Advanced Composition Explorer

Advanced Composition Explorer (ACE) is a NASA Explorers 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 NOAA Space Weather Prediction Center to improve forecasts and warnings of solar storms.[1] The ACE robotic spacecraft was launched August 25, 1997, and entered a Lissajous orbit close to the L1 Lagrangian point (which lies between the Sun and the Earth at a distance of some 1.5 million km from the latter) on December 12, 1997.[2] The spacecraft is currently operating at that orbit. Because ACE is in a non-Keplerian orbit, and has regular station-keeping maneuvers, the orbital parameters in the adjacent information box are only approximate.

As of 2019, the spacecraft is still in generally good condition, and is projected to have enough propellant to maintain its orbit until 2024.[3] NASA Goddard Space Flight Center managed the development and integration of the ACE spacecraft.[4]

Advanced Composition Explorer
Advanced Composition Explorer
An artist's concept of ACE
Mission typeSolar research
COSPAR ID1997-045A
SATCAT no.24912
Mission duration5 years planned
Elapsed: 21 years, 5 months and 20 days
Spacecraft properties
ManufacturerJohns Hopkins Applied Physics Laboratory
Launch mass757 kilograms (1,669 lb)
Dry mass562 kilograms (1,239 lb)
Power444 W End-of-Life (5 years)
Start of mission
Launch dateAugust 25, 1997, 14:39:00 UTC
RocketDelta II 7920-8
Launch siteCape Canaveral LC-17A
Orbital parameters
Reference systemheliocentric
RegimeL1 Lissajous
Semi-major axis148,100,000 kilometers (92,000,000 mi)
Perigee145,700,000 kilometres (90,500,000 mi)
Apogee150,550,000 kilometres (93,550,000 mi)
Period1 year
ACE mission logo
Advanced Composition Explorer in space
ACE in orbit around the Sun–Earth L1 point

Science objectives

ACE observations allow the investigation of a wide range of fundamental problems in the following four major areas:[5]

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 have been 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 have been used to:

  • Search for differences between the isotopic composition of solar and meteoritic material.
  • Determine the contributions of solar–wind and solar energetic particles to lunar 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 are 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 have been 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. [6]

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. [7]

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. [8]

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.[9]

As of 2008, this instrument is no longer functioning due to failed gas valves.[3]

Solar Wind Ions 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.[10][11]

On 23 August 2011, the SWICS time-of-flight electronics experienced an age- and radiation-induced hardware anomaly that increased the level of background in the composition data. To mitigate the effects of this background, the model for identifying ions in the data was adjusted to take advantage of only the ion energy-per-charge as measured by the electrostatic analyzer, and the ion energy as measured by solid state detectors. This has allowed SWICS to continue to deliver a subset of the data products that were provided to the public prior to the hardware anomaly, including ion charge state ratios of oxygen and carbon, and measurements of solar wind iron. The measurements of proton density, speed, and thermal speed by SWICS were not affected by this anomaly and continue to the present day.[3]

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 five 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. [12]

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 ejection, 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. [13]

Magnetometer (MAG)

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. [14]

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. [15]

Science results

The spectra of particles observed by ACE

ACE O Fluence
Oxygen fluences observed by ACE

The figure shows the particle fluence (total flux over a given period of time) of oxygen at ACE for a time period just after solar minimum, the part of the 11-year solar cycle when solar activity is lowest.[16] The lowest-energy particles come from the slow and fast solar wind, with speeds from about 300 to about 800 kilometers per second. Like the solar wind distribution of all ions, that of oxygen has a suprathermal tail of higher-energy particles; that is, in the frame of the bulk solar wind, the plasma has an energy distribution that is approximately a thermal distribution but has a notable excess above about 5 kiloelectron volts, as shown in Figure 1. The ACE team has made contributions to understanding the origins of these tails and their role in injecting particles into additional acceleration processes.

