NASA has paid Russia for seats on the Soyuz spacecraft since the US Space Shuttle was retired in 2011. In 2014, NASA gave contracts to Boeing and Space X, worth a combined total of up to $6.8 billion, to develop crew transportation systems and conduct initial missions to the orbiting laboratory under its Commercial Crew Program.
"The first US spacecraft carrying crew to the IIS is scheduled for a launch after being certified by NASA in May 2020," the source said.
Whether Space X's Crew Dragon or Boeing's Starliner, NASA will first have to certify that the spacecraft is safe for human spaceflight. Initially, both contractors intended to be certified in 2017, now neither is expected to be ready until late 2019 at the earliest.
Space X tested Crew Dragon in an unmanned flight to the ISS in March. Its first flight with crew aboard is scheduled to take place on November 15. On May 27, NASA astronauts Robert Behnken and Douglas Hurley arrived at Russia's Zvyozdny Gorodok for training before their Demo-2 flight on the Crew Dragon.
NASA reportedly plans to conduct an unmanned test flight for Starliner in August. Its first manned mission was initially scheduled for August but was then postponed until November. The company has also revealed plans to increase the duration of Starliner’s first manned flight to the ISS but provided no further details.