C/2009 R1 (McNaught)

Lore & Background

After its discovery on 9 September 2009, earlier images of C/2009 R1 were found from July 20, August 1, and August 18, 2009. The discovery was confirmed the same day at the Optical Ground Station telescope at Tenerife. In early June 2010, the comet was visible with binoculars in the constellations Andromeda and Perseus, and by June 8 it was visible to the unaided eye in a dark sky with little light pollution. Astronomers predicted it would grow brighter and become widely visible in the northern hemisphere by mid- or late-June, appearing between the constellations Auriga and Gemini. The new moon on June 12 provided a particularly dark night sky, making the weekend of June 11–13 the best expected viewing time, and it was described as 'an easy skywatching target for most people.' Late the following week, it remained easy to spot in binoculars.

The comet proved brighter than expected, so much so that Sky and Telescope retitled an online article from 'Faint Comet in the June dawn' to 'Comet in the June dawn.' Predictions expected it to reach a brightness as high as magnitude 2 from June 30 to July 2, 2010, the latter date marking perihelion. However, as it grew brighter, its proximity to the Sun made it difficult to see, likely only visible near the horizon at dawn and dusk. The exception was the total solar eclipse on July 11 in the Southern Hemisphere, which allowed the comet to be seen during the day. The comet was notable for its 'impressive green coma and long ion tail,' which spanned 5 degrees as of 6 June 2010, and its appearance was likened to an 'apple on a stick.' By June 13, a second tail created by dust from the comet was also visible, sharing the same green hue of the coma. The green colors in the coma were caused by the presence of cyanogen and diatomic carbon, while bluish hues in the ion tail were produced by positively charged carbon monoxide and carbon dioxide ions.

Reader's Guide

C/2009 R1 (McNaught) is significant as one of more than fifty comets discovered by Robert H. McNaught, and as a non-periodic comet that is believed to have left the Solar System permanently. Its legacy stems from its unexpected brightness, which defied predictions and made it a notable skywatching event in June 2010. The comet's green coma and dual tails—an ion tail and a dust tail—provided a striking visual, and its appearance during a total solar eclipse on July 11, 2010, allowed daytime observation. The discovery and early images from July and August 2009, along with the confirmation at Tenerife, highlight the collaborative nature of comet detection. The comet's brightness variability underscores the difficulty of predicting cometary behavior, especially for first-known appearances. Its observation history, from binocular visibility to naked-eye sighting, and the scientific details of its coma composition (cyanogen and diatomic carbon for green, carbon monoxide and carbon dioxide ions for blue) contribute to its importance in cometary studies.

Did You Know?

• C/2009 R1 was discovered on 9 September 2009 by Robert H. McNaught using the Uppsala Southern Schmidt Telescope at Siding Spring Observatory.
• The comet's green coma was caused by cyanogen and diatomic carbon, while its blue ion tail came from positively charged carbon monoxide and carbon dioxide ions.
• It was visible during the total solar eclipse on July 11, 2010, in the Southern Hemisphere.