ACE (Advanced Composition Explorer)
A NASA solar-terrestrial space weather mission in the Explorer Program (Explorer-71) with the prime objectives to determine: the elemental and isotopic composition of matter, the origin of the elements, the formation of the solar corona and acceleration of the solar wind. The PI for mission is E. C. Stone of Caltech/JPL. 1) 2)
The ACE spacecraft was designed and built at JHU/APL, Laurel, MD. The S/C structure (decks and panels: honeycomb with aluminum alloy facesheet) has two octagonal decks, 1.6 m across and 1 m high; the overall wing span is about 8.3 m. The S/C is spin stabilized with the spin axis Earth/sun pointing (star and sun sensors). Attitude is measured by a star tracker (CT-632 solid-state scanner of BATC) and sun sensors, attitude knowledge is within ± 0.7º (goal of ± 0.5º for the magnetometer). The nominal spin rate is 5 rpm, the S/C is oriented in the Earth/sun direction.
S/C launch mass = 785 kg (includes 189 kg of hydrazine fuel for orbit insertion and maintenance), peak power = 443 W (EOL with EOL defined as 5 years) from four deployable solar panels (each 86.4 cm x 149.9 cm). A NiCd battery (18 cell 12 Ah) is being used. There are two deployable magnetometer booms (magnetometer sensors on ends of boom). Nominal life of the mission is 2 years with a five-year goal. 3) 4) 5)
Figure 1: Line drawing of the ACE spacecraft
The C&DH (Command and Data Handling) subsystem is of JHU/APL design. It utilizes a Harris RTX2010 processor executing the FORTH language. The RTX2010 is fabricated in a CMOS/SOS process that is exceptionally hard to single-event upsets (SEUs), making it suitable for operation through solar flares. Code is stored in electrically erasable/programmable read-only memory (EEPROM) and downloaded into random-access memory (RAM) for execution. The EEPROM can be reloaded on the ground, and the RAM can be patched in flight. Both RAM and EEPROM utilize error detection and correction (EDAC) circuitry to correct single errors and detect double errors. The FPGAs are designed with triple voting cells to minimize the probability of SEUs.
The ACE propulsion subsystem (of Primex) corrects launch vehicle dispersion errors, injects the spacecraft into the L1 halo orbit, adjusts the orbit and spin axis pointing, and maintains a 5 rpm spin rate. The subsystem is a hydrazine blowdown unit that uses nitrogen gas as the pressurant and is made up of four fuel tanks, four axial thrusters for velocity control along the spin axis, and six radial thrusters for spin plane velocity control and spin rate control.
Figure 2: Block diagram of the ACE observatory (image credit: JHU/APL)
For APL developed spacecraft - the 1st generation autonomy capability began with the ACE mission, during which autonomy was first separated from hard-coded software. The ACE autonomy system, in conjunction with hardware-based fault detection and reaction and together with the command and data handling (C&DH) and power subsystems, formed the overall ACE safing strategy. This autonomy system was responsible for preparing the spacecraft for first contact, monitoring component health, monitoring overall spacecraft attitude and maneuver health, and maintaining proper spacecraft component on/off configurations and other autonomous actions to support the recorder and hardware-based reactions. 6)
The ACE autonomy system, which was a facility of C&DH software, was based on a set of autonomy rules. These rules take the form of "if-then" statements that can be loaded into fixed-size memory locations known as bins. When the autonomy system is running, it scans the rules at a regular interval, evaluating each rule in turn and executing any that evaluate to "true."
To program an autonomous behavior, the autonomy designer would construct a rule by defining the telemetry point (a section of the spacecraft's telemetry data block representing a spacecraft sensor value), defining a mask of the telemetry point if needed, selecting the conditional type, defining the A and B values for the conditional types, defining the number of true evaluations before a command is executed, and selecting the command to issue.
The command selected to issue could be a single command or a call to a block of commands to be run in sequence. The sequence of commands could also include pauses in the sequence to provide relative timing of commands. All commands issued from the autonomy facility, whether single commands or the command sequence from a block, are executed at the same priority. Therefore, only a single autonomy rule could control the spacecraft at one time.
Development of the ACE autonomy system established the separation between rules and hard-coded autonomy at APL. Before this development, autonomous behavior was nonexistent or was directly written into the C&DH software for the spacecraft. This rule-based approach to meeting autonomy requirements allowed C&DH design to proceed, even when autonomy conditions and actions had not been fully specified at the mission level.
Launch: The ACE spacecraft was launched on August 25, 1997 on a Delta II 7920 launch vehicle from the KSC (Kennedy Space Center) at Cape Canaveral, FLA, USA. 7)
Orbit: Initial circular orbit parking orbit of 185 km with an inclination of 28.7º. Then transfer trajectory insertion toward the sun (with re-ignition of 2nd stage). Final halo orbit (Lissajous) about the Lagrangian (or Earth-sun libration) point L1 (250 Earth radii toward the sun, or about 1.5 million km from Earth toward the sun).
Figure 3: Schematic view of the halo orbit of ACE at L1, the point of equilibrium between the Earth and sun's gravitational fields (image credit: NASA, Caltech)
RF communications with ACE are in S-band (2097.9806 MHz for the uplink and 2278.35 MHz for the downlink.). Science and engineering data are collected during one 3-4 hour pass per day. Real-time data is transmitted at a data rate of 6.9 kbit/s or 434 bit/s (to NOAA/SEC at Boulder, CO, supporting the solar wind project). Onboard data may be recorded onto a 1 Gbit solid-state recorder (two recorders available) and played back at a rate of 78 kbit/s. The uplink data rate is 1 kbit/s. The telemetry is designed to be compatible with CCSDS (Consultative Committee for Space Data Systems) protocol standards. - The ACE mission is being monitored by NASA/GSFC. The ACE Science Center is at the SRL (Space Radiation Laboratory) of the California Institute of Technology (Caltech), Pasadena, CA.
