NASA has delayed the final launch of space shuttle Discovery several times in the last week, most recently to no earlier than Nov. 5, due to rainy conditions at the launch site. But the shuttles must fly within a given window, and rescheduling a launch can be a tricky task.

Choosing a new space shuttle launch date means more than simply shifting everything forward a day or two. The liftoff must occur within a designated launch window to keep the spacecraft on target for its intended destination.

For Discovery, that destination is the International Space Station and the launch window opened Nov. 1 and closes on Sunday, Nov. 7. There is a chance the window could be extended to Monday, Nov. 8, NASA spokesman Allard Beutel told SPACE.com.

The window closes around Nov. 8 because after that date, the angle of sunlight on Discovery when it would be docked at the space station becomes unfavorable and could cause some parts to overheat. If Discovery does not launch by the end of the window, NASA would have to wait until Dec. 1 when the sun angles are once again favorable. [INFOGRAPHIC: NASA's Space Shuttle from Top to Bottom]

Shifting launch times

Each day a shuttle launch is delayed, the optimum time to make the next launch attempt is slightly different, typically moving up earlier in the day.

Discovery was originally scheduled to launch Nov. 1 at 4:40 p.m. EDT (2040 GMT). Since then, the launch has been delayed two days due to helium and nitrogen gas leaks, and then another day because of an electrical glitch in a backup computer for one of Discovery's three main engines.

Now, if NASA is able to launch Discovery Friday, Nov. 5, liftoff would occur at 3:04 p.m. EDT (1904 GMT). A launch try on Sunday at the end of the window would occur at 1:15 p.m. EST (1815 GMT). A launch on Sunday would come after the switch from Daylight Savings Time to Standard Time.

Target: International Space Station

To determine the launch window for Discovery's STS-133 mission, there are several critical factors scientists consider in addition to sun angles. Launch delays have ripple effects for the entire mission's timeline, particularly the shuttle's rendezvous and docking times at the International Space Station.

The limited amount of fuel onboard the shuttle means the docking times are calculated based on the station's location above the launch site at Kennedy Space Center here in Florida.

The space station flies over the space center twice a day for about 10 minutes at each pass. Shuttle launch times are set to when the angle of Earth's rotation will allow the orbiter to slip into the same plane that the space station is flying in. Those times shift with each passing day.

For example, Discovery is now slated to lift off at 3:04 p.m. EDT on Friday – 96 minutes earlier in the day than on its original launch target four days earlier.

The shift keeps Discovery on track for its arrival at the station while both fly 220 miles (354 km) above Earth – traveling 17,500 mph (28,164 kph) – two days after launch.

But that's not all.

There are also additional variables that can dictate potential launch windows and launch times, including the astronaut crew's sleep cycle, where the shuttle's external tank and solid rocket boosters will be released, wind and other weather forecasts on launch day, possible abort scenarios, mission-specific requirements, and even desired re-entry and landing times.

This is why when launches are delayed, their liftoff times shift.

On its 11-day mission, Discovery will haul critical spare parts to the space station, including a new storage room and a humanoid robot to assist the crew of the orbiting laboratory.

Discovery's final flight will be NASA's 133rd shuttle mission, before the space agency brings its 30-year space shuttle program to a close in 2011.

NASA will retire the three remaining shuttles in its fleet – Discovery, Atlantis and Endeavour – next year to make way for a new plan aimed at sending astronauts to visit an asteroid and Mars. Discovery is the oldest of NASA's space shuttles currently flying and the first to be retired.

This article was reprinted with permission from SPACE.com.

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