Satellite Shoot-Down

In a first for the Pentagon's Missile Defense Agency, a U.S. Aegis-class cruiser will fire a missile at a falling U.S. intelligence satellite in an attempt to shatter the spacecraft's fuel tank before it comes crashing to Earth. The shoot-down could take place as early as February 20.

The intercept 149 miles above the Pacific Ocean is intended to disperse the satellite's 1,000 pounds of deadly hydrazine fuel, which could otherwise endanger humans and animals over an area the size of two football fields, U.S. officials said at a Pentagon briefing Thursday.

The 40-inch spherical tank is the largest chunk of the 5,000-pound, schoolbus-size satellite expected to survive the fiery re-entry to Earth. Altogether, 2,800 pounds of debris are predicted to result from the demise of the unidentified National Reconnaissance Office (NRO) spacecraft, which failed within hours of reaching orbit on December 14, 2006. Because ground controllers can't communicate with the satellite, they aren't able to steer it to a safe re-entry in the ocean.

"It's the hydrazine aboard that makes this different" from Gen. James Cartwright, vice chairman of the Joint Chiefs of Staff, alluding to blogs that suggest the real reason for the shoot-down was the clandestine nature of the satellite. "It's the hydrazine that is the issue here. It's the only reason we're doing this. We had a similar tank on [space shuttle] Columbia that survived re-entry and came down in a wooded area" (although little hydrazine was left by the end of that mission).

Hydrazine is a colorless liquid with an ammonia-like odor that is routinely used in rocket fuel. Exposure to it can cause damage to the nervous system, liver, and kidneys. No one knows how much of the satellite's hydrazine will be in gas or liquid form when it begins warming up in the atmosphere, Cartwright said.

The plan is to have three Aegis cruisers (one primary, the others backup) stationed at various Northern hemisphere locations in the Pacific. Once the dead satellite drops to an altitude of 149 miles over the Pacific, one cruiser will fire a single missile, which will close in on its target at an estimated speed of 22,000 mph. Pentagon officials then will have two days or so to assess the shot, and if necessary fire again, Cartwright said.

Cartwright said he had "high confidence" that the Standard Missile 3 (SM-3) missile, which he described as "part of the regional/tactical part of MDA," could strike the satellite. "We have a missile that's well-understood. It's the first time we've needed to engage a spacecraft, but it's not the first time we've intercepted something entering the atmosphere," he said. The SM-3 is designed to hit short- and medium-range missiles.

"The worst that can happen is we miss. If we even graze it, it will bring it down sooner," he said. But the likelihood of satellite chunks hitting land is low, he added. Computer models indicate that more than half of all the debris will come down in the satellite's first two circuits around Earth after the missile impact.

"There's a very large amount of uncertainty in predicting a landing zone" for a spacecraft, said NASA Administrator Michael Griffin. A month ahead of time, the best models can only be accurate to within three days. At 10 days out, the uncertainty drops to a day. On landing day, that drops further to within two hours. "Almost anything we can do [on shoot-down day] will turn out to be either neutral…or better. Nothing we can do will make the situation worse."

Griffin and Cartwright both said the satellite shoot-down has no comparison with China's anti-satellite test in January 2007, in which an object fired from Earth smashed into a Chinese weather satellite (see "Satellite Smashers," February/March 2008). "First, we're notifying the world that we're doing this," said Cartwright. "The Chinese test left debris that will be up there for 20 to 40 years. All our debris will be down in a day."