SpaceX's pioneering astronauts board the International Space Station

SpaceX's pioneering astronauts board the International Space Station

The astronauts’ primary mission, called Demo-2, is a final test of the Crew Dragon’s performance before NASA certifies it for ISS crewed transport missions. However, NASA decided to extend their stay so that Cassidy wouldn’t be the lone US astronaut aboard the ISS.

“The SpaceX crew, with Chris Cassidy already on-board the International Space Station, will be doing experiments on physiology, cardiovascular experiments, physical and life sciences, testing out life support systems … and testing out habitation experiments for our future human space exploration,” NASA’s Kirk Shireman said last month. Behnken might also assist Cassidy on a spacewalk to replace batteries on the ISS’s solar power trusses.

The astronauts are expected to remain on the ISS from one to three months, “based on the readiness of the next commercial crew launch,” NASA said earlier in May. The maximum stay will be will be 110 days due to concerns about how the Crew Dragon’s solar panels will hold up in the challenging conditions of space.

Crew Dragon’s mission will only be complete once the astronaut’s safely arrive back on Earth. That reentry is expected to be a particularly tense phase of the mission, despite extensive testing by SpaceX. “I think it was an argument that the return is more dangerous in some ways than the ascent, so in order to declare victory yet, we need to bring them home safely [and] make sure that we're doing everything we can to minimize that risk of reentry and return," Musk said.