Ever since the US retired the Shuttle fleet, we've been bumming rides to the ISS on Russia's old reliable spacecraft, the Soyuz. Technically, this arrangement works well, but tensions with the Russians over the Ukraine, gay rights, and other issues are making the US realize that maybe it's time for a new ride.
There's already been a lot of speculation on what exactly this new way into space could be. The common wisdom lists three main options: NASA's Orion, SpaceX's Dragon, and the Chinese Shenzou. All three could technically work, but the problem is the first two simply aren't finished yet, and the third one isn't something we actually own.
There is, however, one option that hasn't been brought up yet, and it has already flown (well, unmanned), and we own it. It's the Air Force's mostly-secret mini-shuttle, the X-37B.
It's not a perfect solution — there is no really perfect solution here — but when it comes to getting our Astronauts to and from the station soonest, it may prove the best option. Having a finished Orion, of course, would be the ideal solution, but while the plan is for a first manned mission next year, reality reminds us that delays are more than likely to be expected. I'd guess we're at least two years away.
SpaceX has already proven their Dragon capsule with unmanned resupply missions to the ISS, and they do have plans for a manned variant, called the DragonRider. I'm sure their mission shirts will be totally boss and look like an '80s hair-metal album cover. SpaceX is starting initial tests for the DragonRider this year, but realiatically it's still a few years off as well.
The Shenzou is the only option ready to go right now, and as it's derived from the Soyuz, the learning curve shouldn't be too steep. The problems here are more political and demand-related. Currently, China is not an ISS partner, and that was a deliberate decision, though now many people are rethinking that choice. Personally, I'm in favor of global cooperation on scientific space ventures, but it's not certain China is still interesting in joining the team.
Also, China isn't exactly cranking out Shenzous like they crank out, well, pretty everything else we use every day. They've had a very slow, steady, and measured approach to manned space exploration, with around one mission a year at best. So there aren't even any unspoken-for Shenzous for us to buy seats on right now.
So what does that leave us? Doomed to taking Putin's crazy space taxis? I don't think so. The US Air Force has been flying a very scaled-down reusable space plane called the X-37B since 2010, on missions that routinely last over 200 days. So far, all the missions have been unmanned, but manned versions of the X-37B have been proposed. The current X-37 never pulls more than 1.5G, meaning it's already suitable for not turning astronaut brains into custard, which is a huge benefit.
That manned version, called the X-37C, would be about 165% bigger and does not yet exist. But I think the existing X-37B could be modified to carry astronauts to the ISS fairly quickly and easily, and work as a decent stopgap until either the Orion or DragonRider are ready. The launch vehicle for the X-37, the Atlas V, isn't currently man-rated, but plans have been explored to do just that. Other man-rated launch vehicles could be substituted, as well.
If you look at the proposals for the manned X-37 variant, they're essentially a scaled-up X-37B with the cargo area replaced with a cylindrical module holding 6 or so astronauts. The current X-37B should be able to hold a similar module, albeit much smaller. The mini-shuttle is pretty damn mini — I'd guess that the crew size would be 2 or 3 astronauts at best.
The regular Soyuz only holds 3, so even with just 2, it's not a huge loss, and could help if we have no other choice. A small crew module with an ISS-capable docking hatch could be made, and if we really, really needed to go quick and dirty, the astronauts could be required to fly in full spacesuits, eliminating the need to develop any life-support systems— just power and consumables (oxygen) for the suits.
In fact, with a very basic, unpressurized cylindrical crew module, you wouldn't even need an actual docking port. The X-37B could be grappled by the station's robotic arm (much like how cargo modules were grappled from the shuttle) and brought close to the airlock, where a very short spacewalk (think feet) would get them into the station. Return to earth would just reverse these steps, short spacewalk to the X-37B, strap in, and the robotic shuttle takes you home, landing on a runway.
While the astronauts would literally be stuck in a can, they don't have to stay in there long. A 5 hour, 45 minute ground-to-ISS trip was proven by a Soyuz last year, and if they're allowed to listen to audiobooks or something, such a trip could be tolerable for a couple astronauts.
Granted, this is a very last-resort kind of option, but it is one that seems achievable. Using an existing, proven spacecraft with the ability to loiter in space the full duration of a mission and a minimal crew-carrying module, I think we could manage to get our astronauts to the ISS even if we can't buy Soyuz seats anymore.
Now we just have to make sure the Americans and the Russians on the station don't space-shiv each other while they're up there and we'll all be fine.