DYNAMICS OF THE GIOTTO SPACECRAFT IN THE INNER DUST COMA OF COMET-P HALLEY .1. OBSERVATIONS

The deceleration of the GIOTTO spacecraft due to the influx of cometary dust during its flyby at Comet P/Halley was determined by the GIOTTO radio-science experiment (GRE) to be - 23.05 cm/s. A nutation (theta = 1-degree) and a spin-down of the rotating spacecraft by 12 ms was also observed. The deceleration occurred in distinct steps, indicating the presence of mass enhancements along the trajectory. The one-way Doppler data showed a larger frequency shift of the X-band carrier signal (16.9 Hz) than the two-way data (4.64 Hz), implying that the on-board oscillator produced significant pseudo Doppler contributions not associated with the true deceleration. The dust fluence spectrum, measured by in situ detectors, implies that at least 90% of the total cometary mass impacting on the spacecraft is contained in dust particles greater than 10(-8) kg.