The US astronauts Andrew Feustel and Mike Fincke have begun the third spacewalk of their mission servicing the International Space Station, on the final voyage of the space shuttle Endeavour. The astronauts are working on the Russian portion of the space station, where they are adding equipment to provide more power to that area and hooking up an attachment for the station's robotic arm. One more spacewalk is scheduled for this mission before the shuttle returns to Earth in early June. Meanwhile, NASA officials said they will stop trying to contact an exploratory vehicle on the planet Mars that has not returned communications in more than a year. The Spirit rover and its still-functioning twin, Opportunity, landed on Mars in early 2004 on a three-month mission to search for signs that Mars may once have contained water. Spirit developed a series of mechanical and data-storage problems that culminated in its now-dormant state.