Space Dust Measurement

Ever since the beginning of the space age satellites and space probes performed space dust measurements. The goal was, initially, to quantify the hazard of meteoroid impacts onto space vehicles. Samples returned to Earth by the Apollo astronauts demonstrated that the lunar surface is peppered by microcraters generated by high speed impacts of cosmic dust particles. Their analysis provided a measurement of the flux of interplanetary dust particles. The later goal of dust measurements was the exploration of the dust environments throughout interplanetary space, within planetary systems, and, today, it is to analyze planetary bodies from which dust particles originate. The development of instrument swent from simple and small but noisy microphone detectors, over large and reliable multi-coincidence dust detectors to sophisticated dust analyzers that determine the composition of individual dust particles.

Overview

From the ground space dust is observed as scattered sun light from myriads of interplanetary dust particles and as meteoroids entering the atmosphere. By observing a meteor from several positions on the ground the trajectory and the entry speed can be determined by triangulation. Atmospheric entry speeds of up to 72,000 m/s have been observed for Leonid meteors. It was obvious that even sub-millimeter sized meteoroids hitting a spacecraft at speeds much faster than bullets at ~300 m/s can cause significant damage. Therefore, the early US Explorer 1, Vanguard 1 and the Soviet Sputnik 3 satellites carried simple 0.001 m² sized microphone dust detectors in order to detect impacts of micron sized meteoroids. The obtained fluxes were orders of magnitude higher than those estimated from zodiacal light measurements. However, the latter determination had big uncertainties in the assumed size and heliocentric radial dust density distributions. Thermal studies in the lab with microphone detectors suggested that the high count rates recorded were due to noise generated by temperature variations in Earth orbit. An excellent review of the early days of space dust research was given by Fechtig, H., Leinert, Ch., and Berg, O. in the book ''Interplanetary Dust''.

Dust accelerators

A critical facility to develop, test, and calibrate space dust instruments is a dust accelerator. Classic guns have muzzle velocities of a few 100 m/s to about 1 km/s whereas meteoroid speeds range from a few km/s to several 100 km/s for nanometer sized dust particles. Only experimental light-gas guns (e.g. at NASA's Johnson Space Center, JSC) reach projectile speeds of several km/s up to 10 km/s in the laboratory. By exchanging the projectile with a sabot containing any dust particles high speed dust projectiles can be used for impact cratering and dust sensor calibration experiments.
The workhorse for hypervelocity dust impact experiments is the electrostatic dust accelerator.
Nanometer to micrometer sized conducting dust particles are electrically are charged and accelerated by an electrostatic particle accelerator to speeds up to 100 km/s. Currently operational dust accelerators exist at IRS in Stuttgart, Germany (formally at Max Planck Institute for Nuclear Physics in Heidelberg) and at Laboratory for Atmospheric and Space Physics (LASP) in Boulder, Colorado. The LASP dust accelerator facility is operational since 2011, and has been used for basic impact studies, as well as for the development of dust instruments. The facility is available for the planetary and space science communities. Dust accelerators are used for impact cratering studies, calibration of impact ionization dust detectors, and meteor studies. Only electrically conducting particles can be used in an electrostatic dust accelerator because the dust source is located in the high-voltage terminal. James F. Vedder, at Ames Research Center, ARC, used a linear particle accelerator by charging dust particles by an ion beam in a quadrupole ion trap under visual control. This way a wide range of dust materials could be accelerated to high speeds.

Reliable dust detections

Tennis court sized (200 m²) penetration detectors on the Pegasus satellites determined a much lower flux of 100 micron sized particles that would not pose a significant hazard to the manned Apollo missions. First reliable dust detections of micron-sized meteoroids were obtained by the dust detectors on board the Pioneer 8/9 and HEOS 2 spacecraft. Both instruments were impact ionization detectors using coincident signals from ions and electrons released upon impact. The detectors had sensitive areas of ~0.01 m² and detected outside the Earth's magnetosphere on average one impact per ten days.

