What’s a Launch Window?

How NASA decides when it’s OK to take off.

T minus running out of time
        Click image to expand.
T minus running out of time

Last Friday, NASA postponed the launch of the space shuttle Discovery due to an equipment malfunction. The agency may be running out of time to schedule the first launch since 2003’s Columbia disaster. Discovery’s “launch window” ends on July 31; if that deadline expires, there can’t be another countdown until sometime in September. How do you schedule a shuttle launch?

Figure out when the International Space Station will pass over Florida.One of Discovery’s objectives is to dock with the space station, which orbits more than 200 miles above the Earth. The shuttle’s limited fuel supply makes it impossible to cover large distances after the launch. So, flight planners must schedule the takeoff for a precise interval when the station is orbiting directly overhead.

The station passes over Florida’s Kennedy Space Center only twice a day for about 10 minutes each time. Due to the Earth’s rotation, the station flies past the launch site going both northeast and southeast. Restrictions on flights over populated areas require that the shuttle launch toward the northeast. That means Discovery can take off only during one 10-minute  window each day.

For at least the next two missions, the shuttle must also launch during daylight hours. As a result of the Columbia accident, NASA has resolved to photograph and film the Discovery takeoff from as many angles as possible. More than 100 cameras have been set up, and the astronauts will even shoot pictures from inside the shuttle with hand-held digital cameras. Since very few of these cameras will work in the dark, the launch must take place while the sun is out.

The restriction on nighttime launches explains why Discovery’s current launch window will expire on July 31. The space station flies over Cape Canaveral 20 minutes earlier each day. The Discovery’s preferred launch time last Wednesday, for example, was 3:51 p.m., while the window for this Friday would be closer to 12:30 p.m. As the launch date approaches the end of July, the 10-minute window moves closer to sunrise. In August, the windows would occur in the middle of the night, which would make photographing the launch impossible. The launch windows will move back to the daytime in September.

There’s always a chance that bad weather will scuttle the launch. There are eight weather briefings in the 24 hours before takeoff. The event gets canceled if the average temperature dips below 41 degrees or rises above 99 for more than a half hour. It can’t be too windy, and there must not be any rain, thunderclouds, or lightning nearby. Forecasts should indicate good conditions in Cape Canaveral for at least another 20 minutes after launch (in case the shuttle needs to make an emergency landing). It should also be clear in at least one of several emergency “transatlantic abort landing” sites overseas.

Next question?

Explainer thanks Ed Gonzalez of NASA, John Prussing of the University of Illinois, and several Slatereaders for asking the question.