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Not entirely true. Spacecrafts orbiting L2 do not stay still with respect to the Earth; rather, they follow a Lissajous orbit, which is quite efficient given th

Not entirely true. Spacecrafts orbiting L2 do not stay still with respect to the Earth; rather, they follow a Lissajous orbit, which is quite efficient given the gravitational properties of the L2 point. Therefore, they will observe the Sun quite often. (The apparent size of the Sun from L2 is roughly the same as Earth's, so eclipsing the Sun would be too complicated.) And this is not bad, because otherwise solar panels could never produce the power necessary for the spacecraft to work! The real reason why L2 is so interesting is that it violates Kepler's third law: despite being farther from the Sun than Earth, the orbital period is still the same as Earth, because of the gravitational attraction of Earth itself. This allows the spacecraft to keep the same distance from Earth during the year, which eases communications with ground stations. (Disclaimer: I have been part of the Core Team of the ESA Planck mission, which flew around L2 like JWST will do.)

snowwrestler 7y ago
The spacecraft would also keep the same distance from the Earth during the year in Earth orbit, so there has to be another reason they picked L2. The real reason for L2 is that the sun, moon, and Earth are all the same direction from the telescope when it is at L2, so the sunshield can block them all simultaneously. Yes, the spacecraft will receive insolation at L2 (yay solar power), but it will always be from the same direction and therefore the instrument can be persistently shielded. EDIT: from the the horse's mouth: > To have the sunshield be effective protection (it gives the telescope the equivalent of SPF one million sunscreen) against the light and heat of the Sun/Earth/Moon, these bodies all have to be located in the same direction. > This is why the telescope will be out at the second Lagrange point. https://jwst.nasa.gov/orbit.html https://jwst.nasa.gov/orbit.html

snowwrestler 7y ago

The spacecraft would also keep the same distance from the Earth during the year in Earth orbit, so there has to be another reason they picked L2. The real reason for L2 is that the sun, moon, and Earth are all the same direction from the telescope when it is at L2, so the sunshield can block them all simultaneously. Yes, the spacecraft will receive insolation at L2 (yay solar power), but it will always be from the same direction and therefore the instrument can be persistently shielded. EDIT: from the the horse's mouth: > To have the sunshield be effective protection (it gives the telescope the equivalent of SPF one million sunscreen) against the light and heat of the Sun/Earth/Moon, these bodies all have to be located in the same direction. > This is why the telescope will be out at the second Lagrange point. https://jwst.nasa.gov/orbit.html https://jwst.nasa.gov/orbit.html