Comet 46P/Wirtanen
WHEN: All month; brightening towards the end of September
Comet 46P/Wirtanen is set to put on a show for the northern hemisphere over the next few months. A short-period comet orbiting the Sun every 5.4 years, Wirtanen is notorious because it was the original target for ESA’s Rosetta mission. A missed launch window re-directed Rosetta to 67P/Churyumov-Gerasimenko and the rest, as they say, is history.
Discovered on 17 January 1948 by American astronomer Carl A Wirtanen, 46P belongs to the Jupiter family of comets. These have aphelia – greatest distance from the Sun – between 5 and 6 AU and are influenced by Jupiter’s gravity.
Wirtanen’s next perihelion – closest approach to the Sun – occurs on 12 December 2018, coinciding with a particularly favourable 11.7 million km approach to Earth on 16 December. Consequently, 46P/Wirtanen could get as bright as third magnitude; an easy naked-eye object. A world-wide observing campaign has been organised to capitalise on this favourable appearance, Wirtanen retaining target potential for future explorative space missions.
At present Wirtanen is low in Cetus, tracking below mag. +3.5 Tau (o) Ceti. It is currently around mag. +11.8. During September its position worsens, reaching a point 4° west of mag. +4.0 Upsilon (p) Ceti on the night of 13/14 September when it should be a magnitude brighter than it was at the start of the month. From here it dives rapidly southward, all the while getting brighter. It’s most southerly position occurs at the start of November when, at mag. +7.0, it’ll be in Fornax at declination –33°.
For the rest of November it accelerates north, officially leaving Fornax on 28 November and passing back into Cetus. At this time it should be naked-eye visible, around mag. +5.1. Predicted to be mag. +4.5 on 5 December, it passes close to mag. +4.2 Pi (/) Ceti as it moves into Eridanus. On 7 December, rapidly moving north, the mag. +4.3 comet appears close to mag. +3.9 Eta (d) Eridani.
The fourth magnitude comet then passes west of mag. +2.5 Menkar (Alpha (_) Ceti) on the morning of 11 December and slips into Taurus on the morning of 12 December. It tracks between the Pleiades and Hyades open clusters on the morning and evening of 16 December when it’s also closest to Earth. It’s predicted to be an easy naked-eye mag. +3.8 object at this time.
Fading to mag. +4.2, Wirtanen passes close to bright Capella (Alpha (_) Aurigae) on the night of 23 December. It’ll probably remain naked eye until the first week of January 2019 when, at around sixth magnitude, it moves into Ursa Major.