 193, went unresponsive to commands shortly after its launch on December 14, 2006, and is in an uncontrolled decaying orbit, the U.S. Department of Defense announced on February 14.
Left on its own, the school bus-size orbiter would crash on Earth sometime later this month or early next month, the department said in a press release.
According to government computer models, about 56 percent of the 5,000-pound (2,268-kilogram) satellite would survive reentry and hit the Earth.
At a February 14 press conference, officials added that President George W. Bush had authorized the shoot-down to prevent the satellite from releasing a toxic gas from its fuel tank.
Many analysts and bloggers, however, have speculated that the danger from the gas is minor.
"OK, so hydrazine looks pretty toxic. But what would the chance have been of it smashing through your roof?" asks the science and technology blog Exploring our World.
The Navy's real goal, skeptics say, is to counter a similar exercise in China last year and prove that U.S. rockets designed to intercept nuclear missiles can be retrofitted to hit satellites.
Burning Lungs, Seizures
In January 2007 China intentionally shot down one of its own aging weather satellites using a modified ballistic missile.
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.
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 former 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.
Space junk hits Earth often, not people
Seth Borenstein, Associated Press Yahoo News 21 Feb 08;
Giant chunks of manmade space junk — like the dead satellite that the U.S. government is trying to shoot down — regularly fall to Earth. Yet no one has ever been reported hurt by them.
Chunks of debris weighing two tons or more from satellites and rocket parts fall uncontrolled every three weeks or so, according to an analysis by a Harvard University astronomer who tracks satellites and space debris.
And that's just based on the last three years. Go back a decade or so when countries didn't try to control these falling objects. Back then, two-ton chunks fell to Earth much more frequently said Jonathan McDowell, who runs Jonathan's Space Report, which tracks the world's space launches and satellites.
It's likely that 50 to 200 "large" pieces of manmade space debris return to Earth every year, according to the Center for Orbital and Reentry Debris Studies. Bill Ailor, the center's director, like those at NASA's Johnson Space Center, said he was asked by the government not to comment specifically on the current satellite re-entry issue.
In the past 40 years, about 12 million pounds of manmade space junk has survived re-entering Earth's atmosphere, according to the orbital debris center.
Yet experts in the field know of only one report of a person being hit by space debris. Lottie Williams of Tulsa, Okla., was struck on the shoulder in 1997 by a small piece of debris from a discarded piece of a Delta rocket. She was unhurt.
The reason space junk doesn't regularly hit people is simple: About 70 percent of the Earth's surface is water.
And on average there are about 130 people per square mile of land on Earth, but people don't take up a lot of space. Far more than 99.9 percent of the land on Earth is not occupied by a person at a given time, according to rough calculations by researcher Alex de Sherbinin of Columbia University.
There is no one place on Earth that is more prone to space junk than others. Where satellites fall depend on their particular orbit.
So the orbital debris center that studies the issue puts the odds of anyone being hurt by any piece of re-entering space junk at one in a trillion, saying you are far more likely to get hit by lightning.
Using Columbia University's population density maps, McDowell calculated that at the highest possible risk, there's a 1-in-10,000 chance that the dead satellite could hit a person. However, it's probably closer to one in a million, McDowell said.
That doesn't take into account toxic fumes from the ton of frozen and dangerous hydrazine rocket fuel, which is the reason Pentagon officials said they needed to shoot down the dead satellite. The Centers for Disease Control and Prevention put out a notice to local public health officials warning of the toxicity of the fuel.
McDowell is skeptical, however, given the odds.
"My gut reaction is that this is just completely bogus," McDowell said of the decision to shoot down the satellite based on a public health threat. He doesn't completely discount the danger of the rocket fuel, however.
This is the type of risk that shouldn't be reduced to mere numbers, said David Ropeik, a Boston risk communications expert who has consulted with the Bush White House and Department of Homeland Security.
"It's the nature of the risk, not the number," said Ropeik, co-author of the book "Risk." "A good question can be asked whether it is the public's worry that is driving this or whether the government is concerned about the harm that it can cause, even though the chances of that are low."
On the Net:
Jonathan's Space Report: http://www.planet4589.org/space/jsr/jsr.html
Center for Orbital and Reentry Debris Studies:
http://www.aero.org/capabilities/cords/index.html
