622 Encyclopedia of the Solar System
FIGURE 1 A wedge of interplanetary dust. The dusk twilight
sky (pink) toward the northwest shows zodiacal light (blue),
framed by the Pleiades (upper left), Comet Hale–Bopp (upper
right), and Mercury in Aries (left of center above horizon).
(Courtesy M. Fulle.)
interplanetary dust have the difficulty of representing these
particles by simplified models, for example, a spherical par-
ticle of uniform composition and optical properties of a pure
material. True interplanetary dust particles can be very dif-
ferent from these simple models (Fig. 2).
Another practical aspect of dust is its danger to techni-
cal systems. A serious concern of the first spaceflights was
the hazard from meteoroid impacts. Among the first instru-
ments flown in space were simple dust detectors, many of
which were unreliable devices that responded not only to
impacts but also to mechanical, thermal, or electrical inter-
ference. A dust belt around Earth was initially suggested,
which was dismissed only years later when instruments had
developed enough to suppress this noise by several orders
of magnitude. Modern dust detectors are able to reliably
measure dust impact rates from a single impact per month
up to a thousand impacts per second.
In the early days of spaceflight, measures were taken
to protect spacecraft against the heavy bombardment by
meteoroids. The bumper shield concept found its ultimate
verification in the European Space Agency’sGiottomission
to comet Halley. This spacecraft was designed to survive
impacts of particles of up to 1 g mass at an impact speed
of 70 km/s. These grains carry energies comparable to can-
non balls that are 1000 times more massive. Heavy metal
armor was not possible because spacecraft are notoriously
lightweight. TheGiottobumper shield combined a 1-mm-
thick aluminum sheet positioned 23 cm in front of a 7-cm-
thick lightweight composite rear shield. A dust particle that
struck the thin front sheet was completely vaporized. The
vapor cloud then expanded into the empty space between
the two sheets and struck the rear shield, where its energy
was absorbed by being distributed over a large area. In this
way, the 2.7-m^2 front surface of the spacecraft was effec-
tively protected by armor that weighed only 50 kg.
Only recently has the dust hazard become important
again, because of man-madespace debrisin Earth orbit.
Each piece of equipment carried into space becomes, after
disruption by an explosion due to malfunctioning batteries
or fuel systems or by an impact, the source of small projec-
tiles, which endanger other satellites. Some estimates indi-
cate that, in 50 years, the continuous increase in man-made
space activity will lead to a runaway effect that will make
the near-Earth space environment unhabitable to humans
and equipment.
However, we are not concerned with this aspect of inter-
planetary dust; rather, the topic of this chapter is interplan-
etary dust as an exciting object of astrophysical research.
Through its wide distribution over the solar system, cosmic
dust can tell stories about its parents (comets, asteroids,
even interstellar matter) that otherwise are not easily ac-
cessible. This view, however, requires that dust particles be
traced back to their origins. To do this, we must understand
their dynamics. Dust particles not only follow the gravita-
tional pull of the Sun and the planets but also feel the inter-
planetary magnetic field and the electromagnetic radiation
that fills the solar system. In addition, they interact with the
solar wind and with other dust particles that they encounter
in space, generally at high speeds. These collisions lead to
erosion or to disruption of both particles, thus generating
many smaller particles. The dynamics of interplanetary dust
cannot be described solely in terms of position and velocity;
their size or mass must also be considered.2. Observations
Different methods are available to study cosmic dust
(Fig. 3). They are distinguished by the size or mass range
of particles that can be studied. The earliest methods were
ground-based zodiacal light andmeteorobservations. Fifty
years ago, radar observations of meteor trails became avail-
able. With the onset of spaceflight, in situ detection by
space instrumentation provided new information on small
interplanetary dust particles. Among the first reliable instru-
ments were simple penetration detectors; modern impact
ionization detectors allow not only the detection but also the
chemical analysis of micrometeoroids. Deep space probes
have identified micrometeoroids in interplanetary space
from 0.3 to 18 AU from the Sun. Natural (e.g., lunar sam-
ples) and artificial surfaces exposed tomicrometeoroid