Microcrater analyses

Microcraters on lunar samples (Figure 1) provide an extensive record of impacts onto the lunar surface. Uneroded glass splashes from big impacts covering crystalline lunar rocks preserve well microcraters. Microscopic and scanning electron microscopic analyses result in microcrater densities on a single sample of craters with diameters ranging from 10⁻⁸ to 10⁻³ m. Impact crater sizes were related to meteoroid masses based on dust accelerator simulations assuming an effective impact speed onto the lunar surface of 20 km/s. Since no reliable surface exposure age could be determined from solar flare track densities on the same rocks spacecraft measurements were used to determine the interplanetary dust flux at 1 AU (Figure 2). The crater production flux at 100 μm size (~ 10 m⁻⁹ kg) was adjusted to the flux observed by the Pegasus satellites. The flux of micron sized and smaller meteoroids is smaller than the respective cratering flux on the lunar surface because of the significant fast ejecta flux from nearby impacts of bigger meteoroids. This flux was adjusted to the meteoroid flux observed by the HEOS-2 and Pioneer 8/9 space probes.

For 5.7 years from April 1984 to January 1990 the NASA's Long Duration Exposure Facility exposed several passive impact collectors of several square meters area to the space dust environment in low Earth orbit. After recovery of LDEF by Space Shuttle Columbia the instrument trays were returned to and analyzed by the investigators. The results generally confirmed the earlier analysis of lunar microcaters.

Optical and infrared zodiacal dust observations

Zodiacal light observations at different heliocentric distances were performed by the Zodiacal light photometer instruments on Helios 1 and 2 and the Pioneer 10 and 11 space probes between 0.3 AU and 3.3 AU from the sun. This way the heliocentric radial profile was determined which varied by a factor approx. 100 over that distance. The Asteroid Meteoroid Detector, AMD, on Pioneer 10 and 11 used the optical detection and triangulation of individual meteoroids to get information on their sizes and trajectories. Unfortunately, the trigger threshold was set too low that noise corrupted the data. Zodiacal light observations at visible light wavelengths use the light scattered by interplanetary dust particles which is only a few percent of the incoming light. The remainder of ca. 90% is absorbed and reradiated at infrared (IR) wavelengths.

The IR brightness of the Zodiacal dust cloud is much stronger than the brightness at visible wavelengths and dominates the sky brightness. However, on the ground most of the thermal IR sky brightness is blocked by atmospheric absorption bands, therefore, most of IR astronomy observations are done from space observatory satellites. The Infrared Astronomical Satellite, IRAS, mapped the sky at 12, 25, 60 and 100 micrometers wavelength. At 12 to 60 microns wavelengths zodiacal dust was a prominent feature. Later the Diffuse Infrared Background Experiment, DIRBE, on NASA's COBE mission provided a complete high precision survey of the Zodiacal dust cloud at the same wavelengths.

Already IRAS infrared sky maps showed structure in the sky brightness. Within the wide general Zodiacal cloud and a broad central asteroidal band there were several narrow cometary trails. Follow-up observations using the Spitzer Space Telescope showed that at least 80% of all Jupiter family comets had trails. When the Earth passes through a comet trail a meteor stream is observed form the ground. Because the enhanced risk to spacecraft in such meteoroid streams ESA developed the IMEX model which followed the evolution of cometary particles and hence allows us to determine the risk at specific positions and times in the inner solar system.

Penetration detectors

In the early 1960th engineering type pressurized cell micrometeorite detectors were flown on the Explorer 16 and Explorer 23 satellites. Each satellite carried more than 200 individual gas filled pressurized cells of 25 and 50 micron metal wall thickness. A puncture of a cell by a meteoroid impact was detected by a pressure sensor. These instruments provided important measurements of the near Earth meteoroid flux. In 1972 and 1973 the Pioneer 10 and 11 interplanetary spacecraft carried 234 pressurized cell detectors each mounted on the back of the main dish antenna. The stainless steel wall thickness was 25 micron on Pioneer 10, and 50 micron on Pioneer 11. The two instruments characterized the meteoroid environment in the outer solar system as well as near Jupiter and near Saturn.

In preparation of the Apollo Missions to the moon three Pegasus satellites were launched by the Saturn 1 rocket into near Earth orbit. Each satellite carried 416 individual meteoroid detectors totaling approx. 200 m² detection surface. The detectors consisted of 171 m² of 400 micron, 16 m² of 200 micron, and 7.5 m² of 40 micron thick aluminum penetration sheets. Placed behind the penetration sheets were 12 micron thick mylar capacitor detectors that recorded penetrations of the overlying sheet. The results showed that the meteoroid hazard is significant and meteoroid protection methods must be implemented for large space vehicles.

