A commercial spacecraft operated by U.S. space company Orbital Sciences Corp. blasted off on Thursday, flying its first cargo resupply mission to the International Space Station, NASA TV showed. The unmanned Cygnus spacecraft was launched atop the Antares rocket, also built by Orbital Sciences, at 1:07 p.m. EDT (1707 GMT) from the U.S. space agency's Wallops Flight Facility in eastern Virginia."Liftoff!" Orbital Sciences said in a tweet. "Love feeling the roar of a launch."The mission began the company's first contracted cargo delivery flight to the station through a 1.9-billion-U.S.-dollar Commercial Resupply Services contract with NASA. Orbital Sciences will fly at least eight cargo missions to the Space Station through 2016."Today's launch demonstrates how our strategic investments in the American commercial spaceflight industry are helping ... keep the United States the world leader in space exploration," NASA Administrator Charles Bolden said in a statement."American astronauts have been living and working continuously in space for the past 13 years on board the International Space Station, and we're once again sending them supplies launched from U.S. soil," Bolden said.The launch has been delayed for three times, first due to problems with a cooling system on the Space Station last December, then because of cold weather on Tuesday.So it was rescheduled for Wednesday but cancelled again because of what NASA called an " unusually high level of space radiation" caused by a large solar flare.The spacecraft is carrying 2,780 pounds (1,260 kilograms) of supplies to the Space Station, including food, spare part and science experiments, 23 of which are from U.S. and Canadian students. These experiments address life sciences topics ranging from vaccine effectiveness and amoeba reproduction to calcium loss in bones and liquid behavior in space, the U.S. company said.If all goes well, Cygnus is expected to arrive at the Space Station on early Sunday, where it will be captured by astronauts using the orbiting outpost's robotic arm. It will depart the station in February and burn up during reentry in Earth's atmosphere.Orbital Sciences is one of two commercial spaceflight companies selected by NASA to fly unmanned cargo delivery mission to the Space Station. The other company is California-based SpaceX, which has launched two of 12 missions to the station using its Falcon 9 rocket and Dragon spacecraft.In addition to cargo flights, NASA said its commercial space partners are making progress toward a launch of astronauts from U. S. soil within the next three years.Xinhuanet