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.
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.
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