Space Telescope

The James Webb space telescope is now nearing its L2 destination. Hurrah!

Visit https://en.wikipedia.org/wiki/James_Webb_Space_Telescope for an excellent write up about the telescope. 

The gold plated telescope mirror array is 21 ft across. 

From https://scitechdaily.com/webb-space-telescopes-million-mile-journey-to-l2-is-nearly-complete/):

At L2, Earth is far enough away (1 million miles) that the roughly room-temperature heat radiating from it won’t warm up Webb. And because L2 is a location of gravitational equilibrium, it is easy for Webb to maintain an orbit there. Note that it is simpler, easier, and more efficient to orbit around L2 than to dwell precisely at L2. Furthermore, by orbiting rather than being exactly at L2, Webb will never have the Sun eclipsed by Earth, which is necessary for Webb’s thermal stability and for power generation. In fact, Webb’s orbit around L2 is larger in size than the Moon’s orbit around Earth! L2 is also convenient for always maintaining contact with the Mission Operations Center on Earth through the Deep Space Network. Other space-based observatories including WMAP, Herschel, and Planck orbit Sun-Earth L2 for the same reasons. 

While all Lagrange points are gravitational balance points, not all are completely stable. L1, L2, and L3 are “meta-stable’ locations with saddle-shaped gravity gradients, like a point on the middle of a ridgeline between two slightly higher peaks wherein it is the low, stable point between the two peaks, but it is still a high, unstable point relative to the valleys on either side of the ridge. L4 and L5 are stable in that each location is like a shallow depression or bowl atop the middle of a long, tall ridge or hill.

So why send Webb to orbit Sun-Earth L2? Because it is an ideal location for an infrared observatory. At Sun-Earth L2, the Sun and Earth (and Moon, too) are always on one side of space, allowing Webb to keep its telescope optics and instruments perpetually shaded. This enables them to get cold for infrared sensitivity, yet still access nearly half the sky at any given moment for observations. (See video embedded below.) To view any and every point in the sky over the course of time requires merely 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 radiating from it won’t warm up Webb. And because L2 is a location of gravitational equilibrium, it is easy for Webb to maintain an orbit there. Note that it is simpler, easier, and more efficient to orbit around L2 than to dwell precisely at L2. Furthermore, by orbiting rather than being exactly at L2, Webb will never have the Sun eclipsed by Earth, which is necessary for Webb’s thermal stability and for power generation. In fact, Webb’s orbit around L2 is larger in size than the Moon’s orbit around Earth! L2 is also convenient for always maintaining contact with the Mission Operations Center on Earth through the Deep Space Network. Other space-based observatories including WMAP, Herschel, and Planck orbit Sun-Earth L2 for the same reasons. 

The Lagrange points associated with the Sun-Earth system. Lagrange Points are positions in space where the gravitational forces of a two-body system like the Sun and the Earth balance out, allowing a spacecraft to remain in position with reduced fuel consumption. The image includes a small icon representing NASA’s WMAP spacecraft orbiting around L2, which is about 1.5 million km from Earth. Credit: NASA/WMAP Science Team