Why does NASA sometimes schedule a rocket launch for the middle of the night, or aim for a liftoff time when weather is notoriously unlikely to cooperate?

The simplicity of the question belies the complexity of the answer. The best time to start a mission is based on a blend of factors: the flight's target and goals, the needs of the spacecraft, the type of rocket, and the desired trajectory, which refers to the path the vehicle and spacecraft must take to successfully start the mission.

Not only do these variables influence the preferred launch time - the ideal time of departure - but the overall length of the launch window, which can vary from one second to several hours.

The dynamics change from mission to mission, and determining the launch window is an important part of the overall flight design.

"The interesting thing about our job is each mission is almost completely different from any other mission," said Eric Haddox, the lead flight design engineer in NASA's Launch Services Program (LSP), based at Kennedy Space Center in Florida.

Haddox leads the team of agency and contractor personnel overseeing and integrating the trajectory design efforts of the spacecraft team and launch service contractor for each LSP mission.

Once the spacecraft team identifies its needs, a rocket is selected, and the work of hammering out the best launch window and trajectory begins. Ultimately, the launch window and preferred liftoff time are set by the launch service contractor.

"We help everybody understand the requirements of the spacecraft and what the capabilities are of the launch vehicle, and try to mesh the two," Haddox explained.

The most significant deciding factors in when to launch are where the spacecraft is headed, and what its solar needs are. Earth-observing spacecraft, for example, may be sent into low-Earth orbit.

Some payloads must arrive at a specific point at a precise time, perhaps to rendezvous with another object or join a constellation of satellites already in place. Missions to the moon or a planet involve aiming for a moving object a long distance away.

For example, NASA's Mars Science Laboratory spacecraft began its eight-month journey to the Red Planet on Nov. 26, 2011 with a launch aboard a United Launch Alliance (ULA) Atlas V rocket from Cape Canaveral Air Force Station in Florida.

After the initial push from the powerful Atlas V booster, the Centaur upper stage then sent the spacecraft away from Earth on a specific track to place the laboratory, with its car-sized Curiosity rover, inside Mars' Gale Crater on Aug. 6, 2012. Due to the location of Mars relative to Earth, the prime planetary launch opportunity for the Red Planet occurs only once every 26 months.

Additionally, spacecraft often have solar requirements: they may need sunlight to perform the science necessary to meet the mission's objectives, or they may need to avoid the sun's light in order to look deeper into the dark, distant reaches of space.

Such precision was needed for NASA's Suomi National Polar-orbiting Partnership (NPP) spacecraft, which launched Oct. 28, 2011 aboard a ULA Delta II rocket from Vandenberg Air Force Base in California.

The Earth-observing satellite circles at an altitude of 512 miles, sweeping from pole to pole 14 times each day as the planet turns on its axis. A very limited launch window was required so that the spacecraft would cross the ascending node at exactly 1:30 p.m. local time and scan Earth's surface twice each day, always at the same local time.

All of these variables influence a flight's trajectory and launch time. A low-Earth mission with specific timing needs must lift off at the right time to slip into the same orbit as its target; a planetary mission typically has to launch when the trajectory will take it away from Earth and out on the correct course.

According to Haddox, aiming for a specific target - another planet, a rendezvous point, or even a specific location in Earth orbit where the solar conditions will be just right - is a bit like skeet shooting.

"You've got this object that's going to go flying out into the air and you've got to shoot it," said Haddox. "You have to be able to judge how far away your target is and how fast it's moving, and make sure you reach the same point at the same time."

But Haddox also emphasized that Earth is rotating on its axis while it orbits the sun, making the launch pad a moving platform. With so many moving players, launch windows and trajectories must be carefully choreographed.

Of course, weather or technical problems can interfere with the team's best plans. Launch windows are intended to absorb small delays while still offering plenty of chances to lift off on a given day. However, launching at a time other than the preferred time could reduce the rocket's performance, potentially limiting the payload mass.

"To launch at any time other than that optimal time, you're going to have to alter the trajectory, steer the rocket to get back to that point," Haddox said. "So that's where it becomes a trade of, 'Okay, if my window were a half hour long, how much performance would I need to fly at any time within a half hour? Or, if my window were an hour long, how much performance would I be able to get out of the rocket to fly at any time within that one hour?'"

Likewise, if a spacecraft has to use any of its onboard propellant to make up for any difference in the trajectory, that could impact the entire mission.

"The more propellant they have, the longer they can do maneuvers or adjust things" during the flight, Haddox explained. "It basically equates to how long they can stay in orbit and do their science."

These potential give-and-take situations are carefully considered during flight planning. Mission managers must find a way to balance the sacrifices while maximizing the chance of getting off the ground.

Even when the launch and mission teams have chosen the best launch window, they face an additional challenge from the U.S. Air Force: collision avoidance, also called COLA.

The U.S. Air Force's 45th Space Wing controls the Eastern Range surrounding Cape Canaveral Air Force Station in Florida; the 30th Space Wing operates the Western Range, including Vandenberg Air Force Base.

The range determines whether any orbiting spacecraft or debris could strike the vehicle during its climb to space, and cut out portions of the launch window that are too risky.

Collision avoidance can get tricky, because even though the trajectory has been carefully planned, real-time factors result in some uncertainty. For example, during the trajectory design process, the team assumes certain propellant temperatures. But if the temperatures are slightly different on launch day, that will affect the propellant, which in turn alters the efficiency of the rocket's engines or solid rocket motors.

"The navigation system on the rocket is going to do what it needs to do to get the spacecraft where it needs to be, but it's not going to be the same trajectory you looked at before," said Haddox. "When you've got things that are moving seven to eight kilometers a second, half a second can result in a big distance."

"So it just makes things a lot harder to predict," he added.

On launch day, Haddox and other members of the flight design team are involved in the countdown. Even in the final hours before liftoff, they continue to fine-tune the trajectory analysis based on real-time data collected from weather balloons, ensuring the safety of the rocket and spacecraft as the window opens for another successful mission.