An international team of scientists from NASA, the University of Florida at
Gainesville, and the Max
Planck Institute for Nuclear Physics in Heidelberg,
Germany, observed the deflection of galactic dust grains by
solar radiation (Science 17 December 1999). Galactic dust
grains are very small, about four tenth of a micron in
diameter. Due to their small mass their motion towards the
sun is decelerated when the particle is hit by a solar
photon.

Figure: An electron microscope image
of a cosmic dust particle. This is how probably the galactic
dust particles that are detected by Ulysses look like. These
particles are found in Earth’s atmosphere, where they are
collected by high flying aircraft. In order to determine if
cosmic dust particles are of solar origin or if they come
from the Milky Way, their trajectory and velocity have to be
measured directly in space.

Institute of Planetology, University at
Muenster

The discovery of the phenomenon was made
possible by measurements of the ESA/NASA spacecraft
"Ulysses". "Ulysses" is on an orbit about
the Sun since end-1990. It carries a highly sensitive dust
detector that was built at the Max Planck Institute. The
ulysses dust detector can detect dust particles as small as
one tenth of a micron in diameter. Prof. Eberhard
Gruen, the head of the Heidelberg dust group,
leads the Ulysses dust measurements. Regarding the
measurement of galactic dust he remarks: "Galactic dust
particles do not belong to our solar system, they stream into
it from the outside. They are not very abundant, every cubic
kilometer contains about 10 of them. Fortunately, they move
quite fast through the solar system, roughly with 26
kilometer per second. Thanks to the high sensitivity of the
Ulysses instrument we detect about two galctic dust particles
every week."

Because of Ulysses’s elliptic orbit, its distance from the
Sun varies between 1.3 astronomical units (AU, 1AU = distance
of the Earth from the Sun) to 5.4 AU. This allows the
scientists to investigate the properties of galactic dust at
different distances from the Sun. For each dust particle, the
dust detector measures the impact velocity and the mass. In
order to compare the Ulysses measurements with astronomical
observations of galactic dust, the investigators determined
the distribution of grain masses, i.e. how many small and how
many big dust particles hit the detector. They were surprised
to find that particles in a certain mass range were missing
in the data collected by Ulysses close to the Sun, compared
to the number of particles in this mass range that were
collected at larger solar distances.

Dr. Markus
Landgraf of the Johnson
Space Center of NASA, who graduated at the Max Planck
Institute with Prof. Gruen, explains the observed phenomenon:
"In a certain mass range cosmic dust grains absorb or
reflect light very effectively. This is the case when the
grains’s sizes are compareable to the average wavelength of
the radiation. According to Newton’s princple of actio equals
reactio, every absorbed or reflected photon transfers
momentum to the dust grain. For the galactic grains that we
find missing at small distances from the Sun, this repelling
force, also called radiation pressure, is larger than solar
gravity. Therefore the grains move slower and slower as they
approach the Sun, until they stop and start moving into the
opposite direction. They may also be deflected to the side,
if they do not approach the Sun head-on." The minimal
distance that can be reached by a dust grain depends on the
grain’s initial velocity and the strength of radiation
pressure that the grain experiences. From the observation
that grains were missing inside 4AU, but could be detected
outside 4AU, the team determined that for these grains
radiation pressure is 40 to 80% stronger than solar gravity.

Galactic dust is a indigenous part of the galactic
interstellar medium. It provides the substance from which
stars and planets are formed. The analysis of galactic dust
grains can reveal basic information about the early phases of
the planetary formation process. Despite the astronomical
observations of galactic dust that are conducted since the
1930ies, not much is known about these enigmatic constituents
of the Milky Way. For this reason, the Max Planck Institute
for Nuclear Physics proposes a space mission named DUNE (DUst
Near Earth), in order to measure the chemical
composition of galactic grains directly. As a first step to
realize DUNE, the European Space Operation Center (ESOC) in Darmstadt,
Germany, performs a mission analysis.

Published: 14-12-99
Contact: Markus Landgraf
ESA/ESOC
Darmstadt/Germany
E-Mail: mlandgra@esoc.esa.de
Phone: +49-6151-90-3627
Fax: +49-6151-90-2625