With the successful landing of space shuttle Atlantis this morning, U.S. Defense Department officials said the window of opportunity is now open for attempting to shoot down a wayward spy satellite.
But experts question the stated motives and whether the potential benefits outweigh the risks.
The satellite USA-193, also known as NROL-21, was launched into orbit Dec. 14, 2006. Shortly after it reached orbit, ground controllers lost contact with it. Though the satellite's objective is secret, many figure it is a high-resolution radar satellite intended to produce images for the National Reconnaissance Office (NRO).
The Pentagon plans to shoot it down, convinced its toxic hydrazine fuel poses an unacceptable risk to people on the ground. Currently, three U.S. Navy ships ? the cruiser USS Lake Erie and the destroyers USS Decatur and USS Russell ? are posted in the Pacific Ocean waiting for an optimal time to launch. The Erie will get one 10-second window each day over the next 10 days or so to fire one of its two SM-3 missiles before the defunct satellite tumbles to Earth, according to the American Forces Press Service. The Decatur is also fitted with an SM-3 missile.
But former assistant secretary of defense Philip Coyle doesn't buy that rationale.
"The spy agency doesn't want some part of the satellite to fall into the wrong hands," said Coyle, now a senior advisor to the Center for Defense Information. "I don't think that's being emphasized enough as a motivation for NRO to want this thing to be shot down."
In order of importance to the NRO, Coyle named two other reasons for the attempted shoot-down:
"Number two, poke the Chinese, because we're showing them not only that we can shoot down a satellite in a test without creating a lot of debris like they did. But we're also showing them we can do it any place in the world, because we're doing it from the ocean," Coyle said, referring to a similar satellite destruction by the Chinese. "And a third reason is to show off our missile-defense capabilities such as this, though this is much easier than hitting an enemy warhead."
Warheads move more erratically and are much trickier targets than a predictably moving satellite.
Even so, there is a chance of missing the satellite all together, for one because it's moving so fast.
"The closing speed between the missile and the satellite is going to be very high, much higher than they might have in a missile engagement," said Coyle, who is also past director of the Operational Test and Evaluation program in the Department of Defense. "So that's another reason why they might miss is because they've never had experience trying to hit something moving so fast before. Still I think it should be relatively straightforward."
The satellite and missile would close on one another at a velocity of about 22,783 mph (36,667 kph).
Hitting the bus-sized target is just half the battle. To be completely successful, the missile must also destroy the satellite?s fuel tank, which holds about 1,000 pounds (454 kilograms) of toxic hydrazine.
Pentagon officials have argued that if the satellite were to fall through the atmosphere with no missile interference the hydrazine tank could survive the fiery descent to reach Earth's surface intact, spewing toxic gas over an area about the size of two football fields. Those who inhaled it would need medical attention.
?In this case, we have some historical background that we can work against for the tank that contains the hydrazine," said Marine Gen. James E. Cartwright during a Feb. 14 press briefing. "We had a similar one on Columbia that survived re-entry. We have a pretty reasonable understanding that, if the tank is left intact, it would survive the re-entry.?
However, destroying the fuel tank and dispersing the hydrazine requires a direct hit on the possibly tumbling satellite. The high closing speeds for the satellite intercept and the uncertainty of puncturing the fuel tank could make that goal questionable, according to an analysis done by Geoffrey Forden, an MIT physicist and space expert.
"If they do shoot at it, even if they hit it, there's just a 30 percent chance that the shrapnel connected by the intersection hits the hydrazine tank," Forden said.
What if the missile misses?
?As we reviewed the data, if we fire at the satellite, the worst that could happen is that we miss,? Cartwright said. ?Then we have a known situation, which is where we are today.? He continued, ?If we hit the hydrazine tank, then we?ve improved the potential to mitigate that threat. The regret factor of not acting clearly outweighed the regret factor of acting.?
They wouldn't know for at least a day whether the tank had been destroyed, said Pentagon Press Secretary Geoff Morrell.
Coyle, the former Defense Department official, doesn't think the hydrazine tank is a big enough safety issue in the first place, stating that the U.S. produces about 36 million pounds (about 16 million kilograms) of hydrazine each year.
"If we're so worried about hydrazine we oughtn't to be trucking it around on U.S. highways and on rail cars the way we do, if that's really our concern," Coyle told SPACE.com. "So if we're really worried about safety, putting a few dollars into a school-bus crossing some place would probably be a better investment than $60 million for this test."
During a Tuesday press briefing, Morrell denied that this is a test of the United State?s anti-satellite capabilities.
?This operation is designed to alleviate a threat to human beings on this planet. There is a large tank of hydrazine fuel onboard that satellite that would pose a significant threat to people within the immediate vicinity of it if it were to hit land,? Morrell said. ?So not wishing to take that risk, the president has asked ? ordered ? this department to shoot down that satellite."
The plan comes on the heels of the intentional destruction last year of China's Fengyun-1C weather satellite, which produced a flurry of concern over the hostile-or-not nature of the firing as well as a serious load of shrapnel littering Earth orbit. That debris is still in space, frustrating mission managers and satellite operators forced to dodge the potentially debilitating bits.
The U.S. Defense Department plan, however, was made public to the international community.
?This announcement demonstrates openness about a sensitive subject that is appropriate to a democratic society,? said Ray Williamson, Executive Director of the Secure World Foundation in Colorado.
However, Williamson added, ?I fear that using a ballistic missile in this manner after the United States roundly chastised the Chinese government for its anti-satellite (ASAT) test just over a year ago sends completely the wrong message to the world community.?
The attempted shoot down of the satellite will undoubtedly send a political message. Both Russia and China have expressed concerns regarding the U.S. attempt, with Russia labeling it a of the missile defense system.
"The timing of it is very interesting, coming after [the Russia-China] proposal on banning space weapons," said Roger Launius, National Air and Space Museum senior curator. He added that the U.S. attempt could be a response to China's anti-satellite test last year that "we can do this too," and take out satellites if necessary.
"The potential political cost of shooting down this satellite is high," said Laura Grego, an astrophysicist with the Union of Concerned Scientist's Global Security Program. "Whatever the motivation for it, demonstrating an anti-satellite weapon is counterproductive to U.S. long-term interests, given that the United States has the most to gain from an international space weapons ban. Instead, it should be taking the lead in negotiating a treaty."
A U.S. attempt that fails to destroy the satellite could also send a message ? although not one the U.S. would like. China and the rest of the world could assume the miss was a fluke, or they could also see failure as evidence that the U.S. technological lead in space has declined, according to Launius.
Weighing the balance
The U.S. may find itself in a difficult position by going ahead with the attempt, even given the real danger of the satellite's hazardous payload falling to Earth.
Forden calculated the risks of the hydrazine tank killing or injuring someone at 3.5 percent if it survived re-entry. However, he stated his belief that the political consequences of the attempted shoot-down could be worse, by further opening up the international arena for future anti-satellite tests and possible conflict in space.
"You have to weigh the chance of [the satellite] killing or injuring someone against legitimizing China's ASAT [anti-satellite] test," Forden said. "A three percent chance of killing or injuring someone is large, but the consequences of allowing China to go ahead?I still come down and say it's a bad idea."
The future of space as a battlefield could mean clouds of debris from destroyed satellites. That would add to some 17,000-plus objects that are already being tracked by the U.S. Space Surveillance Network. According to the NASA Orbital Debris Program Office at the space agency?s Johnson Space Center in Houston, Texas, for the past 45 years, the average number of cataloged object re-entries has been one per day.
"Stuff will hang up there until gravity brings it down," noted Launius. "If you get enough of that up there, just getting through it could be an issue for mission launches."