At energies higher than those of the solar wind particles, ACE observes particles from regions known as corotating interaction regions (CIRs). CIRs form because the solar wind is not uniform. Due to solar rotation, high-speed streams collide with preceding slow solar wind, creating shock waves at roughly 2–5 astronomical units (AU, the distance between Earth and the Sun) and forming CIRs. Particles accelerated by these shocks are commonly observed at 1 AU below energies of about 10 megaelectron volts per nucleon. ACE measurements confirm that CIRs include a significant fraction of singly charged helium formed when interstellar neutral helium is ionized.[17]

At yet higher energies, the major contribution to the measured flux of particles is due to solar energetic particles (SEPs) associated with interplanetary (IP) shocks driven by fast coronal mass ejections (CMEs) and solar flares. Enriched abundances of helium-3 and helium ions show that the suprathermal tails are the main seed population for these SEPs.[18] IP shocks traveling at speeds up to about 2000 kilometers per second accelerate particles from the suprathermal tail to 100 megaelectron volts per nucleon and more. IP shocks are particularly important because they can continue to accelerate particles as they pass over ACE and thus allow shock acceleration processes to be studied in situ.

Other high-energy particles observed by ACE are anomalous cosmic rays (ACRs) that originate with neutral interstellar atoms that are ionized in the inner heliosphere to make “pickup” ions and are later accelerated to energies greater than 10 megaelectron volts per nucleon in the outer heliosphere. ACE also observes pickup ions directly; they are easily identified because they are singly charged. Finally, the highest-energy particles observed by ACE are the galactic cosmic rays (GCRs), thought to be accelerated by shock waves from supernova explosions in our galaxy.

Other findings from ACE

Shortly after launch, the SEP sensors on ACE detected solar events that had unexpected characteristics. Unlike most large, shock-accelerated SEP events, these were highly enriched in iron and helium-3, as are the much smaller, flare-associated impulsive SEP events.[19][20] Within the first year of operations, ACE found many of these “hybrid” events, which led to substantial discussion within the community as to what conditions could generate them.[21]

One remarkable recent discovery in heliospheric physics has been the ubiquitous presence of suprathermal particles with common spectral shape. This shape unexpectedly occurs in the quiet solar wind; in disturbed conditions downstream from shocks, including CIRs; and elsewhere in the heliosphere. These observations have led Fisk and Gloeckler [22] to suggest a novel mechanism for the particles’ acceleration.

Another discovery has been that the current solar cycle, as measured by sunspots, CMEs, and SEPs, has been much less magnetically active than the previous cycle. McComas et al.[23] have shown that the dynamic pressures of the solar wind measured by the Ulysses satellite over all latitudes and by ACE in the ecliptic plane are correlated and were declining in time for about 2 decades. They concluded that the Sun had been undergoing global change that affected the overall heliosphere. Simultaneously, GCR intensities were increasing and in 2009 were the highest recorded during the past 50 years.[24] GCRs have more difficulty reaching Earth when the Sun is more magnetically active, so the high GCR intensity in 2009 is consistent with a globally reduced dynamic pressure of the solar wind.

ACE also measures abundances of cosmic ray nickel-59 and cobalt-59 isotopes; these measurements indicate that a time longer than the half-life of nickel-59 with bound electrons (7.6 × 104 years) elapsed between the time nickel-59 was created in a supernova explosion and the time cosmic rays were accelerated.[25] Such long delays indicate that cosmic rays come from the acceleration of old stellar or interstellar material rather than from fresh supernova ejecta. ACE also measures an iron-58/iron-56 ratio that is enriched over the same ratio in solar system material.[26] These and other findings have led to a theory of the origin of cosmic rays in galactic superbubbles, formed in regions where many supernovae explode within a few million years. Recent observations of a cocoon of freshly accelerated cosmic rays in the Cygnus superbubble by the Fermi gamma-ray observatory[27] support this theory.

