The Advanced Composition Explorer was launched August 25, 1997 carrying six high resolution spectrometers that measure the elemental, isotopic, and ionic charge state composition of nuclei from H to Ni (1 Z 28) from solar wind energies (~1 keV/nuc) to galactic cosmic ray energies (~500 MeV/nuc). Data from these instruments is being used to measure and compare the elemental and isotopic composition of the solar corona, the nearby interstellar medium, and the Galaxy, and to study particle acceleration processes that occur in a wide range of environments. ACE also carries three instruments that provide the heliospheric context for ion composition studies by monitoring the state of the interplanetary medium. From its orbit about the Sun-Earth libration point ~1.5 million km sunward of Earth, ACE also provides real-time solar wind measurements to NOAA for use in forecasting space weather. ))
About the space craft: The Z axis is the spin axis of the observatory and will be aligned within 20 degrees of the Earth-Sun line. Six of the scientific instruments are on the +Z (sunward) deck, two are on side-panels, and the ninth is mounted on the Y-axis booms. Only the four propellent tanks of the propulsion subsystem are mounted in the spacecraft interior. The -Z (earthward) deck has the fixed high gain antenna for communication.