Advanced Composition Explorer
||This article contains orbital elements but does not include an epoch, or date when those elements, which typically vary over time, were correct.|
An artist's concept of ACE
|Mission type||Solar research|
|Mission duration||5 years planned
Elapsed: 17 years, 1 month and 5 days
|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, 14:39:00UTC|
|Rocket||Delta II 7920-8|
|Launch site||Cape Canaveral LC-17A|
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. 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. NASA Goddard Space Flight Center managed the development and integration of the ACE spacecraft.
- 1 Science Objectives
- 2 Instrumentation
- 2.1 Cosmic Ray Isotope Spectrometer (CRIS)
- 2.2 Solar Isotope Spectrometer (SIS)
- 2.3 Ultra Low Energy Isotope Spectrometer (ULEIS)
- 2.4 Solar Energetic Particle Ionic Charge Analyzer (SEPICA)
- 2.5 Solar Wind Ion Mass Spectrometer (SWIMS) and Solar Wind Ion Composition Spectrometer (SWICS)
- 2.6 Electron, Proton, and Alpha-particle Monitor (EPAM)
- 2.7 Solar Wind Electron, Proton and Alpha Monitor (SWEPAM)
- 2.8 Magnetometer (MAG)
- 2.9 ACE Real Time Solar Wind (RTSW)
- 3 See also
- 4 References
- 5 External links
ACE observations allow the investigation of a wide range of fundamental problems in the following four major areas:
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. 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
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
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
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)
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. 
Solar Isotope Spectrometer (SIS)
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. 
Ultra Low Energy Isotope Spectrometer (ULEIS)
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. 
Solar Energetic Particle Ionic Charge Analyzer (SEPICA)
The Solar Energetic Particle Ionic Charge Analyzer (SEPICA) was the instrument on the Advanced Composition Explorer (ACE) that determined the ionic charge states of solar and interplanetary energetic particles in the energy range from ≈0.2 MeV nucl-1 to ≈5 MeV charge-1. The charge state of energetic ions contains key information to unravel source temperatures, acceleration, fractionation and transport processes for these particle populations. SEPICA had the ability to resolve individual charge states with a substantially larger geometric factor than its predecessor ULEZEQ on ISEE-1 and -3, on which SEPICA was based. To achieve these two requirements at the same time, SEPICA was composed of one high-charge resolution sensor section and two low- charge resolution, but large geometric factor sections.
As of 2008, this instrument is no longer functioning due to failed gas valves.
Solar Wind Ion Mass Spectrometer (SWIMS) and Solar Wind Ion Composition Spectrometer (SWICS)
The Solar Wind Ion Composition Spectrometer (SWICS) and the Solar Wind Ions Mass Spectrometer (SWIMS) on ACE are instruments optimized for measurements of the chemical and isotopic composition of solar and interstellar matter. SWICS determined uniquely the chemical and ionic-charge composition of the solar wind, the thermal and mean speeds of all major solar wind ions from H through Fe at all solar wind speeds above 300 km s-1 (protons) and 170 km s-1 (Fe+16), and resolved H and He isotopes of both solar and interstellar sources. SWICS also measured the distribution functions of both the interstellar cloud and dust cloud pickup ions up to energies of 100 keV e-1. SWIMS measures the chemical, isotopic and charge state composition of the solar wind for every element between He and Ni. Each of the two instruments are time-of-flight mass spectrometers and use electrostatic analysis followed by the time-of-flight and, as required, an energy measurement.
As of August, 2011, the SWICS time-of-flight has failed, limiting the composition measurements to the abundant species.
Electron, Proton, and Alpha-particle Monitor (EPAM)
The Electron, Proton, and Alpha Monitor (EPAM) instrument on the ACE spacecraft is designed to measure a broad range of energetic particles over nearly the full unit-sphere at high time resolution. Such measurements of ions and electrons in the range of a few tens of keV to several MeV are essential to understand the dynamics of solar flares, co-rotating interaction regions (CIR’s), interplanetary shock acceleration, and upstream terrestrial events. The large dynamic range of EPAM extends from about 50 keV to 5 MeV for ions, and 40 keV to about 350 keV for electrons. To complement its electron and ion measurements, EPAM is also equipped with a Composition Aperture (CA) which unambiguously identifies ion species reported as species group rates and/or individual pulse-height events. The instrument achieves its large spatial coverage through fife telescopes oriented at various angles to the spacecraft spin axis. The low-energy particle measurements, obtained as time resolutions between 1.5 and 24 s, and the ability of the instrument to observe particle anisotropies in three dimensions make EPAM an excellent resource to provide the interplanetary context for studies using other instruments on the ACE spacecraft. 
Solar Wind Electron, Proton and Alpha Monitor (SWEPAM)
The Solar Wind Electron Proton Alpha Monitor (SWEPAM) experiment provides the bulk solar wind observations for the Advanced Composition Explorer (ACE). These observations provide the context for elemental and isotopic composition measurements made on ACE as well as allowing the direct examination of numerous solar wind phenomena such as coronal mass ejections, interplanetary shocks, and solar wind fine structure, with advanced, 3-D plasma instrumentation. They also provide an ideal data set for both heliospheric and magnetospheric multi-spacecraft studies where they can be used in conjunction with other, simultaneous observations from spacecraft such as Ulysses. The SWEPAM observations are made simultaneously with independent electron (SWEPAM-e) and ion (SWEPAM-i) instruments. In order to save costs for the ACE project, SWEPAM-e and SWEPAM-i are the recycled flight spares from the joint NASA/ESA Ulysses mission. Both instruments had selective refurbishment, modification, and modernization required to meet the ACE mission and spacecraft requirements. Both incorporate electrostatic analyzers whose fan-shaped fields of view sweep out all pertinent look directions as the spacecraft spins. 