In 1986 John Simpson flew on the Vega 1 and 2 missions a new dust detector using polyvinylidene difluoride PVDF films. This material responds to dust impacts by generating electrical charge due to impact cratering or penetration. Since PVDF detectors are also sensitive to mechanical vibrations and energetic particles, detectors using PVDF work acceptably well as High rate dust detectors, HRD, in high dust density dust environments like in cometary comae or on the Cassini Cosmic Dust Analyzer in planetary rings. The Dust Flux Monitor Instrument, DFMI, on the Stardust mission used also PVDF detectors to study dust in the coma of comet Wild 2. In interplanetary space, however, the PVDF detectors on the Venetia Burney Student Dust Counter needed reference detectors which are shielded from dust impacts in order to determine the background noise rate.

Modern microphone detectors

A 1 mm thick front shield (Whipple shield of 1.85 m diameter) together with the 12 mm thick Kevlar rear shield protected the Giotto spacecraft from dust from Halley's Comet during its flyby at 600 km distance. Mounted on the front dust shield were three piezoelectric momentum sensor of the Dust Impact Detection System, DIDSY. A fourth momentum sensor was mounted on the rear sheet. These microphone detectors together with other detectors measured the dust distribution within the inner coma of comet 1P/Halley. This instrument measured also dust during Giotto encounter with comet 26P/Grigg–Skjellerup.

On the Mercury Magnetospheric Orbiter of the BepiColombo mission the Mercury Dust Monitor (MDM) will measure the interplanetary and Mercury dust environments. MDM is composed of four piezoelectric ceramic sensors made of lead zirconate titanate, PZT. Waveforms of the impact signals will be recorded and analyzed.

Chance dust detectors

Most instruments on a spacecraft flying through a dense dust environment will see some effects of dust impacts. A prominent example of such an instrument was the Plasma Wave Subsystem, PWS, on Voyager 1 and 2 spacecraft. PWS provided useful information on the local dust environment. Initially, the Asteroid Meteoroid Detector, AMD, previously flown on Pioneer 10 and 11 was preliminarily selected for the Voyager payload. However, because there were doubts about its performance the instrument was deselected and, hence, no dedicated dust instrument was carried bý the Voyager 1 and 2 spacecraft.

During the Voyager 2 flythrough through of the Saturn system PWS detected intense impulsive noise centered on the ring plane at 2.88 Saturn radii distance, slightly outside of the G ring. This noise was attributed to small micron-sized particles hitting the spacecraft. In situ dust detections by the Cassini Cosmic Dust Analyzer and camera observations of the outer rings confirmed the existence of an extended G ring. Also during Voyager's flybys of Uranus and Neptune dust concentrations in the equatorial planes were observed. During the flyby of comet 21P/Giacobini-Zinner by the ICE spacecraft dust impacts were observed by the plasma wave instrument. In the meantime plasma wave instruments on various spacecraft claimed the detection of dust but only in 2021 a model for the generation of signals on plasma wave antennas by dust impacts was presented based on dust accelerator tests.

Impact ionization detectors

Impact ionization detectors are the most successful dust detectors in space. With help of these detectors the interplanetary dust environment from Venus to Jupiter was explored. Impact ionization detectors use the simultaneous detection of positive ions and electrons upon dust impact on a solid target. This coincidence provides a means to discriminate from noise signals on a single channel. The first successful dust detector in interplanetary space at ~ 1AU was flown on the Pioneer 8 and 9 space probes. The Pioneer 8 and 9 detectors had sensitive target areas of 0.01 m². Besides interplanetary dust on eccentric orbits it detected dust on hyperbolic orbits leaving the planetary system. The HEOS 2 dust detector was the first detector that employed a hemispherical geometry like all the following Galileo, Ulysses, and LDEX detectors on the LADEE mission. The hemispherical target of 0.01 m² area collected electrons from the impact and the ions were collected by the central ion collector. These signals served to determine the mass and speed of the impacted meteoroid. The HEOS 2 dust detector explored the Earth dust environment within 10 Earth radii.

