BBC News - SpaceX Falcon rocket set for second launch attempt

By Jonathan AmosScience correspondent, BBC News

SpaceX will try again on Tuesday to launch its re-supply mission to the space station.

The Dragon capsule on top of the Falcon rocket is carrying half a tonne of supplies

The first attempt on Saturday was thwarted by a faulty valve in one of the engines on the California company's Falcon 9 rocket.

The lift-off at the Cape Canaveral Air Force Station in Florida was aborted just half a second before the vehicle was due to leave the pad.

Tuesday's attempt has been scheduled for 03:44 EDT (07:44 GMT; 08:44 BST).

SpaceX is endeavouring to become the first commercial concern to carry cargo to the International Space Station (ISS).

Its Falcon rocket will loft an unmanned capsule called Dragon, which has been loaded with half a tonne of food and spares for the platform's astronauts.

Once in orbit, the Dragon will chase down the station and berth with it. After a week, the ship will return to Earth with equipment that needs to be repaired.

To date, only government-owned vehicles have been used to run this freight service.

But Nasa wants in future to contract out the role to US companies. In a few years, the private sector could even take over the task of ferrying astronauts to and from the station.

The agency hopes these changes will save it money that can then be invested in exploration missions far beyond Earth, at destinations such as asteroids and Mars.

Tough task

SpaceX acted quickly on Saturday to correct the problem in the Falcon's propulsion system.

During the final moments of the weekend's countdown, a flight computer detected slightly high pressures inside the thrust chamber of the vehicle's number five engine - one of nine that power the first stage of the ascent - and commanded a shut-down.

Technicians traced the issue to a check valve that has since been exchanged.

Much is riding on the mission. Many in the US Congress remain to be convinced that the path set for Nasa is the right one.

These politicians doubt the private sector is ready to assume important roles in America's human spaceflight programme; and if SpaceX falls short of its objectives in the coming days, these critics are likely to argue that their position has been vindicated.

But both SpaceX and Nasa have tried to lower expectations ahead of the mission, repeating often that it represents a test flight.

The company has also been stressing the difficulty of what it is trying to achieve.

"Sending a spacecraft to the space station has only ever been accomplished by four entities - the United States, Russia, Japan and the European Union," it said on the eve of Tuesday's launch attempt.