Chandra not quite where expected

The Chandra X-ray telescope drifts away from the space shuttle Columbia after its deployment July 23
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August 3, 1999
Web posted at: 3:10 p.m. EDT (1910 GMT)

By Robin Lloyd
CNN Interactive Senior Writer

(CNN) -- Two engines on NASA's powerful X-ray telescope put it slightly lower in space than expected, so engineers will rely instead on back-up thrusters over the next several days to push the spacecraft into its working orbit.

An engine firing Saturday raised the high point of the Chandra X-ray Observatory's orbit but left it a few hundred kilometers short of its target of 140,000 km (about 87,000 miles).

"The whole process is pretty highly controlled but it's not 100 percent precise nor is it intended to be," said Harvey Tanenbaum, Chandra science center director. "We had set a range of 140,000 plus or minus 5,000 kilometers. We ended up at 139,000 and change."

"It's amazingly close to hitting the mark dead center," he said.

Still, engineers noticed that the one of the engines ran a bit hotter than expected, so they will use a backup pair of engines for the last two planned thrusts to finalize the low-point of Chandra's orbit, said NASA Program Manager Fred Wojtalik. A five-minute burn is set for Wednesday, with an eight-minute burn to follow early Saturday morning.

The engines, built by TRW, burn a mixture of hydrazine and nitrogen tetra-oxide at temperatures reaching 2,500 degrees C, and engineers suspect too much of the latter oxidizing agent got into the mix. That might have been the reason for the slightly weaker thrust of the engine burns, Tanenbaum said.

"The idea is to switch to the cleaner system or the system that hasn't been used and get back to the one to one ratio of hydrazine and oxidizer," he said. "It would have been nice if we could have finished orbital maneuvers on engines we already know and love and have fired three times but the more conservative approach is to rely on the other two engines."

The final two burns should raise the low-point of Chandra's orbit from 6,000 km (3,700 miles) to 9,500 km (5,900 miles). Raising the orbit's high and low points improves the opportunities for Chandra to collect data.

Astronauts released the $1.5 billion telescope from space shuttle Columbia on July 23. Chandra is designed to study the highest energy radiation sources in the universe, including black holes, colliding galaxies and quasars at the edge of time.

Initially, the plan was to raise the low-point of Chandra's orbit on Monday but the engine performance led mission operators to move that firing to Wednesday to give them time to switch over to the backup engines and re-route fuel and other protective software.

Now, the mission team is anxious about the switch-over to new engines, Tanenbaum said. "That involves more work and slightly more uncertainty, and certainly we will all feel a lot better after we fire them for first time on Wednesday," he said.

Engineers will remain anxious about Chandra until all its instrument covers are removed and it successfully takes in its first test data in about two weeks, he said.

"Until the door opens and we act have X-rays coming onto detectors and we focus X-rays and we see a focused image, it's real hard to celebrate," Tanenbaum said.