Astronomers spy rocky and icy wanderers of all shapes and sizes zipping past Earth all the time. But earlier this month, they were flabbergasted when they caught sight of the largest comet they'd ever seen.
One of its discoverers, Pedro Bernardinelli, an astrophysicist at the University of Pennsylvania, conservatively estimates the object's dusty, icy nucleus is between 100 and 201km long. That means this comet is as small as five Manhattan Islands, or it's larger than the Island of Hawaii. Hale-Bopp, which lit up night skies in the late 1990s with its 40.23km nucleus, was long perceived to be a giant among comets. But the nucleus of this comet, Comet C/2014 UN271, "is still two or three Hale-Bopps across," said Teddy Kareta, a planetary astronomy graduate student at the University of Arizona. "It's just wild."
"With a reasonable degree of certainty, it's the biggest comet that we've ever seen," said Colin Snodgrass, an astronomer at the University of Edinburgh.
The comet is currently inside Neptune's orbit. Over the next decade, it will scoot toward the inner solar system. More of its ices will be vaporised by the sun's glare, causing it to effervesce and brighten. In 2031, it will get within a billion miles of the sun — almost but not quite making it to Saturn — before journeying back to the coldest, darkest fringes of our galactic neighbourhood.
Although it's unlikely a spacecraft will be able to rendezvous with the comet, spotting it while it's still two billion miles away means that astronomers can train their telescopes on it and watch it flare, then fade, in staggering detail over the next 20 years.