By Dr David Whitehouse BBC News Online science editor

Now that they have studied the foam impact theory, US space agency (Nasa) investigators do not think that it was the cause of the Columbia accident.

A new theory is emerging which Nasa says now stands alongside the foam theory.

Did we take some hit? That's a possibility. Something was breached Milt Heflin Nasa shuttle flight director

Could the fatal damage to Columbia's heat-resistant tiles have been caused by an impact by space debris while in orbit?

"Did we take some hit?" asked Nasa's shuttle flight director Milt Heflin. "That's a possibility. Something was breached."

Nasa engineers say they are considering the possibility that a small piece of space debris could have struck the shuttle, damaging or loosening tiles just enough to start a chain reaction once the craft re-entered the Earth's atmosphere.

During orbit, Nasa advises the shuttle how to avoid man-made debris of which, by some estimates, there are more than a million pieces circling the Earth.

All but 9,000 of the pieces are smaller than a tennis ball.

But Mr Heflin said his engineers had no hard evidence Columbia was struck by a piece or a micro-meteoroid.

Theory's problems

But there are problems with the space debris theory.

Before each mission, the US Air Force (USAF) and Nasa perform an analysis of the shuttle's orbit to ensure that it will not encounter any known debris.

SHUTTLE BREAK-UP

Re-entered atmosphere at 12,500 mph (20,000 km/h) Disintegrated 40 miles (65 kilometres) above the Earth Debris scattered over Texas and Louisiana - reports now being checked of sightings in California and Arizona

Click here for a step-by-step guide to the disaster

USAF radar and optical monitoring has the ability to pinpoint the location of space debris that is just centimetres in size.

There are also zones that are relatively clear of debris, including the one that is used by the shuttle.

Also, if something struck the shuttle, its crew and computers would have known.

In the past, Nasa has had to adjust the flight path of space shuttles on at least eight occasions to avoid debris.

Striking micro-debris should have had little effect.

One study showed that a craft that had been in space for more than five years was struck by these particles more than 30,000 times, with no ill effect.

Collision risk

In 1997, the US National Research Council said that Nasa should carry out a thorough risk assessment to determine the likelihood that the space shuttle could be severely damaged by meteoroids and orbital debris.

It said that, although Nasa was taking steps to protect the shuttle from orbital debris such as spent rocket bodies, satellite fragments and paint chips - there was a real risk that a collision could cripple the shuttle or threaten the safety of the crew.

It went on to say that space debris could pose risks "during launch or re-entry to the Earth's atmosphere" and that there was a risk that "the crew will be harmed or that the spacecraft will incur major damage. Nasa should consider reducing its allowable level of risk from orbital debris."

The report said that Nasa's space shuttles, designed in the 1970s, were not built to withstand bombardment by orbital debris because such objects were not then recognised as a substantial threat.

It remains to be seen if space debris was the cause of the accident, as Columbia's crew compartment has not been recovered in any identifiable form.