WASHINGTON (Reuters) - A Northrop Grumman Corp U.S. military satellite used to track enemy missiles stopped working in mid-September, underscoring the urgent need to keep a program for replacement satellites on track, a defense official and several analysts said on Monday.

The U.S. Air Force had no comment, but Space News reported on Monday that the Pentagon’s chief weapons buyer John Young has signed a memorandum asking Congress to provide $117 million in funding in fiscal 2009 for a new satellite to hedge against a potential gap in satellite coverage around 2014.

“There is no gap today in U.S. missile warning, but the apparent loss of a satellite means there is an increased danger of a gap down the road because the redundancy of the existing constellation has been diminished,” said Loren Thompson at the Virginia-based Lexington Institute.

Thompson said he had learned of the problem with the Defense Support Program (DSP) satellite from several sources, and it underscored the urgency of getting the new Space Based Infrared Satellite (SBIRS) system being developed by Lockheed Martin Corp into orbit.

“There’s a sense of renewed purpose. We need to be sure that the SBIRS launches proceed as scheduled,” agreed one defense official, who asked not to be named, adding that the loss of the satellite posed challenges for the DSP constellation.

Theresa Hitchens, who heads the Washington-based Center for Defense Information, said several amateur astronomers had reported that DSP 23, the last of the U.S. missile-warning satellites built by Northrop, and launched in November 2007, had stopped transmitting in mid-September.

The defense official said it was unclear what caused the failure, but said the explanations could range from defective parts to natural phenomenon, and possibly, although unlikely, an intentional attack. There was also a chance that the problem stemmed from the satellite being hit by debris in space.

The U.S. government has launched 23 DSP missile-warning satellites into space since 1970, and experts estimate that six to 10 are still working, about double the number needed to watch the entire Earth at once.

The satellites have generally lasted longer than initially expected, which makes it even more troubling that the newest of the DSP satellites would have developed trouble a year after its launch, said the official and the analysts.

SBIRS PROBLEMS

“The DSP constellation is starting to degrade now, that’s not a surprise. But it wouldn’t even be a concern if SBIRS weren’t so screwed up,” said Jonathan McDowell, astronomer with the Harvard-Smithsonian Center for Astrophysics.

The SBIRS program has seen its share of problems, including delays due to technology challenges, and Lockheed is rewriting software for the satellites after software problems prevented communication with a smaller satellite once it went into orbit in late 2006. The U.S. government shot down that satellite last February, saying its toxic fuel tank posed a potential threat.

The first SBIRS sensor is already in space on board a classified satellite in highly elliptical orbit, but the first dedicated satellites are not due to be launched until 2010.

The SBIRS program was launched in 1996 with an eye to launching the first satellites in 2004 at a cost of $4.2 billion. The program has been restructured several times and its price tag is now seen at well over $11 billion.

The defense official said the success of the first SBIRS payload in space was good news for the next satellites being developed for geosynchronous orbit.

But it was prudent to secure additional funding to ensure that the U.S. military was able to maintain its ability to spot enemy missiles, the official added.

Hitchens said some amateur astronomers, who use optical and radio telescopes to track objects in space, suggested the DSP satellite may have been adrift in geosynchronous orbit, which could pose a danger to other satellites operating in that orbit, she said.

The defense official downplayed that possibility, and said the failed satellite also posed no imminent threat to human life, as was the case with the NRO satellite.