We all know that James Webb Space telescope is planned to be launched in 2018. It has been decided that the orbit of JWST will be elliptical around the Lagrange point L2, which has been declared as one of the unstable points (L1, L2, L3).
So then, why should it orbit the point?
I've got a few points. The telescope's instruments are quite sensitive and should always be maintained away from Sun (Really? Is that so?) and also in a cold environment - both these are satisfied by L2. Wikipedia says this:
The combined gravitational forces of the Sun and the Earth can hold a spacecraft at this point, so that in theory it takes no rocket thrust to keep a spacecraft in orbit around L2. In reality, the stable point is comparable to that of a ball balanced upon a saddle shape. Along one direction any perturbation will drive the ball toward the stable point, while in the crossing direction the ball, if disturbed, will fall away from the stable point. Thus some station-keeping is required, but with little energy expended (only 2–4 m/s per year, from the total budget of 150 m/s)
First, is it really stable at all? If that is so unstable in reality, why should it be positioned there? I mean, L4 & L5 points are fine. Why can't the telescope be positioned in such a way that it's always facing outward from the Sun? (Earth too, if they don't want the reflection from it to crack itself)