A halo orbit around the first Lagrangian point of the Earth-Sun system.
A halo orbit is an orbit in which a spacecraft will remain in the vicinity of a Lagrangian point, following a circular or elliptical loop around that point. The first mission to take advantage of such an orbit was ISEE-3, launched in 1978. ISEE-3 was later renamed ICE when it was sent away from L1 (the first Lagrangian point) of the Sun-Earth system to encounter a comet's tail.
Halo orbits are special cases of Lissajous orbits where the in-plane and out-of-plane frequencies are the same.