NASA has cleared a second commercial company to launch a cargo ship to the ISS, with blast-off slated for this week.
If successful, Orbital Sciences, would join privately owned Space Exploration Technologies, also known as SpaceX, in flying US supplies to the space station, a partnership of 15 nations orbiting about 250 miles above Earth.
Orbital Sciences' two-stage Antares rocket, which made a successful debut flight in April, is scheduled to lift off at 10.50am EDT (2.50pm GMT) on Wednesday from the Virginia-owned Mid-Atlantic Regional Spaceport, which operates under a lease agreement with NASA's Wallops Flight Facility.
The 40.5m tall rocket will be carrying the company's first Cygnus cargo capsule. Like SpaceX's Dragon capsules, which so far have made three flights to the space station, Cygnus is intended to restore a US supply line to the station following the retirement of NASA's space shuttles in 2011.
"We have them lined up to use them fairly regularly," NASA's space station program manager Mike Suffredini told reporters during a prelaunch press conference. "This is what we said was going to be the fleet to take care of the US segment (of the space station) and we need to have it."
Unlike traditional government contracts, NASA provided $684m (£430m) in seed funds as well as technical support to SpaceX and Orbital Sciences to develop their rockets, capsules and launch facilities. The firms also hold a combined $3.5bn in contracts to fly cargo to the station for NASA.
SpaceX, which was awarded its development contract in 2006, is preparing to debut an upgraded version of its Falcon 9 rocket later this month and NASA wants SpaceX to have two or three missions under its belt with the new rocket before resuming supply runs to the station, Suffredini said.
Orbital Sciences, which began its partnership with NASA 18 months later, stands to collect a final $2.5m development payment from NASA upon completion of its demonstration flight to the station.
If the launch occurs as planned on Wednesday, astronauts aboard the station on Sunday plan to use a robotic crane to pluck the Cygnus capsule from orbit and attach it to a docking port.
Unlike Dragon capsules, Cygnus spacecraft are designed to burn up in the atmosphere after they are loaded with trash and depart the station.
For its orbital debut, Cygnus will be carrying a half-load of about 700kg of food and other cargo considered "non-essential" by NASA in case the rocket or spacecraft encounters problems and cannot reach the station.
"For a demo flight, we don't typically fill them up," Suffredini said.
Cygnus is expected to remain docked at the station for about a month. Should the mission be successful, Orbital Sciences plans to return to that station in December for the first flight under a $1.9bn cargo resupply contract with NASA.
For now, NASA is the only customer for Cygnus, but Orbital Sciences expects new business as the US and other countries launch exploration initiatives beyond the space station's orbit.
"We think Cygnus has the capability to do a lot more than just deliver cargo to the station," said Frank Culbertson, a former astronaut who now serves as Orbital Science's executive vice president.