Past Experiments

For the last 40 years or so, efforts have been made to characterise the mass distribution of the dust in near Earth space, and its spatial distribution. Sampling the entire size range of dust (which can broadly be described by the 0.01 fim to cm size regime) is almost impossible for a single detector, and so building a picture of the dust complex at 1 AU has required a combination of data, taking due account of differing thresholds, velocity biases and exposure geometries in space. Without attempting a complete review of all past dust detectors, it is worth considering some of the more important missions that have contributed to the understanding of the near Earth dust flux to date (see McDonnell

Some early attempts at measuring fluxes, using piezo-electric microphone detectors on Explorer 8 [2], proved unreliable as the microphones were susceptible to thermal changes associated with low Earth orbit [3], thus giving apparent fluxes that were much too high (by ~3 orders of magnitude). However, more reliable data were obtained by Explorer 16 [4-7] and Explorer 23 [8] which were Earth orbiting satellites (with altitudes <1200 km) that carried experiments comprising large arrays (1.6 m2 and 2.1 m2 respectively) of pressurised canisters, or 'beer cans'. A pressure-sensing switch capable of measuring a 'once only' leak was activated after the (first) perforation of any can (thicknesses 25-55 fim; thus the experiments were sensitive to particles in the ~10 fim size regime). Clearly the array of canisters had a finite lifetime, although calibration of the ballistic limit of the canister wall gives this sort of sensor a fairly reliable mass threshold for a given impact velocity. The Explorer 16 and 23 experiments recorded 55 and 124 impacts respectively.

Extremely large area (~200 m2) penetration sensitive capacitance detectors were flown on three Pegasus satellites in low Earth orbit (altitude ~580 km) [9-11]. Penetrations in the charged capacitors were registered by discharges through the dielectric layers (i.e. again the detectors rely on ballistic limit calibrations). Two thicknesses of 2024-T3 aluminium (203 fim and 406 //m) gave reliable data, with sampled particles in the ~10—100 firn size regime (thus overlapping with the radar meteor size regime). Around 2000 impacts were recorded with these detectors.

Pioneers 8 and 9 were spacecraft in heliocentric orbits sampling space between 0.75 AU and 1.08 AU. They carried dust instruments based on impact plasma detection, and thus sampled down to the sub-micron particle size regime. The detectors were mounted on the sides of the spinning spacecraft, which had their spin axes perpendicular to the ecliptic plane. Thus the detectors 'scanned' the ecliptic plane. The data comprised a total of about 800 particle impacts, most of which appeared to come from the solar direction — the small (and possibly very fast) ß meteoroids (see [12,13]).

The HEOS-2 spacecraft had a highly eccentric orbit (altitude 350-240,000 km) giving it extended periods away from the increasingly debris-contaminated low Earth orbit region. The dust experiment [14,15] was also based on an impact plasma detection system (which sampled down to the sub-micron particle size regime). As the spacecraft span, with its spin axis along the Earth-Sun line, the detector would scan a plane perpendicular to the Earth-Sun line. Thus the solar direction was not well sampled (inhibiting ß meteoroid detection), but the Earth-apex direction and antiapex direction fluxes could be resolved. It was found that many more impacts were detected from the Earth apex direction. Several hundred particles were detected in total.

These spacecraft data (i.e. the Pegasus, Pioneer and HEOS data) offer excellent 'tie points' for combination with lunar microcrater data (and indeed meteor data at the larger size regime). This was done by Grün et al. [16] who built on previous work, to define a mean interplanetary dust flux at 1 AU over a wide size regime. One of the key elements of this flux definition was to recognise the 'true' contribution of lunar rock sample microcraters (in the ~ <10 fim size regime). The apparent excess of these small craters [17] was in fact due to secondary impact craters being more prevalent than had first been realised (see [18,19]) and would thus lead to an over-estimation of the deduced flux (by ~ 2 orders of magnitude) if taken at 'face value'. Grün et al. [16] concluded that the interplanetary flux was not represented by the smaller lunar microcraters, and used the spacecraft data instead. The 'Grün flux' is often used, and has become the definitive representation of the mean interplanetary flux at 1 AU.

Anisotropics in the near Earth dust environment (such as the Earth apex-antiapex bias, and the ß meteoroids) as identified by the Pioneer and HEOS detectors, has been confirmed by more recent experiments such as those on Helios (see [20]), and Hiten. For example, the impact plasma experiment, Munich Dust Counter (MDC) [21] on the Hiten satellite sampled space from a few thousand km from the Earth, to beyond the Moon. The data again identified fast (and small) ß meteoroids from the solar direction, superimposed upon an overall bias towards the Earth apex direction.

