'Johannes Kepler' space freighter ready for launch

Europe's unmanned space freighter is on the launch pad in French Guiana, ready to begin its voyage into orbit.

At more than 20 tonnes, the truck will be the heaviest payload an Ariane 5 rocket has ever lifted into the sky.

The robotic ship, dubbed Johannes Kepler, will be delivering food, air, fuel, experiments and other equipment to the International Space Station.

Another key task for the freighter in the coming months will be to push the station to a higher altitude.

The ISS has a tendency to lose height over time as it brushes through the residual atmosphere still present 350km above the Earth.

Johannes Kepler will dock with the rear of the platform and use its propulsive might to boost the station's orbit.

Lift-off for the freighter and its carrier Ariane from the Kourou spaceport is timed for 1913 local time (2213 GMT) on Tuesday.

The launch is doubly significant because it marks also the 200th mission of an Ariane rocket since the vehicle series was introduced in 1979.

"Kepler's mass at lift off will be 20,062kg. This is a record for the launcher and for the European Space Agency (Esa)," said Nico Dettmann, who runs the freighter programme at Esa.

"The integration and launch campaign have been very smooth, but still we have to be thorough and careful," he told BBC News.

An artist's impression of ATV2 Johannes Kepler flying over France

Kepler is due to make a docking with the orbiting platform next Wednesday.

The spacecraft - also known by the generic name of Automated Transfer Vehicle (ATV) - is the second such craft to be sent to the ISS by Europe.

The first, "Jules Verne", had a near-flawless flight to the station in 2008.

Esa trucks are part of the barter arrangement the 18-nation alliance has with its international partners on the ISS project.

Instead of handing over cash to cover station running costs, Europe has taken on the major responsibility of resupplying the platform.

In return it gets residency rights for its astronauts, and current flier, Italian Paolo Nespoli, will be on hand at the ISS to unload Kepler when it docks.

Much the cargo was loaded into Kepler months ago but workers at Kourou used a special elevator in recent days to add some final items.

"We've loaded 430kg of late cargo," said Olivier de la Bourdonnaye, the ATV project manager at the truck's manufacturer, Astrium.

"This is something new we can offer to the partners so that they can decide - not at the last minute, but - in the last weeks before launch what they want to put in the spacecraft for the mission. This includes food which must be eaten soon by the astronauts and not kept stored for a long time."

Three further freighters have already been ordered to help fulfil Esa's commitments through to about 2016. But with the station set to stay in orbit until at least 2020, Europe will need to decide soon how to meet its "subscription" during the extension.

This could involve the procurement from European industry of additional ATVs, either in their current configuration or perhaps in a form modified to fulfil tasks additional to simply hauling cargo.

The truck is the biggest, most sophisticated vehicle Europe has ever flown in space.

Its automatic rendezvous and docking technology allows it to find its own way to the station and attach without any human intervention.

Esa believes the vehicle's capabilities will feed into many other exploration activities, at the Moon, Mars and other Solar System destinations.

It currently has no return capability, however, and is simply ditched in Earth's atmosphere to burn up at the end of a mission.

Feasibility studies have looked at how a return capsule could be added, even one capable of carrying astronauts.

Member states may be reluctant to commit at this stage to such an expensive development.

"Budget constraints are tremendous as you know, but more fundamentally I think that first Europe has probably to build a consistent strategy for exploration," observed Francois Auque, Astrium's chief executive officer.

"And today they have not reached this stage; some conferences have occurred, but the vision about exploration for Europe I think is not mature enough to strongly trigger tracks of development," he told BBC News.

Commentators expect this vision to become clearer in the coming months.

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