Enhanced Hubble Telescope image of ISON from April 2013, showing a dusty tail and some evidence of volatile sublimation around the nucleus (NASA, ESA)

Back in February these pages discussed a newly discovered long-period comet, ISON (otherwise known as C/2012 s1), that is falling sunwards for what is probably its first passage through the inner solar system later this year - on a beautiful near parabolic orbit. At its closest point it will pass a mere 700,000 miles from the solar surface, on November 28th 2013.

Pristine cometary nuclei, volatile rich lumps, offer the best chances of flaring into spectacular forms as they near the Sun. And ISON might be a biggie, or it might just be, well, ordinary. Regardless of the display it eventually puts on, like any new cometary body it's a veritable treasure trove for science.

Images of ISON at 3.6 and 4.5 microns (infrared). Left panel shows fine dust in a tail pointing away from the Sun. Right panel shows the glow of neutral gas, likely CO2, that is sublimating off the warming nucleus (Credit: NASA/JPL-Caltech/JHUAPL/UCF)

Images taken by NASA's Spitzer Space Telescope over a month ago show the infrared glow of dust and carbon dioxide already pouring from ISON into a tail more than 180,000 miles long. This translates into a daily production of some 2.2 million pounds of gaseous carbon dioxide and about 120 million pounds of dust. That may sound impressive, but at roughly 3 miles across ISON's nucleus is estimated to weigh as much as 7 trillion pounds, and even after it rounds the Sun it might end up shedding no more than about 10% of its total mass - assuming it remains intact during that close pass.

But, by studying the composition of the material being shed, we gain insight to some of the fundamental building blocks of our solar system (and perhaps even other solar systems). Comets can carry some remarkably complex organic chemistry, the kind of 'prebiotic' mix that may have existed on the surface of a youthful planet Earth.

Right now ISON is a little more than 2.7 times further from the Sun than the Earth (you can track it with this neat calculator). It's worth keeping an eye on its progress, since humans may never witness it again.