It sounds like a story line from a summer action-adventure motion picture: A spy satellite the size of a bus with a large supply of toxic fuel on board is hurtling toward Earth. The president has ordered the Navy to shoot it down before it crashes to the ground next month.

But it's not a movie script. It's for real. And depending on whom you ask, a lot is riding on a successful mission.

The official government line is that the intercept mission is necessary to protect people from exposure to hydrazine, a hazardous rocket fuel. Others speculate that the disabled satellite provides the perfect opportunity for the United States to test its weapons and to show other nations — particularly China, which conducted its own such successful test in January 2007 — that the United States dominates space.

Still others say the mission is necessary so that the spy satellite, dubbed US 193, does not fall into the hands of an adversary. The Pentagon insists it would be too damaged to be of benefit to an adversary.

Whatever the case, there is intense pressure for this mission to succeed. After the successful intercept of a Chinese satellite by the Chinese government, American intelligence analysts publicly expressed that the purpose of that test was to give the Chinese military the ability to blind American imaging satellites and hamper military operations if there were a confrontation over Taiwan, the New York Times reported on April 23, 2007. This sounds very much like President Ronald Reagan's "Star Wars" missile defense program. In fact, the last time the United States shot down a satellite was two decades ago during the Reagan administration.

Some scientists say the risk of the hydrazine tank landing near people is very small because three-fourths of the Earth is covered in water. Moreover, it would likely explode while re-entering the Earth's atmosphere. But scientists making those predictions are not privy to the satellite's design specifications.

It makes sense to minimize any risk to life, however small. And Pentagon officials say the malfunctioning satellite has no aerodynamic properties, therefore its trajectory is not predictable. Breaking it apart seems a sensible thing to do. According to the Los Angeles Times, ships in the North Pacific plan to fire a tactical missile at the satellite when it reaches a low orbit of about 130 nautical miles. The mission will take place after the space shuttle Atlantis, in orbit on a mission to the International Space Station, returns to Earth.

To reduce the risk of US 193 endangering human life — and in the process show off the nation's military capabilities in outer space — the missile must hit its mark. Very clearly, substantially more is at stake than simply taking out a disabled satellite.