WASHINGTON — NASA released Sept. 25 a request for proposals (RFP) for a second round of contracts to transport cargo to and from the international space station, with both current providers and new entrants expected to compete.

NASA plans to award one or more Commercial Resupply Services (CRS)2 contracts as a successor to its existing CRS contracts with Dulles, Virginia-based Orbital Sciences Corp. and Hawthorne, California-based Space Exploration Technologies Corp.

Like the existing CRS contracts, CRS2 awards will cover the transport of pressurized and unpressurized cargo to the ISS, and the disposal or return to Earth of cargo from the station.

In a Sept. 26 press release, NASA said the CRS2 contracts will cover ISS cargo transportation through 2020, with options through 2024. The solicitation requests companies provide pricing information for between one and five missions per year for 2018 through 2024. Each CRS2 contract will cover a minimum of six missions, according to the RFP.

Orbital Sciences and SpaceX are currently carrying out their existing CRS contracts. SpaceX launched the fourth of its twelve CRS missions Sept. 21, while Orbital is scheduled to launch the third of its eight CRS missions no earlier than Oct. 20. On NASA’s current ISS manifest, both companies are scheduled to fly their final CRS missions by late 2016, although NASA has options for additional missions to 2018.

While Orbital and SpaceX are expected to submit CRS2 proposals, they will likely face competition from other companies. Sierra Nevada Corp. said Sept. 24 that they planned to submit a CRS2 proposal involving their Dream Chaser vehicle after failing to win a commercial crew contract. Boeing, who won a commercial crew contract along with SpaceX Sept. 16, has previously expressed interest in bidding on commercial cargo contracts with its CST-100 spacecraft.

Responses to the RFP are due to NASA by Nov. 14, and the agency expects to award contracts in May 2015.

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...