Advanced Composition Explorer

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Advanced Composition Explorer
Advanced Composition Explorer.jpg
An artist's concept of ACE
Mission type Solar research
Operator NASA
COSPAR ID 1997-045A
SATCAT № 24912
Mission duration 5 years planned
Elapsed: 17 years and 24 days
Spacecraft properties
Bus Custom
Manufacturer Johns Hopkins Applied Physics Laboratory
Launch mass 757 kilograms (1,669 lb)
Dry mass 562 kilograms (1,239 lb)
Power 444 watts End-of-Life (5 years)
Start of mission
Launch date August 25, 1997, 14:39:00 (1997-08-25UTC14:39Z) UTC
Rocket Delta II 7920-8
Launch site Cape Canaveral LC-17A
Orbital parameters
Reference system [heliocentric]
Regime [L1 Lissajous]
ACE in orbit around the Sun–Earth L1 point

Advanced Composition Explorer (ACE) is a NASA Explorer program Solar and space exploration mission to study matter comprising energetic particles from the solar wind, the interplanetary medium, and other sources. Real-time data from ACE is used by the Space Weather Prediction Center to improve forecasts and warnings of solar storms.[1] The ACE robotic spacecraft was launched August 25, 1997 and is currently operating in a Lissajous orbit close to the L1 Lagrange point (which lies between the Sun and the Earth at a distance of some 1.5 million km from the latter). The spacecraft is still in generally good condition, and has enough fuel to maintain its orbit until 2024.[2] NASA Goddard Space Flight Center managed the development and integration of the ACE spacecraft.[3]

Science Objectives[edit]

ACE observations allow the investigation of a wide range of fundamental problems in the following four major areas:[4]

Elemental and Isotopic Composition of Matter[edit]

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. These observations will be used to:

  • 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”, which represents a sample of the local interstellar medium.

Origin of the Elements and Subsequent Evolutionary Processing[edit]

Isotopic “anomalies” in meteorites indicate that the solar system was not homogeneous when formed. Similarly, the Galaxy is neither uniform in space nor constant in time due to continuous stellar nucleosynthesis. ACE measurements will be used to:

  • Search for differences between the isotopic composition of solar and meteoritic material.
  • Determine the contributions of solar–wind and solar energetic particles tolunar and meteoritic material, and to planetary atmospheres and magnetospheres.
  • Determine the dominant nucleosynthetic processes that contribute to cosmic ray source material.
  • Determine whether cosmic rays are a sample of freshly synthesized 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.

Formation of the Solar Corona and Acceleration of the Solar Wind[edit]

Solar energetic particle, 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 be used to:

  • Isolate the dominant coronal formation processes by comparing a broad range of coronal and photospheric abundances.
  • Study plasma conditions at the source of solar wind and 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.

Particle Acceleration and Transport in Nature[edit]

Particle acceleration is ubiquitous in nature and understanding its nature is one of the fundamental problems of space plasma astrophysics. The unique data set obtained by ACE measurements will be used to:

  • Make direct measurements of charge and/or mass–dependent fractionation during solar energetic particle and interplanetary acceleration events.
  • Constrain solar flare, coronal shock, and interplanetary shock acceleration models with charge, mass, and spectral data spanning up to five decades in energy.
  • Test theoretical models for 3He–rich flares and solar γ–ray events.


Cosmic Ray Isotope Spectrometer (CRIS)[edit]

The Cosmic Ray Isotope Spectrometer covers the highest decade of the Advanced Composition Explorer’s energy interval, from 50 to 500 MeV/nucleon, with isotopic resolution for elements from Z ≈ 2 to 30. The nuclei detected in this energy interval are predominantly cosmic rays originating in our Galaxy. This sample of galactic matter investigates the nucleosynthesis of the parent material, as well as fractionation, acceleration, and transport processes that these particles undergo in the Galaxy and in the interplanetary medium. Charge and mass identification with CRIS is based on multiple measurements of dE/dx and total energy in stacks of silicon detectors, and trajectory measurements in a scintillating optical fiber trajectory (SOFT) hodoscope. The instrument has a geometrical factor of 250 cm2 sr for isotope measurements. [5]

Solar Isotope Spectrometer (SIS)[edit]

