| Advanced Composition Explorer (ACE) |
| A US spacecraft, launched in August 1997, carrying nine scientific instruments for determining the isotopic and elemental composition of the solar corona, the interplanetary medium, the local interstellar medium and galactic material. It was placed in solar orbit, at one of the Earth's Lagrangian points, 1.5 million km nearer the Sun than the Earth, where it will maintain an almost constant position relative to the Earth. |