Stuff already at Lagrangia?

>
> Eric Dahlstrom wrote,
>
> "
> I had looked into this as a possible Ph.D topic many moons ago. (I was
> thinking about infrared searches of Sun-Earth L4&L5.)
> "
>
> Eric: Are the Sun-Earth L4/L5 points observable in visible light from the
> surface of the Earth?
>

Yes ( if I understand your question ).

It depends on your defintion of "observable".

They are always 60 degrees away from the sun in the sky, sitting on the Ecliptic. So they can be above the horizon when the sky is dark, if that is your concern.

Objects at L4/5 will not appear 100% illuminated, they will always have a "gibbous" phase, that is about 66% of the hemisphere facing the Earth is illuminated.

i.e. similar to the appearance of the Moon midway between full and first (or last) quarter.