ISON

In the autumn of 2012, a faint smudge of light found beyond the orbit of Jupiter set the astronomical world alight. Catalogued as C/2012 S1 but known to everyone as Comet ISON, it was a pristine visitor from the Oort Cloud — the vast shell of icy bodies surrounding the Solar System — falling almost straight toward the Sun for the first time after a journey of millions of years.

Because it was a sungrazer on course to skim extraordinarily close to the Sun, and because it was already visible so far out, some hoped ISON would become the "Comet of the Century". Instead it became a cautionary tale about how unpredictable comets can be.

Discovery and the hype

ISON was discovered on 21 September 2012 by Vitaly Nevsky and Artyom Novichonok using a 0.4-metre telescope of the International Scientific Optical Network (ISON) near Kislovodsk, Russia — the network that gave the comet its name. Early projections, based on its distance and apparent brightness, suggested it might one day rival the Moon. Astronomers urged caution: fresh Oort Cloud comets, laden with volatile ices, are notorious for over-promising.

A first-time sungrazer

ISON was almost certainly a dynamically new comet — making its very first trip into the inner Solar System, its ices never before exposed to strong sunlight. Its orbit was set to carry it to within roughly a million kilometres of the Sun's surface at perihelion: close enough to be classed a sungrazer, and close enough that survival was never guaranteed.

Destruction at perihelion

On 28 November 2013, ISON rounded the Sun, watched live by a fleet of solar observatories. It faded and flared alarmingly as it approached, then emerged from behind the Sun's glare as little more than a smeared trail of dust: the nucleus had broken apart under the Sun's ferocious heat and tidal forces. A small fragment briefly raised hopes of survival before it, too, dissipated. The Comet of the Century was gone.

What ISON taught us

Though it never delivered its promised show, ISON was one of the most intensively observed comets in history, tracked from the ground and from space across its entire plunge toward the Sun. It gave scientists a rare, detailed look at how a pristine comet behaves — and disintegrates — on a first and final passage, and a lasting lesson in the humility comets demand of anyone who tries to predict them.