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Last Updated: Monday, 17 July 2006, 13:51 GMT 14:51 UK
Europe set for bigger station role
By Irene Klotz
Johnson Space Center, Houston, Texas

The ISS (Nasa)
The ISS: Construction work will now resume in earnest

As the US space agency (Nasa) prepares to resume assembly of the International Space Station (ISS) following the success of the shuttle Discovery mission, Europe is about to take on an enhanced role in maintaining and supplying the orbiting outpost.

Even though the Discovery mission suggested that Nasa had finally licked the fuel tank problems which triggered the 2003 Columbia accident, the shuttles' days are numbered.

After 16 scheduled missions to complete construction of the $100bn ISS, Nasa plans a 2010 retirement date for the space shuttle fleet.

Russian Soyuz and Progress ships will continue flying to the station, as they have been since the programme began. The capsules and cargo vessels were the only vehicles to fly to and from the station during the years Nasa grounded its shuttle fleet for post-Columbia safety upgrades.

But next year, if all goes as planned, there will be a new vehicle on the orbital scene.

The European Space Agency's (Esa) Automated Transfer Vehicle (ATV) is scheduled for a debut flight to the station.

Restraining hole

In all, Esa has promised five ATV cargo ships to the space station project through to 2015.

With the shuttles' retirement, that may not be enough. Nasa already is looking into transferring cargo originally earmarked for the shuttle to other carriers, including the ATV, which was designed to dock at the station's Russian ports.

Japan also is building a space station cargo carrier called the H-2 Transfer Vehicle (HTV).

Artist's concept of the ATV (Esa)
The ATV is set to become a key feature of the ISS project
"We're figuring out how to integrate the ATV and the HTV into our plans," said Joy Bryant, the space station programme manager for Boeing, Nasa's prime contractor on the project.

The ATV can haul about 7,000kg (15,000 pounds) of cargo to the ISS, but payloads are largely limited to items that go inside the station.

They must be small enough to pass through its hatch, which is a bit bigger than shoulder-width. Some items earmarked for addition to the outside of the station could fly on the ATV, but they would have to be taken out through an airlock during spacewalks, adding time and complexity during what already is among the busiest times of station operations.

Nasa plans to use ATVs, HTVs and possibly even commercial vessels well before the shuttles' retirement.

And Columbus, too

Nasa consolidated its shuttle manifest to reduce the number of station assembly flights as much as possible, with the intention of finishing construction by the time the shuttles retire.

There are no other vehicles that can carry to orbit and assemble the station's trusses, power modules and laboratories.

The ATV is limited by its internal space and the size of its hatch

As a result, there is little room to spare on most flights for carrying spare parts and extra cargo to the outpost, a need Nasa hopes to fill by buying rides on other spaceships.

The ATV's biggest contribution is likely to be fuel. The shuttle cannot carry propellants used by the station to maintain itself in orbit, so it is solely reliant on Russian Progress ships for resupply.

Once ATV is operational, the partners will have a choice of using a Progress or an ATV for deliveries.

Though Nasa plans an extensive and extended checkout before allowing an ATV to dock, cosmonaut Mikhail Tyurin, who is scheduled to begin a six-month station mission in September, expects no problems with the vehicle.

"I think people make too much [of it]. The [system] is much safer than what people would expect. The motion control system is made of proven technology. I have no concerns," Tyurin said in an interview.

Europe has another station debut planned next year as well. The Columbus laboratory module is scheduled for launch aboard the shuttle shortly after the ATV's first mission.

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