BOSTON — U.S. Air Force Space Command has stopped using a satellite that was launched more than a decade ago as a 15- to 18-month missile tracking experiment but found new life in an operational role as part of the military’s space surveillance infrastructure.

The Midcourse Space Experiment (MSX) satellite, which was launched aboard a Delta 2 rocket in April 1996, has operated more than seven years past its expected lifetime, according to Col. Shawn Barnes, command lead and requirements division chief for space situational awareness and command and control at Air Force Space Command in Colorado Springs, Colo. Space situational awareness refers to the ability to detect and track objects in orbit that might pose a threat or otherwise be of some military significance.

The Johns Hopkins University’s Applied Physics Laboratory of Laurel, Md., built the MSX satellite for the Ballistic Missile Defense Organization—predecessor to the Missile Defense Agency—which turned it over to Air Force Space Command in 2000. The Air Force began using the spacecraft for space situational awareness operations in 1998 as part of an Advanced Concept Technology Demonstration, Barnes said in a June 2 interview.

The space surveillance data came from the MSX’s Space Based Visible sensor, which was built by the Massachusetts Institute of Technology’s Lincoln Laboratory in Cambridge and was the spacecraft’s only working sensor in recent years. The other sensors were infrared sensors, which require a coolant whose supply on MSX has long since been exhausted, he said.

The Air Force made the decision to shut down the satellite after determining that the Space Based Visible sensor had degraded to the point where it no longer could be relied upon, Barnes said.

The satellite ceased military operations June 1, but Space Command has not yet shut it down because it is gathering data related to the Space Based Visible sensor’s degradation on orbit that could help with the development of future systems, Barnes said.

The Air Force in recent years has developed tactics, techniques and procedures with its ground- based optical space surveillance telescopes that will help mitigate the impact of shutting down MSX, Barnes said. While these telescopes are impeded by cloud cover and nightfall, Space Command is not concerned about its ability to monitor the geostationary-orbit arc between now and when the Space-Based Space Surveillance System (SBSS) pathfinder satellite is launched in early 2009, he said.

Theresa Hitchens, president of the Center for Defense Information, a Washington-based think tank, said the immediate impact of shutting down MSX is not clear. “Any time you lose a sensor that you’re not replacing, you have to wonder what the impact would be,” said Hitchens, a vocal advocate of enhancing space security through better space situational awareness—as opposed to deploying anti-satellite weapons. “I don’t know that it’s going to be a problem, but it raises a question about gaps in coverage.”

The SBSS pathfinder satellite originally was expected to launch in 2008; the Air Force in 2003 unsuccessfully tried to move the date to 2006 to avoid a gap in coverage when MSX reached the end of its life. The SBSS pathfinder, being built by Boeing Space and Intelligence Systems of Seal Beach, Calif., has been plagued by technical difficulty, poor subcontractor management, and congressional budget reductions that have driven its price tag from $350 million to $815 million, according to Joe Davidson, a spokesman for the Air Force Space and Missile Systems Center.

The first SBSS satellite currently is slated for launch in February, Davidson said. The Air Force will decide this summer on its plans for a follow-on constellation that could consist of satellites similar to the pathfinder, or more advanced spacecraft, he said.

The service is not likely to try and accelerate the SBSS launch following the decommissioning of MSX because the current launch schedule already is considered aggressive, Barnes said.

The Air Force is planning to work with the Missile Defense Agency’s Space Tracking and Surveillance System demonstration satellites, slated to launch in November, to see if they could immediately contribute to the space situational awareness mission, Barnes said. That work could result in design specifications that might enable future operational missile tracking satellites to perform space surveillance as well, he said.

Hitchens hailed the Air Force’s work with the MSX satellite, which cost $325 million to build and launch, as a positive example of improving space situational awareness by leveraging an existing asset.