James Webb About to Orbit

NASA James Webb Space Telescope
NASA James Webb Space Telescope - (Photo: QAI Publishing/Universal Images Group via Getty Images)

The James Webb Telescope is nearly complete in orbiting L2. By reading these lines, the orbital process will most likely be completed. On Monday, January 24, engineers plan to order one final fix to NASA's James Webb Space Telescope.

This will put it in the desired orbit at about 1 million miles from Earth, at what's called the second Sun-Earth Lagrange point, or "L2" for short.

Mathematically, Lagrangian points are solutions to what's called the "constrained three-body problem." Any two massive, gravitationally significant objects in space form five special locations – Lagrangian points – where gravitational forces and the centrifugal force of motion of a small, third body such as a spacecraft are in balance.

What are Lagrange Points?

Lagrangian points are positions in space where objects sent there tend to stay in place. At Lagrangian points, the gravitational force of two large masses is exactly equal to the centripetal force required for a small object to move with them. These points in space can be used by spacecraft to reduce the fuel consumption required to stay in position.

The Lagrangian points are labeled L1 through L5 and precede the names of the two gravitational bodies (the larger one first) that compose them.

While all Lagrangian points are gravitational equilibrium points, not all are completely stable. L1, L2, and L3 are “meta-stable” locations with saddle-shaped gravity gradients, like a point in the middle of a hill between two slightly higher peaks, where the low, fixed point between the two peaks but still high relative to the valleys on either side of the ridge, an unstable point.

L4 and L5 are stable in that each location is like a shallow depression or bowl in the middle of a long, high ridge or hill.

So why send Webb into orbit of Sun-Earth L2?

Because it's an ideal location for an infrared observatory. In Sun-Earth L2, the Sun and Earth (and the Moon too) are always on one side of space, allowing Webb to constantly eclipse his telescope optics and instruments. This allows them to cool down for infrared sensitivity, but still reach almost half the sky for observations at any one time. Seeing any point in the sky in time requires only waiting a few months to travel farther around the Sun and reveal more of the sky that was previously "behind" the Sun.

Moreover, at L2, Earth is far enough away that the roughly room-temperature heat emitted from it does not warm Webb. And because L2 is a place where there is a gravitational balance, it's easy for Webb to maintain an orbit there. Note that turning around L2 is simpler, easier, and more efficient than stopping exactly at L2. Also, by orbiting instead of being exactly at L2, Webb will never have the Earth-eclipse of the Sun necessary for Webb's thermal stability and power generation. In fact, Webb's orbit around L2 is larger in size than the Moon's orbit around Earth!

L2 is also suitable for maintaining contact at all times with the Mission Operations Center on Earth via the Deep Space Network. Other space-based observatories such as WMAP, Herschel, and Planck orbit Sun-Earth L2 for the same reasons.

Generally speaking, bringing a spacecraft to Sun-Earth L2 is pretty straightforward, but Webb's architecture has added a wrinkle.

Karen Richon, Webb's chief engineer of Flight Dynamics, describes bringing Webb to L2 and keeping it there:

“Imagine throwing a ball into the air as hard as you can; It starts very fast, but slows down as gravity pulls it back to Earth, eventually stopping at its peak and then back to the ground. Much like your arm energizes it to rise a few feet above the Earth's surface, the Ariane 5 rocket energized Webb to travel the great distance of 1,1 million kilometers, but not enough to escape Earth's gravity. Like the ball, Webb slows down and would eventually stop and fall back towards Earth if we let it. Unlike the ball, Webb would not 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.

Using every three weeks or more of thrust from the small rocket motors at Webb will keep it orbiting L2's orbit, semi-annually.

“Well, why didn't Ariane give Webb more energy and why did Webb need course correction? If Ariane had given Webb just a little more energy than needed to get him to L2, when he got there he would have gone too fast and crossed his desired science trajectory. Webb would have to make a significant braking maneuver, pushing towards the Sun to slow down. Not only would this massive burn cost a lot of propellant, it would also be impossible, since Webb would have to rotate 180 degrees to be pushed toward the Sun, which would expose the telescope optics and instruments directly to the Sun, thereby causing them to overheat. would be. It literally melts the structures and the glue that holds them together.

Mounting thrusters on the telescope as a way of directing braking thrust was not possible for a number of reasons and was never a design option.

“So Webb wanted enough energy from the Ariane rocket to ensure we never had to do a retrograde combustion, but it would always require a combustion from the observatory to fully compensate for the difference and place it in the desired orbit.

Ariane 5 targeted Webb so accurately that our first and most critical aspect was smaller than we had to plan and design, leaving more fuel for a long mission!”

Source: NASA – Alise Fisher January 21, 2022

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