Astronauts Sunita Williams (L) and Michael Lopez-Algeria are shown during a space walk on the International Space Station, Jan. 31, 2007. (NASA TV) Photo Gallery>>>

    BEIJING, March 16 (Xinhuanet) -- Recent NASA tracking data reveals the international space station has fallen to 207 miles (332) kilometers, the lowest average altitude in the nine-year history of the project.

    The U.S. space agency said the station's orbit has decayed because of air drag. A short rocket blast this week from a docked cargo ship will boost the orbit a bit, but since the orbit is contually dropping about 300 feet (90 meters) a day, the boost will be negated within several weeks.

    A graph released by NASA shows the loss of the shuttle Columbia in early 2003 marks when a gradual decline became more severe. Until that time, the altitude had been maintained at a fairly high level, thanks mainly to reboosts from visiting shuttles. Since the Columbia disaster, all reboosts have been the responsibility of Russian spacecraft.

    "Our altitude is driven by shuttle rendezvous altitude limits," Mission Control expert Ainsley Collins explained by telephone. Collins is head of the space station's Trajectory Operations Office (TOPO), which coordinates with corresponding specialists in the Moscow Mission Control Center.

    The U.S. and Russian teams specialize in calculating the future paths of the space station as well as all objects flying toward it or away from it, including orbital debris. They then plan the occasional small propulsive maneuvers that line the station up with other vehicles -- or, in the case of space debris, will dodge them.

    NASA orbital trajectory experts insist the station's orbit is under control. They say the decline is part of a long-range plan for the current phase of orbital assembly that involves particularly heavy payloads for shuttle missions.

    They say the orbit is still stable, and scheduled Russian reboost rocket firings will not allow it to decay any further. Bbeginning late next year, when the delivery of the station's heaviest structural elements will be completed, the orbit will be reboosted to greater and greater altitudes.

    "The lowest the station orbit has ever been was on May 23, 2000," Collins recalled.

    After launch into a higher orbit of 250 miles (400 kilometers), it had dropped to 207 miles (331.5 kilometers) before it was pushed higher during a shuttle mission. Throughout 2001 and 2002, the station hovered at around 246 miles (395 kilometers) before beginning its long slide to the current low point, below 207 miles.

    (Agencies)