Space station gets free boost from shuttle

James Oberg NBC News space analyst

SPACE CENTER, Houston - NASA officials have confirmed a bizarre observation by amateur satellite trackers following the flight of the international space station and the space shuttle Discovery. Far from causing the 200-ton space station to lose altitude, the space shuttle has actually pushed up the station’s flight path by almost a mile since it docked there late last week.

"The shuttle has raised the station’s orbit by about 4,000 feet (1,220 meters)," Paul Hill, the mission's chief flight director, said Monday. This contrasts sharply with the average daily altitude loss of about 100 feet (30 meters).

Deputy shuttle program manager Wayne Hale, who heads NASA's mission management team, confirmed Hill’s assessment on Tuesday. "We got about seven-tenths of a meter per second speed boost," he told MSNBC.com.

During normal cruise mode, the station is hitting the very thin air at its height of 220 miles (350 kilometers). This very slight — but non-zero — drag is enough to make it lose energy, and as a result it drops into a slightly lower orbit. Due to conservation laws of the physics of motion, this causes the station to slightly speed up as it drops.

Normal space practice is for docked visiting spacecraft to fire their own rocket engines to push the station back up, every few months. The special reboost maneuvers were scheduled when the docked vehicles ignited their engines, using propellant saved during the regular docking maneuvers.

Prior to the loss of Columbia, about half of the total reboost came from shuttles, with the other half coming from Russian robot freighters.

No such reboost maneuver was scheduled for this current flight. Nevertheless, over the weekend, outside observers realized that something was pushing the station higher.

The puzzle surfaces
Last Sunday, amateur satellite observer Kevin Fetter posted a query on See-Sat online discussion group, which brings together an international fraternity of space observers. "When did the shuttle boost the orbit of the ISS?" he asked.

Since no reboost maneuver had been planned or announced, Fetter admitted puzzlement. "So either I am seeing things, or space tracking data is faulty, or I wasn’t paying attention," he concluded.

Fetter’s See-Sat colleague Ted Molczan, widely considered the world’s leading guru of amateur satellite observations, added a message that the orbit had gotten about half a mile (870 meters) higher over the weekend. "I suspect it was a collision avoidance maneuver," he speculated, describing the occasional sudden rocket firings that must be made with only a few hours' notice to steer the station away from nearby space junk.

When a check with ISS activity logs showed no such rocket firing, these observers grew even more baffled. Perhaps, they suspected, a computer problem in the Defense Department space tracking computers was distorting the released values.

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