Solar System Dust 629
FIGURE 7 TheCassinicosmic dust analyzer consists of two
types of dust detectors—the high rate detector (HRD) and the
dust analyzer (DA). The cylindrical DA (upper center) has a
diameter of 43 cm. The bottom of the sensor contains the
hemispherical impact target; in the center are charge-collecting
electrodes and the multiplier for measurement of the mass
impact spectrum. Two entrance grids sense the electric charge of
incoming dust grains. The detector records impacts of
submicrometer- and micrometer-sized dust particles above an
impact speed of 1 km/s. HRD consists of two circular film
detectors that record impacts of micrometer-sized dust particles
at a rate of 10,000 per second. The detectors are carried by the
electronics box that is mounted on top of a turntable bolted to
the spacecraft.
pulses on these electrodes signal the impact of a high-
velocity dust particle. The strength and the wave form of the
signal are measures of the mass and speed of the impact-
ing particle. The small central part of theCassinidetector
is a time-of-flight mass spectrometer: A high electric field
between the target and a grid 3 mm in front of the target ac-
celerates the ions at high speed. During the flight between
the grid and the ion collector, ions of different masses sep-
arate and arrive at different times at the multiplier. The
lightest ions arrive first and the heavier ones appear later.
In this way, a mass spectrum that represents the elementary
composition of the dust grain is measured. Entrance grids in
front of the target pick up any electric charge of dust parti-
cles. Measurements of the electric charge on interplanetary
dust particles have been accomplished for the first time by
theCassinidetector. Dust detectors incorporating a mass
spectrometer have been flown on theHeliosspacecraft, the
GiottoandVEGAmissions to Comet Halley, theStardust
mission to Comet Wild 2, and theCassinimission to Saturn.
Electrostatic dust accelerators are used to calibrate these
detectors with micrometer- and submicrometer-sized pro-
jectiles at impact speeds of up to about 100 km/s.2.5.1 INTERPLANETARY DUST
The radial profile of the dust flux in the inner solar system
between 1 and 0.3 AU from the Sun has been determined
by theHelios 1and 2 space probes. Three dynamically dif-
ferent interplanetary dust populations have been identified
in the inner solar system. First, particles that orbit the Sun
in low-eccentricity orbits had already been detected by the
Pioneer 8/9andHEOS 2dust experiments. They relate to
particles originating in the asteroid belt and spiraling under
thePoynting–Robertsoneffect toward the Sun. Second,
there are particles on highly eccentric orbits that have, in
addition, large semimajor axes and that derive from short-
period comets. Third, thePioneer 8/9dust experiments de-
tected a significant flux of small particles, which were called
β-meteoroids, from approximately the solar direction. Ex-
istence of these particles was recently confirmed by mea-
surements with the JapaneseHitensatellite.
Recently, theGalileoandUlysses spacecraft carried dust
detectors through interplanetary space between the orbits
of Venus and Jupiter and above the ecliptic plane. Swing-
bys of Venus and Earth (two times) were necessary to give
the heavyGalileospacecraft (mass of 2700 kg) the necessary
boost to bring it to Jupiter within 6 years of flight time,
where it became the first man-made satellite of this giant
planet. TheUlyssesspacecraft, being much lighter (mass of
375 kg), made the trajectory to Jupiter within 1.5 years. In
a swing-by of Jupiter, theUlyssesspacecraft was brought
into an orbit almost perpendicular to the ecliptic plane that
carried it under the South Pole, through the ecliptic plane,
and over the North Pole of the Sun.
Interplanetary dust measurements were obtained by the
Galileospacecraft in the ecliptic plane between Venus’s or-
bit and the Asteroid Belt. The dust impact rate was generally
higher closer to the Sun than it was farther away. After all
planetary flybys, the spacecraft moved away from the Sun.
At these times, the impact rate was more than an order of
magnitude higher than before the flyby when the spacecraft
moved toward the Sun. This observation is explained by the
fact that interplanetary dust inside the asteroid belt orbits
the Sun on low-inclination (< 30 ◦) and in low-eccentricity
bound orbits.Thus, the detector that looks away from
the Sun all the time, detects more dust impacts when the