One of the most complex and expensive construction projects in history could inch ever closer to completion April 5, when space shuttle Discovery is scheduled to lift off on a mission to the International Space Station (ISS). The orbiter and its seven-member crew will deliver additional sleeping quarters, exercise equipment and racks for science experiments to the station.

After the 13-day mission, officially designated STS-131, the shuttle program has just three remaining scheduled flights, all to the ISS, before the orbiters are retired. NASA is within range of meeting a timeline laid out in 2004, following the Columbia disaster, that specified the U.S. construction of the ISS be wrapped up and the shuttle retired by the end of 2010. By that time, the U.S. will have spent an estimated $50 billion over more than 15 years on developing the station, according to a NASA estimate cited by the U.S. Government Accountability Office.

The final planned shuttle launch is currently scheduled for September, but a March 25 report (pdf) from the NASA inspector general concluded that "it will most likely take NASA until early 2011 to complete the last of the four remaining flights." Given NASA's poor track record in recent decades for delivering projects on time or on budget, however, even an early 2011 phaseout of the shuttle would have to count as a surprisingly good showing.

Discovery's crew for STS-131 comprises commander Alan Poindexter, pilot Jim Dutton, and mission specialists Rick Mastracchio, Dottie Metcalf-Lindenburger, Stephanie Wilson, Naoko Yamazaki and Clay Anderson. Yamazaki is a member of the Japan Aerospace Exploration Agency astronaut corps; the other six are NASA astronauts.

Photo of Discovery on the launch pad at Kennedy Space Center in Florida: NASA