Space shuttle Endeavour is scheduled to blast off from NASA Kennedy Space Center's Launch Pad 39A at 3:47 P.M. local time on Friday en route to the International Space Station (ISS)—the second-to-last mission for NASA's 34-year-old shuttle program. On board this time will be a six-member crew commanded by astronaut Mark Kelly (husband of Arizona Rep. Gabrielle Giffords, who survived an assassination attempt in January).

During the Endeavour's 14-day farewell mission its crew will be delivering to the ISS the Alpha Magnetic Spectrometer (AMS), two communications antennas, a high-pressure gas tank, an ammonia tank assembly, circuit breaker boxes, a computer for the station's Canadarm2 robotic system and a spare arm for the Dextre robot.

The AMS, which weighs nearly 7,000 kilograms, is a particle physics detector to be operated over the next decade by an international team of 60 institutes from 16 countries and organized under United States Department of Energy sponsorship. The AMS will study high-energy cosmic ray particles from low Earth orbit, without interference from the planet's atmosphere, to help scientists better understand and perhaps locate antimatter and dark matter.

As particles enter the AMS—at a rate of about 25,000 per second—layers of detectors will determine each particle's mass, energy and velocity. Other detectors will measure particle trajectory as they pass through the AMS.

It is not unusual for shuttle launches to be delayed for safety, weather or mechanical reasons, but Friday's weather forecast currently calls for a 70 percent chance of favorable conditions at launch time. Endeavour was commissioned in 1992 by President George H.W. Bush as a replacement for the Challenger shuttle, which blew up shortly after launch in January 1986.

Image of Endeavour on its launch pad courtesy of George Musser

Image of the AMS courtesy of NASA