The Galileo and Ulysses Dust Detectors were optimized for outer planet system dust measurements. The twin instruments were optimized for interplanetary dust measurements in the outer planetary system. The sensitive target areas were 10 fold increased to 0.1 m² in order to cope with the expected low dust fluxes. In order to provide reliable dust impact data even within the harsh Jovian environment an electron channeltron was added in the center of the ion grid collector. This way an impact was detected by triple coincidence of three charge signals. The 2.5 tons Galileo spacecraft was launched in 1989 and cruised for 6 years interplanetary space between Venus’ and Jupiter's orbit and measured interplanetary dust. The 370 kg Ulysses spacecraft was launched a year later and went on a direct trajectory to Jupiter which it reached in 1992 for a swing-by maneuver which put the spacecraft on a heliocentric orbit of 80 degrees inclination. In 1995 Galieo started its 7-year path through the Jovian system with several fly-bys of all Galilean moons. After Jupiter fly-by Ulysses identified a flow of interstellar dust sweeping through the Solar System and hyper-velocity streams of nano-dust which are emitted from Jupiter and then couple to the solar magnetic field. In addition the Galileo instrument detected ejecta clounds around the Galilean moons.

The Lunar Dust Experiment (LDEX) on board the Lunar Atmosphere and Dust Environment Explorer (LADEE) Mission is a smaller version of the Galileo and Ulysses dust detectors. The most sensitive impact charge detector is a microchannel plate (MCP) behind the central focusing grid. LDEX has a sensitive area of 0.012 m². The objective of the instrument was the detection and analysis of the lunar dust environment. From 16 October 2013 to 18 April 2014 LDEX detected about 140 000 dust hits at an altitude of 20 to 100 km above the lunar surface. It found a tenuous and permanent, asymmetric ejecta cloud around the Moon that is caused by meteoroid impacts onto the lunar surface. From this data it was found that approximately 40 μm/Myr of lunar regolith is redistributed due to meteoritic bombardment. Besides a continuous meteoroid bombardment meteoroid streams cause temporary enhancements of the ejecta cloud

Dust composition analyzers

The Helios Micrometeoroid Analyzer was the in situ instrument to analyze the composition of cosmic dust. In 1974 the instrument was carried by the Helios spacecraft inside the Earth's orbit down to 0.3 AU from the sun. The goal of the Micrometeoroid Analyzer was to determine the spatial distribution of the dust in the inner planetary system, and to search for variations of the compositional and physical properties of micrometeoroids. The instrument consisted of two impact ionization time-of-flight mass spectrometers (Ecliptic and South sensor) of total ~0.01 m² target area. One sensor was shielded by the spacecraft rim from direct sun light, whereas the other sensor was protected by a thin aluminized parylene film from intense solar radiation. The Helios micrometeoroid analyzers were calibrated with a wide range of materials at the dust accelerators of the Max Planck Institute for Nuclear Physics in Heidelberg and of the Ames Research Center, ARC, in Moffet Field. The mass resolution of the mass spectra of the Helios sensors was low $R=\cfrac$ ~ 10. There was an excess of impacts recorded by the South sensor over the impacts on the Ecliptic sensor. On the basis of the penetration studies with the Helios film this excess was interpreted to be due to low density, $\rho$ < 1000 kg/m³ meteoroids that were shielded from entering the Ecliptic sensor. Helios mass spectra range from those with dominant low masses up to 30 u that are compatible with silicates to those with dominant high masses between 50 and 60 u of iron and molecular ion types. Meteoroid streams and even interstellar dust particles were identified in the data.

Twin dust mass analyzers were flown on the 1986 comet Halley missions Vega 1, Vega 2, and Giotto. The spacecraft flew by Halleys comet at 600 to 10,00 km distance at 70 to 80 km/s. The twin PUMA (Vega) and PIA (Giotto) instruments were developed by Jochen Kissel of the Max Planck Institute for Nuclear Physics in Heidelberg. Dust particle hitting the small (~ 5 cm²) impact target generated ions by impact ionization. The instruments were high mass resolution (''R'' ~ 100) reflectron type time-of-flight mass spectrometers. The instruments could record up to 500 impacts per second. During Halley fly bys the instruments recorded and abundance of small (<10⁻¹⁴ g) particles. Besides unequilibrated silicates many of the particles were rich in light elements such as H, C, N and O. This suggests that most particles consisted of a predominantly chondritic core with a refractory organic mantle.

