Spherical glass spacecraft BLITS

The BLITS spacecraft is a retroreflector system made of a central ball and two external glass spherical menisci. 

The menisci and the central lens have significantly different refractive indices in such satellites (a ball having a high refractive index and a meniscus having a low one). That makes it possible to focus the radiation incident on them to one point on the reflecting meniscus (Luneberg lens). 

The BLITS spacecraft was launched into orbit in 2009 and confirmed the possibility of using zero-signature satellites, which provide submillimeter target error.