Together, the companies have contracts worth $3.5 billion to keep the station stocked with food, clothing, supplies and science experiments.
Wednesday's test flight is intended to demonstrate Orbital Sciences' Antares rocket. The Cygnus cargo capsule will debut on second flight this summer.
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Antares, like SpaceX's Falcon 9 booster, consumes liquid fuel, but there the similarities end. SpaceX designed nearly everything on its rocket in-house. Orbital, which started as a commercial space company in 1982, turned to well-established aerospace contractors to supply the rocket's engines and support systems.
The differences between SpaceX's Dragon and Orbital Sciences' Cygnus capsules are even more pronounced.
Dragon, which SpaceX is upgrading to carry people as well, is designed to return to Earth. Cygnus, like the Russian, European and Japanese cargo ships that also service the station, burns up in the atmosphere after it departs.
But the company is developing plans to keep a Cygnus in orbit for up to a year to host science instruments, sensors and other experiments.