The relay satellite included two micro-satellites for the moon, only one of them survived: http://www.planetary.org/blogs/guest-blogs/2018/20180615-que...
[edit] I mean I was aware of Lagrange points, but not the practicality of the halo orbit. This is a nice explanation:
http://hyperphysics.phy-astr.gsu.edu/hbase/Mechanics/lagpt.h...
>A full n-body dynamical system such as the Solar System does not contain these periodic orbits, but does contain quasi-periodic (i.e. bounded but not precisely repeating) orbits following Lissajous-curve trajectories. These quasi-periodic Lissajous orbits are what most of Lagrangian-point space missions have used until now. Although they are not perfectly stable, a modest effort of station keeping keeps a spacecraft in a desired Lissajous orbit for a long time.