The object will make its closest approach tomorrow — right in the middle of a planned spacewalk. News of the space junk came soon after yesterday's docking by Atlantis to the International Space Station. Atlantis is making the last shuttle flight.
Mission management team leader LeRoy Cain said the size of the space junk is unknown. The object might stay at a safe distance but experts won't know until today. If necessary, Atlantis' thrusters will move the linked craft.
Less than two weeks ago, space station astronauts had to take shelter in their lifeboats because of a piece of junk. It missed by 1,100 feet, the closest encounter yet.
"We will work through our normal procedures and processes for dealing with that," LeRoy Cain, chair of Atlantis' mission management team, said in a news briefing today.
"What we were told today was very preliminary."
This potentially threatening piece of space junk is debris from a defunct Russian satellite that was launched in 1970, and is part of a known catalog of derelict objects in orbit.
After further analysis, NASA will better understand the size and orbit of the debris, and whether or not it poses a threat to the shuttle and station, Cain said.
Based on initial assessments, the object is expected to make its closest approach at 12:59 pm EDT (1659 GMT) tomorrow, during a scheduled spacewalk by station astronauts Ron Garan and Mike Fossum.
If deemed necessary, mission managers will use small thrusters aboard Atlantis to manoeuvre the shuttle and station out of harm's way, and allow both the shuttle and station crews to press ahead with their mission objectives.
"In all likelihood, it would not interfere with what we're doing on the spacewalk," Cain said.
Space junk, which consists mostly of spent rocket parts and pieces of broken satellites ranging in size, is an ongoing threat to spacecraft and satellites in orbit.
Today, more than 22,000 pieces of space junk are constantly being tracked in Earth orbit. NASA and its space station partners have procedures in place to deal with potentially threatening pieces of orbital debris if they fly within a preset safety perimeter around the station and any attached spacecraft. NASA maintains a pizza box-shaped safety zone that measures just over 15 miles (25 kilometres) around the space station and about a half-mile (0.75 km) above and below the outpost.
"It's not uncommon," Cain explained."There's a lot of junk in orbit. We have a very good process for knowing where they are and how to avoid them in cases where we need to avoid them. It's not unusual to have to deal with it."
Meanwhile, the shuttle docked flawlessly to the station this morning, its final rendezvous with the orbiting outpost.
Among the tasks ahead for the mission is a Tuesday spacewalk to remove a failed pump from the station, planned to occur during the passage of the orbital debris, preliminarily estimated to happen at 9:59 pm PST.
"In all likelihood, we will not need to initiate debris avoidance. But if necessary, the manoeuvring rockets on the space shuttle latched to the station would be briefly fired to move the orbiting lab out of harm's way, without disrupting crew activities," Cain said.
"The space station crew was forced to take refuge last month following a close pass by a piece of debris, often paint chips or small parts of discarded rockets and satellites.
"Because objects in orbit travel very fast, where the space station passes around the Earth at about 17,000 miles per hour, even a collision with something as small as a screw represents a safety concern," Cain added.
The 8:07 am PST docking followed a series of small rocket firings that raised the shuttle to the station's 240-mile altitude.
After a back flip that allowed station cameras to inspect heat shield tiles, shuttle commander Chris Ferguson perfectly docked the space shuttle for the last time.
"It's great to be here," Ferguson said at the docking, Atlantis' 12th connection to the orbiting lab.
In all, space shuttles have docked 37 times to the 460-ton station, largely built from trusses and modules hauled to space by Atlantis and its now-retired sister space shuttles.
"It was a powerful moment," said shuttle flight director Kwatsi Alibaruho, of the final shuttle docking.
"I know the significance of that was felt by the astronauts on board. Astronauts embraced their space station counterparts from the US, Russian and Japanese space agencies after opening connecting hatches."
The visit increases the station's population to 10 men and women. The combined crew will remove the Rafaello supply module from the shuttle and dock it to the station, temporarily adding a supply closet stuffed with 8,000 pounds of supplies to the orbiting lab.
Along with experiments and the planned Tuesday spacewalk, 160 hours of the shuttle astronauts' remaining eight days will be devoted to moving supplies onto the station.
