Discovered in September, 2012, Comet C/2012 S1 ISON is just a few months away from its rendezvous with the inner solar system. Since its discovery and our initial article in October 2012, the comet has been photographed by the Hubble Space Telescope and we know a lot more about this potentially exciting object.
What We Know For Sure
In April, Comet ISON passed within Jupiter’s orbit and astronomers released refined estimates of its size. The comet’s nucleus – the solid, central component – is estimated to be 5 to 6.5 kilometres (3 to 4 miles) across. This is ten times larger than Comet Lovejoy, which put on an amazing show for the Southern Hemisphere in 2011, but Hubble researchers say that ISON’s nucleus is “…remarkably small considering the high level of activity observed in the comet so far.”
As for the rest of the comet, its coma – the bright, dusty portion around the nucleus – is approximately 3,100 miles across (which is wider than Australia), and its dust tail extends more than 80,000 kilometres behind it.
ISON is hurtling towards the Sun and will make a very close pass by our star in late November, missing it by only one million kilometres (700,000 miles) on November 28th. If it survives this encounter, which is very close in astronomical terms, it will whip around the Sun and be well positioned for viewing from Earth throughout December. It will be closest to Earth on December 26th but miss us by a comfortable 64 million km (40 million miles).
Path of Comet ISON courtesy of NASA
What We Don’t Know
Whether Comet ISON will remain intact past perihelion (its closest approach to the Sun) is still an open question. Physicist Ignacio Ferrin has noted that ISON will pass within the Sun’s Roche Limit – the minimum distance (in theory) in which a satellite can approach its parent body without being torn apart by tidal forces. Ferrin believes that unless ISON has very strong internal cohesion, it has a high probability of disintegrating due to its close approach and the heat and radiation it will encounter near the Sun.
On the other hand, Comet Lovejoy was well within the Sun’s Roche limit (just 140,000 km) in 2011 and surprised everyone by surviving the encounter and putting on a spectacular show. Astronomers hope that Comet ISON will do the same, but it’s impossible to predict how it will behave. We’ll literally have to wait and see.
As for the comet’s brightness, the jury is still out on that question as well. While there are still predictions that Comet ISON’s brightness could peak at an extremely bright magnitude between -8 (as bright as the two-day old moon) and -12 (nearly as bright as the full Moon), these predictions are misleading. At most, the comet would only be this bright for a few hours during daylight on November 28th and it would be so close to the Sun that observing it will be challenging and potentially dangerous for inexperienced observers!
Where and When to Look
Comet Pan-STARRS demonstrated the difficulty of observing a “naked-eye” comet when it is close to the Sun and low to the horizon. If Comet ISON rounds the Sun in one piece, the best observations for Northern Hemisphere observers will likely come in the morning sky in early- to mid-December. The comet will rise higher in the sky each morning through the month and will hopefully sport a brighter, longer tail than Pan-STARRS that will make it easier to spot.
If you don’t want to wait until December to catch ISON, you can try to observe it in November before perihelion with the aid of binoculars or a telescope. It will probably still be quite dim in early November but its magnitude could rise to magnitude +3 or brighter by mid-month which would make it a very nice binocular target before sunrise.
Comet “Meteor Shower”?
In April, University of Western Ontario meteor researcher Paul Wiegert suggested that in mid-January, 2014, Earth will likely pass through tiny dust particles left behind by Comet ISON. This report has been misinterpreted by at least one news outlet as an indication of a new meteor shower, when in fact Weigert notes that “Instead of burning up in a flash of light, (the particles) will drift gently down to the Earth below." At best, the only visible sign of this interaction would be noctilucent clouds (NLCs) over Earth’s polar regions, so this isn’t likely to be a widely-visible event.
As Comet ISON moves into the inner solar system, predictions on its potential brightness will become more firm and we will report on the latest findings. Be sure to check back for updates and follow us on Twitter!