NASA has issued a request for proposal (RFP) for private sector firms to deliver supplies to the International Space Station.
Under the Commercial Resupply Services 2 RFP, NASA intends to award contracts with one or more companies for six or more flights per contract.
As with current resupply flights, these missions will launch from US spaceports and include logistical cargo and research experiments. The contracts will run until 2020 with the option to buy additional launches to 2024, following the Obama administration’s decision to extend the life of the space station until at least 2024.
William Gerstenmaier, associate administrator for human exploration and operations at NASA, said: "The International Space Station is vital to the United States' exploration efforts, a laboratory in orbit where we can work off the earth, for the earth.
"To push beyond low-earth orbit and on to Mars, we rely on American industry to keep the station supplied through cargo deliveries."
Proposals are due by 14 November and NASA anticipates making a selection in May 2015.
Under the previous two cargo resupply contracts, let after the space shuttle programme ended in 2011, NASA ordered eight flights from Orbital Sciences Corporation valued at $1.9 billion (£1.2 billion) and 12 from SpaceX at a cost of $1.6 billion (£986 million). Five of these flights have taken place with a sixth under way.
NASA announced on 16 September that US astronauts will travel to and from the space station on flights provided under commercial contracts with Boeing and SpaceX, “with the goal of ending the nation’s sole reliance on Russia in 2017”.