Two American astronauts are carrying out a seven-hour spacewalk on the International Space Station (ISS) to fit a massive steel beam that holds some giant solar wings.
A problem with the ISS' existing solar panels means the new ones have to work perfectly, so the astronauts have got a really tricky job ahead.
Inside the ISS, the other astronauts are doing their bit to help by using a robotic arm to hook the beam to the back of the ISS. Then the beam's huge solar panels can be opened up.
The crew of the space shuttle Discovery has already installed a new room on the ISS. Called Harmony, it's the first time the living and working space has been made bigger on the ISS since 2001.