Follow-on space weather observatory

On February 11, 2015, the Deep Space Climate Observatory (DSCOVR)—with several similar instruments including a newer and more sensitive instrument to detect Earth-bound coronal mass ejections—successfully launched by NOAA and NASA aboard a SpaceX Falcon 9 launch vehicle from Cape Canaveral, Florida. The spacecraft arrived at L1 by 8 June 2015, just over 100 days after launch.[28] Along with ACE, both will provide space weather data as long as ACE can continue to function.[29]

See also


  1. ^ "Satellite to aid space weather forecasting". USA Today. June 24, 1999. Archived from the original on October 18, 2009. Retrieved October 24, 2008.
  2. ^
  3. ^ a b c Christian, Eric R.; Davis, Andrew J. (February 10, 2017). "Advanced Composition Explorer (ACE) Mission Overview". California Institute of Technology. Retrieved December 14, 2017.
  4. ^ NASA - NSSDC - Spacecraft - Details
  5. ^ 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.
  6. ^ 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.
  7. ^ 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.
  8. ^ 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.
  9. ^ 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.
  10. ^ 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.
  11. ^ "ACE/SWICS & ACE/SWIMS". The Solar and Heliospheric Research Group. Archived from the original on August 10, 2006. Retrieved June 30, 2006.
  12. ^ 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.
  13. ^ 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.
  14. ^ 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.
  15. ^ 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.
  16. ^ Mewaldt, R.A.; et al. (2001). "Long-term fluences of energetic particles in the heliosphere". AIP Conf. Proc. 86: 165–170. Bibcode:2001AIPC..598..165M. doi:10.1063/1.1433995.
  17. ^ Möbius, E.; et al. (2002). "Charge states of energetic (~ 0.5 MeV/n) ions in corotating interaction regions at 1 AU and implications on source populations". Geophys. Res. Lett. 29 (2): 1016. Bibcode:2002GeoRL..29.1016M. doi:10.1029/2001GL013410.
  18. ^ Desai, M.I.; et al. (2001). "Acceleration of 3He nuclei at interplanetary shocks". Astrophysical Journal. 553 (1): L89–L92. Bibcode:2001ApJ...553L..89D. doi:10.1086/320503.
  19. ^ Cohen, C.M.S.; et al. (1999). "Inferred charge states of high energy solar particles from the solar isotope spectrometer on ACE". Geophys. Res. Lett. 26 (2): 149–152. Bibcode:1999GeoRL..26..149C. doi:10.1029/1998GL900218.
  20. ^ Mason, G.M.; et al. (1999). "Particle acceleration and sources in the November 1997 solar energetic particle events". Geophys. Res. Lett. 26 (2): 141–144. Bibcode:1999GeoRL..26..141M. doi:10.1029/1998GL900235.
  21. ^ Cohen, C.M.S.; et al. (2012). "Observations of the longitudinal spread of solar energetic particle events in solar cycle 24". AIP Conf. Proc. 1436: 103–109. Bibcode:2012AIPC.1436..103C. doi:10.1063/1.4723596.
  22. ^ Fisk, L.A.; et al. (2008). "Acceleration of suprathermal tails in the solar wind". Astrophysical Journal. 686 (2): 1466–1473. Bibcode:2008ApJ...686.1466F. doi:10.1086/591543.
  23. ^ McComas, D.J.; et al. (2008). "Weaker solar wind from the polar coronal holes and the whole Sun". Geophys. Res. Lett. 35 (18): L18103. Bibcode:2008GeoRL..3518103M. doi:10.1029/2008GL034896.
  24. ^ Leske, R.A.; et al. (2011). "Anomalous and galactic cosmic rays at 1 AU during the cycle 23/24 solar minimum". Space Sci. Rev. 176 (1–4): 253–263. Bibcode:2013SSRv..176..253L. doi:10.1007/s11214-011-9772-1.
  25. ^ Wiedenbeck, M.E.; et al. (1999). "Constraints on the time delay between nucleosynthesis and cosmic-ray acceleration from observations of 59Ni and 59Co". Astrophysical Journal. 523 (1): L61–L64. Bibcode:1999ApJ...523L..61W. doi:10.1086/312242.
  26. ^ Binns, W.R.; et al. (2005). "Cosmic-ray neon, Wolf-Rayet stars, and the superbubble origin of galactic cosmic rays". Astrophysical Journal. 634 (1): 351–364. arXiv:astro-ph/0508398. Bibcode:2005ApJ...634..351B. doi:10.1086/496959.
  27. ^ Ackermann, M.; et al. (2011). "A cocoon of freshly accelerated cosmic rays detected by Fermi in the Cygnus superbubble". Science. 334 (6059): 1103–7. Bibcode:2011Sci...334.1103A. doi:10.1126/science.1210311. PMID 22116880.
  28. ^ "Nation's first operational satellite in deep space reaches final orbit". NOAA. June 8, 2015. Archived from the original on June 8, 2015. Retrieved June 8, 2015.
  29. ^ Graham, William (8 February 2015). "SpaceX Falcon 9 ready for DSCOVR mission". Retrieved 8 February 2015.

