The second U.S. commercial cargo carrier launched this morning on its inaugural voyage to the International Space Station, clearing its new launch pad here after mission managers overcame a potential range safety issue.

Liftoff of the Orbital Science Corp. Antares launch vehicle, with its pressurized Cygnus cargo capsule on top, came at 10:58 a.m. EDT to start a four-day demonstration mission to the ISS. Orbital is working to join SpaceX as a NASA commercial cargo carrier.

Low clouds lay offshore of the seaside launch pad where the first Antares test flight lifted off in April, but a larger concern was a pressure inversion that posed a threat to four nearby houses. Had the rocket exploded early in ascent, there would have been a danger of the blast overpressure breaking windows. Four houses were evacuated to clear the range.

At ignition the rocket’s two AJ26 kerosene-fueled engines – surplus Soviet-era power plants upgraded by Aerojet Rocketdyne – generated 720,000 lb. thrust and pushed the 13-story-tall rocket off the pad. At 3:53 into the flight the first stage shut down as planned, and separated 5 sec. later. The fairing dropped away at 5:20 mission elapsed time (MET) to expose the Italian-built Cygnus capsule and its service module, supplied by Orbital Sciences. The interstate followed 5 sec. later, and the solid-fuel ATK Castor 30 upper stage ignited at 5:29 MET for a 153-sec. burn.

Spacecraft separation came at 10:02 MET, leaving the Cygnus in an initial orbit of 151-by-186 mi., inclined 51.6 deg. to catch the space station. The vehicle will gradually maneuver toward the station over the next four days, demonstrating that it can be controlled safely before moving close enough to be grappled by station crew with the robotic arm and berthed to a common berthing mechanism.

Successful completion of the mission will clear Orbital to begin fulfilling its eight-flight, $1.9 billion commercial resupply service contract with NASA. The first of those flights is planned before the end of the year.