The U.S. Navy is specially modifying three advanced SM3 anti-ballistic missile interceptors to shoot down an electronically dead, intelligence-gathering satellite that was launched into space for the National Reconnaissance Organization (NRO).

Aviation Week broke the news on AviationWeek.com Feb. 12 that the U.S. was planning the shootdown.

Communications with the satellite were lost almost immediately, which means there’s no way of guiding the spacecraft to a predictable crash site as it returns from orbit, says Marine Corps Gen. James Cartwright, vice chairman of the Joint Chiefs of Staff. The extraordinary decision to shoot it down was the result of analyses that show the satellite’s 40-in.-dia. hydrazine tank—now holding a 1,000-lb. frozen sphere of maneuvering propellant—will survive the descent. It will pose a lethal danger when it strikes the Earth, cracks open, and the frozen slush turns into a toxic gas, says James Jeffrey, White House deputy national security adviser. The effect on human lungs would be similar to ammonia or chlorine gas.

A hydrazine tank on the Columbia space shuttle survived its crash in Texas, but it was almost empty when it struck. The NRO satellite’s fuel, by comparison, has the potential to spread over an area as large as two football fields, they say. Odds are that the satellite will reenter the atmosphere and crash into the ocean, which makes up 70% of the Earth’s surface under the satellite’s orbit, the areas between 58.5 deg. N. and S. Lat. But U.S. officials, with the approval of President Bush, decided not to take the chance, since a 2,500-2,800-lb. mass from the satellite is expected to survive descent, Cartwright says.

The three Aegis ships involved in the intercept, from a launch site in the northern Pacific, will be “reconfigured on a one-time, reversible basis,” says Jeffrey. Even if the space defense missiles miss or misfire, the threat will be no greater, says NASA Administrator Michael Griffin. However, even if the missile only grazes the errant satellite, it will fall out of orbit faster, analysts contend. If they make a direct hit, the spacecraft is expected to fall into an unpopulated area, Cartwright says.

If the SM3 missile hits the satellite as it nears the atmosphere, more than 50% of the debris will reenter within two orbits, about 10-15 hr. Most of the remaining pieces would fall within a month, Cartwright says. It will be critical to hit the satellite before it enters the atmosphere, where its nonaerodynamic shape will cause it to tumble and be almost impossible to engage, he says. If the first SM3 misses, operators will reassess and try again with the backup missiles.

It is officially denied that debris from the payload could reveal secret new U.S. national security capabilities if satellite wreckage were recovered by another nation. But some analysts are less certain, given the experimental nature of the intelligence-gathering payload.

Pentagon officials and industry analysts are circumspect about the satellite’s mission capabilities. At least two sources with insight into the program say it’s not an imaging radar as some believed. The satellite was launched Dec. 14, 2006, on a Delta II, one of the smaller family of vehicles in the U.S. launcher inventory; key parts of this particular package are not sized for a traditional imaging radar satellite. “It’s not a radar,” says a longtime Pentagon radar expert. The Space Radar, for example—a program that was shifted from the U.S. Air Force to NRO in last year’s budget—was criticized because it would need a heavy booster, which would drive up costs.

It’s also thought not to be a product of NRO’s traditional imagery, signals or communications intelligence shops. Instead, it’s a smaller passive system—“not radar, optical or ladar”—that was cobbled together by the organization’s fast-development, advanced science and technology office, the aerospace official says. It also appears to be an offshoot of earlier NRO work on small, cheap, fast-reaction, imaging satellites. The U.S. is admittedly looking hard at how it could rapidly repopulate its intelligence-gathering satellite network if the current constellations are targeted by antisatellite weapons or network and electronic attack.

Nonetheless, the issue of the payload’s classification alone was not enough to justify breaking up the satellite with an interceptor missile, Cartwright says.