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Generally speaking, getting a spacecraft to Sun-Earth L2 is fairly straightforward, but Webb’s architecture added a wrinkle. Karen Richon, Webb’s Flight Dynamics lead engineer, describes getting Webb to L2 and keeping it there:
“Think about throwing a ball straight up in the air, as hard as you can; it starts out very fast, but slows down as gravity pulls it back towards Earth, eventually stopping at its peak and then returning to the ground. Similar to your arm giving the ball energy to go up a few meters from the Earth’s surface, the Ariane 5 rocket gave Webb energy to go the great distance of 1.1 million kilometers, but not quite enough energy to escape Earth’s gravity. Just like the ball, Webb is slowing down, and, if we allowed it, would eventually stop and fall back towards Earth. Unlike the ball, Webb wouldn’t return to the Earth’s surface, but would be in an extremely elliptical orbit, with a perigee altitude of 300 kilometers and an apogee altitude of 1,300,000 kilometers. Utilizing thrust every three weeks or so from small rocket engines aboard Webb will keep it orbiting L2, looping around it in a halo orbit once every six months.
“So, why did the Ariane not give Webb more energy and why did Webb need course correction? If the Ariane had given Webb even a little bit too much energy than needed to get it to L2, it would be going too fast when it got there and would overshoot its desired science orbit. Webb would have to do a significant braking maneuver by thrusting toward the Sun to slow down. Not only would that big burn cost a lot of propellant, it would be impossible because it would require Webb to turn 180 degrees in order to thrust toward the Sun, which would have exposed its telescope optics and instruments directly to the Sun, thus overheating their structures and literally melting the glue that holds them together. Mounting thrusters on the telescope as a way to direct braking thrust was infeasible for a number of reasons and was never a design option.
“Therefore, Webb requested just enough energy from the Ariane rocket to ensure that we would never have to do a retro burn, but would always require a burn from the observatory to precisely make up the difference and place it in the desired orbit. The Ariane 5 targeted Webb so accurately that our first and most critical burn was smaller than we had to plan and design for, leaving more fuel for an extended mission!”
—Karen Richon, Webb Flight Dynamics lead engineer, NASA’s Goddard Space Flight Center