March 24, 2020: No one knows how big the icy core of Comet ATLAS (C/2019 Y4) might be–possibly no wider than a few kilometers. One thing’s for sure, though, the comet’s atmosphere is huge. New images from amateur astronomers around the world show that ATLAS’s gaseous envelope has ballooned in diameter to ~720,000 km–about half as wide as the sun.

“Comet ATLAS’s coma (atmosphere) is approximately 15 arcminutes in diameter,” reports Michael Jäger of Weißenkirchen, Austria, who took the picture, above, on March 18th. “Its newly-formed tail is about the same size.”

Other astronomers are getting similar results. 15 arcminutes = a quarter of a degree. Given Comet ATLAS’s distance of 1.1 AU on March 18th, that angle corresponds to a physical size of 720,000 km.

On the scale of big things in the solar system, Comet ATLAS falls somewhere between the sun (1,392,000 km  diameter) and Jupiter (139,820 km). It’s not unusual for comets to grow so large. While their icy solid cores are typically mere kilometers in diameter, they can spew prodigious amounts of gas and dust into space, filling enormous volumes. In the fall of 2007, Comet 17P/Holmes partially exploded and, for a while, had an atmosphere even larger than the sun. The Great Comet of 1811 also had a sun-sized coma. Whether Comet ATLAS will eventually rival those behemoths of the past remains to be seen.

Right now, Comet ATLAS is certainly the biggest green thing in the Solar System. Its verdant hue comes from diatomic carbon, C₂, a molecule commonly found in comets.  Gaseous C₂ emits a beautiful green glow in the near-vacuum of space.

Currently, Comet ATLAS is shining like an 8th magnitude star–invisible to the unaided eye but an easy target for backyard telescopes. The comet is brightening rapidly as it comes closer to Earth and the sun. By late May it could rival Venus in the evening twilight sky. Stay tuned!

Comet ATLAS resources: sky map; 3D orbit; ephemeris, light curve.