The magnetic field experiment on ACE provides continuous measurements of the local magnetic field in the interplanetary medium. These measurements are essential in the interpretation of simultaneous ACE observations of energetic and thermal particles distributions. The experiment consists of a pair of twin, boom- mounted, triaxial fluxgate sensors which are located 165 inches (=4.19 m) from the center of the spacecraft on opposing solar panels. The two triaxial sensors provide a balanced, fully redundant vector instrument and permit some enhanced assessment of the spacecraft's magnetic field. 
ACE Real Time Solar Wind (RTSW)
The Advanced Composition Explorer (ACE) RTSW system is continuously monitoring the solar wind and producing warnings of impending major geomagnetic activity, up to one hour in advance. Warnings and alerts issued by NOAA allow those with systems sensitive to such activity to take preventative action. The RTSW system gathers solar wind and energetic particle data at high time resolution from four ACE instruments (MAG, SWEPAM, EPAM, and SIS), packs the data into a low-rate bit stream, and broadcasts the data continuously. NASA sends real-time data to NOAA each day when downloading science data. With a combination of dedicated ground stations (CRL in Japan and RAL in Great Britain), and time on existing ground tracking networks (NASA's DSN and the USAF's AFSCN), the RTSW system can receive data 24 hours per day throughout the year. The raw data are immediately sent from the ground station to the Space Weather Prediction Center in Boulder, Colorado, processed, and then delivered to its Space Weather Operations Center where they are used in daily operations; the data are also delivered to the CRL Regional Warning Center at Hiraiso, Japan, to the USAF 55th Space Weather Squadron, and placed on the World Wide Web. The data are downloaded, processed and dispersed within 5 min from the time they leave ACE. The RTSW system also uses the low-energy energetic particles to warn of approaching interplanetary shocks, and to help monitor the flux of high-energy particles that can produce radiation damage in satellite systems. 
- Tom Krimigis
- Cluster (spacecraft)
- Helios (spacecraft)
- MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging), launched 2004, still operational.
- Solar Dynamics Observatory (SDO), launched 2010, still operational.
- Solar and Heliospheric Observatory (SOHO), launched 1995, still operational.
- Solar Maximum Mission (SMM), launched 1980, decommissioned 1989.
- Solar Orbiter (SOLO), set to launch in 2017.
- Solar Probe Plus, set to launch in 2018.
- STEREO (Solar TErrestrial RElations Observatory), launched 2006, still operational.
- Transition Region and Coronal Explorer (TRACE), launched 1998, decommissioned 2010.
- Ulysses (spacecraft), launched 1990, decommissioned 2009.
- Van Allen Probes
- "Satellite to aid space weather forecasting". USA Today. June 24, 1999. Retrieved October 24, 2008.
- "Advanced Composition Explorer (ACE) Home Page". Retrieved June 29, 2009.
- NASA - NSSDC - Spacecraft - Details
- 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.
- 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.
- 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.
- 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.
- Moebius, E.; et al. (July 1998). "The Solar Energetic Particle Ionic Charge Analyzer (SEPICA) and the Data Processing Unit (S3DPU) for SWICS, SWIMS and SEPICA". Space Science Reviews 86: 449–495. Bibcode:1998SSRv...86..449M. doi:10.1023/A:1005084014850.
- Gloeckler, G.; et al. (July 1998). "Investigation of the composition of solar and interstellar matter using solar wind and pickup ion measurements with SWICS and SWIMS on the ACE spacecraft". Space Science Reviews 86: 497–539. Bibcode:1998SSRv...86..497G. doi:10.1023/A:1005036131689.
- "ACE/SWICS & ACE/SWIMS". The Solar and Heliospheric Research Group. Archived from the original on 10 August 2006. Retrieved June 30, 2006.
- Gold, R.E.; et al. (July 1998). "Electron, Proton, and ALpha Monitor on the Advanced Composition Explorer Spacecraft". Space Science Reviews 86: 541–562. Bibcode:1998SSRv...86..541G. doi:10.1023/A:1005088115759.
- McComas, D.J.; et al. (July 1998). "Solar Wind Electron Proton Alpha Monitor (SWEPAM) for the Advanced Composition Explorer". Space Science Reviews 86: 563–612. Bibcode:1998SSRv...86..563M. doi:10.1023/A:1005040232597.
- Smith, C.W.; et al. (July 1998). "The ACE Magnetic Fields Experiment". Space Science Reviews 86: 613–632. Bibcode:1998SSRv...86..613S. doi:10.1023/A:1005092216668.
- Zwickl, R.D.; et al. (July 1998). "The NOAA Real-Time Solar-Wind (RTSW) System using ACE Data". Space Science Reviews 86: 633–648. Bibcode:1998SSRv...86..633Z. doi:10.1023/A:1005044300738.
|Wikimedia Commons has media related to ACE (spacecraft).|
- "Advanced Composition Explorer (ACE) Home Page". ACE Science Center. July 1, 2003. Archived from the original on 5 July 2006. Retrieved June 30, 2006.