Nasa will study whether the space shuttle can operate beyond its planned retirement in 2010, reports say.

The agency will look at what might be required to delay the retirement of its fleet until the shuttle's replacement - Ares-Orion - begins flying in 2015.

The exercise is aimed at answering questions it expects on the matter from Congress and the incoming president.

News of the study comes from a leaked internal email obtained by a Florida-based newspaper.

Nasa chief Michael Griffin, who is reported to have ordered the study, had previously opposed extending the shuttle programme.

The agency's administrator argued that the money and effort required to do so would stymie progress on the Ares rockets and the Apollo-style Orion capsules that will succeed the shuttle.

These are being developed by Nasa as part of its "Constellation" programme. The system is expected to carry astronauts to the Moon under the Vision for Space Exploration plans announced by President George W Bush in 2004.

Russian flights

In April, Dr Griffin told a Senate sub-committee: "The shuttle is an inherently risky design. We currently assess the per-mission risk as about one in 75 of having a fatal accident.

"If one were to do, as some have suggested, fly the shuttle for an additional five years - say two missions a year - the risk would be about one in 12 that we would lose another crew."

But an e-mail obtained by the Orlando Sentinel suggests Nasa will now research this option.

In it, John Coggeshall, manifest and schedules manager at Nasa's Johnson Space Center in Houston, writes: "The [shuttle] programme in conjunction with [Constellation] and [space station] have been asked by the administrator to put together some manifest options to assess extending shuttle flights to 2015.

He added: "We want to focus on helping bridge the gap of US vehicles travelling to the [space station] as efficiently as possible."

But Nasa spokesman John Yembrick described the e-mail as "premature".

"The parameters of the study have not yet been defined," he said.

The agency remains committed to retiring the shuttle in 2010.

Five-year gap

In the five-year gap between the retirement of the shuttle and the first flights of the Orion capsule, Nasa will be reliant on Russia's Soyuz system for transporting astronauts to the International Space Station (ISS).

But some are now concerned about the wisdom of this plan to purchase seats aboard the Soyuz, given the diplomatic tension between the US and Russia over the conflict in Georgia.

Nasa's Orion ship will not be ready until 2015

Last week, Republican presidential candidate John McCain, and other senators, sent a letter urging President Bush to "direct Nasa to take no action for at least one year from now that would preclude the extended use of the space shuttle beyond 2010".

This letter said Russia's conduct during the Georgia conflict "raised concerns about the reliability of Russia as a partner for the International Space Station".

It added: "Our concern is that we do not have a guarantee that such co-operative and mutually beneficial activity will continue to be available, and the successful utilisation of the ISS may thus be jeopardized."

The Democrats' presidential candidate Barack Obama has also talked about the possibility of additional shuttle flights to close the five-year gap.

Nasa currently has an agreement with Russia to fly astronauts to the ISS aboard the Soyuz spacecraft until 2011. After that, the agency would have to seek approval from Congress for an extension.

Nasa has previously said it would cost between $2.5bn and $4bn per year to keep the shuttles flying past 2010.

The agency has also given seed money to a commercial venture to develop a spacecraft for transporting crew and cargo to the space station.