Update: First, everything is fine! The ISS is safe, and the astronauts are alive and back to work. Here's the story. Today at 16:39 (UT), a small piece of space debris passed by the International Space Station. Orbit predictions put it close enough to the ISS to be a real danger, though the actual collisions odds were low. Astronauts closed all the hatches in the station and moved to the Soyuz escape module to wait it out. After the debris passed safely, they emerged, opened up all the hatches, and got back to work. It's still not known as I write this just how close the debris passed. It was a piece of a used booster called a Payload Assist Module, a device used to boost satellites from low Earth orbit up to geosynchronous orbit. Robert Pearlman from CollectSpace told me:

This object is the "yo weight" from a Delta PAM-D stage (used to launch GPS 37 in 1993). The yo weight is a small mass attached to a 1-meter-long cable, used to tumble the stage after separation from the payload so it doesn't recontact. The mass is probably less than 1 kg, but because it's basically a piece of string, the cross-section is large for its mass.

A 1 kg mass moving at a relative speed of several kilometers per second could have done vast damage to the ISS, especially since it was not compact like a chunk of metal, but extended. It could have torn a huge hole in the station, and I imagine the astronauts are breathing a huge sigh of relief. The good news here is that this object, though small, was tracked well enough to give the astronauts on the ISS plenty of time to evacuate... and don't overlook the fact that the astronauts did a great job handling the situation (whereas I would be very busy screaming and wetting myself). This was an extraordinary event handled by extraordinary people. My sincere thanks to those who took care of it, and congratulations as well. Also, my thanks to Nancy Atkinson of Universe Today for breaking this story and updating Twitter so fervently. Here is my original post on this:

Nancy Atkinson is on top of the story that a small piece of orbital debris might hit the space station at 11:39 Central (US) time, less than 40 minutes from the time I write this. Astronauts on board are preparing for this potential impact. Follow her Twitter feed for more info. You can also watch NASA TV for info.