On January 21, 1998, NOAA/SEC (Space Environment Center) at Boulder, CO, and the ACE project opened up the ACE Real Time Solar Wind (RTSW) monitoring capability to the public. The ACE RTSW network uses a beacon to deliver an operationally useful subset of its space physics data to various ground stations around the world in real-time. The intent is to provide 24 hour coverage of the solar wind parameters and solar energetic particle intensity. The position of ACE at 1.5 million km upstream of Earth offers an hour's warning time of CME (Coronal Mass Ejection) events that can cause geomagnetic storms on Earth. Four ACE instruments supply data to NOAA/SEC for RTSW processing.
The instruments are:
• EPAM (Electron, Proton, and Alpha-particle Monitor) for energetic ions and electrons
• MAG (Magnetic Field Monitor) for magnetic field vectors
• SIS ((Solar Isotope Spectrometer) for high energy particle fluxes
• SWEPAM (Solar Wind Electron, Proton, and Alpha Monitor) for solar wind ions.
Figure 4: Alternate illustration of ACE (image credit: NASA)
Figure 5: Expanded view of the ACE spacecraft structure (image credit: NASA)
Status of the ACE mission:
• April 21, 2016: Most of the cosmic rays that we detect at Earth originated relatively recently in nearby clusters of massive stars, according to new results from NASA's ACE (Advanced Composition Explorer) spacecraft. ACE allowed the research team to determine the source of these cosmic rays by making the first observations of a very rare type of cosmic ray that acts like a tiny timer, limiting the distance the source can be from Earth. 8)
- "Before the ACE observations, we didn't know if this radiation was created a long time ago and far, far away, or relatively recently and nearby," said Eric Christian of NASA's Goddard Space Flight Center in Greenbelt, Maryland. Christian is co-author of a paper on this research published on April 21 in Science. 9)
- Cosmic rays are high-speed atomic nuclei with a wide range of energy — the most powerful race at almost the speed of light. Earth's atmosphere and magnetic field shield us from less-energetic cosmic rays, which are the most common. However, cosmic rays will present a hazard to unprotected astronauts traveling beyond Earth's magnetic field because they can act like microscopic bullets, damaging structures and breaking apart molecules in living cells. NASA is currently researching ways to reduce or mitigate the effects of cosmic radiation to protect astronauts traveling to Mars.
- Cosmic rays are produced by a variety of violent events in space. Most cosmic rays originating within our solar system have relatively low energy and come from explosive events on the sun, like flares and coronal mass ejections. The highest-energy cosmic rays are extremely rare and are thought to be powered by massive black holes gorging on matter at the center of other galaxies. The cosmic rays that are the subject of this study come from outside our solar system but within our Galaxy and are called galactic cosmic rays. They are thought to be generated by shock waves from exploding stars called supernovae.
- The galactic cosmic rays detected by ACE that allowed the team to estimate the age of the cosmic rays, and the distance to their source, contain a radioactive form of iron called Iron-60 (60Fe). It is created inside massive stars when they explode and then blasted into space by the shock waves from the supernova. Some 60Fe in the debris from the destroyed star is accelerated to cosmic-ray speed when another nearby massive star in the cluster explodes and its shock wave collides with the remnants of the earlier stellar explosion.
- 60Fe galactic cosmic rays zip through space at half the speed of light or more, about 150,000 km/s. This seems very fast, but the 60Fe cosmic rays won't travel far on a galactic scale for two reasons. First, they can't travel in straight lines because they are electrically charged and respond to magnetic forces. Therefore they are forced to take convoluted paths along the tangled magnetic fields in our Galaxy. Second, 60Fe is radioactive and over a period of about 2.6 million years, half of it will self-destruct, decaying into other elements (Cobalt-60 and then Nickel-60). If the 60Fe cosmic rays were created hundreds of millions of years or more ago, or very far away, eventually there would be too little left for the ACE spacecraft to detect.
- "Our detection of radioactive cosmic-ray iron nuclei is a smoking gun indicating that there has likely been more than one supernova in the last few million years in our neighborhood of the Galaxy," said Robert Binns of Washington University, St. Louis, Missouri, lead author of the paper.
- "In 17 years of observing, ACE detected about 300,000 galactic cosmic rays of ordinary iron, but just 15 of the radioactive Iron-60," said Christian. "The fact that we see any Iron-60 at all means these cosmic ray nuclei must have been created fairly recently (within the last few million years) and that the source must be relatively nearby, within about 3,000 light years, or approximately the width of the local spiral arm in our Galaxy." A light year is the distance light travels in a year, almost six trillion miles. A few thousand light years is relatively nearby because the vast swarm of hundreds of billions of stars that make up our Galaxy is about 100,000 light years wide.
- There are more than 20 clusters of massive stars within a few thousand light years, including Upper Scorpius (83 stars), Upper Centaurus Lupus (134 stars), and Lower Centaurus Crux (97 stars). These are very likely major contributors to the 60Fe that ACE detected, owing to their size and proximity, according to the research team.
