Orbital Sciences, one of two private US firms chosen by NASA to shuttle cargo to the International Space Station, is preparing for the first launch of its Antares rocket Wednesday.

The launch over the Atlantic ocean is scheduled for 5 pm (2100 GMT) from the Wallops flight facility from an island off the coast of Virginia about 270 kilometers from Washington.

The two-stage launch vehicle, 131 feet (40 meters) tall and 12.8 feet (3.9 meters) in diameter, aims to hit an orbit of 155 miles (250 kilometers) some ten minutes after take-off.

Weather forecasts give a 45 percent change of favorable conditions.

As this is a test mission, Antares will not be transporting the company's Cygnus capsule but will instead carry a simulation of an equivalent payload.

If the launch is a success, Virginia-based Orbital Sciences plans a demonstration run to the space station with the Cygnus capsule in three months, followed by its first delivery mission before the end of the year.

"We see this as a key milestone in proving what can be done in an industry-government partnership," Mike Pinkston, the program manager for Antares at Orbital Sciences, said in a conference call Tuesday.

The US space agency, in offering its expertise and equipment to partners in the private sector, aims to achieve "safe, reliable and cost-effective access to space and to low orbit and to the ISS in particular," said Alan Lindenmoyer, manager of NASA's commercial crew and cargo program.

"We also recognize the need to create a market environment that will sustain these capabilities, and the ISS provides a perfect market for this new capability with reliable and predictable needs."

The $1.9 billion contract requires Orbital Sciences to deliver freight to the ISS over the course of eight flights by the beginning of 2016. Cygnus has a capacity of two tons.

Unlike the Dragon capsule developed by rival SpaceX, Cygnus cannot return to Earth and will be destroyed upon re-entry after its mission is complete.

SpaceX made history when Dragon became the first commercial spacecraft in history to successfully attach to the International Space Station in a May 2012 test flight.

Previously only four governments -- the United States, Russia, Japan and the European Space Agency -- had achieved this challenging technical feat.

Dragon has already successfully completed three missions to the space station and must complete another 10 to fulfill California-based SpaceX's $1.6 billion contract.

NASA retired its fleet of space shuttles in July 2011 and has been relying on Russia's Soyouz spacecraft to transport its astronauts to the ISS at a cost of $63 million a seat.

US cargo was being transported to the ISS by European, Japanese and Russian shuttles which, like Cygnus, cannot return to Earth and are destroyed upon re-entry.

SpaceX, Boeing and Sierra Nevada have also been retained by NASA to develop ships to transport astronauts to the ISS and other destinations.