More recently, results from the SPADUS experiment aboard the Earth orbiting ARGOS satellite (altitude ~850 km), has multilayer penetration sensors to determine high accuracy velocities (and trajectories). Results to date [22], have produced 24 coincident penetrations showing a mixture of meteoroid and debris particles (as might be expected).

Dust measurements in geostationary orbit have also been obtained. Time of flight data obtained via the penetration of thin dielectric films have been made in GEO from the GORIZONT-41 and GORIZONT-43 communication satellites [23]. Velocities were measurable for 76 impacts from particles of sizes 3-100 fim of which 80% were inferred as natural meteoroids (i.e. >12 km s_1). The Geostationary Orbit Impact Detector (GORID), a flight-quality engineering model of the Ulysses impact ionization detector [24] was launched with the Russian Express-2 communications satellite into GEO in 1996, and has since been detecting typically a few impacts per day, many of which are consistent with GEO debris [25] (see also [26] for a consideration of bound and unbound dust particles in GEO in relation to the GORID detections).

As well as remotely operated dust sensors, an extremely powerful technique for understanding the near Earth dust environment, is to retrieve surfaces that have spent significant periods of time in space, and interrogate their surfaces to deduce mean crater fluxes and where possible, the chemical signatures of the impactor.

Retrieval of the SMM (Solar Maximum Mission) spacecraft for repairs in the mid-1980s allowed the study of multi-layer thermal insulation and aluminium louvres. These data [27] covered a wide range of crater diameters from sub-^m to mm dimensions. The precise pointing direction of the louvres is largely undefined, but assumed to be effectively random with respect to Earth. Chemical analysis of crater residues showed that debris particles were important at the smaller size regimes. However, while the geocentric 'random' orientation of the SMM spacecraft gave a good 'snapshot' of mean LEO flux environment, is not ideal for resolving the discrete contributions of debris and meteoroids that can impact from different directions in orbit. This information is better served by a spacecraft that maintains its orientation with respect to Earth. Thus the LDEF dataset (see below) essentially supersedes the SSM data in terms of its overall usefulness (or at least, a fuller understanding of the fluxes is obtained by considering SMM and LDEF data).

Probably the most expansive and statistically reliable LEO dust impact dataset, has come from the Long Duration Exposure Facility (LDEF) which was deployed in 1984, and stationed at an altitude of ~470 km for 5.8 years. LDEF's large area-time product, the wide range of materials deployed on it, and its gravity gradient stabilised orbit which maintained the geocentric exposure geometry of 14 discrete sides, has meant that the data is of great importance in 'decoding' LEO particulate fluxes.

The small-particle data was best defined by the MAP experiment [28] which comprised foils with thicknesses in the range 2 to 30 fim, located on the north, south, east, west (N, S, E, W) and space faces. Thick target (i.e. non-penetrating) measurements [29] and data from the thermal control surfaces and the longerons and intercostals of the LDEF frame collated by the LDEF Meteoroid and Debris Special Investigator Group (M&D-SIG; e.g. [30,31]) have provided a database of impacts on spacecraft surfaces at a range of sizes. The IDE experiment [32] provided valuable time resolved fluxes of small particles (micron sized and smaller) for the first 10 months of the mission, from capacitor discharge detectors located on the N, S, E, W, space and Earth faces. Intense, but shortlived, 'spikes' exceeding the background by several orders of magnitude and 'multiple orbit event sequences' (MOES) recurring for large numbers of spacecraft orbits were detected [33,34], and interpreted as small debris particulates.

Since LDEF's recovery from orbit in late 1989, other complementary sources of impact data have also been obtained. ESA's EuReCa spacecraft provided impact data on large areas of thermal blanket, solar cell arrays and the science experiment, TiCCE (Timeband Capture Cell Experiment [35]). While the time resolution aspect of TiCCE did not function correctly, it did provide high reliability penetration data from 2.5-9.2 ¡im foils. EuReCa was oriented with pseudo-fixed Sun and Earth-apex pointing faces, and the data complements the LDEF dataset, and demonstrates a flux bias towards the Earth apex of motion (see McBride et al. [36]).

The recovery of one solar panel after 3.6 years in space from the Hubble Space Telescope (HST) on its servicing mission in December 1993 has provided an additional large area of exposed material to complement the EuReCa solar cell array data (see e.g. Taylor et al.

The MIR space station has also provided a platform for retrievable experiments such as Echantillons [38] with capture cells, capacitor discharge detectors and aerogel cassettes.

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