The Solar Isotope Spectrometer (SIS) provides high resolution measurements of the isotopic composition of energetic nuclei from He to Zn (Z = 2 to 30) over the energy range from ~10 to ~100 MeV/nucleon. During large solar events SIS measures the isotopic abundances of solar energetic particles to determine directly the composition of the solar corona and to study particle acceleration processes. During solar quiet times SIS measures the isotopes of low-energy cosmic rays from the Galaxy and isotopes of the anomalous cosmic ray component, which originates in the nearby interstellar medium. SIS has two telescopes composed of silicon solid-state detectors that provide measurements of the nuclear charge, mass, and kinetic energy of incident nuclei. Within each telescope, particle trajectories are measured with a pair of two-dimensional silicon strip detectors instrumented with custom very-large- scale integrated (VLSI) electronics to provide both position and energy-loss measurements. SIS was especially designed to achieve excellent mass resolution under the extreme, high flux conditions encountered in large solar particle events. It provides a geometry factor of 40 cm2 sr, significantly greater than earlier solar particle isotope spectrometers. [6]

Ultra Low Energy Isotope Spectrometer (ULEIS)[edit]

The Ultra Low Energy Isotope Spectrometer (ULEIS) on the ACE spacecraft is an ultra-high-resolution mass spectrometer that measures particle composition and energy spectra of elements He–Ni with energies from ~45 keV/nucleon to a few MeV/nucleon. ULEIS investigates particles accelerated in solar energetic particle events, interplanetary shocks, and at the solar wind termination shock. By determining energy spectra, mass composition, and their temporal variations in conjunction with other ACE instruments, ULEIS greatly improves our knowledge of solar abundances, as well as other reservoirs such as the local interstellar medium. ULEIS combines the high sensitivity required to measure low particle fluxes, along with the capability to operate in the largest solar particle or interplanetary shock events. In addition to detailed information for individual ions, ULEIS features a wide range of count rates for different ions and energies that allows accurate determination of particle fluxes and anisotropies over short (few minutes) time scales. [7]

Solar Energetic Particle Ionic Charge Analyzer (SEPICA)[edit]

As of 2008, this instrument is no longer functioning due to failed gas valves.[2]

Solar Wind Ion Mass Spectrometer and Solar Wind Ion Composition Spectrometer (SWICS)[edit]

These two instruments are time-of-flight mass spectrometers, each tuned for a different set of measurements. They analyze the chemical and isotopic composition of solar wind and interstellar matter.[8]

Electron, Proton, and Alpha-particle Monitor (EPAM)[edit]

Solar Wind Electron, Proton and Alpha Monitor (SWEPAM)[edit]

Magnetometer (MAG)[edit]

ACE Real Time Solar Wind (RTSW)[edit]

See also[edit]


  1. ^ "Satellite to aid space weather forecasting". USA Today. June 24, 1999. Retrieved October 24, 2008. 
  2. ^ a b "Advanced Composition Explorer (ACE) Home Page". Retrieved June 29, 2009. 
  3. ^ NASA - NSSDC - Spacecraft - Details
  4. ^ Stone, E.C.; et al. (July 1998). "The Advanced Composition Explorer". Space Science Reviews 86: 1–22. Bibcode:1998SSRv...86....1S. doi:10.1023/A:1005082526237. 
  5. ^ Stone, E.C.; et al. (July 1998). "The Cosmic-Ray Isotope Spectrometer for the Advanced Composition Explorer". Space Science Reviews 86: 285–356. Bibcode:1998SSRv...86..285S. doi:10.1023/A:1005075813033. 
  6. ^ Stone, E.C.; et al. (July 1998). "The Solar Isotope Spectrometer for the Advanced Composition Explorer". Space Science Reviews 86: 357–408. Bibcode:1998SSRv...86..357S. doi:10.1023/A:1005027929871. 
  7. ^ Mason, G.M.; et al. (July 1998). "The Ultra Low Energy Isotope Spectrometer (ULEIS) for the Advanced Composition Explorer". Space Science Reviews 86: 409–448. Bibcode:1998SSRv...86..409M. doi:10.1023/A:1005079930780. 
  8. ^ "ACE/SWICS & ACE/SWIMS". The Solar and Heliospheric Research Group. Archived from the original on 10 August 2006. Retrieved June 30, 2006. 

External links[edit]