CAPE CANAVERAL, Fla. - A pair of backup steering thrusters boosted the low end of the Chandra X-ray Observatory's orbit by another 1,373 miles on Wednesday, with just one more engine firing left until the telescope reaches its final perch.

The five-minute firing, which occurred at 12:36 p.m. EDT, placed the observatory into an orbit that varies between 3,535 miles and 86,448 miles from Earth.

The maneuver had been delayed to allow time for flight controllers to switch to a backup engine system. The telescope, which was carried into space by the shuttle Columbia on July 23, ended up slightly short of its intended orbit after a positioning maneuver over the weekend.

"Initial indications are that today's firing went very well," said Fred Wojtalik, NASA's Chandra program manager at the Marshall Space Flight Center in Huntsville, Ala. The telescope is the third member of a quartet of major space observatories to reach orbit. It joins the Hubble Space Telescope and the Compton Gamma Ray Observatory in space. An infrared telescope is scheduled to be launched in late 2001.