The much-postponed launch of space shuttle Discovery on a mission to the International Space Station (ISS) finally looks set to go. The shuttle is scheduled to blast off from Kennedy Space Center in Florida tonight at 9:20 (Eastern Daylight Time), delivering the final pieces of the ISS's solar arrays and helping to boost the station's capacity from three to six crew members.

The shuttle's climb into space should be visible—weather permitting—in the night sky along the East Coast, according to Joe Rao of the Hayden Planetarium in New York City. The glow of Discovery's three main engines, he writes in his blog, will look like "a very bright, pulsating, fast-moving star, shining with a yellowish-orange glow." In the Southeast, the bright light of the solid-rocket boosters will be visible as they burn for the first two minutes of flight.

Rao's blog has a map showing where and when the shuttle can be found in the sky post-liftoff, but he notes that anywhere outside the neighborhood of Cape Canaveral it will be quite low—five to 10 degrees above the horizon. Shuttle viewing opportunities in the Northeast, where the shuttle should be visible six to eight minutes after liftoff, may be spotty due to possible cloud cover, but much of the rest of the Eastern seaboard has a relatively clear forecast around launch time.

NASA pushed back Discovery's launch date nearly a month to evaluate possible safety concerns surrounding three valves that control the flow of hydrogen gas from the main engines to the external fuel tank. After the previous shuttle mission in November, engineers discovered that one of the valves on that shuttle, Endeavour, had broken during launch. The space agency inspected Discovery's valves but wanted to further investigate the cause of the breakage in the Endeavour mission and the problems such an occurrence might cause. NASA has issued a fact sheet on the valves (pdf) with more details and photos of the damaged part from Endeavour.

Photo of Discovery in transit to the launch pad at Kennedy Space Center: NASA/Troy Cryder