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: