File:Comet Halley by Pioneer 12 118532main AC86-0107-5.jpg

English: Comet Halley in Lyman Alpha light from hydrogen ions. These images were created from data collected by the Ultra-Violet Spectrometer on the Pioneer Venus Orbiter spacecraft during the days of February 2-6, 1986. During the six days, the spacecraft was maintained in a fixed attitude while the comet moved past, and the instrument scanned across the coma.

During the six days, telemetry data was collected from Deep Space Network (DSN) stations in Spain, Australia and California. The data was processed to assemble the scans into false color images where the lightest colors indicate the strongest signal.

These images show the cloud of hydrogen ions, which surrounded the comet during February 2-6. These images are actually a composite because different parts of the cloud were scanned each day, and also because individual ions moved from the nucleus to the edge in about four days. At the time that these images were acquired, the hydrogen coma was the largest object in the Solar System with a radius of about 0.1 Astronomical Unit. In ultra-violet light, the comet was the second brightest object in the Solar System (exceeded only by the Sun).

These images show asymmetry caused by radiation pressure from the Sun. Hydrogen ions initially leave the nucleus in all directions but eventually are deflected by radiation pressure and move away from the Sun. The size of the Hydrogen coma indicates that water was being evaporated from the nucleus at a rate of about 40 tons per second.