External links

David J. McComas

David John McComas (born May 22, 1958) is an American space plasma physicist, Vice President for Princeton Plasma Physics Laboratory, and Professor of Astrophysical Sciences at Princeton University. He had been Assistant Vice President for Space Science and Engineering at the Southwest Research Institute, full Adjoint Professor of Physics at the University of Texas at San Antonio (UTSA), and was the founding director of the Center for Space Science and Exploration at Los Alamos National Laboratory. He is noted for his extensive accomplishments in experimental space plasma physics, including leading instruments and missions to study the heliosphere and solar wind: Ulysses/SWOOPS, ACE/SWEPAM, IBEX, TWINS, and Parker Solar Probe. He received the 2014 COSPAR Space Science Award and the NASA Exceptional Public Service Medal.

Halloween solar storms, 2003

The Halloween solar storms were a series of solar flares and coronal mass ejections that occurred from mid-October to early November 2003, peaking around October 28–29. This series of storms generated the largest solar flare ever recorded by the GOES system, modeled as strong as X45 (initially estimated at X28 due to saturation of GOES' detectors). Satellite-based systems and communications were affected, aircraft were advised to avoid high altitudes near the polar regions, and a one-hour-long power outage occurred in Sweden as a result of the solar activity. Aurorae were observed at latitudes as far south as Texas and the Mediterranean countries of Europe.The SOHO satellite failed temporarily, and the Advanced Composition Explorer (ACE) was damaged by the solar activity. Numerous other spacecraft were damaged or experienced downtime due to various issues. Some of them were intentionally put into safe mode in order to protect sensitive equipment. Astronauts aboard the International Space Station (ISS) had to stay inside the more shielded parts of the Russian Orbital Segment to protect themselves against the increased radiation levels. Both the Ulysses spacecraft which was near Jupiter at the time, and Cassini, approaching Saturn, were able to detect the emissions. In April 2004, Voyager 2 was also able to detect them as they reached the spacecraft. One of the solar storms was compared by some scientists in its intensity to the Carrington Event of 1859.These events occurred during solar cycle 23, approximately three years after its peak in 2000, which was marked by another occurrence of solar activity known as the Bastille Day Flare.


The term heliophysics means "physics of the Sun" (the prefix "helio", from Attic Greek hḗlios, means Sun), and appears to have been used only in that sense until quite recently. In the early times, heliophysics was concerned principally with the superficial layers of the star, and was synonymous with what is now more commonly called "solar physics". Usage was extended explicitly in 1981 to its literal meaning, denoting the physics of the entire Sun: from center to corona, and has been used in that sense since. As such it was a direct translation from the French héliophysique, which had been introduced to provide a distinction from physique solaire (solar physics). It thus became a subdiscipline of heliology. Early in the 21st century the meaning of the term was extended by Dr George Siscoe of Boston University to include the physics of the heliosphere (the space around the Sun beyond the corona, in principle out to the shock where the solar wind encounters the interstellar medium, but excluding the planets and other condensed bodies), although Siscoe's view of the discipline appears not to contain most of the true realm of endeavour. The term was adopted in Siscoe's restricted sense by the NASA Science Mission Directorate to denote the study of the heliosphere and the objects that interact with it—most notably planetary atmospheres and magnetospheres, the solar corona, and the interstellar medium. Heliophysics combines several other disciplines, including solar physics, and stellar physics in general, and also several branches of nuclear physics, plasma physics, space physics and magnetospheric physics. Solar wind interaction with magnetized planets, Solar wind propagation, Solar activity effects on planetary magnetospheres. Solar magnetic field configuration from the Sun to the Heliopause. The recent extension of heliophysics is closely tied to the study of space weather and the phenomena that affect it, and consequently to space climate and to terrestrial climatology. To quote Siscoe from a recent conference presentation:

Heliophysics [encompasses] environmental science, a unique hybrid between meteorology and astrophysics, comprising a body of data and a set of paradigms (general laws—perhaps mostly still undiscovered) specific to magnetized plasmas and neutrals in the heliosphere interacting with themselves and with gravitating bodies and their atmospheres.

