NASA joins ESA dark stuff hunt NASA has signed on for a European Space Agency project to send a telescope to the L2 earth-sun Lagrange point in 2020, to investigate dark matter and dark energy. L2 is a spot where the gravity of sun and Earth will keep the spacecraft in a stable orbit in Earth’s shadow behind the Earth (relative to the Sun). Once stationed …
"L2 is a spot where the gravity of sun and Earth will keep the spacecraft in a stable orbit in Earth’s shadow."
Except that it's not in shadow: L2 is too far away from Earth for the Sun to be hidden behind Earth. L2 sees a bright ring of Sun around Earth all the time, a permanent annular eclipse. L2 is in Earth's antumbra.
I suspect that it's not really relevant that it may be shaded, the real purpose of placing it at L2 is that it is gravitationally stable so the reflectors and sensors in the telescope are not subjected to gravitational distortion (and so are more accurate).
The reduction in overspill radiation from the sun is a bonus, but not really the main reason as it could be fairly easily filtered form any data collected.
L2 is not stable, although a halo orbit close to (not around) L2 is. At L2 a modest expenditure of delta-V is occasionaly required to nudge it back into place. The same applies to L1 and L3.
In contrast, L4 and L5 are far more stable. Not perfectly so, because of perturbations (mainly from Jupiter and Saturn), but far less correction is required than for L1, L2 and L3.
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