Space dust measurement refers to the study of small particles of extraterrestrial material, known as micrometeoroids or interplanetary dust particles (IDPs), that are present in the Solar System. These particles are typically of micrometer to sub-millimeter size and are composed of a variety of materials including silicates, metals, and carbon compounds. The study of space dust is important as it provides insight into the composition and evolution of the Solar System, as well as the potential hazards posed by these particles to spacecraft and other space-borne assets. The measurement of space dust requires the use of advanced scientific techniques such as secondary ion mass spectrometry (SIMS), optical and atomic force microscopy (AFM), and laser-induced breakdown spectroscopy (LIBS) to accurately characterize the physical and chemical properties of these 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.
Even sub-millimeter sized meteoroids hitting spacecraft at speeds around 300 m/s (much faster than bullets) 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 dust accelerator is a critical facility to develop, test, and calibrate space dust instruments. Classic guns have muzzle velocities between just a few 100 m/s and 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 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 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 the Laboratory for Atmospheric and Space Physics (LASP) in Boulder, Colorado. The LASP dust accelerator facility has been 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 crewed Apollo missions. The first reliable dust detections of micron sized meteoroids were obtained by the dust detectors on board the Pioneer 8 and 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 approximately 0.01 m2 and detected outside the Earth's magnetosphere on average one impact per ten days.
Microcrater analyses
Microcraters on lunar samples provide an extensive record of impacts onto the lunar surface. Uneroded glass splashes from big impacts covering crystalline lunar rocks preserve microcraters well.
The number of microcraters was measured on a single rock sample using microscopic and scanning electron microscopic analyses. The craters ranged in size from 10−8 to 10−3 m, and were correlated to the mass of meteoroids based on impact simulations. The impact speed onto the lunar surface was assumed to be 20 km/s. The age of the rocks on the surface could not be determined through traditional methods (counting the solar flare track densities), so spacecraft measurements by the Pegasus satellites were used to determine the interplanetary dust flux, specifically the crater production flux at 100 μm size. The flux of smaller meteoroids was found to be smaller than the observed cratering flux on the lunar surface due to fast ejecta from impacts of bigger meteoroids. The flux was adjusted using data from the HEOS-2 and Pioneer 8/9 space probes.
From April 1984 to January 1990, NASA's Long Duration Exposure Facility exposed several passive impact collectors (each a few square meters in area) to the space dust environment in low Earth orbit. After recovery of LDEF by the Space Shuttle Columbia, the instrument trays were analyzed. The results generally confirmed the earlier analysis of lunar microcraters.
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 Pioneer 11 space probes, ranging between 0.3 AU and 3.3 AU from the sun. This way, the heliocentric radial profile was determined, and shown to vary by a factor of about 100 over that distance. The Asteroid Meteoroid Detector (AMD) on Pioneer 10 and Pioneer 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, and noise corrupted the data. Zodiacal light observations at visible light wavelengths use the light scattered by interplanetary dust particles, which constitute only a few percent of the incoming light. The remainder (over 90%) is absorbed and reradiated at infrared wavelengths.
The zodiacal dust cloud is much brighter at infrared wavelengths than visible wavelengths. However, on the ground, most of these infrared wavelengths are blocked by atmospheric absorption bands. Therefore, most infrared astronomy observations are done from space observatory satellites. The Infrared Astronomical Satellite (IRAS) mapped the sky at wavelengths of 12, 25, 60, and 100 micrometers. Between wavelengths of 12 and 60 microns, 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.
IRAS sky maps showed structure in the sky brightness at infrared wavelengths. In addition to 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 shower is observed from the ground. Due to the enhanced risk to spacecraft in such meteoroid streams, the European Space Agency developed the IMEX model, which follows the evolution of cometary particles and hence allows us to determine the risk of collision at specific positions and times in the inner Solar System.
Penetration detectors
In the early 1960s, 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 with metal walls of 25 and 50 microns thick. A puncture of a cell by a meteoroid impact could be detected by a pressure sensor. These instruments provided important measurements of the near-Earth meteoroid flux. In 1972 and 1973, the Pioneer 10 and Pioneer 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 microns on Pioneer 10, and 50 microns 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 for 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 with a total detection surface of about 200 m2. The detectors consisted of aluminum penetration sheets of various thicknesses: 171 m2 of 400 micron-thick, 16 m2 of 200 micron-thick, and 7.5 m2 of 40 micron-thick. Placed behind these 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, the Vega 1 and Vega 2 missions were equipped with a new dust detector, developed by John Simpson, which used 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 in very dusty environments, like cometary comae or planetary rings (as was the case for the Cassini–Huygens Cosmic Dust Analyzer). For example, on the Stardust mission, the Dust Flux Monitor Instrument (DFMI) used PVDF detectors to study dust in the coma of Comet Wild 2. However, in low-dust environments such as interplanetary space, this sensitivity makes the detectors susceptible to noise. Because of this, the PVDF detectors on the Venetia Burney Student Dust Counter also needed shielded reference detectors in order to determine the background noise rate.
Modern microphone detectors During its flyby of Halley's Comet at a distance of 600 km, the Giotto spacecraft was protected from space dust by a 1 mm-thick front Whipple shield (1.85 m diameter) and a 12 mm-thick rear Kevlar shield. Mounted on the front dust shield were three piezoelectric momentum sensors of the Dust Impact Detection System (DIDSY). A fourth momentum sensor was mounted on the rear shield. These microphone detectors, together with other detectors, measured the dust distribution within the inner coma of the comet. These instruments also measured dust during Giotto's encounter with the comet 26P/Grigg–Skjellerup.
On the Mercury Magnetospheric Orbiter of the BepiColombo mission, the Mercury Dust Monitor (MDM) will measure the dust environments of interplanetary space and Mercury. MDM is composed of four piezoelectric ceramic sensors made of lead zirconate titanate, from which impact signals will be recorded and analyzed.
Chance dust detectors