Updated Feb. 28 at 3:49 p.m. EDT

WASHINGTON — NASA is preparing to take bids on a second round of cargo deliveries to the international space station in a follow-up to the Commercial Resupply Services (CRS) contracts Orbital Sciences Corp. and Space Exploration Technologies Corp. are now flying for the agency.

NASA announced the plan in a request for information released Feb. 21. Responses from industry are due March 21. The document, which NASA posted online, did not say when the agency would solicit bids, or when it would make an award for the CRS 2 contract.

The expected budget for CRS 2 is between $1 billion and $1.4 billion a year from 2017 to 2024, NASA said. NASA envisions four to five flights a year under CRS 2. The White House announced in January it wants to extend space station operations through 2024. Congress has currently committed to fund the space station through 2020.

The CRS 2 contract calls for delivery of 14,250 to 16,750 kilograms per year of pressurized cargo, and delivery of 1,500 to 4,000 kilograms per year of unpressurized cargo.

The contract also requires cargo return and disposal services for 14,250 to 16,750 kilograms a year of pressurized cargo. The agency did not specify what portion of this cargo must be disposed of, and which must be returned to Earth. CRS 2 would also require disposal of 1,500 to 4,000 kilograms per year of unpressurized cargo, according to the request for information.

In 2008, Orbital Sciences received a $1.9 billion cargo delivery contract that called for eight flights. SpaceX got a $1.6 billion contract for 12 flights. Currently, only SpaceX performs cargo-return service, and only Orbital offers trash disposal. Both companies’ contracts run through 2016. Orbital completed its first paid cargo run on Feb. 19. SpaceX is preparing to launch its third paid cargo run March 16.

For follow-on cargo services, “NASA may elect to have one contract or multiple contracts to meet the requirements,” according to the request for information.

The document released Feb. 21 offers a glimpse of what it will take to turn NASA’s plans for ramped-up ISS research beyond 2020 into reality.

According to the request for information, cargo haulers must be able to transport 24 to 30 powered lockers a year. Each of these units, which could contain freezers for science experiments or animal habitats for lab rats, requires continuous power of 55 to 120 watts at 28 volts, as well as cooling and two-way communications services, NASA said.

Sam Scimemi, director of the international space station office at NASA headquarters here, told the NASA Advisory Council’s research subcommittee Feb. 24 that SpaceX is in the middle of getting certified to transport rodents to ISS. The company is set to blast live animals to station on its sixth cargo run in early December. Some of the habitat technology will be taken for a test flight on SpaceX’s fourth run, scheduled for early June.

Although SpaceX is taking steps to accommodate the type of payloads called for under CRS 2, spokeswoman Emily Shanklin declined comment about CRS 2 in a Feb. 26 email, citing company policy not to discuss “pending bids [or] requests for information.”

SpaceX could soon be competing for two ISS contracts. The company has its sights set sending astronauts to the space station under NASA’s Commercial Crew Transportation Capability contract, an award for which is expected in August or September. NASA wants at least one of the three companies that bid to begin routine astronaut flights by late 2017 or early 2018.

Meanwhile, Orbital spokesman Barron Beneski did not reply to a request for comment about CRS 2. However, Dulles, Va.-based Orbital has said it intends to compete for this work.

With NASA now moving forward with CRS 2, Orbital faces mounting pressure to find a long-term source of engines for its Antares rocket. The AJ-26 engines it uses now, which are refurbished Soviet era engines called NK-33 by their Russian manufacturer, suffer from corrosion. Aerojet, which refurbishes the engines for sale in the United States, says it can restore the engines, but that has not stopped Orbital from pursuing a replacement.

In a Feb. 13 conference call, Orbital Chief Executive David Thompson declined to say whether the company planned to stick with the AJ-26, or whether a lawsuit against United Launch Alliance, for the right to purchase the Russian-made RD-180 engine used on that company’s Atlas 5 rocket, might bear fruit.

Thompson did tell investors that the engine problem could be settled this year. That might leave Orbital time to craft a CRS 2 proposal that does not leave  unanswered questions about the long-term viability of Antares’ propulsion system.

Dan Leone is a SpaceNews staff writer, covering NASA, NOAA and a growing number of entrepreneurial space companies. He earned a bachelor’s degree in public communications from the American University in Washington.