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Cosmic Dust

The Ulysses dust experiment provides direct observations of dust grains in interplanetary space and allows for investigation of their physical and dynamical properties as functions of heliocentric distance and ecliptic latitude. Of special interest is the question of what portion is provided by comets, asteroids and interstellar particles.

The Ulysses dust detector is a descendant of the dust detector flown on the HEOS-2 satellite. This instrument carried out measurements in the near-Earth space and observed various effects of the Earth's magnetosphere and the Moon on the interplanetary dust population.

DUST is a multicoincidence detector with a mass sensitivity 106 times higher than that of previous in-situ experiments which measured dust in the outer Solar System. It consists of an impact ionization sensor and the appropriate electronics and allows for measuring the mass, speed, flight direction and electric charge of individual dust particles.

Positively or negatively charged particles entering the sensor are first detected via the charge QP which they induce to the charge grid while flying between the entrance and shield grids. All dust particles - charged or uncharged - are detected by the ionization they produce during the impact on the hemispherical impact sensor. After separation by an electric field, the ions and electrons of the plasma are accumulated by charge sensitive amplifiers (CSA), thus delivering two coincident pulses QE and QI, of opposite polarity. The rise times of the pulses, which are independent of the particle mass, decrease with increasing particle speed. From both the pulse heights and rise times, the mass and impact speed of the dust particle are derived by using empirical correlations between these four quantities. A third independent signal originates from part of the positive impact charge which is detected and amplified (~ 100×) by an electron multiplier (channeltron). This signal QC serves as a control for the identification of dust impacts.

Summary of Objectives

The overall objective of the Ulysses dust experiment is the investigation of the physical and dynamical properties of small dust particles (10-16 - 10-6 g) as a function of ecliptic latitude and heliocentric distance, and the study of their interrelation with interplanetary/interstellar phenomena. The parameters to be measured include the mass, speed, flight direction and electric charge of individual particles. The impact rate, size frequency, and the distribution of flight directions and electric charges will be determined. Specific objectives are:

  • To allow us to classify particle orbits into bound orbits around the Sun or hyperbolic orbits leaving or entering the Solar System. The distributions of orbital elements (semi-major axis, eccentricity, inclination) of particles in bound orbits will be studied
  • To determine as functions of heliocentric distance and ecliptic latitude the spatial density of the interplanetary large particle population which generally moves on bound orbits around the Sun, and to determine the relative significance of comets and asteroids as sources for these zodiacal dust particles
  • To measure the flux and velocity of particles coming on hyperbolic orbits from the general direction of the Sun
  • To identify interstellar dust particles and perform direct measurements of the spatial density, heliocentric distribution, velocity and mass of interstellar grains transiting the Solar System
  • To observe enhancements of cometary dust particles during the transit of the spacecraft through the plane of a comet's orbit
  • To investigate the spatial density of dust particles within the asteroid belt and determine the dust production by collisions in the asteroid belt
  • To investigate the influence of the Jovian gravitational field on the interplanetary dust population
  • To measure electric charges of dust particles and establish the relationship of these charges with properties of the ambient plasma (plasma density, energy spectrum), the solar radiation spectrum and magnetic fields

Last Update: 08 December 2006

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