The Cometary and Interstellar Dust Analyzer (CIDA) was flown on the Stardust mission. In January 2004 Stardust flew by comet comet Wild 2 in a distance of 240 km at a relative speed of 6.1 km/s. In February 2011 Stardust flew by comet Tempel 1 in a distance of 181 km at a speed of 10.9 km/s. During interplanetary cruise between the comet encounters there were favorable phases to analyze the interstellar dust stream that was earlier discovered by the Ulysses space probe. CIDA is a derivative of the impact ionization mass spectrometers flown on the Giotto and Vega 1 and Vega 2 missions. The impact target is peeking out to the side of the spacecraft while the main part of the instrument is protected from high speed dust. It has a sensitive area ~100 cm² and a mass resolution ''R'' ~ 250. Besides the positive ion mode CIDA has also a negative ion mode for better sensitivity for organic molecules. The 75 spectra obtained during the comet flybys indicate a dominance of organic matter; also sulfur ions were detected in one spectrum. In the 45 spectra obtained during the cruise phase favorable for detection of interstellar particles, derivates of quinone were suggested as constituents of the organic component.

The Cosmic Dust Analyzer (CDA) was flown on the Cassini mission to Saturn. CDA is a large-area (0.1 m² total sensitive area) multi-sensor dust instrument that includes a 0.01 m² medium resolution (''R'' = 20 to 50) chemical dust analyzer, a 0.09 m² highly reliable impact ionization detector, and two high rate polarized polyvinylidene fluoride (PVDF) detectors with sensitive areas of 0.005 m² and 0.001 m², respectively
. During its 6-year cruise to Saturn CDA analysed interplanetary dust, the stream of interstellar dust, and Jupiter dust streams. A highlight was the detection of electrical dust charges in interplanetary space and in Saturn's magnetosphere
During the following 13 years Cassini completed 292 orbits around Saturn (2004 to 2017) and measured several million dust impacts that characterize dust mostly in Saturn's E ring
. In 2005 during Cassini's close flyby of Enceladus within 175 km from the surface CDA discovered active ice geysers
. Detailed compositional analyses found salt-rich water ice grains close to Enceladus that led to the discovery of large reservoirs of liquid water oceans below the icy crust of Enceladus. Analyses of interstellar grains at Saturn's distance suggest magnesium-rich grains of silicate and oxide composition, some with iron inclusions

Dust Telescopes

A Dust Telescope is an instrument to perform Dust Astronomy that not only analyses the signals and ions that are generated by a dust impact on the sensitive target but also determines the dust trajectory prior to the impact. The latter is based on the successful measurement of the dust electric charge by CDA. A dust Trajectory Sensor consists of four planes of parallel position sensing wire electrodes. Dust accelerator tests show that dust trajectories can be determined to an accuracy of 1% in velocity and 1° in direction. The second element of a Dust Telescope is a Large-area Mass Analyzer. A Large-area Mass Analyzer is a reflectron type time-of-flight mass analyzer of up to 0.2 m² sensitive area of mass resolution ''R'' > 150. It consists of a circular plate target with the ion detector behind the center hole. In font of the target is an acceleration grid. Ions generated by an impact are reflected by a paraboloid shaped grid onto the center ion detector. Prototypes of dust telescope have been built at the Laboratory for Atmospheric and Space Physics (LASP) of the University of Colorado in Boulder (USA)
and at the Institute of Space Systems of the University of Stuttgart and tested at their dust accelerators.

The Surface Dust Analyser to asteroid 3200 Phaethon. Phaethon is believed to be the origin of the Geminids meteor stream that can be observed from the ground every December. DDA development is led by Ralf Srama and colleagues from the Institute of Space Systems (IRS) at the University of Stuttgart in cooperation with von Hoerner & Sulger GmbH (vH&S) company. DDA's will analyze interstellar and interplanetary dust on cruise to Phaethon and will study its dust environment during the encounter; of particular interest is the proportion of organic matter. Launch is planned for 2024.

