Attempt to measure the cometary mass fluence from atmospheric drag

Depending upon the density of the dust and gas in the cometary coma, a spacecraft like Giotto will suffer a deceleration when travelling through the cometary atmosphere. This velocity loss can be determined directly by measuring the Doppler shift of the spacecraft signals during the Halley encounter and indirectly by comparing the range before and after flyby, if the spacecraft survives. Doppler-shift measurements require a correction for the dispersive effects of the cometary plasma in addition to the correction for ionospheric and interplanetary influences. A very high measurement accuracy is also necessary, because the estimated Doppler shift may amount to less than 1 Hz. Since the accumulated range difference is expected to increase by a couple of km for each day after encounter, postencounter ranging may provide an independent determination of the cometary mass fluence.