Astronauts have been carrying out the third and final spacewalk of their current mission. Joe Tanner and Heidemarie Stefanyshyn-Piper are completing work on new solar energy panels to help restart construction of the half-finished International Space Station. The new solar array will double the amount of electricity available on the station as it expands. Construction of the 100-billion euro complex came to a halt in 2003, when NASA grounded its space shuttles after the Columbia disaster. Now, after more than a billion spent in safety upgrades and two test flights, NASA has an ambitious programme of at least 15 missions to finish the station before the shuttles are retired in 2010.