The change was made to allow the flight operations team in Cambridge, Mass., time to configure the observatory's redundant set of liquid apogee engines. A decision to use the redundant engines was made after a review of orbital parameters and Integral Propulsion System performance data from Saturday's 21-and-a-half-minute burn.

Saturday's burn raised the high-point, or apogee, of Chandra's orbit to 86,433 miles (139,100 kilometers). This is very slightly -- about 559 miles (900 kilometers) -- lower than the predicted 86,992 miles (140,000 kilometers), but is well within specifications, emphasized Chandra managers. Specifications call for the observatory's operating apogee to be between 83,885 and 90,099 miles (135,000 and 145,000 kilometers).

"While the propulsion system performed within specifications and has delivered us to a completely acceptable apogee altitude, the performance of Chandra's engine number 3 was slightly below expectations," said NASA Program Manager Fred Wojtalik. "That is leading us to take a conservative approach and switch to the redundant system as a precaution."

NASA's Marshall Space Flight Center in Huntsville, Ala., manages the Chandra X-ray Observatory for NASA's Office of Space Science, NASA Headquarters, Washington, D.C. The Smithsonian Astrophysical Observatory in Cambridge, Mass., manages the Chandra science program and controls the observatory for NASA. TRW Space and Electronics Group of Redondo Beach, Calif., leads the contractor team that built Chandra.