Comet ISON has dramatically brightened to naked eye visibility, rising in a short space of time from magnitude +8.5 to fifth magnitude by 14 November. This is a dramatic increase in brightness as ISON races towards its close encounter with the Sun on 28 November.

Comet ISON imaged on November 15 by Damian Peach.

Our previous report on 12 November described ISON as appearing with a "soft glow" in 10 x 50 binoculars. Reports from observers since then state that by 13 November ISON had reached magnitude +7.3 and twenty-four hours later was as bright as magnitude +5.4 according to some, making it feasible to see the comet with the naked eye from a dark site in the direction of the constellation Virgo. However, to the unaided eye it merely appears as a faint smudge in the morning sky (around 5am) at present - its tail will grow more dominant around perihelion (its closest point to the Sun) and the days that follow. Recent images from astrophotographers such as Damian Peach do show the tail to now be over 3.5 degrees in length, while its coma - the cloudy head of the comet - appears quite uniform, indicating the comet is still likely in one piece, despite its outburst.

Comet outbursts are not unusual. In 2007 comet 17P/Holmes underwent a huge outburst when it brightened from magnitude +17 to +2.8 in two days. Comet ISON's outburst is nowhere near as brilliant, but the causes are probably similar - a pocket of gas below the surface, warmed by the Sun's heat, pushing its way through the comet's crust and bursting out into space, carrying glistening particles of water vapour into the comet's coma that cause it to brighten. Holmes was unusual in that it was moving away from the Sun at the time, whereas ISON is hurtling headlong for it and, on 28 November, will pass within 1.86 million kilometres of our star.

ISON's track across the sky. AN graphic by Greg Smye-Rumsby.

Estimates vary wildly regarding how bright ISON will become at this time, but there are reasons for this uncertainty. Firstly, it is a comet fresh from the Oort Cloud, a distant repository of comets on the very edge of our Solar System. As such, this is the first time it will have experienced heating by the Sun since its formation four and a half billion years ago, so we do not know how it is going to act. Secondly, it is passing the Sun at a distance we have never seen a comet pass at before - other 'sungrazing comets' get much closer - and the effects on the comet at this distance remain to be seen. Combining these two factors, ISON is the first time we have seen a comet direct from the Oort Cloud graze the Sun, so whatever happens we may not see its like again for some time. Astronomer Matthew Knight of Lowell Observatory in the United States estimates that the heat from the Sun will strip 250 metres off the surface of the comet, which is a huge amount for a comet that is only a few kilometres across at best. If ISON does survive its encounter with the Sun it will have been irrevocably changed and its tail could grow to be quite impressive. As this recent outburst shows, however, ISON may still have a few surprises in store for us yet.