Orbital Sciences Corp. of Dulles plans to launch its Antares rocket Wednesday from Wallops Island, Va., and it could be visible in cities from Washington to New York.
But NASA said Tuesday there's only a 45 percent chance weather conditions would allow a launch Wednesday. Orbital would try again Thursday if need be.
If successful in sending a practice payload into orbit, Orbital aims to send its cargo ship to dock with the International Space Station this summer on a test run.
Orbital and California-based SpaceX have contracted to make space station supply runs with NASA, which ended its shuttle program in 2011.