Figure 6: A cluster of massive stars seen with the Hubble Space Telescope. The cluster is surrounded by clouds of interstellar gas and dust called a nebula. The nebula, located 20,000 light-years away in the constellation Carina, contains the central cluster of huge, hot stars, called NGC 3603. Recent research shows that galactic cosmic rays flowing into our solar system originate in clusters like these 8image credit: NASA/U. Virginia/INAF, Bologna, Italy/USRA/Ames/STScI/AURA)
Figure 7: This is a mosaic image — one of the largest ever taken by NASA's Hubble Space Telescope — of the Crab Nebula, a six-light-year-wide expanding remnant of a star's supernova explosion. Recent research shows that galactic cosmic rays flowing into our solar system originate in clusters like these (image credit: NASA/ESA/Arizona State University)
• October 2015: Fuel distribution study of ACE (Advanced Composition Explorer). The spin period to precession period ratio of a non-axisymmetric spin-stabilized spacecraft, the ACE, was used to estimate the remaining mass and distribution of fuel within its propulsion system. This analysis was undertaken once telemetry suggested that two of the four fuel tanks had no propellant remaining, contrary to pre-launch expectations of the propulsion system performance. Numerical integration of possible fuel distributions was used to calculate moments of inertia for the spinning spacecraft. A FFT (Fast Fourier Transform) of output from a dynamics simulation was employed to relate calculated moments of inertia to spin and precession periods. The resulting modeled ratios were compared to the actual spin period to precession period ratio derived from the effect of post-maneuver nutation angle on sun sensor measurements. A Monte Carlo search was performed to tune free parameters using the observed spin period to precession period ratio over the life of the mission. This novel analysis of spin and precession periods indicates that at the time of launch, propellant was distributed unevenly between the two pairs of fuel tanks, with one pair having approximately 20% more propellant than the other pair. Furthermore, it indicates the pair of the tanks with less fuel expelled all of its propellant by 2014 and that approximately 46 kg of propellant remains in the other two tanks, an amount that closely matches the operational fuel accounting estimate. 10)
- ACE is a spin-stabilized spacecraft which orbits the Sun-Earth L1 libration point. Four blow-down fuel tanks are onboard the spacecraft, split into two pairs, pair A and pair B. The propulsion system performed nominally for the first 16 years of the mission. In September 2013 the B1 tank began to report increased temperatures. Subsequently, B1 and B2 tank temperatures increased by 5° C and maneuvers began to periodically underperform by as much as 50% in 2014. It is believed that these performance issues were caused by pressurant escaping into the fuel lines as two of the four onboard fuel tanks ran dry.
- The fuel lines from all four tanks are interconnected; the pressurant lines, on the other hand, only connect tanks in a given pair. Sun sensor data is used to determine if a fuel imbalance was present at the time of launch and if the B tanks expended their fuel in 2014. The ideal gas law is applied, showing the ratio of pressurant in one pair of tanks to that in the other pair, VN2_BtoA, is constant while fuel remains in all tanks and that — given a small initial imbalance in fuel mass but equal at pressure — one pair of tanks could run dry significantly earlier than the other.
- A finite element model generates spacecraft moments of inertia over the past 13 years. An FFT model using sun sensor data translates the history of moments of inertia into a history of predicted spin period to precession period ratios. The predicted ratios are compared to the observed ratios and the coefficient of determination, R2, is calculated to determine the degree of fit. Trials with R2 ≥ 0.988 were collected as ‘successful' trials. A 12,000,000-trial Monte Carlo search contained 175 successful trials. For all successful trials, VN2_BtoA is contained in [1.531, 1.706], indicating that the fuel was unevenly distributed through the life of the mission and that the B tanks became empty in 2014. A second, finer Monte Carlo search is employed to provide best estimates of the fuel distribution, fitting uncertain parameters as necessary.
- In September 2013 a small (0.01%) but unexplained increase in the spacecraft spin rate occurred. Also in September, the B1 tank began to report increased temperatures, as shown in Figure 8. The B1 tank temperature continued to increase to the point where in December 2013 the tank heaters — which had always been on for the length of the mission — began cycling on and off, causing the B1 tank temperature data to be less static. At this point, the mission began investigating a tank thermistor anomaly.
Figure 8: Daily average fuel tank temperature from telemetry (image credit: Honeywell Technology Solutions, NASA)
- However in May 2014, maneuvers began to periodically underperform by 10 – 20%. A retrospective investigation of the earlier increased temperatures and the response of tank B1 to heater cycling as provided by Figure 9 suggested, that tank B1 may be nearly devoid of fuel. This was initially met with some skepticism because the spacecraft propulsion system CDR (Critical Design Review) was interpreted at the time as implying the fuel should remain balanced onboard. 11)
- In November 2014, tank B2 began reporting increased temperatures and by December 2014 was reporting temperature data similar to tank B1. The spacecraft design is such that all four tank heaters are either on or off. Thus, the temperature responses shown in Figure 9 illustrate that both tanks in the B pair are acting in a manner consistent with having little to no fuel to act as a heat sink. Finally, in February 2015, maneuvers began to periodically underperform by as much as 50%.
Figure 9: Tank temperatures during heater cycling (image credit: Honeywell Technology Solutions, NASA)
- The ACE propulsion system utilizes gaseous nitrogen to act as a pressurant in the blow-down hydrazine tanks. Figure 10 is an illustration of the tanks, with the A-pair being the ones more closely aligned with the spacecraft BCS (Body Coordinate System) y axis. The system uses conispherical tanks, with the spacecraft spin forcing the fuel radially outward toward the tank nozzle.
