WASHINGTON - Orbital Sciences Corp., one of two private companies that currently hold a contract with US space agency NASA to fly unmanned cargo missions to the International Space Station, is set for the first test flight of its Antares rocket on Wednesday.

The launch is scheduled for 5 pm EDT (2100GMT) with a launch window that runs until 8 pm EDT (2400GMT), from NASA's Wallops Flight Facility on Wallops Island, Virginia.

The Antares test flight, which will carry a simulated payload to a target orbit, is the first of two missions Virginia-based Orbital is scheduled to conduct in 2013.

If successful, the company will carry out a full flight demonstration of its new Antares and Cygnus cargo delivery system to the space station around mid-year.

Orbital signed a 1.9-billion-US-dollar deal with NASA to fly eight unmanned supply missions using the Antares and Cygnus delivery system. The company said it plans to launch the first of eight operational cargo resupply missions before the end of the year.

Another US company, California-based SpaceX made history when its Dragon spacecraft became the first commercial vehicle in history to successfully attach to the space station in a May 2012 test flight.

Dragon has already made three trips to the space station and it still need to complete another ten to fulfill its 1.6-billion- dollar contract with NASA. The next SpaceX cargo run is scheduled for September.

Before Dragon's liftoff, flights to the space station have always been a government-only affair. Until their retirement, US space shuttles carried most of the gear and many of the astronauts to the orbiting outpost. Since then, American astronauts have had to rely on Russian capsules for rides.

NASA is looking to the private sector, in this post-shuttle era, to get American astronauts launching again from US soil. It will be at least four to five years before the country's private operator is capable of flying astronauts.