Advanced Composition Explorer (ACE) is an Explorer mission

The Advanced Composition Explorer (ACE) is an Explorer mission that was managed by the Office of Space Science Mission and Payload Development Division of the National Aeronautics and Space Administration (NASA). 

ACE launched on a McDonnell-Douglas Delta II 7920 launch vehicle on August 25, 1997 from the Kennedy Space Center in Florida.

The Earth is constantly bombarded with a stream of accelerated particles arriving not only from the Sun, but also from interstellar and galactic sources. Study of these energetic particles contributes to our understanding of the formation and evolution of the solar system as well as the astrophysical processes involved. The Advanced Composition Explorer (ACE) spacecraft carrying six high-resolution sensors and three monitoring instruments samples low-energy particles of solar origin and high-energy galactic particles with a collecting power 10 to 1000 times greater than past experiments.

ACE orbits the L1 libration point which is a point of Earth-Sun gravitational equilibrium about 1.5 million km from Earth and 148.5 million km from the Sun. From its location at L1 ACE has a prime view of the solar wind, interplanetary magnetic field and higher energy particles accelerated by the Sun, as well as particles accelerated in the heliosphere and the galactic regions beyond.

ACE also provides near-real-time 24/7 continuous coverage of solar wind parameters and solar energetic particle intensities (space weather). When reporting space weather ACE provides an advance warning (about one hour) of geomagnetic storms that can overload power grids, disrupt communications on Earth, and present a hazard to astronauts.

The spacecraft has enough propellant on board to maintain an orbit at L1 until ~2024.

The prime objective of ACE is to measure and compare the composition of several samples of matter, including the solar corona, the solar wind, and other interplanetary particle populations, the local interstellar medium (ISM), and galactic matter. While there has been great progress addressing these objectives, the changing conditions over the solar cycle present new opportunities. In addition, new observations and theoretical advances, new missions, and the evolving goals of NASA and the Heliophysics Theme have introduced new challenges, including the goal of achieving the scientific understanding needed to forecast space weather in the coming years when humans will venture beyond Earth's protective magnetosphere. 

The nine scientific instruments on ACE are performing:

Comprehensive and coordinated composition determinations

Elemental

Isotopic

Ionic charge state

Observations spanning broad dynamic range

Solar wind to galactic cosmic ray energies

(~100 eV/nucleon to ~500 MeV/nucleon)

Hydrogen to Zinc (Z = 1 to 30)

Solar active and solar quiet periods

Investigations of the origin and evolution of solar and galactic matter

Elemental and isotopic composition of matter

Origin of the elements and subsequent evolutionary processing

Formation of the solar corona and acceleration of the solar wind

Particle acceleration and transport in nature.

Source: NASA