Launching a rocket into space is a bit like jumping off a moving merry-go-round. You wait until you see the exit coming around, factor in the time it’ll take you to coil up and launch yourself, the speed you’re already traveling, the merry-go-round’s general dimensions, and your air time after leaping—and then jump at just the right moment and scoot out the gate. If you wait until the exit is right in front of you, you’ll overshoot it and have to turn around and walk back. If you jump as soon as it comes into view you’ll have to walk (a bit sheepishly) that extra 15 feet.
Space shuttle launches work on the same principle, just on a much larger scale and with many more variables. Engineers calculate how much time they have—down to the minute or even second—to launch and reach a target for rendezvous. This determination is called the launch window, and it has become such a critical part of spaceflight operations that NASA now devotes teams of engineers to the task of getting it just right. “Before the space shuttle, you had only a few guys figuring out when to launch, how to rendezvous, and when to reenter,” says NASA flight dynamics officer Phillip Burley, who works on shuttle missions at Johnson Space Center in Houston. “But the shuttle is much more complicated, and it has broken down into teams of specialists for each phase. I specialize in rendezvous—making sure the spacecraft gets precisely where it needs to go once in orbit.”
His colleague Richard Jones, also a flight dynamics officer, or FDO (prounced “fido”), specializes in the ascent, and the two, with other team members, work closely to factor in all the variables and plan the mission’s timing from start to finish. Basically, Jones explains, a launch window is the overlap of two time periods, known as the plane window and the phase window. When the space shuttle is to rendezvous with the International Space Station, engineers calculate when the orbital path of the ISS passes directly over or near the latitude and longitude coordinates of Kennedy Space Center in Florida—that’s the plane window (the orbit is the edge of a circular plane that passes through the center of Earth). “If it passes directly over Kennedy Space Center, that’s our optimal time for launch,” Jones says. “If it passes a few miles to the east or west, that’s okay but it will require some additional steering. And that takes additional fuel and adds stress to the external tank, which we want to minimize.”
When visualizing plane window scenarios, it’s important to remember that Earth rotates at 1,035 mph, but an object’s orbit is fixed in space. That means that the orbital path of the ISS passes over a different part of Earth on each 90-minute, 17,000-mph orbit—the station’s “ground track” is always sliding to the west. The ground track of the ISS may cross near Kennedy Space Center on one orbit, but when it comes back around 90 minutes later, Earth will have rotated and the orbit will cross at a point about 1,000 miles due west. The result: There is only one plane window per day for a rendezvous mission, because it takes about 24 hours (Earth’s circumference is approximately 25,000 miles, divided by a rotation rate of 1,035 mph) for the target orbit to return.
The other component of the launch window calculation is the phase window, the period during which launching the shuttle will place it in line behind the ISS and on schedule to rendezvous at a specified time, usually three days after the launch. Fuel constraints limit the launch window to between 2.5 minutes and 10 minutes for a rendezvous. Any longer and the shuttle will not have enough fuel to catch the station. (The shuttles that have held on the pad for hours before launching didn’t have a scheduled rendezvous.)
Though there may be many plane and phase window intersections over a several-week period, there are other variables that might further reduce the number of potential launch windows. These include the crew’s sleep cycles (so a rendezvous is not scheduled “at night”), where the external tank and the solid rocket boosters will be dropped off (never over land), wind characteristics on launch day, possible abort scenarios, certain types of mission-specific requirements, or even the desired reentry and landing times.
When the window is finally selected, the launch calculations can be further adjusted by computers even during the final seconds of the countdown. If, because of delays or other schedule changes, the launch occurs earlier or later in the 10-minute launch window, the orbiter will have to steer east or west to match the shift in the orbital path of the ISS).
Finally, though the launch window coincides with the space station’s orbital path, launch isn’t necessarily timed for the exact moment the station flies over Kennedy. Engineers factor in multiple orbits over several days before the meeting. A rendezvous could happen on launch day, but the crew needs time to adjust to weightlessness and prepare for their mission, so the orbiter takes a lower orbit at higher speed, possibly lapping the ISS, and then burns its engines to increase altitude, slow down, and creep up on the station from below and behind on the third day. The shuttle’s stealthy approach may sound sneaky, but don’t worry. The station crew usually knows it’s coming.