Figure 10: Fuel tank configuration and BCS axes (image credit: Honeywell Technology Solutions, NASA)
- The fuel lines from all four tanks are interconnected. The pressurant lines, on the other hand, only connect tanks in a given pair. This arrangement allows one pair of tanks to have a different amount of pressurant than the other. A difference in the amount of pressurant obscures imbalances in fuel. While the four fuel tanks may report equal pressures, this does not equate to equal fuel masses; rather, it simply means more pressurant is present in the tanks with less fuel. Had both the fuel and pressurant lines been interconnected, the fuel would be equally divided if all pressures were equal.
- Once all fuel has been expelled from a pair of tanks and the corresponding fuel lines, pressurant escapes from the thrusters, causing marked underperformance. Since the fuel lines are interconnected, once a sufficient pressure difference exists between the two pairs of tanks, fuel from one pair can migrate to the empty fuel tanks and/or fuel lines. Once that fuel is expended, the cycle repeats itself. Hence, the design of the propulsion system allows for one pair of tanks to become empty, which then can cause periodic maneuver underperformance. This study uses attitude telemetry to determine if this is the cause of the telemetry and maneuver performance exhibited by ACE.
- Both star scanner and sun sensor telemetry are available from the ACE attitude control system. The sun sensor data is provided more frequently, though. The spin period is determined by the time between sun pulses. A sun pulse is generated by the sun sensor once per spin when the sensor's X-axis crosses zero. The sun pulse timing accuracy is approximately 0.7 ms. On the other hand, the star scanner reports 4 stars every 64 s; for 20 minutes after each maneuver, 10 stars are reported every 16 s. Fine time resolution is needed in this study; consequently, sun sensor data is used and not star scanner data. Figure 11 provides diagrams showing the locations of the sun sensors, star scanner, thrusters.
Figure 11: Locations of sun sensors (SS), star scanner (ST), and thrusters (image credit: Honeywell Technology Solutions, NASA)
- Based on the results of this study, it is very likely that at the time of launch the fuel was not evenly distributed among the four fuel tanks. The pressurant line design and unequal loading of pressurant hid this discrepancy until anomalous propulsion system telemetry began in 2013. All Monte Carlo trials that match (R2 ≥ 0.988) observed spin period to precession period ratios have the B tanks becoming empty in 2014. All spacecraft configuration values perturbed in the Monte Carlo searches performed in this study remain, for successful cases, within one sample standard deviation of pre-launch documented values with one exception — the fuel distribution. The Monte Carlo search estimates 1.621 times as much nitrogen existed in the B tanks than in the A tanks at the time of launch. This corresponds to 55.1% of the fuel being in the A tanks and 44.9% being in the B tanks at launch. Based on the findings of this study, the ACE mission closed the B-side latch valves on March 17, 2015 to prevent pressurant from escaping through the thrusters which thereby causes maneuver underperformance. Since doing so, maneuvers have returned to historically nominal performance (Ref. 10).
• July 2015: The ACE mission is approved to continue planning against the current budget guidelines. Any changes to the guidelines will be handled through the budget formulation process. The ACE mission will be invited to the 2017 Heliophysics Senior Review. 12)
- In August 2015, the ACE mission is completing its 18th year on orbit measuring key parameters of the solar wind from the first Lagrangian orbit point (L1), which is about 1% of the way toward the sun on the sun-Earth line. The ACE instruments measure the IMF (Interplanetary Magnetic Field) as well as the elemental, isotopic, and ionic charge state composition of the energetic nuclei in interplanetary space from the low energies of the solar wind up to the high energies of the galactic cosmic rays. These data are used to study the origins and acceleration of the particles in interplanetary space.
• June 2015: The ACE related research proposed for the 2016–2020 era focuses on five topic areas: (1) solar wind, (2) solar energetic particles, (3) heliospheric and interstellar medium, (4) space weather, and (5) supporting the heliospheric systems observatory. There are four PSGs (Prioritized Science Goals) proposed for the next phase of the ACE mission. These are: (1) Discover the nature and consequences of the changes in the space environment in this unusual solar cycle; (2) Determine where and how energetic particle populations are accelerated; (3) Determine the spatial variations of particles and field in the inner heliosphere with multi-spacecraft studies; and (4) Develop new space weather warnings using L1 data. These topic areas and PSGs are all consistent with the goals and objectives of NASA science and heliophysics research. 13)
- The proposal justifies the request for continued operations in several ways. There is the need to better understand the consequences of the declining phase of the solar cycle and a very low solar minimum (as is anticipated) on the acceleration and transport of solar energetic particles and GCRs (Galactic Cosmic Rays) in the heliosphere. There is new science that can be accomplished with three solar wind satellites near L1. For ACE there is the additional and compelling justification of providing context and supporting data to nearly all other solar, heliosphere, magnetosphere, and ionosphere/thermosphere missions both current and planned.
- ACE value to the HSO (Heliophysics System Observatory): ACE observations contribute to nearly every aspect of heliophysics research making these observations some of the most important to the HSO. The observations at L1 are used to validate models of the acceleration and propagation of CMEs(Coronal Mass Ejections) and SEPs (Solar Energetic Particles). They provide key monitoring of the solar wind and the IMF, which in turn feed the models of the magnetosphere and the near-Earth space environment. Every other mission within the senior review references ACE data as important or even critical to their mission. And the ACE data will provide the context for all of the upcoming missions from Solar Probe and Solar Orbiter, the MMS (Magnetospheric MultiScale) mission in the magnetosphere, and down to the ICON (Ionospheric Connection Explorer) and GOLD (Global-scale Observations of the Limb and Disk) missions in the ionosphere and thermosphere. - ACE data have been used extensively by other non-ACE scientists in research areas that span the entire HSO system. Models of solar wind, CMEs and coronal holes, use ACE data for validation. Models of the magnetosphere and the ionosphere/thermosphere system use ACE solar wind and IMF data as input drivers. The importance of solar wind and IMF data at L1 cannot be over stated.
