A Pearl Harbor-based Navy ship may be called upon within days to perform a first: shoot down a failing satellite while it's still in space.

The bus-sized satellite holds more than 1,000 pounds of hazardous hydrazine fuel.

CNN reported that the cruiser USS Lake Erie was expected to fire the SM-3 missile to demolish the 5,000-pound military spy satellite over the Pacific. Other reports said the attempt would be made northwest of Hawai'i.

Pentagon officials announced the plan at a news conference yesterday but did not say which ships would be used or where in the Pacific it would take place.

"We'll use one missile with two backups. We'll have three ships on station, but it'll be one shot," Marine Gen. James Cartwright, vice chairman of the Joint Chiefs of Staff, said at the Pentagon news conference.

The window to accomplish the mission will open in three to four days, and remain open for about a week after that, officials said. The Pentagon said the Navy will not fire until after the shuttle Atlantis mission ends next Wednesday.

President Bush ordered the shootdown attempt after security advisers said the satellite re-entry posed potential danger to civilian populations.

James Jeffrey, a deputy national security adviser, said if the satellite is not shot down, it could release more than 1,000 pounds of hydrazine fuel as a toxic gas. The fuel could spread across an area equal to two football fields.

Cartwright said hydrazine is similar to chlorine or ammonia and affects lung tissue with symptoms including a burning sensation. If a lot is inhaled, it's deadly, he said.

About 2,800 pounds of the satellite typically would survive re-entry, officials said.

"The likelihood of the satellite falling in a populated area is small, and the extent and duration of toxic hydrazine in the atmosphere would be limited," Jeffrey said at the Pentagon news conference. "Nevertheless, if the satellite did fall in a populated area, there was a possibility of death or injury to human beings beyond that associated with the fall of satellites and other space objects normally."

There was enough of a risk for Bush to decide on the shootdown attempt, the Pentagon said.

The goal is to hit the satellite just before it enters Earth's orbit so that the hydrazine tank explodes. The US-193 satellite belongs to the National Reconnaissance Office and was launched on Dec. 14, 2006. It malfunctioned almost immediately.

Maj. Gen. Robert G.F. Lee, the Hawai'i adjutant general, said he has confidence the shootdown will work. But should it not, there is a small probability that Hawai'i could be one of the impact areas for satellite debris.

He said there's no indication that Hawai'i would be a likely spot for falling debris.

"Absolutely not. We've got just as good of a chance as anybody else in the world to be the low-probability bad penny," he said.

REDUCING DEBRIS RISK

Intercepting the satellite at about 130 nautical miles and "absolutely low as possible" just prior to it hitting the Earth's atmosphere will reduce the risk of debris in space as well as minimize risk to people from falling debris, Cartwright said.

He would not say where the ships would fire from, saying only it will be from the northern hemisphere and the Pacific.

The military will have as many as two days to assess the success before a decision is made whether to take a second shot.

If the satellite is hit, officials said "well over" 50 percent of the debris will fall to Earth in the first two orbits and weeks, "maybe a month" for some of the smaller debris to come down, Cartwright said.

The Pentagon also wants to intercept the satellite at a point just above the atmosphere so there would be a high likelihood of bringing it down in an unpopulated area.

Analysis done by the military shows that the frozen hydrazine fuel would survive the fall intact if the satellite were allowed to descend normally out of orbit.

In January, the U.S. government notified other nations that the satellite was unresponsive and would make an uncontrolled re-entry in late February or early March, the Pentagon's news service said.

SUCCESSFUL INTERCEPTS

The Pearl Harbor-based USS Lake Erie has been used as a test bed for the nation's still-developing ballistic missile defense shield.

In a November test, the Lake Erie's Aegis weapons system detected and tracked two simulated ballistic missile targets. Minutes later, Lake Erie's crew fired two SM-3 missiles, and two minutes after that, the missiles successfully intercepted the targets outside the Earth's atmosphere more than 100 miles above the Pacific Ocean and 250 miles northwest of Kaua'i.

Cartwright said the SM-3 is "well understood."

"It has the ability to get up just beyond the atmosphere, so it has the kinetic energy to be able to reach this satellite as it prepares to re-enter.

The Missile Defense Agency and Navy are modifying 15 destroyers and three cruisers to have Aegis ballistic missile defense capabilities.

Sixteen of the 18 Aegis ships with missile shootdown capability will be based in the Pacific. At least four will be based at Pearl Harbor, including the cruisers Lake Erie and Port Royal, and destroyers Russell and Hopper.

Cartwright said the planned shootdown will be the first time the U.S. has used a tactical missile to engage a spacecraft. Closing velocities, the speed at which two objects approach each other, will be roughly 22,000 mph.

The U.S. had to make modifications to missile defense interceptors, the three ships and software, but the Pentagon said the shootdown attempt is not being made to test anti-satellite capabilities, and no additional SM-3 missiles or Aegis ships will be modified for satellite engagement.

The U.S. had criticized China in January after China used a ground-based missile to hit and destroy one of its aging satellites in orbit.

There are more than 9,600 objects orbiting the Earth, and since tracking began with Sputnik, more than 17,000 man-made objects have re-entered the Earth's atmosphere, according to the Pentagon.

The Federal Emergency Management Agency is in contact with the Pentagon and will notify all states with information about the re-entry of the satellite and debris field once it's known.

Reach William Cole at wcole@honoluluadvertiser.com.