The sudden appearance of the tail of Comet Halley

An outburst event on Comet Halley, which was photographed in November 1985 when the comet was very near the ecliptic plane at 1.82 AU from the sun, is discussed. An irregular mass of material was observed, reaching about 1 arcmin south of the comet, approximately 70 degrees from the expected angle of the tail. Brightness changes during the event, up to a factor of 20, were shown in 2.3-10.3-micron IR measurements. The behavior of the outburst is consistent with the rapid expulsion of volatile ice particles. It is shown that the tail formed rather suddenly, though somewhat episodically. It is suggested that the outburst was not caused by a meteorite impact. It is concluded that the probable cause of the outburst was thermal heating of the nuclear crust with modest heat storage.