- The importance of L1 solar wind monitors cannot be over emphasized as is evident by the newly launched DSCOVR spacecraft (launch on Feb. 11, 2015), which will provide operational solar wind and IMF data for the NOAA SWPC (Space Weather Prediction Center) and other operational space weather forecast entities around the world. Having three solar wind monitors as L1 (WIND, ACE, and DSCOVR) emphasizes the importance of the observations but de-emphasizes the contribution of any single observation of these key parameters. However, the research objectives that will be addressed by ACE in the coming years are unique to ACE or they take advantage of the multi-spacecraft constellation that now exists at the L1 point. In addition, ACE solar wind and IMF data will be critical for cross-calibration of the DSCOVR solar wind sensors during the first few years of the DSCOVR mission.
- ACE spacecraft / instrument health and status: For the most part, the ACE spacecraft and the ACE sensors continue to perform well and have exceeded expectations for the satellite. Several of the sensors and detectors have either been adjusted or simply turned off to compensate for this degradation. There has been degradation to the sensitivity of SWEPAM (the primary solar wind sensor) however, through modified procedures and periodic spacecraft maneuvers, the impact of this degradation has been minimized. The operation and analysis of SWICS data had to be modified significantly, but since the modifications were performed, SWICS has provided excellent solar wind composition information. There are no issues with the ACE satellite or with the sensors that would degrade the value and utility of these data for the research that is proposed.
• In early 2015, ACE is in its 18th year on orbit; it is the only current satellite providing realtime solar wind observations from the L1 orbit (Lagrangian point 1), approximately 1.5 million km away from Earth, and it is well past its design life (nominal life of 2 years with a design goal of 5 years).
- The DSCOVR (Deep Space Climate Observatory) mission of NOAA, the successor spacecraft for ACE, was launched on Feb. 11, 2015 and is on its way to L1. DSCOVR is expected to reach L1 in June 2015. Service provision will start some time thereafter when all operational functions have been verified.
• As part of the Space Weather Prediction Center's rollout of our improved website, the content from the ACE realtime Solar Wind Lists page is being provided in a new way. 14)
Figure 12: ACE mission realtime solar wind acquired on Feb. 23, 2015 (image credit: NOAA)
• In 2014, the ACE mission is operating nominally (however, without the SEPICA instrument).
Since January of 1998, ACE has been in orbit around the L1 Lagrangian point, ~1.5 million km sunward of Earth. At this location it has made measurements of the elemental, isotopic, and ionic charge-state composition of energetic nuclei from solar wind to galactic cosmic ray energies. These observations have been used to enhance our understanding of the sources, composition and processes related to the solar wind, solar energetic particles (SEPs), and galactic and anomalous cosmic rays. In addition, continuous measurements of the background and disturbed solar wind, provided by solar wind plasma, energetic particles and magnetic field instruments on ACE, are crucial for space science, as well as for space weather. 15)
In spite of being one of the "older" missions, ACE continues to make significant contributions to new and emerging scientific problems on topics related to the solar wind and ICME (Interplanetary Coronal Mass Ejections), solar and interplanetary energetic particles, cosmic rays and heliosphere/interstellar interactions, and space weather and the science behind space weather. During the next few years, new observations by ACE will be crucial for understanding changes in the solar corona as the recent unusual solar cycle conditions (weak solar minimum and solar maximum) evolve.
• Dec. 2, 2013: The last solar minimum, which extended into 2009, was especially deep and prolonged. Since then, sunspot activity has gone through a very small peak while the heliospheric current sheet achieved large tilt angles similar to prior solar maxima. The solar wind fluid properties and IMF (Interplanetary Magnetic Field) have declined through the prolonged solar minimum and continued to be low through the current "mini" solar maximum. Compared to values typically observed from the mid-1970s through the mid-1990s, the proton parameters are lower on average from 2009 through the day 79 of 2013 by: solar wind speed and beta (~11%); temperature (~40%); thermal pressure (~55%); mass flux (~34%); momentum flux or dynamic pressure (~41%); energy flux (~48%); IMF magnitude (~31%), and radial component of the IMF (~38%). These results have important implications for the solar wind's interaction with planetary magnetospheres and the heliosphere's interaction with the local interstellar medium, with the proton dynamic pressure remaining near the lowest values observed in the space age: ~1.4 nPa, compared to ~2.4 nPa typically observed from the mid-1970s through the mid-1990s. The combination of lower magnetic flux emergence from the Sun (carried out in the solar wind as the IMF) and associated low power in the solar wind points to the causal relationship between them. Our results indicate that the low solar wind output is driven by an internal trend in the Sun that is longer than the ~11-year solar cycle, and suggest that this current weak solar maximum is driven by the same trend. 16) 17)
Figure 13: Weakest solar wind of the Space Age and the current "Mini" Solar Maximum (image credit: CalTech)
Legend to Figure 13: Top — Solar wind dynamic pressure in the ecliptic plane at ~1 AU, taken from IMP-8, Wind, and ACE and inter-calibrated through OMNI-2. Means (red), medians (blue), 25%-75% ranges (dark grey), and 5%-95% ranges (light grey) are shown averaged over complete solar rotations from 1974 through the first quarter of 2013. Bottom — monthly (black) and smoothed (red) sunspot numbers and the current sheet tilt (blue) derived from the WSO radial model [Hoeksema, SSRv, 72, 137, 1995].