The Interstellar Dust Experiment (IDEX) will fly on the Interstellar Mapping and Acceleration Probe (IMAP) in orbit about the Sun–Earth L1 Lagrange point. IDEX is developed by Mihaly Horanyi and colleagues at the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado Boulder. IDEX is a large-area (0.07 m²) dust analyzer that provides the mass distribution and elemental composition of interstellar and interplanetary dust particles. A laboratory version of the IDEX instrument was used at the dust accelerator facility operated at University of Colorado to collect impact ionization mass spectra for a range of dust samples of known composition. Launch is planned for 2025.

Collected dust analyses

The importance of lunar samples and lunar soil for dust science was that they provided a meteoroid impact cratering record. Even more important are cosmochemical aspects. From their isotopic, elemental, molecular, and mineralogical compositions important conclusions about e.g. the origin of the Moon like the giant-impact hypothesis can be drawn. From 1969 to 1972 six Apollo missions collected 382 kilograms of lunar rocks and soil. These samples are available for research and teaching projects. From 1970 to 1976 three Luna spacecraft returned 301 grams lunar material. Recently, in 2020 Chang'e 5 collected 1,7 kg of lunar material.

In 1950 Fred Whipple showed that micrometeoroid smaller than a critical size (~100 micrometers) are decelerated at altitudes > 100 km slowly enough to radiate the frictional power away without melting. Such micrometeorites sediment through the atmosphere and, finally, deposit on the ground. The most efficient method to collect micrometeorites is by high (~20 km) flying aircraft with special silicon oil covered collectors that capture this dust. At lower altitudes these micrometeorites become mixed with Earth dust. Don Brownlee first reliably identified the extraterrestrial nature of collected dust particles by their chondritic composition. These stratospheric dust samples are available for further research.

Stardust was the first mission to return samples from a comet (Wild 2) and from interstellar space. In January 2004, Stardust flew by Comet Wild 2 at a distance of 237 km with a relative velocity of 6.1 km/s. One side of the 0.104 m² aerogel and 0.015 m² aluminium foil dust collector was exposed to the flow of cometary dust.
The Stardust cometary samples were a mix of different components that included presolar grains like ¹³C-rich SiC grains, a wide range of chondrule-like fragments, and high temperature condensates like calcium-aluminum inclusions (CAIs) found in primitive meteorites that were transported to cold nebular regions.

From March to May 2000 and from July to December 2002 the spacecraft was in favourable position to collect interstellar dust on the back side of the sample collector.
After return of the sample return capsule in January 2006 the collector trays were inspected and thousands of grains from Comet Wild 2 and seven probable interstellar grains were identified. These grains are available for teaching and research from the NASA Astromaterials Curation Office

The first asteroid samples were returned by the JAXA Hayabusa missions. Hyabusa encountered asteroid 25143 Itokawa in November 2005, picked up surface samples, and returned to Earth in June 2010. Despite some problems during sample collection thousands 10 to 100 micron sized particles are available for research in the laboratories. The second Hayabusa2 mission rendezvoused with asteroid 162173 Ryugu in June 2018. About 5 g surface and sub-surface material from this primitive C-type asteroid were returned. About 10% of the collected samples JAXA is sharing with NASA sample curation.

The Rosetta space probe orbited comet 67P/Churyumov–Gerasimenko from August 2014 to September 2016. During this time Rosetta's instruments analyzed the nucleus, the dust, gas, and plasma environment. Rosetta carried a suite of miniaturized sophisticated lab instruments to study collected cometary dust particles. Among them was the high-resolution secondary ion mass spectrometer ''COSIMA'' (Cometary Secondary Ion Mass Analyzer) that analyzed the rocky and organic composition of collected dust particles, an atomic force microscope ''MIDAS'' (Micro-Imaging Dust Analysis System) that investigated morphology and physical properties of micrometer-sized dust particles that were deposited on a collector plate, and the double-focus magnetic mass spectrometer (DFMS) and the reflectron type time of flight mass spectrometer (RTOF) of ''ROSINA'' (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) to analyze cometary gas and the volatile components of cometary particulates. Rosetta's Philae lander carried the gas chromatography–mass spectrometry COSAC experiment to analyze organic molecules in the comet's atmosphere and on its surface

See also

* Cosmic Dust Analyzer
* Galileo and Ulysses Dust Detectors
* Helios Dust Instrumentation
* Surface Dust Analyser

References

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Space exploration
Cosmic dust
Astrobiology
Astrochemistry
Extragalactic astronomy
Galactic astronomy
Planetary science