In Medieval times comets were thought to foretell plagues, wars, and the deaths of kings, so a pandemic should be just the time for one to show up. These days we know comets are dirty snowballs leftover from the material that formed the Solar System and not related to terrestrial events, but it seems we are getting a particularly unusual one anyway, and it may put on a quite spectacular show.
The inner Solar System is visited by dozens of comets each year, but most are so faint it takes a telescope to see them. When C/2019 Y4 ATLAS was discovered back in December (coincidentally around the time news of the new coronavirus reached beyond Wuhan) it looked to be just another of these. ATLAS's brightness was anticipated to peak around magnitude 9, some 40 times too faint to see with the naked eye.
It has now exceeded that brightness two months early, and is getting brighter every night. Cometary luminosity is notoriously hard to predict, as those who have been bitterly disappointed by one comet after another failing to live up to expectations can attest. With ATLAS already behaving in unexpected ways, it could either be the brightest comet in 13 years or another dud.
The two best comets of recent years were hidden from most of the world's population deep in the southern skies when at their best, so a generation has grown up in the northern hemisphere without the chance to witness the true glory of visitors brighter than Venus.
Instead of brightening at a relatively steady rate of a magnitude every week or two, ATLAS suddenly jumped in magnitude a hundred-fold in brightness in a few days. Comets often brighten as the Sun vaporizes their volatile elements (their ice), but such a rapid change is hard to explain. If the current trend continues it will be visible to the naked eye by early April and spectacular in early May.
Unfortunately, by late May, as it approaches peak brightness it will be so close to the Sun it will be visible for a decreasing amount of time after sunset until it vanishes altogether. It's visibility after it reemerges from the glare is anyone's guess.
Anyone in the northern hemisphere with binoculars or a telescope can track ATLAS’s movement already – it’s currently crossing in Ursa Major and moving slowly, although its apparent motion will speed up dramatically.
Experienced comet-watchers are already expressing surprise at its appearance. At two-thirds of the diameter of the Moon it's much larger and more diffuse than would be expected of an object at this distance. Again, we don’t know why.
A more explicable feature is that ATLAS's 5,467-year orbit is almost identical to that of the Great Comet of 1844, a visitor so impressive it had a town in Queensland, Australia named after it. It’s thought both were once part of a larger object that broke up, probably when the ice holding it together melted on a previous close approach to the Sun. These two, and possibly other components, continued to trace the same orbit, but with the distance between them growing with time.