Yasushi Aoshima of Ishikawa, Japan, took the picture using a 12-inch telescope. It shows the comet's green atmosphere which is, impressively, almost twice as wide as the planet Jupiter. The green color comes from diatomic carbon (C2)--a gaseous substance common in comet atmospheres that glows green in the near-vacuum of space.
At the moment, the integrated brightness of the comet is similar to a 10th magnitude star--that is, dim. However, forecasters expect it to brighten more than 200-fold by December. If current trends hold, 46P could ultimately reach magnitude +3, making it not a Great Comet but a very good one, visible to the unaided eye and an easy target for binoculars or small telescopes.
Comet Wirtanen passes through the inner solar system every 5.4 years. Right now it is near the orbit of Mars, and it is heading in our direction.
Click on the image above to explore the comet's approach, courtesy of NASA/JPL.
Are you new to comet photography? Get Astro-imaging Tutorials from award-winning astrophotographer Damian Peach.
More resources: sky map; ephemeris; 3D orbit; light curve.
Comment: Close calls are becoming increasingly common, and, while they may be a awesome sight, historically they portend disaster: