Advanced Composition Explorer (ACE) observes particles of solar, interplanetary, interstellar, and galactic origins, spanning the energy range from solar wind ions to galactic cosmic ray nuclei.
The Earth is constantly bombarded with a stream of accelerated particles arriving not only from the Sun, but also from interstellar and galactic sources. The study of these energetic particles will contribute 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 carries six high-resolution sensors and three monitoring instruments that sample low-energy particles of solar origin and high-energy galactic particles with a collecting power 10 to 1000 times greater than past or planned experiments. From a vantage point approximately 1/100 of the distance from the Earth to the Sun, ACE performs measurements over a wide range of energy and nuclear mass, under all solar wind flow conditions and during both large and small particle events including solar flares.
ACE provides near-real-time solar wind information over short time periods. When reporting space weather, ACE can provide 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 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 Sun-Solar- System Connection (S3C) 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.