NASA is considering launching two Space Shuttle missions to the International Space Station (ISS) in 2005 and 2008 for two-week stays using the Extended Duration Orbiter fuel-cell system.

The missions would be used to conduct intensive science experiment programmes to catch up on a backlog and for maintenance. The current three-person crew aboard the ISS can only devote 20h a week to scientific research.

Meanwhile, Boeing and Lockheed Martin are investigating the potential of man-rating the Delta IV and Atlas V boosters to carry a crew vehicle to deliver six people to the ISS and return them to Earth routinely, or as a crew lifeboat. Such missions are unlikely to take place until at least 2010, however, says NASA.

To have a six-person crew operating on the ISS sooner than this to increase scientific productivity would require ordering two Russian Soyuz TMA spacecraft to act as lifeboats, rather than the present one.

The concerns about the station's science capabilities come as the ISS marked its second year of permanent occupation earlier this month.

Experiments have attained more than 90,000h of operating time and the station has grown in weight by 90,800kg (200,000lb).

Source: Flight International