"Heliophysics" is now the name of one of four divisions within NASA's Science Mission Directorate (Earth Science, Planetary Science, Heliophysics, and Astrophysics). The title was used to simplify the name of the "Sun--Solar-System Connections" Division (and before that, the "Sun-Earth Connections" Division).

NASA's restricted use of the term heliophysics has also been adopted in naming the International Heliophysical Year in 2007-2008.

Heliophysics Science Division

The Heliophysics Science Division of the Goddard Space Flight Center (NASA) conducts research on the Sun, its extended solar system environment (the heliosphere), and interactions of Earth, other planets, small bodies, and interstellar gas with the heliosphere. Division research also encompasses geospace—Earth's uppermost atmosphere, the ionosphere, and the magnetosphere—and the changing environmental conditions throughout the coupled heliosphere (solar system weather).

Scientists in the Heliophysics Science Division develop models, spacecraft missions and instruments, and systems to manage and disseminate heliophysical data. They interpret and evaluate data gathered from instruments, draw comparisons with computer simulations and theoretical models, and publish the results. The Division also conducts education and public outreach programs to communicate the excitement and social value of NASA heliophysics.

Lagrangian point

In celestial mechanics, the Lagrangian points ( also Lagrange points, L-points, or libration points) are the points near two large bodies in orbit where a smaller object will maintain its position relative to the large orbiting bodies. At other locations, a small object would go into its own orbit around one of the large bodies, but at the Lagrangian points the gravitational forces of the two large bodies, the centripetal force of orbital motion, and (for certain points) the Coriolis acceleration all match up in a way that cause the small object to maintain a stable or nearly stable position relative to the large bodies.

There are five such points, labeled L1 to L5, all in the orbital plane of the two large bodies, for each given combination of two orbital bodies. For instance, there are five Lagrangian points L1 to L5 for the Sun-Earth system, and in a similar way there are five different Langrangian points for the Earth-Moon system. L1, L2, and L3 are on the line through the centers of the two large bodies. L4 and L5 each form an equilateral triangle with the centers of the large bodies. L4 and L5 are stable, which implies that objects can orbit around them in a rotating coordinate system tied to the two large bodies.

Several planets have trojan satellites near their L4 and L5 points with respect to the Sun. Jupiter has more than a million of these trojans. Artificial satellites have been placed at L1 and L2 with respect to the Sun and Earth, and with respect to the Earth and the Moon. The Lagrangian points have been proposed for uses in space exploration.

Lissajous orbit

In orbital mechanics, a Lissajous orbit (pronounced [ʒu]), named after Jules Antoine Lissajous, is a quasi-periodic orbital trajectory that an object can follow around a Lagrangian point of a three-body system without requiring any propulsion. Lyapunov orbits around a Lagrangian point are curved paths that lie entirely in the plane of the two primary bodies. In contrast, Lissajous orbits include components in this plane and perpendicular to it, and follow a Lissajous curve. Halo orbits also include components perpendicular to the plane, but they are periodic, while Lissajous orbits are not.In practice, any orbits around Lagrangian points L1, L2, or L3 are dynamically unstable, meaning small departures from equilibrium grow over time. As a result, spacecraft in these Lagrangian point orbits must use their propulsion systems to perform orbital station-keeping. Although they are not perfectly stable, a modest effort of station keeping keeps a spacecraft in a desired Lissajous orbit for a long time.

In the absence of other influences, orbits about Lagrangian points L4 and L5 are dynamically stable so long as the ratio of the masses of the two main objects is greater than about 25. The natural dynamics keep the spacecraft (or natural celestial body) in the vicinity of the Lagrangian point without use of a propulsion system, even when slightly perturbed from equilibrium. These orbits can however be destabilized by other nearby massive objects. For example, orbits around the L4 and L5 points in the Earth–Moon system can last only a few million years instead of billions because of perturbations by the planets.

