Advanced Composition Explorer orbit. Illustration of the location and orbit of NASA's Advanced Composition Explorer (ACE) spacecraft (centre right). The Earth and Moon are at far right, the Sun at centre. The four Lagrangian points (L1, L2, L3, L4) are shown for the Sun-Earth system. ACE was launched in 1997 and occupies a Lissajous orbit (blue circle) at the L1 point. Here, it is outside Earth's magnetosphere and can observe material streaming off the Sun before it enters near-Earth space. The prime objective is to measure and compare the composition of the solar corona, the solar wind, and other interplanetary particle populations.