A NASA spacecraft to study the composition of the solar wind and cosmic rays, launched 1997 August. It is stationed in a halo orbit around the L1*Lagrangian point 1.5 million km sunwards of the Earth. ACE carries nine instruments to sample low-energy particles of solar origin and high-energy particles from the Galaxy. From its position upstream in the solar wind it provides advance warning of geomagnetic storms.
http://www.srl.caltech.edu/ACE/ Official mission website.
Subjects: Astronomy and Astrophysics.