List of heliophysics missions

This is a list of missions supporting heliophysics, including solar observatory missions, solar orbiters, and spacecraft studying the solar wind.

List of objects at Lagrangian points

This is a list of known objects which occupy, have occupied, or are planned to occupy any of the five Lagrangian points of two-body systems in space.

Orbital station-keeping

In astrodynamics, the orbital maneuvers made by thruster burns that are needed to keep a spacecraft in a particular assigned orbit are called orbital station-keeping.

For many Earth satellites the effects of the non-Keplerian forces, i.e. the deviations of the gravitational force of the Earth from that of a homogeneous sphere, gravitational forces from Sun/Moon, solar radiation pressure and air drag, must be counteracted.

The deviation of Earth's gravity field from that of a homogeneous sphere and gravitational forces from Sun/Moon will in general perturb the orbital plane. For a sun-synchronous orbit the precession of the orbital plane caused by the oblateness of the Earth is a desirable feature that is part of the mission design but the inclination change caused by the gravitational forces of Sun/Moon is undesirable. For geostationary spacecraft the inclination change caused by the gravitational forces of the Sun & Moon must be counteracted by a rather large expense of fuel, as the inclination should be kept sufficiently small for the spacecraft to be tracked by a non-steerable antenna.

For spacecraft in low orbits the effects of atmospheric drag must often be compensated for. For some missions this is needed simply to avoid re-entry; for other missions, typically missions for which the orbit should be accurately synchronized with Earth rotation, this is necessary to avoid the orbital period shortening.

Solar radiation pressure will in general perturb the eccentricity (i.e. the eccentricity vector), see Orbital perturbation analysis (spacecraft). For some missions this must be actively counter-acted with manoeuvres. For geostationary spacecraft the eccentricity must be kept sufficiently small for a spacecraft to be tracked with a non-steerable antenna. Also for Earth observation spacecraft for which a very repetitive orbit with a fixed ground track is desirable, the eccentricity vector should be kept as fixed as possible. A large part of this compensation can be done by using a frozen orbit design, but for the fine control manoeuvres with thrusters are needed.

For spacecraft in a halo orbit around a Lagrangian point station-keeping is even more fundamental, as such an orbit is unstable; without an active control with thruster burns the smallest deviation in position/velocity would result in the spacecraft leaving the orbit completely.


An orbiter is a space probe that orbits a planet or other astronomical object.


The RTX2010 manufactured by Intersil is a radiation hardened stack machine microprocessor which has been used in numerous spacecraft.

Solar Maximum Mission

The Solar Maximum Mission satellite (or SolarMax) was designed to investigate Solar phenomena, particularly solar flares. It was launched on February 14, 1980. The SMM was the first satellite based on the Multimission Modular Spacecraft bus manufactured by Fairchild Industries, a platform which was later used for Landsats 4 and 5 as well as the Upper Atmosphere Research Satellite.

After an attitude control failure in Nov 1980 it was put in standby mode until April 1984 when it was repaired by a Shuttle mission.

The Solar Maximum Mission ended on December 2, 1989, when the spacecraft re-entered the atmosphere and burned up over the Indian Ocean.

Solar and Heliospheric Observatory

The Solar and Heliospheric Observatory (SOHO) is a spacecraft built by a European industrial consortium led by Matra Marconi Space (now Astrium) that was launched on a Lockheed Martin Atlas II AS launch vehicle on December 2, 1995 to study the Sun. SOHO has also discovered over 3,000 comets. It began normal operations in May 1996. It is a joint project of international cooperation between the European Space Agency (ESA) and NASA. Originally planned as a two-year mission, SOHO continues to operate after over 20 years in space: the mission is extended until the end of 2020 with a likely extension until 2022.In addition to its scientific mission, it is the main source of near-real-time solar data for space weather prediction. Along with the GGS Wind, Advanced Composition Explorer (ACE) and DSCOVR, SOHO is one of four spacecraft in the vicinity of the Earth–Sun L1 point, a point of gravitational balance located approximately 0.99 astronomical unit (AU)s from the Sun and 0.01 AU from the Earth. In addition to its scientific contributions, SOHO is distinguished by being the first three-axis-stabilized spacecraft to use its reaction wheels as a kind of virtual gyroscope; the technique was adopted after an on-board emergency in 1998 that nearly resulted in the loss of the spacecraft.

