Cygnus would re-supply the space station

The Cygnus spacecraft would take food, water, air, fuel and equipment to the orbiting platform. A demonstration flight is expected in late 2010.

Nasa needs logistics support for the ISS while it produces a successor to the space shuttle.

It plans to hand routine low-Earth operations to the private sector.

This would allow the agency to concentrate its efforts on getting humans back to the Moon.

Nasa's new lunar ship, known as Orion, is not expected to begin service until 2015.

With the shuttle due to retire in two years' time, this leaves a sizeable gap in which the US would be reliant on Russian, European and Japanese cargo missions unless it can find a domestic commercial carrier.

"Nasa plans to get out of low-Earth orbit and focus on going back to the Moon to prepare explorers for a future voyage to Mars," said Rick Gilbrech, from the agency's Exploration Systems Mission Directorate.

"Being able to buy safe, reliable and economical service to low-Earth orbit will help us achieve our national goals."

Payload potential

The latest contract is part of the agency's Commercial Orbital Transportation Services (COTS) project.

Nasa already has a similar agreement with SpaceX, the company headed by Elon Musk, who founded the internet banking system PayPal.

Orbital is based in Dulles, Virginia, and is perhaps best known as a commercial communications satellite producer. It plans to launch Cygnus from the Wallops Island complex off the state coast.

The unmanned ship will be based on the MultiPurpose Logistics Module, an Italian-made vessel used currently by the shuttle to package supplies for the International Space Station (ISS).

Cygnus will launch on the Taurus II rocket and should be capable of delivering just over two tonnes to the platform.

Unlike Europe's new Automated Transfer Vehicle which will execute a docking at the platform, Cygnus will instead place itself next to the platform so that the station's robotic arm can grab and berth it.

Orbital hopes its new Taurus launcher can be used by Nasa, other government agencies and private-sector concerns to loft a range of scientific, defence and commercial payloads - not just Cygnus.

"By serving as an anchor mission for Orbital's Taurus II rocket, the COTS project will not only benefit Nasa's ISS operations with reliable commercial cargo service once the system is fully operational, but will also aid Nasa's Earth and space science and planetary exploration programmes with lower-cost launches of medium-class satellites," said Dr Antonio Elias, from Orbital's Advanced Programs Group.

Europe's Thales Alenia Space, producers of the MPLM, will assist Orbital in the Cygnus project.