DYNAMICS OF THE GIOTTO SPACECRAFT IN THE INNER DUST COMA OF COMET P/HALLEY .2. INTERPRETATIONS

The mass density along the trajectory and the dust jet emission features around the nucleus of Comet P/Halley at the time of the GIOTTO flyby are reconstructed from the deceleration profile determined from calibrated Doppler frequency data. The simulation of the signal level shows that the large nutation of the spacecraft was most likely generated during the blackout interval by a shower of large dust particles, and not, as previously supposed, by one large particle impact at 7.6 sec before encounter. The signal drop and loss of telemetry for 22 sec is due to an off-pointing of the High Gain Antenna (HGA) from the Earth direction caused by a counting error in the HGA Despin Control System.