Solar wind

The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma consists of mostly electrons, protons and alpha particles with kinetic energy between 0.5 and 10 keV. Embedded within the solar-wind plasma is the interplanetary magnetic field. The solar wind varies in density, temperature and speed over time and over solar latitude and longitude. Its particles can escape the Sun's gravity because of their high energy resulting from the high temperature of the corona, which in turn is a result of the coronal magnetic field.

At a distance of more than a few solar radii from the Sun, the solar wind is supersonic and reaches speeds of 250 to 750 kilometers per second. The flow of the solar wind is no longer supersonic at the termination shock. The Voyager 2 spacecraft crossed the shock more than five times between 30 August and 10 December 2007. Voyager 2 crossed the shock about a billion kilometers closer to the Sun than the 13.5-billion-kilometer distance where Voyager 1 came upon the termination shock. The spacecraft moved outward through the termination shock into the heliosheath and onward toward the interstellar medium. Other related phenomena include the aurora (northern and southern lights), the plasma tails of comets that always point away from the Sun, and geomagnetic storms that can change the direction of magnetic field lines.

Space physics

Space physics is the study of plasmas as they occur naturally in the Earth's upper atmosphere (aeronomy) and within the Solar System. As such, it encompasses a far-ranging number of topics, such as heliophysics which includes the solar physics of the Sun: the solar wind, planetary magnetospheres and ionospheres, auroras, cosmic rays, and synchrotron radiation. Space physics is a fundamental part of the study of space weather and has important implications in not only to understanding the universe, but also for practical everyday life, including the operations of communications and weather satellites.

Space physics is distinct from astrophysical plasma and the field of astrophysics, which studies similar plasma phenomena beyond the Solar System. Space physics utilizes in situ measurements from high altitude rockets and spacecraft, in contrast to astrophysical plasma that relies deduction of theory and astronomical observation.

Stamatios Krimigis

Stamatios (Tom) M. Krimigis (Greek: Σταμάτιος Κριμιζής) is a Greek-American scientist in space exploration. He has contributed to many of the United States' unmanned space exploration programs of the Solar System and beyond. He has contributed to exploration missions to almost every planet of the Solar System. In 1999, the International Astronomical Union named the asteroid 8323 Krimigis (previously 1979 UH) in his honor.

Unmanned spacecraft

Unmanned or uncrewed spacecraft are spacecraft without people on board, used for unmanned spaceflight. Uncrewed spacecraft may have varying levels of autonomy from human input; they may be remote controlled, remote guided or even autonomous, meaning they have a pre-programmed list of operations, which they will execute unless otherwise instructed. Many habitable spacecraft also have varying levels of robotic features. For example, the space stations Salyut 7 and Mir, and the ISS module Zarya were capable of remote guided station-keeping, and docking maneuvers with both resupply craft and new modules. The most common uncrewed spacecraft categories are robotic spacecraft, uncrewed resupply spacecraft, space probes and space observatories. Not every uncrewed spacecraft is a robotic spacecraft; for example, a reflector ball is a non-robotic uncrewed spacecraft.

Whistler (radio)

A whistler is a very low frequency or VLF electromagnetic (radio) wave generated by lightning. Frequencies of terrestrial whistlers are 1 kHz to 30 kHz, with a maximum amplitude usually at 3 kHz to 5 kHz. Although they are electromagnetic waves, they occur at audio frequencies, and can be converted to audio using a suitable receiver. They are produced by lightning strikes (mostly intracloud and return-path) where the impulse travels along the Earth's magnetic field lines from one hemisphere to the other. They undergo dispersion of several kHz due to the slower velocity of the lower frequencies through the plasma environments of the ionosphere and magnetosphere. Thus they are perceived as a descending tone which can last for a few seconds. The study of whistlers categorizes them into Pure Note, Diffuse, 2-Hop, and Echo Train types.

Voyager 1 and 2 spacecraft detected whistler-like activity in the vicinity of Jupiter, implying the presence of lightning there.

This page is based on a Wikipedia article written by authors (here).
Text is available under the CC BY-SA 3.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.