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Galileo

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"Galileo" was a mission devoted to the largest planet Jupiter. It was launched on October 18th, 1989, and spent 8 years in an orbit, beginning in 1995. It carried also an entry probe for atmospheric measurement to study the outer layers. Unfortunately, the high-gain antenna failed to fully deploy, and the communication had to take place via a less effective low-gain antenna. The data yield became much lower than expected. However, the mission was equipped with a dust detector DDS that measured the mass, electric charge, and velocity of incoming particles.



Detector description

The sensor consists of a grid system for the measurement of the particle charge, an electrically grounded target (hemisphere) and a negatively biased ion collector. A charged dust particle entering the sensor will induce a charge to the charge grid, which is connected to a charge sensitive amplifier. The output voltage of this amplifier rises until the particle passes this grid, and falls off to zero when it reaches the shield grid. The peak value (Q_p) is stored for a maximum of 600 microseconds and is only processed if an impact is detected by the impact ionization detector within this time.

A dust particle hitting the hemispherical target produces electrons and ions, which are separated by the electric field between the hemisphere and ion collector into negative charges (electrons and negative ions) and positive ions. The negative charges are collected at the hemisphere and measure by a charge sensitve amplifier (Q_e). Positive ions are collected and measured at the negatively biased ion collector with a charge sensitive amplifier (Q_i). Some of the ions penetrate the ion collector, which is partly transparent (total transmission approximately 40%), are further accelerated, and hit the entrance cone of an electron multiplier (channeltron). Secondary electrons are produced, amplified, and measured by a charge sensitive amplifier (Q_c). Other quantities measured, are the rise times of both the positive and negative charge pulses (Q_i and Q_e). The measurement of the time delay between electron pulse and ion pulse serves as a means for distinuguishing impact events from noise. Impact events have time delays of 2-50 microseconds, while mechanical noise has a time delay of milliseconds. These signal amplitudes and times of a single recorded event are digitized and stored in an Experiment Data Frame (EDF).

A measurement cycle is initiated if either the negative charge Q_e on the hemispherical target, or the positve charge on the ion-collector Q_i, or Q_c exceeds a threshold. Since the hemisphere has a large area which is directly exposed to interplanetary plasma and high-energy radiation, this may cause some interferences for the Q_e measurement. To avoid these interferences during high activity times, it is possible to swithch by command to a mode in which a measurement cycle is initiated if only the charge on the ion collector Q_i (small area and not directly exposed) or channeltron signal Q_c exceeds the threshold. If more than one event occur within the transmission time of one EDF, then these events are counted by several amplitude-dependent counters. The dead-time caused by the measurement cycles is 5 milliseconds.

The signals from the sensor are conditioned and analysed. The microprocessor coordinates the experiment measurement cycle, collects the buffered measurement data and processes the data according to a program stored in the memory.

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Overview of the dust instrument

The instrument consist of a 0.1 mm thick gold foil of hemispherical shape with three grids at the entrance (entrance grid, charge grid, and shield), as well as an ion collector and channeltron detector. The maximum sensitive area (for particles moving parallel to the sensor axis) is 0.1 m^2. Upon impact the particle produces a plasma, whose charge carriers are separated by an electric field between the target and the ion collector. Negative charges (mainly electrons) are collected at the target, the positive charges are collected partly by the ion collector and partly by a channeltron. The channeltron is used as it is insensitive to electric and vibrational noise. See (Grunetal 1992b) for more details.

The objective of the Galileo dust experiment is to investigate the physical and dynamical properties of small dust particles (10^-16 - 10^-6g) in the Jovian environment. The parameters to be determined include the mass, speed, flight direction and electric charge of individual particles. Specific objectives are:

  • To investigate the interaction of the Galilean satellites with their dust environment in order to study the relationship between dust influx on satellites and their surface properties, and to perform direct measurements of ejecta particles from the satellites;
  • To study the interaction between dust particles and magnetospheric plasma, high-energy electrons and protons, and magnetic field, to determine the relationship between dust concentrations and attenuation of the radiation belts, and to investigate the effects of the Jovian magnetic field on the trajectories of charged dust particles;
  • To investigate the influence of the Jovian gravitational field on the interplanetary dust popultaion and to search for rings around Jupiter.

 

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