Mission Type: Orbiter
NASA Center: Goddard Space Flight Center, Johns Hopkins University Applied Physics Laboratory
Spacecraft Mass: 752 kg
Spacecraft Instruments: 1) SWIMS solar wind ion mass spectrometer; 2) SWICS solar wind ion composition spectrometer; 3) ULEIS ultra-low-energy isotope spectrometer; 4) SEPICA solar energetic-particle ionic charge analyzer; 5) SIS solar isotope spectrometer; 6) CRIS cosmic-ray isotope spectrometer; 7) SWEPAM solar wind electron, proton, and alpha monitor; 8) EPAM electron, proton, and alphaparticle monitor; 9) MAG magnetometer and 10) RTSW real-time solar wind experiment
References:
Deep Space Chronicle: A Chronology of Deep Space and Planetary Probes 1958-2000, Monographs in Aerospace History No. 24, by Asif A. Siddiqi

National Space Science Data Center, http://nssdc.gsfc.nasa.gov/

The Advanced Composition Explorer (ACE) spacecraft was designed to study spaceborne energetic particles from the L1 Libration Point, about 1.4 million kilometers from Earth. Specifically, the spacecraft was launched to investigate the matter ejected from the Sun to establish the commonality and interaction among the Sun, Earth, and the Milky Way galaxy.

In addition, ACE also provides real-time space weather data and advanced warning of geomagnetic storms. ACE's nine instruments have a collecting power that is 10 to 10,000 times greater than anything previously flown. After launch, the spacecraft's Delta 2 launch vehicle's second stage reignited (for 4 hours) to insert the satellite into a 177 x 1.37-million-kilometer orbit.

After reaching apogee a month after launch, ACE inserted itself into its halo orbit around the L1 point. The spacecraft was declared operational on 21 January 1998 and has operated far beyond its expected lifetime.