Max Planck Society

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. have to be measured directly in space.

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

Phone: +49-6151-90-3627
Fax: +49-6151-90-2625