Officials said that they had no control over the nonfunctioning satellite and that it was unknown where the debris might land.
“Appropriate government agencies are monitoring the situation,” Gordon Johndroe, a spokesman for the National Security Council, said in a statement. “Numerous satellites over the years have come out of orbit and fallen harmlessly. We are looking at potential options to mitigate any possible damage this satellite may cause.”
Specialists who follow spy satellite operations suspect it is an experimental imagery satellite built by Lockheed Martin and launched from Vandenberg Air Force Base in California in December 2006 aboard a Delta II rocket. Shortly after the satellite reached orbit, ground controllers lost the ability to control it and were never able to regain communication.
“It’s not necessarily dead, but deaf,” said Jonathan McDowell, an astronomer at the Harvard-Smithsonian Center for Astrophysics and an analyst of various government space programs.
It is fairly common for satellites to drop out of orbit and enter Earth’s atmosphere, but most break up before they reach the surface, Mr. McDowell said. Such incidents occur every few months, and it is often difficult to control the satellite’s trajectory or its re-entry into the atmosphere. The debris, if any survives the fiery descent, typically lands in remote areas and causes little or no harm.
“For the most part,” Mr. McDowell said, “re-entering space hardware isn’t a threat because so much of the Earth is empty. But one could say we’ve been lucky so far.”
Of particular concern in this case, however, is that the debris from the satellite may include hydrazine fuel, which is typically used for rocket maneuvers in space.
Much of the fuel on the experimental satellite may not have been used and, should the tank survive re-entry into the atmosphere, the remaining fuel would be hazardous to anyone on the ground. It is likely, however, that the tank may rupture on re-entry, and that the fuel would burn off in a fiery plume that would be visible to the naked eye.
John E. Pike, the director of Globalsecurity.org in Alexandria, Va., said that if the satellite in question was a spy satellite, it was unlikely to have any kind of nuclear fuel, but that it could contain toxins, including beryllium, which is often used as a rigid frame for optical components.
Since it was launched, the experimental satellite has been in a slowly decaying orbit. As of Jan. 22, it was moving in a circular orbit at about 275 kilometers above the Earth, Mr. McDowell said. In the last month, its orbit has declined by 15 to 20 kilometers.
“If you plot the curve, it’s now just a matter of weeks before it falls out of orbit,” he said.
The largest uncontrolled re-entry by a NASA spacecraft was that of Skylab, the 78-ton abandoned space station that fell from orbit in 1979.
