We’re a little over a month away from what might be the greatest sky-show of the year. On 28 November, Comet ISON will reach perihelion – its closest point to the Sun – making it shine so brightly in the sky it will be visible with the naked eye. Or not: the comet might not survive its close encounter with the Sun. In any case, ISON’s entrance into the inner solar system has been the most highly anticipated move by a comet since Hale-Bopp’s 1997 visit, and it’s been keeping amateur and professional astronomers alike busy for months. And it seems like the best is yet to come.
Comet ISON is a fairly recent object from our Earthly perspective. It was discovered just last year, on September 21, by Vitali Nevski and Artyom Novichonok, two astronomers at the International Scientific Optical Network (ISON) near Kislovodsk, Russia using a 0.4 meter reflector telescope.
But ISON is far from a young object, and it’s been on its path for quite a while. ISON originated in the Oort cloud, that loose disk of rocky and icy objects that orbits far beyond the orbit of Pluto and the other Kuiper belt objects. And this is ISON’s first visit to the inner Solar System, which means it’s in pristine condition. It’s passed out from behind the Sun (again, from our Earth-based perspective) this summer making it visible in the dawn sky for the last few months. And it will continue to get brighter as it moves closer to the Sun.
Hubble observations made in April revealed that ISON’s pole is constantly facing the Sun. This continual heat bombardment by Solar wind, radiation, and tidal forces on one hemisphere could tear ISON apart before perihelion. But the good news for scientists is that the hemisphere that has never been exposed to the Sun remains almost as untouched as it was out in the Oort cloud.
Though the Sun’s already having an effect on ISON, the real survival challenge for the comet will come at perihelion. ISON is a sun-grazer. At its closest point it will pass just 0.0124 AU from the centre of the Sun. That’s about 1,860,000 km: the sun’s surface is about 700,000 km away from its core. So ISON isn’t going to fall into the Sun, but it is going to get extremely hot – hot enough to maybe melt and crack into pieces.
For those of you interested in looking for ISON or just tracking its progress, there are a few key dates to be aware of. After passing by Mars earlier this month, ISON will finally be visible to the naked eye in the beginning of November if you are lucky enough to have really dark skies. On 5 November, ISON will cross out of the constellation Leo and into Virgo; it will pass through the equinoctial point in Virgo on the eighth. On 18 November, ISON will pass close to the bright star Spica (just 0.38 degrees north) before leaving Virgo and passing into Libra on the 22nd. On 27 November, ISON will pass into the field of view of NASA’s SOHO’s LASCO C3 coronagraph.
The next day, just before midnight UTC, ISON will hit perihelion.
If ISON does survive its near-Solar encounter, it will become a stunning comet in the pre-dawn skies for in the weeks leading up to the new year.
Image: Comet ISON as photographed by the Hubble Space Telescope in April 2013. Credit: NASA, ESA, and the Hubble Heritage Team