Aiming for an Open Window

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.

Juno Ready to Launch to Jupiter

The Juno spacecraft will soon be on its way to Jupiter on a mission to look deep beneath the planet's swirling curtain of clouds to find out what lies beneath. The answer might confirm theories about how the solar system formed, or it may change everything we thought we knew.

"The special thing about Juno is we're really looking at one of the first steps, the earliest time in our solar system's history," said Scott Bolton, the principal investigator for the Juno mission. "Right after the sun formed, what happened that allowed the planets to form and why are the planets a slightly different composition than the sun?"

Starting the 4-ton spacecraft on its five-year journey to the largest planet in the solar system is the job of a United Launch Alliance Atlas V equipped with five solid-fueled boosters. Even with that much power, Juno will still require a flyby of Earth to get up enough energy to swing out to Jupiter.

With three 34-foot-long solar arrays and a high-gain antenna in the middle, the spacecraft is reminiscent of a windmill. It even spins slowly as it goes through its mission. Those arrays will be the sole power source for Juno as it conducts its mission, a first for a spacecraft headed beyond the asteroid belt between Mars and Jupiter.

The Atlas V has proven a reliable option for NASA's Launch Services Program, or LSP, the organization that oversees NASA launches and chooses the best launchers for different spacecraft.

"It's flown 28 times, pretty challenging missions, pretty challenging payloads," said Omar Baez, launch director for Juno. "It's got a heritage that goes back to the Atlas I in some of the components and in the upper stage, so it's an evolution of a family in its current configuration and shape and form. I'd say it's pretty robust."

The spacecraft is to lift off at 11:34 a.m. on Aug. 5 from Space Launch Complex 41 at Cape Canaveral Air Force Station in Florida. The mission faces a limited launch window to get off Earth before Jupiter's orbit took it out of alignment.

"Juno only has a 22-day launch window, or else we're down for another 13 months until our next opportunity," said John Calvert, mission manager for Juno. "So it's those kinds of challenges with making sure you do all the little things necessary to maximize the opportunities you get for those 22 days."