Australian Sky & Telescope


FREQUENT FLYER Comet 21P/Giacobini-Zinner was captured by the Kitt Peak 0.9-metre telescope on October 31, 1998. North is up with east to the left. (N. A. SHARP/NOAO/AURA/NSF)

August and September will be pretty good for time southern comet observers, with three comets of note to keep us occupied. First up is C/2016 M1 (PANSTARRS), which will reach its rather large perihelion distance of 2.21 a.u. from the Sun on August 10. Beginning the month close to the boundary of the far southern constellations Norma, Circinus and Lupus, the comet will continue to slowly drift southward during the late winter and early spring. It will reach Centaurus during the final quarter of August and, throughout September, will linger close to Alpha Centauri as it slowly moves away from the inner Solar System.

On present indications, C/2016 M1 is expected to be about magnitude 9.5 to 10 during early August, fading by approximately half a magnitude by the end of September.

Early August will find another comet, C/2017 T3 (ATLAS), sinking into both the morning and evening twilight, although suitably placed observers should have a better chance of following it in the early evening skies. Having passed perihelion (at 0.82 a.u.) on July 19 and making its closest approach to Earth (1.35 a.u.) on August 1, the comet will be fading as it moves away from both Earth and Sun. Trekking from Puppis into Pyxis during the first week of the month, C/2017 T3 is unlikely to be brighter than magnitude 10. The waning Moon will not make observing any easier for morning viewing, but the comet may still be worth a try if you have a clear eastern horizon, and before it is completely overtaken by the morning twilight.

The real cometary ‘star’ of the period, however, will be the short-period (6.55 years) 21P/Giacobini-Zinner, which this year is making a very favourable return. Trekking southward from high northerly declinations, the comet will be visible in the early morning sky from mid-southern latitudes. Beginning early September in the constellation Auriga, 21P will cross into Gemini on September 13 and into Monoceros some ten days later. Perihelion and closest passage of Earth (1.014 a.u. and 0.39 a.u. respectively) will occur on the same day — September 10. Around this time the comet is expected to be about magnitude 7.5, fading to 8 or a little fainter by the end of the month.

21P is famous for having produced the spectacular Draconid meteor storms of October 1933 and 1946, when at least 6,000 meteors per hour were recorded. The comet also made history by becoming the first to be visited by a spacecraft, albeit rather distantly. On September 11, 1985, the International Comet Explorer (ICE) flew through 21P’s ion tail, making the very first in situ measurements of a comet’s magnetic and plasma environment.

According to comet expert Z. Sekanina, 21P’s nucleus — thought to be about two kilometres in diameter — is a rapidly-rotating body shaped somewhat like a pancake. Maybe we should call it a cometary ‘flying saucer’?

• DAVID SEARGENT is the discoverer of comet 1978 XV. His most recent book,

Snowballs in the Furnace, is available from Amazon.com