Advanced Composition Explorer (ACE)

Advanced Composition Explorer (ACE or Explorer 71) is a NASA Explorer program satellite and space exploration mission to study matter comprising energetic particles from the solar wind, the interplanetary medium, and other sources.

Real-time data from ACE are used by the National Oceanic and Atmospheric Administration (NOAA) Space Weather Prediction Center (SWPC) to improve forecasts and warnings of solar storms. The ACE robotic spacecraft was launched 25 August 1997, and entered a Lissajous orbit close to the L1 Lagrange point (which lies between the Sun and the Earth at a distance of some 1,500,000 km (930,000 mi) from the latter) on 12 December 1997. The spacecraft is currently operating at that orbit. Because ACE is in a non-Keplerian orbit, and has regular station-keeping maneuvers, the orbital parameters in the adjacent information box are only approximate.

As of 2022, the spacecraft is still in generally good condition, and is projected to have enough propellant to maintain its orbit until 2024. NASA Goddard Space Flight Center managed the development and integration of the ACE spacecraft.