The U.S. Navy, acting on orders from the Bush administration, is finalizing plans to fire a modified tactical missile at a falling 2.5-ton spy satellite in an unprecedented attempt to break up the dead spacecraft and disperse its load of toxic hydrazine rocket fuel before it can re-enter on its own and possibly pose a threat to the public. The attempt is expected before the end of the month, but after the shuttle Atlantis returns to Earth next Wednesday.

The satellite in question, a classified spacecraft now known as USA 193, was launched in December 2006 from Vandenberg Air Force Base, Calif., by a Delta 2 rocket. The spacecraft failed shortly after reaching an orbit measuring 217 by 227 statute miles tilted 58 degrees to the equator (see Heavens Above for tracking maps: http://www.heavens-above.com/).

Out of contact and out of control, the National Reconnaissance Office satellite (also known as NROL-21) will re-enter the atmosphere with virtually a full load of now-frozen hydrazine rocket fuel inside a spherical tank. While the odds are the tank will survive re-entry heating and make it to the surface, the probability of impact in a populated area is considered remote.

But if the tank did, in fact, defy the odds, "we're talking an area, say, roughly the size of two football fields that the hydrazine could be dispersed over and you would at least incur something that would make you go to the doctor," said Marine Gen. James Cartwright, vice chairman of the Joint Chiefs of Staff. "If you stayed inside that zone, if you got very close to it and stayed, you could get to exposures that would be deadly. So that's a sense of what we're dealing with here."

Ted Molczan, an experienced, widely respected amateur satellite observer, told CBS News today the last verifiable sighting of the spacecraft was on Feb. 11. Based on tracking data at that time, he predicted re-entry on March 18, plus or minus one week, if nothing was done to hasten the event. And that projection includes assumptions about atmospheric behavior that could change closer to re-entry.

Concerned about the potential health threat, the Bush administration approved a plan to fire a standard tactical missile from an Aegis cruiser in an attempt to hit the falling satellite around the end of February. By attempting an intercept at a relatively low altitude - about 160 statute miles - half the resulting debris could be expected to burn up within hours with the rest following suit within a few weeks.

"What to me was compelling as we reviewed the data was that if we fire at the satellite, the worst case is we miss and then we have a known situation which is where we are today," Cartwright said. "If we graze the satellite, we're still better off because likely we'll still bring it down sooner and therefore more predictably. If we hit the hydrazine tank, then we've improved our potential to mitigate that threat. So the regret factor of not acting clearly outweighed the regret factors of acting."

NASA Administrator Mike Griffin said debris from a successful intercept would not pose a significant threat to the international space station, which flies at an altitude of about 210 statute miles and is permanently staffed by rotating three-person crews. The planned intercept will not be attempted until the shuttle Atlantis returns to Earth next Wednesday.

"We have a shuttle on orbit and a space station on orbit permanently with a permanent crew," he said. "So we looked very carefully at increased risk to shuttle and station and, broadly speaking, they are negligible. They are at least a factor of 10 smaller than the risks we take just being in space anyway in the shuttle. On the space station, of course, it's a different issue. The space station is much more robust than the shuttle. But even there, the risk posture does not increase significantly. And so we are very comfortable that this is a decision made carefully and objectively and safely."

He did not mention what a low-altitude cloud of debris might mean for the planned launch of the shuttle Endeavour March 11 on the next space station assembly mission.

The Chinese government was subjected to widespread international criticism when it destroyed a defunct weather satellite in January 2007 in a dramatic test of anti-satellite technology. The Feng Yun 1C was at an altitude of some 530 miles when it was destroyed, creating some 2,400 pieces of trackable debris, Molczan said.

Asked how U.S. plans to destroy the falling NRO satellite are different from what China did, Cartwright said the United States is notifying other nations in advance and "this is right at the surface of the atmosphere. Other intercepts that have occurred have occurred substantially higher than the space station, for example, and that means the debris is up there for 20 to 40 years and has to migrate down through both manned space platforms and unmanned space platforms. That will not be the case here."

Griffin agreed, saying "the Chinese ASAT test was conducted against a satellite in a circular orbit at around 850 kilometers of altitude. ... All of the debris from this encounter, as carefully designed as it is, will be down at most within weeks, and most will be down in the first couple of orbits afterward. There's an enormous difference."

Backing up that position, Molczan said only 1 percent - about 25 pieces of debris out of a population of more than 2,400 - have re-entered in the wake of the Chinese anti-satellite test. The rest is still in orbit.

He said the additional risk of debris that might get knocked out of USA 193's orbit into a more elliptical path with a high point, or apogee, above the space station's orbit is relatively small and that any pieces that did get knocked into such orbits would decay and re-enter within a few weeks.

"You're talking about intercepting this at 139 nautical miles, or 240 kilometers," he said. "Obviously, at that point the thing would be within three weeks of (a natural) decay if we're talking the end of February. That means at worse case, you engage this thing and most pieces are going to come down in the original timeframe."

While not disputing U.S. government projections of debris behavior, Molczan said "I just have a hard time being worried about a 40-inch sphere of hydrazine. I normally try not to get political in this stuff, but the thought that crossed my mind was ... is this in some measure a PR exercise to boost the concept of missile defense?"

Others have wondered if the Pentagon wants to ensure that no classified systems make it to the ground where they could possibly be recovered and subjected to analysis.

But Cartwright said the only concern was the threat posed by the satellite's load of hydrazine fuel.

"It's the hydrazine here that's the distinguishing characteristic," he said. "I read the blogs, there's some question about the classified side of this. That is really not an issue. Once you go through the atmosphere and the heating and the burning, that would not be an issue in this case. That would not justify using a missile to take it and break it up further."

Griffin said enough uncertainty exists with frozen hydrazine to warrant the extraordinary intervention.

"Solid as it is now, not all of it will melt, OK?" he said. "So you will land on the ground with a tank full of slush hydrazine that would then later evaporate. The tank will have been breached. Not probably, the tank will have been breached because the fuel lines will have been ripped out of the main spacecraft and so that hydrazine will vent.

"It's hard to find areas that have any significant population to them where you could put a toxic substance down across a couple of football fields and not have somebody at risk. And so, we didn't want to create a situation like that. So in brief, the tank will survive, it will be breached, the hydrazine will reach the ground. And that's not an outcome we want to see."