Since the end of its own space shuttle program in 2011, the US has used Russian Soyuz spacecraft
Every time it wants to transport humans to and from space. While NASA is busy working on its own solution, the agency has increasingly turned to US private companies, such as Elon Musk's SpaceX, to meet its off-planet transportation needs. Although SpaceX has successfully flown cargo(货物) flights for NASA since 2012 via the reusable Dragon spacecraft, the company has yet to launch humans in its so-called Crew Dragon capsule. Originally scheduled to launch in 2017, the program has seen multiple delays. After all, it is rocket science and the path has not been easy.
But 2019 seemed different. NASA's pre-purchased Soyuz flights had run out by the end of 2019, and SpaceX, along with Boeing, had scheduled all its final tests to achieve certification in time to pick up where Soyuz left off. On March 2, the Crew Dragon capsule was launched on a run without crew toward the International Space Station (ISS). The flight, known as Demo-1, went successfully.
The next major milestone scheduled was the in-flight abort test(飞行中止测试), anotherdrymission meant to test the reliability of the abort system that would save the crew in case of any accident during launch. The plan was to use the same capsule from the Demo-1 flight. But SpaceX never got the chance.
In April 2019, the company ran what was supposed to be a routine test, firing certain rockets with the capsule anchored firmly to the ground. Some 100 milliseconds before the engines fired, a leak of oxidizer caused an explosion that completely destroyed the capsule.
SpaceX spent the rest of spring and early summer figuring out what had gone wrong and pushing preparations for a replacement capsule. While the problem is now solved and the Crew Dragon spacecraft for Demo-2 has arrived at the launch site, NASA and SpaceX haven't set an exact date for the first test flight with astronauts to the ISS.
