628 Encyclopedia of the Solar System
FIGURE 6 Cumulative flux of interplanetary meteoroids on a
spinning flat plate at 1 AU distance from the Sun. The solid line
has been derived from lunar microcrater statistics, and it is
compared with satellite and spaceprobe measurements.
to the mass of a single object (comet or asteroid) of about
20 km in diameter.
In low Earth orbit the meteoroid flux is about a fac-
tor of two higher than in deep space because of the
Earth’s gravitational concentration. However, micrometer-
sized natural meteoroids are outnumbered (by a factor of
three) by man-made space debris. Craters produced by
space debris particles are identified by chemical analyses
of residues in the craters. Residues have been found from
space materials and signs of human activities in space, such
as paint flakes, plastics, aluminum, titanium, and human
excretion.
2.5 In Situ Dust Measurements
Complementary to ground-based and astronomical dust ob-
servations are in situ observations by dust impact detectors
on board interplanetary spacecraft. In situ measurements
have been performed in interplanetary space between 0.3
and 18 AU heliocentric distance (Table 3).
Two types of impact detectors were mainly used for in-
terplanetary dust measurements: penetration detectors and
impact ionization detectors. Penetration detectors record
the mechanical destruction from a dust particle’s impact,
for example, a 25- or 50-μm-thick steel film has a detection
threshold of 10−^9 or 10−^8 g (approx. 10 or 20μm radius) at
a typical impact speed of 20 km/s. At lower impact speeds
the minimum detectable particle mass is bigger and vice
versa. A more sensitive penetration detector is the PVDF
(PolyVinylidine Fluoride) film. PVDF is a polarized mate-
rial (i.e., all dipolar molecules in the material are aligned so
that they are pointing in the same direction). When a dust
particle impacts the film, it excavates some polarized mate-
rial. This depolarization generates an electric signal, which
is then detected. The pulse height of the signal is a function
of the mass and speed of the dust particle. A typical mea-
surement range is from 10−^13 to 10−^9 g (1–10μm radius).
The most sensitive dust detectors are impact ionization
detectors. Figure 7 shows a photo of the dust detector flown
on theCassinispacecraft. The detector has an aperture of
0.1 m^2 and is based on the impact ionization effect: A dust
particle that enters the detector and hits the hemispherical
target in the back at speeds above 1 km/s will produce an
impact crater and part or all of the projectile’s material will
vaporize. Because of the high temperature at the impact
site some electrons are stripped off atoms and molecules
and generate a vapor that is partially ionized. These ions
and electrons are separated in an electric field within the
detector and collected by electrodes. Coincident electricTABLE 3 In Situ Dust Detectors in Interplanetary Space: Distance of Operation, Mass Sensitivity, and Sensitive
Area.
Spacecraft Year of Launch Distances (AU) Mass Threshold (g) Area (m^2 )Pioneer 8 1967 0.97–1.09 2 × 10 −^13 0.0094
Pioneer 9 1968 0.75–0.99 2 × 10 −^13 0.0074
HEOS 2 1972 1 2 × 10 −^16 0.01
Pioneer 10 1972 1–18 2 × 10 −^9 0.26
Pioneer 11 1973 1–10 10 −^8 0.26
Helios 1/2 1974/76 0.3–1 10 −^14 0.012
Galileo 1989 0.7–5.3 10 −^15 0.1
Hiten 1990 1 10 −^15 0.01
Ulysses 1990 1–5.4 10 −^15 0.1
Cassini 1997 0.7–10 2 × 10 −^16 0.1
Nozomi 1998 1–1.5 10 −^15 0.01