NASA said late Tuesday that it has ruled out the possibility of either a spacewalk repair session or the Atlantis crew seeking refuge at the international space station after an unknown object was detected outside the orbiter.

The discovery postponed a landing originally set for 6 a.m. ET Wednesday. Atlantis has been tentatively scheduled to land at 6:22 a.m. Thursday, but shuttle program manager Wayne Hale said it could be delayed until Friday.

A TV camera aboard the space shuttle Atlantis spotted this object, seen floating in orbit with the shuttle. The camera is pointed toward the Earth. The object can be seen as the black speck inside the white circle.
(NASA TV/CBC)

A TV camera in the shuttle's cargo bay spotted an object in orbit near the shuttle and moving with it.

Hale said the object, which could be anything from a piece of ice to a piece of the shuttle's heat shield, may have come loose during the test firing of the craft's manoeuvring jets early Tuesday morning.

Sensors on the right wing of the shuttle registered an impact around the same time that the object was spotted, but NASA officials said the sensor was likely triggered by the vibrations caused during the test firing.

Hale said the TV camera that spotted the object lacks the resolution to identify it. Astronauts aboard Atlantis took more photos of the object, but weren't able to identify it either.

"There is absolutely no scale in space to tell, so we're left with a bit of a mystery," he said.

Commander Brent Jett said the object was "fairly small" and was moving away from the shuttle at about half a metre per second.

Officials will now use Wednesday to try and identify the object and whether it caused significant damage to the craft. The shuttle has enough supplies to stay in space through Saturday.

It was initially believed the shuttle would be delayed because of a poor weather forecast Wednesday for the Kennedy Space Center in Florida.

This shuttle mission has had its fair share of delays. Atlantis was set to launch on Aug. 27, but was delayed when lightning struck the space centre two days earlier. The launch was rescheduled, but had to be postponed again because of the approach of tropical storm Ernesto.

On Aug. 29, NASA decided to move Atlantis off the launch pad and put it back into its barn for the duration of the storm.

During the shuttle's trip back to the vehicle assembly building, though, the forecast for Ernesto improved and NASA engineers gave the order to return the shuttle to the launch pad.

The shuttle launch was delayed again on Sept. 6 when a problem with the coolant system on one of the shuttle's fuel cells was found.

First addition since 2003

Finally on Sept. 9, Atlantis was able to lift off.

The Altantis mission included installing the first addition to the orbiting station since Columbia disintegrated upon re-entry in 2003.

In three spacewalks after the shuttle docked on Sept. 11, the astronauts unpacked and installed a truss containing a pair of solar wings more than 70 metres long. Scientists hope the solar panels will ultimately generate a quarter of the space station's power.

Astronaut Steve MacLean is on board the mission and became just the second Canadian to walk in space.

Also in orbit is a Russian Soyuz capsule carrying the world's first female space tourist.

Fourth amateur astronaut

Iranian-American Anousheh Ansari, 40, who lives in Texas, is reported to be paying an estimated $20 million US to become the fourth amateur astronaut to visit the orbiting station.

The Soyuz capsule is scheduled to dock with the ISS early Wednesday.

Ansari and her husband run a venture capital and home-networking technology firm. She is set to return to Earth on Sept. 28, along with the station's two current inhabitants, Jeffrey Williams of the United States and Pavel Vinogradov of Russia.

Ansari's two travel companions, Mikhail Tyurin and Michael Lopez-Alegria, will remain on the station to help German astronaut Thomas Reiter, who arrived in July, with a series of construction projects.

With files from Associated Press