• The ACE mission is operating nominally (however, without the SEPICA instrument) in 2013 at L1 for over 15 years. According to SRL (Space Radiation Laboratory) of Caltech, the ACE spacecraft has enough propellant on board to maintain an orbit at L1 until ~2024 (Ref. 22). 18)
• The solar CME (Coronal Mass Ejection) that erupted from the sun on Oct. 4, 2012 at 11:24 p.m. EDT, arrived at Earth on Oct. 8 at 12:30 a.m. EDT, as observed by instruments aboard NASA's ACE (Advanced Composition Explorer) spacecraft. At Earth, when the CME connected up with Earth's magnetic environment, the magnetosphere, it caused a space weather phenomenon called a geomagnetic storm. This storm was categorized by NOAA as a G2 – on a scale from G1 to G5. A storm at this level is considered reasonably mild. Auroras did appear in the north, including Canada, due to this storm. 19)
• The ACE mission is operating nominally in 2012 - providing continuous, real-time space weather data. ACE has been at the L1 point for over 14 years (ACE was 15 years on orbit on August 25, 2012), and the spacecraft and instruments are still working very well, with the exception of the SEPICA instrument (Ref. 21).
- Due to a failure of the valves that control the gas flow through the instrument, active control of the SEPICA proportional counter was lost, and delivery of science data from SEPICA ended on Feb 4 2005. The SEPICA instrument was turned off permanently on April 20, 2011 (Ref. 21).
ACE is a crucial component of NASA's fleet, but its job as sentinel is, in fact, just a small piece of what ACE has accomplished since it launched on August 25, 1997. In its first 15 years, the spacecraft has helped determine the composition of the vast sea of flowing particles surrounding Earth. ACE also serves as a sentinel that helps measure the input — the solar wind — that drives the dynamics of the magnetosphere. 20)
• The ACE mission is operating nominally in 2011.
• The ACE mission is operating "nominally" in 2010 (after > 12 years in orbit at L1). The spacecraft and instruments are still working very well, with the exception of the SEPICA instrument. 21) 22) 23)
- 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 only spacecraft anomalies have been rare, inconsequential, single-bit errors in the solid-state recorders. All propellant line and tank temperatures and pressures have been within nominal limits. Although warmer than predicted, temperatures of the spacecraft sunward top deck and thermal blankets are within specifications. Solar array performance is currently declining by ~1%/year; the power output is predicted to be adequate until ~ 2025. The attitude control, propulsion, RF, and command and data handling systems have all performed nominally.
- Three types of maneuvers (attitude, orbit and spin) have been used since July 2001 to control ACE. Orbit maneuvers use ~1.5 kg/year of fuel per year and keep the spacecraft bound to the L1 libration point. Attitude maneuvers use ~ 3 kg/year and are required to maintain the HGA antenna constraint. With this strategy, fuel use is 4.5 kg/year total, and the 70 kg of fuel remaining as of October 2007 will be consumed by 2024.
- The instruments on the sun-facing deck have experienced higher operating temperatures than expected, due to degradation of the thermal blankets. However, they are still operating nominally and returning excellent science data, and the thermal blanket degradation is a process that slows over time, so we expect temperatures for these instrument to rise only a few degrees over the next 10 years (Ref. 21).
• SEPICA failure in 2005.
• In 2002 the SEPICA instrument experienced problems with the gas flow regulation of its proportional counters and with a high-voltage power supply. Two thirds of the instrument is non-functional, but the third counter is returning good science data. Data of SEPICA is only available until Feb. 4, 2005 when further problems arose resulting in a de facto retirement of the instrument.
• The spacecraft has enough propellant on board to maintain an orbit at L1 until about 2019. Lately, a fuel use strategy has been implemented that will allow continued operations through the year 2022.
Sensor complement: (SWIMS, SWICS, ULEIS, SEPICA, SIS, CRIS, EPAM, SWEPAM, MAG)
To meet observing requirements and to simplify access to the instruments and spacecraft subsystems, all components except the propulsion system are mounted on the external surfaces of the body. Six of the instruments are mounted on the top (sunward facing) deck, and two are mounted on the sides. 24) 25)
Table 1: ACE instrument summary
Figure 14: Overview of the sensor complement on ACE (image credit: JHU/APL)
SWIMS (Solar Wind Ion Mass Spectrometer):
PI: G. Gloeckler, University of Maryland. Objectives of SWIMS: Measurement of solar wind composition data over a wide range of solar wind bulk speeds and for all solar wind conditions. Abundances of most of the elements and several isotopes in the mass range from 4 - 60 amu (atomic mass unit) every few minutes. SWIMS uses a time-of-flight (TOF) measurement technique to determine the mass of a solar wind ion with high accuracy. SWIMS consists of the Wide-Angle, Variable Energy/charge (WAVE) three chamber parallel-plate electrostatic analyzer, the time-of flight High-Mass Resolution Spectrometer (HMRS), high-voltage supplies, and analog and digital electronics.
Figure 15: View of the SWIMS instrument (image credit: NASA)
SWICS (Solar Wind Ion Composition Spectrometer):
PI: G. Gloeckler, U. of Maryland. Objective: measurement of the elemental and ionic-charge composition and the temperature and mean speeds of all major solar wind ions from H through Fe at solar wind speeds ranging from 145 km/s (for protons) to 1532 km/s (for Fe+8). The instrument, which covers an energy per charge range from 16 - 60 keV/Q, combines an electrostatic analyzer with post-acceleration, followed by a time-of-flight (TOF) and energy measurements.
