Comet ISON was an ordinary-sized comet with an extraordinary story. It started out millions of years ago in the faraway Oort cloud (SN: 10/19/13, p. 19), where remnants of the early solar system hang out in deep freeze. A passing star may have given one of these remnants a nudge, launching it on a journey that would bring it to within 1.2 million kilometers of the sun in 2013.
Two astronomers with the International Science Optical Network discovered the remnant in September 2012, while it was still outside Jupiter’s orbit. Scientists quickly realized that ISON would be the first known Oort cloud object to pass near the sun. The sun’s light and heat would vaporize this fossil of the primordial solar system layer by layer, in full view of the world’s most powerful telescopes.
From early observations, scientists learned that ISON was already throwing off lots of dust and ice. This finding further confirmed that ISON was on its first trip to the inner solar system; a regular visitor like Halley’s comet would have lost its surface ice long ago (SN: 11/16/13, p. 14).
But the real spectacle came around Thanksgiving. As ISON rounded the sun, solar observatories got a close look at the comet. By December 3, most astronomers agreed that ISON had lost its nucleus, and all that remained of the comet was dust.
Though initial forecasts that ISON would outshine the full moon went unfulfilled, the combination of the comet’s unique trajectory, its early discovery and today’s sophisticated telescopes made ISON one of history’s most studied comets.