Tensions between the US and Russia reach the space station

The International Space Station. (Photo by Paolo Nespoli - ESA/NASA via Getty Images)

In May, tensions between the US and Russia over the latter's invasion of Crimea threatened to affect the two countries' chief partnership: the International Space Station (ISS). In response to economic sanctions, Russia threatened to stop ferrying NASA astronauts to the ISS beginning in 2020.

Tensions have cooled slightly, and Russia has appeared to back off that threat, but it does expose a huge liability in NASA's crewed space program. After the retirement of the space shuttle, the agency put a plan in the works to hand off transport to the ISS to private companies, but delays have forced NASA to be entirely reliant on Russia for human transport. SpaceX and others are currently delivering cargo to the ISS, but they won't be ready to carry humans until 2017 at the earliest.