The US and Russia aren't really on speaking terms these days, with one big exception — space.

The Space Station is one of the few places where Russia and America still talk

After Russia's invasion of Crimea, the US government imposed economic sanctions on Russia. Russia subsequently left the G-8 (an informal group of the world's biggest economies), further straining diplomatic ties.

All this has led to fears that the relationship might disintegrate entirely, especially when it comes to space. Last week, the Russian Deputy Prime Minister angrily tweeted veiled threats to exclude NASA astronauts from future trips to the International Space Station.

But the US and Russia have to keep talking in space. That's because there are currently two US astronauts and three Russian cosmonauts aboard the International Space Station, orbiting about 260 miles above Earth's surface.

Russia, for its part, depends on the US segment of the space station for the electricity provided by its solar panels. And NASA, for its part, has no way of getting its American astronauts to and from the space station — or anywhere else in space, for that matter. So it pays $70.7 million for each one-way ticket on Russian rockets. The earliest that could possibly change? 2017.

How the US and Russia became strange space bedfellows

Back in 2004, President Bush announced that NASA's aging space shuttle program would be retired in 2010 and — eventually — replaced by a plan to return to the moon. At the time, NASA realized there would be a four-year gap between the space-shuttle retirement and when the new manned space transport system would be in place.

But at that point, it didn't seem like a big problem for NASA to ask Russia to transport US astronauts to and from the space station in the interim. Relations between the two countries were friendly — Bush was telling reporters that he'd looked into Putin's eyes and "got a sense of his soul." What's more, NASA had relied on Russian transport for 29 months after the Columbia disaster in 2003, when the shuttle program was put on hold.

At the time, it didn't seem like a big problem to ask Russia to transport NASA astronauts

Development of NASA's replacement vessels, however, has taken much longer than anticipated — the agency won't have a replacement for the shuttle until 2017. There are a few reasons for that. Bush's moon program was cancelled by Obama in 2010 and replaced with a plan for private companies to shuttle astronauts. Meanwhile, NASA's budget requests to pay for the new program were repeatedly underfunded by Congress.

"It's put us in a vulnerable position," says John Logsdon, former director of the Space Policy Institute.

This isn't the only way that the US spaceflight program is dependent on Russia. The Atlas V rocket — built by Lockheed Martin and used to launch American military satellites and civilian payloads — runs on a Russian-built engine.

When the Atlas V was being designed in the 1990s, Lockheed Martin got a waiver from the usual Defense Department requirement that critical components be manufactured in the US, partly because the Russian engines were better and less expensive than American options, and partly because of political motivations.

"There was a fear that if we didn't find some way of keeping Russian rocket scientists employed, they would go off and work for Iran or North Korea," says James Lewis, a national security and space analyst.

The Space Station brings the US and Russia together

When construction of the International Space Station was begun in 1998, it was seen as an embodiment of the new age of cooperative space exploration — a permanent scientific alliance between a number of countries, but especially longtime rivals US and Russia. The two countries launched separate segments of the station that were assembled in orbit.

Obviously, the tenor of Russian-American relations has changed, but in space, the two countries are still codependent. NASA astronauts travel to and from the station — used mostly for scientific experiments on long-term human spaceflight — on Russian Soyuz rockets, launched from the Baikonur Cosmodrome, in Kazakhstan, the cradle of Soviet space exploration since the 1957 launch of Sputnik.

The whole thing is designed to operate with both American and Russian involvement

The next launch is scheduled for May 28, when a rocket carrying American, Russian, and German astronauts will take off to replace the current crew. The astronauts will live together in extremely close quarters for about six months, and if anything goes wrong, they'll depend on a Russian spacecraft docked at the station to serve as an emergency lifeboat.

But the Russian astronauts are also heavily dependent on NASA. The US segment makes up the majority of the space station, by area. Its solar panels provide most of the electricity, and its gyroscopes are crucial in controlling the station in orbit. On a broader level, the space station represents one of the few prestigious international projects that Russia is currently involved with, something that probably appeals to Russian leaders.

As a whole, the space station takes instructions from command centers in Houston and Korolyvov. The whole thing is designed to operate with both American and Russian involvement, and it's hard to imagine it surviving otherwise.

But are NASA and Roscosmos heading for a breakup?

A few recent events have raised the possibility that political strife between US and Russia may spill over into the space relationship.

Last month, as part of the sanctions process, NASA announced that it would be suspending all non-essential contact with Russia. That order had lots of exceptions — including anything space station-related — but it's still not conducive to a good working relationship in space.

A russian minister tweeted veiled threats to exclude nasa astronauts from future trips

In response to the sanctions, Russian Deputy Prime Minister Dimitry Rogozin tweeted "I suggest to the USA to bring their astronauts to the International Space Station using a trampoline," earlier this week.

Russia hasn't moved forward with any plans to withhold space equipment or delay the May 28 launch, but Rogozin's comment is an obvious concern. "That's the first high-level negative comment from either side regarding space," Logsdon says. "It's certainly not a good sign."

These events — and the Ukraine crisis as a whole — seem to have made the need to develop American manned space transport more urgent. The current plan is for NASA to award a grant to one of three private companies (Boeing, Sierra Nevada, or SpaceX), so they can upgrade their cargo-carrying rockets for human use.

The day after Rogozin's tweet, a House subcommittee fully approved a $785 million NASA funding request for the private space flight plan — an increase from recent years — and added money to the Defense Department's 2015 budget specifically to develop an American replacement for the Russian-made Lockheed Martin rocket engines.

To make a difference right now, though, these appropriations would've needed to come years ago. Even with more money, the private companies likely won't be ready to carry humans to space until 2017. As for the rocket engines, Lockheed Martin currently has a two-year supply stockpiled, but it'd probably take much longer than that to develop a homegrown alternative.

What's more, the International Space Station currently represents the entirety of manned space activities by NASA. The US can figure its own ways of putting astronauts in space, but it doesn't really have any destinations that don't involve Russia. When Obama cancelled the Moon program, he outlined vague plans to send a manned mission to an asteroid by 2025 and Mars sometime thereafter, but many argue that these goals are unrealistic given current NASA funding levels.

For now, it seems, if NASA wants to have humans in space, it's stuck being dependent on Russia. "All this ought to serve as a wakeup call for Congress," Logsdon says. "If they don't provide more funding, we're going to continue along in this vulnerable situation."