On the heels of awarding groundbreaking contracts to U.S. commercial space companies to ferry American astronauts to the International Space Station, NASA has released a request for proposals (RFP) for the next round of contracts for private-sector companies to deliver experiments and supplies to the orbiting laboratory.

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 would launch from U.S. spaceports, and the contracted services would include logistical and research cargo delivery and return to and from the space station through fiscal year 2020, with the option to purchase additional launches through 2024.

Earlier this year, the Obama Administration decided to extend the life of the International Space Station until at least 2024.

The ability to continue commercial deliveries to the station is critical to continuing the use of the station as a platform for discovery that improves life on Earth, expands the commercial use of low-Earth orbit, and helps advance America's journey to Mars through high-quality scientific research and technology development.

"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," said William Gerstenmaier, associate administrator for Human Exploration and Operations and NASA Headquarters.

"To push beyond low-Earth orbit and on to Mars, we rely on American industry to keep the station supplied through cargo deliveries."

This RFP is open to companies able to demonstrate safe, reliable launch and rendezvous capabilities with the station. The contract will fulfill NASA's need to procure cargo delivery services for pressurized and unpressurized cargo delivery, disposal, return, or any combination, to the space station using U.S. commercial carriers after the initial Commercial Resupply Service contracts conclude.

The goal of the RFP is to foster a full and open competition that provides the most complete set of services, providing the best value to American taxpayers. Proposals are due Nov. 14. The awarded contracts will be firm-fixed price, indefinite-delivery/indefinite quantity. NASA anticipates making a selection in May 2015.

A little more than one year after the Space Shuttle program ended in 2011, NASA returned Space Station cargo resupply missions to the U.S. under two contracts -- one with Orbital Sciences Corp. of Dulles, Virginia, and one with Space Exploration Technologies (SpaceX) of Hawthorne, California.

At the time of award, NASA ordered eight flights valued at about $1.9 billion from Orbital and 12 flights valued at about $1.6 billion from SpaceX through December 2016. SpaceX has completed three of the contracted delivery missions with a fourth currently underway, and Orbital has completed two.

On Sept. 16, NASA announced U.S. astronauts once again will travel to and from the International Space Station from the United States on American spacecraft under groundbreaking commercial contracts. The agency unveiled its selection of Boeing and SpaceX to transport U.S. crews to and from the space station using their CST-100 and Crew Dragon spacecraft, respectively, with a goal of ending the nation's sole reliance on Russia in 2017.