Figure 16: View of the SWICS instrument (image credit: NASA)
SWICS uses 4 components to study the mass and isotopic composition of incoming ions: a) E/q filtering by electrostatic deflection, b) post-acceleration of the filtered ions by up to 30 kV, c) TOF measurement, and d) E measurement using ion-implant solid-state detectors (SSDs).
Knowing the E/q, E, and TOF, one knows the mass (M) and the mass per charge (M/q) since E = (M/2) x v 2. SWICS was the flight spare of the Ulysses GLG experiment, launched in October 1990.
ULEIS (Ultra-low Energy Isotope Spectrometer):
PI: G. Mason, U. of Maryland, R. Gold, JHU/APL. Objective: measurement of ion fluxes over the charge range from He through Ni from about 20 keV/n to 10 MeV/n (superthermal and energetic particle ranges). ULEIS is a time-of-flight (TOF) mass spectrometer which identifies incident ion mass and energy by simultaneously measuring the time-of-flight, τ, and residual kinetic energy, E, of particles which enter the telescope cone and stop in one of the six detectors in the telescope. 26) 27)
Figure 17: View of the ULEIS instrument (image credit: JHU/APL)
SEPICA (Solar Energetic Particle Ionic Charge Analyzer):
PI: E. Möbius, University. of New Hampshire (UNH) and MPE Garching; D. Hovestadt, MPE Garching. Objective: measurement of the ionic charge state, Q, the energy, E, and the nuclear charge, Z, above 0.2 MeV/n. - Energetic particles entering the multi-slit collimator will be electrostatically deflected between the six sets of electrode plates which are supplied with variable high voltages up to 30 kV. The deflection, which is inversely proportional to energy per charge, E/Q, is determined in the back portion of the instrument (dE/dX device and a position-sensitive silicon solid-state detector). The residual energy of the particle, Eres, and the amount of electrostatic deflection is directly determined in the detector, thus yielding the energy per ionic charge, E/Q, of the incoming particle, and its energy, E. 28) 29) 30)
SEPICA consists of three independent sensor units, called "fans". Each of the three fans is symmetric about the plane with the high voltage deflection plate. A schematic view of one individual sensor unit is shown in Figure 18 together with the basic measuring principles. Shown is a single side of one SEPICA fan. Energetic particles enter a multi-slit collimator, which selects those incoming particles that target a narrow line in the detector plane (indicated by F, the "focal line"). They are electrostatically deflected between a set of electrode plates. The curved plate is on ground potential, while the flat center plate is supplied with a positive high voltage up to 30 kV (to be set by telecommand).
Figure 18: Schematic view and principles of operation of the SEPICA instrument (image credit: UNH)
Figure 19: Functional block diagram of SEPICA (image credit: UNH)
Figure 20: View of the SEPICA instrument (image credit: UNH)
Figure 21: SEPICA during the hoist on ACE (image credit: UNH)
SIS (Solar Isotope Spectrometer):
PI: A. Cummings, California Institute of Technology (CalTech). Objective: measurement of elemental and isotopic composition of solar energetic particles, anomalous cosmic rays, and interplanetary particles from He to Zn over the energy range from 10 - 100 MeV/nucleon. Measurements by a technique considering a particle's energy loss ΔE in a detector (multiple ΔE versus residual energy E). SIS has a geometry factor of ~40 cm2 sr, which is significantly larger than previous satellite solar particle isotope spectrometers. It is also designed to provide excellent mass resolution during the extremely high particle flux conditions which occur during large solar particle events.
Spectroscopic observations of solar isotopes are very difficult; there are isotopic observations for only a few elements and the uncertainties are large. With its greatly improved collecting power over other instruments, it is hoped that SIS can make a major advance in our knowledge of SEP (Solar Energetic Particle) isotopic composition. 31)
Anomalous cosmic rays: During solar minimum conditions there are seven elements (H, He, C, N, O, Ne, and Ar) whose energy spectra have shown anomalous increases in flux above the quiet time galactic cosmic ray spectrum. This so-called "anomalous cosmic ray" (ACR) component is now thought to represent neutral interstellar particles that have drifted into the heliosphere, become ionized by the solar wind or UV radiation, and then been accelerated to energies >10 MeV/nucleon, most likely at the solar wind termination shock.
Figure 22: SIS instrument illustration (image credit: NASA)
ACR measurements, free from contamination of solar and interplanetary particles at lower energy, and free from GCR contamination at higher energies, are best made in the energy interval from ~5 to 25 MeV/nucleon, where the flux is a decreasing function of energy. Similarly, SEP spectra typically decrease rather steeply with increasing energy. It follows that to maximize the number of detected particles for both of these species requires the use of thin detectors with as low a threshold for penetration as possible, combined with a large geometry factor. For this reason SIS has two telescopes composed of the largest area devices available (~65 cm2 each).
It is also of interest to extend the SEP measurements to as high an energy as possible to understand the acceleration process in these events. The SIS detector stack is composed of devices of graduated thicknesses to cover a broad energy range. There are two identical telescopes in SIS, each composed of 17 high-purity silicon detectors (Figure 23).
The first two detectors, M1 and M2, are position-sensitive "matrix" devices (Figure 24) that form the hodoscope measuring the trajectory and energy loss of incident nuclei. The matrix detectors are octagonal in shape, 70 to 80 µm in thickness, and have 34 cm2 active areas that are divided into 64 strips. Each of the strips on M1 and M2 is individually pulse-height analyzed with its own 12 bit ADC (Analog Digital Converter) when an event occurs so that the trajectory of heavy ions traversing the system can be separated from the tracks of low energy H or He that might happen to hit one of these detectors at the same time. Detectors M1 and M2 are separated by 6 cm; the resulting rms angular resolution of the system is ~0.25 degrees, averaged over all angles.
