ACE Mission Description

(A translation into plain English is also available)

Brief Summary*

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 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 carrying six high-resolution sensors and three monitoring instruments will 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 will perform 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 will provide 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.

Mission Goals **

The observations from ACE instruments will allow the investigation of a wide range of fundamental problems in the following major areas:

1. The Elemental and Isotopic Composition of Matter:
   A major objective is the accurate and comprehensive determination of the elemental and isotopic composition of the various samples of "source material" from which nuclei are accelerated. Thus, ACE measurements will:
   • Generate a set of solar isotopic abundances based on direct sampling of solar material.
   • Determine the coronal elemental and isotopic composition with greatly improved accuracy.
   • Establish the pattern of isotopic differences between galactic cosmic ray and solar system matter.
   • Measure the elemental and isotopic abundances of interstellar and interplanetary "pick-up ions".
   • Determine the isotopic composition of the "anomalous cosmic ray" component thought to represent a sample of the very local interstellar medium.

2. Origin of the Elements and Subsequent Evolutionary Processing:
   Isotopic "anomalies" in meteorites indicate that the solar system was not homogeneous when formed, while other data suggest that the solar composition continues to evolve. Similarly, the galaxy is neither uniform in space nor constant in time due to continuous stellar nucleosynthesis. ACE measurements will:
   • Search for additional differences between the isotopic composition of solar and meteoritic material.
   • Determine the contributions of solar-wind and solar flare nuclei to lunar and meteoritic material, and to planetary atmospheres and magnetospheres.
   • Determine the dominant nucleosynthesic processes that contribute to cosmic ray source material.
   • Determine whether cosmic rays are a sample of freshly sythesized material (e.g., from supernovae), or of the contemporary interstellar medium.
   • Search for isotopic patterns in solar and galactic material as a test of galactic evolution models.

3. Formation of the Solar Corona and Acceleration of the Solar Wind:
   Solar energetic particles, solar wind, and spectroscopic observations show that the elemental composition of the corona is differentiated from that of the photosphere, although the processes by which this occurs, and by which the solar wind is subsequently accelerated, are poorly understood. The detailed composition and charge-state data provided by ACE will:
   • Isolate the dominant coronal formation processes by comparing a broad range of coronal and photospheric abundances.
   • Study plasma conditions at the source of the solar wind and the solar energetic particles by measuring and comparing the charge states of these two populations.
   • Study solar wind acceleration processes and any charge or mass-dependent fractionation in various types of solar wind flows.

4. Particle Acceleration and Transport in Nature:
   Particle acceleration is ubiquitous in nature and is one of the fundamental problems of space plasma astrophysics. The unique data set that will be obtained by ACE measurements will:
   • Make direct measurements of charge and/or mass-dependent fractionation during solar flare and interplanetary acceleration.
   • Constrain solar flare and interplanetary acceleration models with charge, mass, and spectral data spanning up to five decades in energy.
   • Test theoretical models for 3He-rich flares and solar gamma ray events.
   • Measure cosmic ray acceleration and propagation time scales using radioactive "clocks".

*From the JHU/APL ACE Fact Sheet

**From the GSFC ACE Scientific Goals page