Hyper Suprime-Cam on the Subaru Telescope serendipitously captured high-resolution images of Comet 67P/Churyumov-Gerasimenko during the inaugural queue-mode observations last March. The camera was scheduled to observe a distant compact galaxy when the chance came up to capture a view of the comet, and the images reveal fine details in the comet's coma and tails (Figure 1).
Comet 67P is a short-period comet that orbits the Sun every 6.57 years. It is known world-wide as the target of the European Space Agency's comet-chasing Rosetta mission, launched in March 2004. The spacecraft has returned thousands of close-up images and data of the comet and continues to keep pace with the nucleus as it moves away from the Sun.
Capturing a Comet on the RunSubaru Telescope observed Comet 67P when it was about 200 million km (100 million miles) from the Earth, between the orbits of Mars and Jupiter. Subaru images clearly show a bright coma, long tails, and a faint but clearly visible dust trail.
Comet 67P was not the main goal of the observations in which it was captured. The telescope was actually performing a study of the 59th object in the Hickson's Compact Groups of Galaxies Catalog (HCG 59). By a sheer chance, the comet was also visible in the same wide field of view (the largest available among the world's 8- to 10-meter class telescopes) (Figure 2).
Timing is EverythingThe telescope was pointed at the particular region of sky during the inaugural observation mode called "queue mode". During queue mode, targets are chosen from a pre-selected list with respect to the sky conditions and the scientific priority of the targets in the list. As it turned out, the night of March 7 was the very first night of the queue mode observation for HSC on Subaru.
Originally another target was in the queue at the time of the observation. However, the transparency of the atmosphere for that target was not ideal because of thin clouds. The HCG 59 observation was then moved to the top of the queue since it did not require such exacting conditions. As it turned out, Comet 67P was in the same wide field of view as the HCG 59.
The timing was perfect. Due to its relatively rapid motion, 67P would have not shown up in the HCG 59 field if the galaxy group observation had not been moved up in the queue in that week (Figure 3).
Dr. Masafumi Yagi of the National Astronomical Observatory of Japan and Hosei University, who proposed the observation of the galaxy group, commented on the lucky turn of events. "I was pleasantly surprised to hear that our target was in the queue even though it did not have the higher priority," he said. "Even more so to see this interesting object in the field of view!"
Thanks to this serendipitous set of observations Subaru Telescope and HSC are contributing welcome ground-based data to the study of Comet 67P. Because the image of the 67P/Churyumov-Gerasimenko shows a dust trail in addition to the dust tail, the research group is now in discussion with comet experts. Even though the presence of this dust trail was already known, there could well be an interesting finding from these HSC images about the nature of the dust trails.