The astronauts on the ISS often remain on board for quite some time, which means that regular deliveries of food, water, fuel and various other materials and equipment are necessary. How are the supplies delivered?

Robert Lainé: There are several different ways that supplies can reach the ISS. Here is a quick rundown:

• The US Space Shuttle, which can carry both equipment and astronauts to the ISS.
• The Russian Soyuz, for carrying astronauts.
• The Progress space vehicle, which uses the same service vehicle as Soyuz, only with a different container for carrying equipment and fuel, but cannot return to the Earth after it completes its mission.
• The Japanese have developed a spacecraft called HTV, which can also carry supplies to the ISS. It has no automatic docking capability however, and cannot carry astronauts.
• Europe has developed the ATV, a much larger vehicle that can automatically dock with the space station.

What types of missions do these spacecraft perform? Does each one play a specific role?

RL: Together, these spacecraft provide the ISS and its crew with all the services they need: water, oxygen, fuel and propellant for the station. Only Progress and the ATV can provide the full range of services. The other vessels do not have the same capacity for refuelling the station. The impressive size of the ATV allows it to carry three times more fuel than Progress – nine metric tonnes of payload for the ATV, versus three tonnes for Progress. This expanded capacity means that the ATV can remain docked with the ISS for longer periods of time than the others – six or seven months.

How is it decided which type of vessel to send, and the number of flights for each one?

RL: In actual fact, it is a bit like a co-ownership arrangement: each participant has to pay their fees to use the station, and each flight could be considered as each country’s contribution towards enabling the astronauts to use the pooled resources.

Some of the vessels were developed quite recently. Is the next generation of these spacecraft already in the pipeline?

RL: The Shuttle programme is scheduled to be phased out, leaving the Soyuz as the only remaining vehicle capable of carrying astronauts to and from the ISS. This is a major change. The United States has now decided to work through private industry to develop new means for carrying crews and cargo to the ISS. Two projects are currently underway: Dragon being developed by the company Space X, and the Cygnus project by the company Orbital Science . Europe’s ATV may continue to evolve as well, as the ARV (Advanced Re-entry Vehicle) project bears witness. The major advancement of the ARV will be its capacity to return to Earth from the station with cargo, once the Shuttle is out of service. Over time, why not see if we can make the ARV a manned spacecraft?