SpaceX is one of several private companies competing to build “space taxis” for Nasa to replace the retired space shuttle fleet. It previously flew four cargo missions to the space station, delivering food and supplies.

Since the shuttle fleet retired in 2011, Nasa has depended on Russian rockets to transport astronauts to orbit and back, paying nearly US$71 million a seat.

In the latest mission, a Russian Soyuz-FG rocket booster lifted a space ship carrying a new crew to the International Space Station  from the Russian-leased Baikonur cosmodrome in Kazakhstan on Thursday.

Nasa wants US companies to fill the void by 2017 and has doled out seed money to spur innovation.

SpaceX – short for Space Exploration Technologies Corp. – has made four cargo runs to the giant orbiting outpost 320km above earth. Just last month, its Dragon capsule splashed into the Pacific, returning nearly 2 tonnes of science experiments and old equipment.

Companies competing for the right to ferry station astronauts need to design a spacecraft that can seat a crew of four or more and be equipped with life support systems and an escape hatch in case of emergency. SpaceX has said it’s designing a seven-seat spacecraft.

SpaceX and longtime Nasa contractor Boeing are “more or less neck and neck” in the competition, but experts say there’s a long way to go before astronauts can rocket out of the atmosphere on private spacecraft.

John Logsdon, professor emeritus of political science and international affairs at George Washington University, said progress was slower than anticipated, mainly because Congress had not fully funded Nasa’s budget request for the effort.

He said it was important for the US to wean its off reliance on Russia given the political tension over the annexation of Crimea.