Figure 23: Scale drawing of one of the two SIS telescopes (image credit: CalTech)
Figure 24: Illustration of matrix detectors M1 and M2 (image credit: CalTech)
CRIS (Cosmic Ray Isotope Spectrometer):
PI: A. Cummings, California Institute of Technology; T. von Rosenvinge, GSFC; R. Binns, Washington U.; M. Wiedenbeck, JPL. Objective: measurements of all stable and long-lived isotopes of galactic cosmic ray nuclei from He to Zn over the energy range from ~100 to 600 MeV/nucleon. CRIS also provides limited measurements of low energy H isotopes and data for exploratory studies of the isotopes of "ultra-heavy" (UH) nuclei. Measurements by a technique considering a particle's energy loss ΔE in a detector ( multiple ΔE versus residual energy E). CRIS is of CRRES, ISEE-3 and SAMPEX heritage. 32)
The fully assembled CRIS instrument consists of two boxes bolted together (Figure 25). The upper box contains the SOFT system, while the lower box contains the Si(Li) detector stacks with their pulse-height analysis electronics, as well as the main CRIS control electronics. The large window on the top of the SOFT box is the CRIS entrance aperture. The two smaller patches are thermal radiators for cooling the two CCD cameras.
Figure 25: Photo of the CRIS instrument (image credit: CalTech)
Figure 26: Block diagram of CRIS electronics (image credit: CalTech)
EPAM (Electron, Proton, and Alpha-particle Monitor):
PI: R. Gold, JHU/APL. The EPAM instrument is the flight spare unit of the HI-SCALE instrument flown on Ulysses. 33)
Objective: measurement of solar and interplanetary particle fluxes with a wide dynamic range and a directional coverage of nearly a full unit sphere. EPAM consists of five apertures in two telescope assemblies and an associated instrument electronics box.. The EPAM detectors consist of three silicon solid-state detector systems: 1) LEMS (Low Energy Magnetic Spectrometers); 2) LEFS (Low Energy Foil Spectrometers); and 3) CA (Composition Aperture). The LEMS/LEFS provide pulse-height-analyzed single-detector measurements with active anticoincidence. The CA provides elemental composition in an energy range similar to LEMS/LEFS, plus Helium isotope resolution.
EPAM measures ions (Ei ≳50 keV) and electrons (Ee >30 keV) with essentially complete pitch angle coverage from the spinning ACE spacecraft. It also has an ion elemental abundance aperture using a delta-E versus E technique in a three-element telescope. The telescopes use the spin of the spacecraft to sweep the full sky. Solid-state detectors are being used to measure the energy and composition of the incoming particles.
Figure 27: Illustration of the EPAM instrument (image credit:JHU/APL)
SWEPAM (Solar Wind Electron, Proton, and Alpha Monitor):
PI: D. McComas, LANL. SWEPAM is of SWOOPS heritage flown on the Ulysses spacecraft. Objective: high quality measurements of electron and ion fluxes in the low energy solar wind range (electrons: 1 - 1240 eV; ions: 0.26 - 35 keV). SWEPAM is of Ulysses mission heritage. SWEPAM makes simultaneous and independent electron and ion measurements with two separate sensors. Both sensors make use of curved-plate electrostatic analyzers which are spherical sections cut off in the form of a sector. 34) 35)
Figure 28: View of the SWEPAM instruments (image credit: NASA, LANL)
MAG (Magnetic Field Monitor):
PI: N. Ness, U. of Delaware. Objective: measurement of the three components of the magnetic field. MAG is triaxial fluxgate magnetometer, boom-mounted. MAG provides continuous data at 3, 4 or 6 vectors/sec, and snapshot memory data and FFT (Fast Fourier Transform) data based on 24 vectors/sec. acquired on board, working synchronously with blocks of 512 samples (FFT only) each. 36)
Table 2: Summary of MAG instrument characteristics
Figure 29: The MAG instrument (image credit: University of New Hampshire)
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32) E. C. Stone, C. M. S. Cohen,W. R. Cook, A. C. Cummings, B. Gauld, B. Kecman, R. A. Leske, R. A. Mewaldt, M. R. Thayer, B. L. Dougherty, R. L. Grumm, B. D. Milliken, R. G. Radocinski , M. E. Wiedenbeck, E. R. Christian, S. Shuman, H. Trexel, T. T. von Rosenvinge, W. R. Binns, D. J. Crary, P. Dowkontt, J. Epstein, P. L. Hink, J. Klarmann, M. Lijowski, M. A. Olevitch, "The Cosmic Ray Isotope Spectrometer for the Advanced Composition Explorer," URL: http://www.srl.caltech.edu/ACE/CRIS_SIS/cris-ssr-paper.pdf
33) R. E. Gold, S. M. Krimigis, S. E. Hawkins, D. K. Haggerty, D. A. Lohr, E. Fiore, T. Armstrong, G. Holland, L. J. Lanzerotti, "Electron, Proton and Alpha Monitor on the Advanced Composition Explorer spacecraft," Space Science Reviews., Vol. 86, No 1-4, July1998, pp. 541-562.
34) D. J. McComas, S. J. Bame, P. Barker, W. C. Feldman, J. L. Phillips, P. Riley, J. W. Griffee, "Solar Wind Electron Proton Alpha Monitor (SWEPAM) for the Advanced Composition Explorer," Space Science Reviews, Vol. 86, No 1-4, July 1998, pp. 563-612
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).