NASA has awarded contracts to three companies to deliver cargo to the orbiting International Space Station (ISS), the US space agency said Thursday.
The resupply contracts run from 2019-24, as NASA continues a policy of cultivating a private space launch industry.
The contract winners are Orbital ATK of Dulles, Virginia; Sierra Nevada Corporation of Sparks, Nevada; and SpaceX of Hawthorne, California, which is owned by investor Elon Musk, whose ventures include electric car company Tesla Motors.
Under NASA's first round of ISS resupply contracts, Orbital and SpaceX have already lifted nearly 16,000 kilograms of cargo in recent years to the orbiting space station.
The ISS, which has been occupied since November 2000, was assembled by the United States and Russia and is operated jointly by those two countries along with Japan, Canada and the European Space Agency.
Sierra Nevada will be carrying out its first cargo mission. Orbital and SpaceX each lost an unmanned cargo spaceship since 2014 on ISS launches.
"Selecting multiple providers assures access to ISS," NASA said in a statement.
"Awarding multiple contracts provides more options and reduces risk through a variety of launch options and mission types, providing the ISS program a robust portfolio of cargo services that will be necessary to maximize the utility of the station."
Each company is guaranteed at least six cargo launches, NASA said. Under the contracts, the private resupply missions will also dispose of unneeded ISS cargo and return other cargo including research samples to NASA.
NASA chief Charles Bolden said the agency is "firmly on track to return launches of American astronauts to the ISS from American soil on American commercial carriers."
The first 2008 resupply contract was for 3.6 billion dollars. NASA gave no value for the new contract, saying it would depend on the details of the missions requested.