So what’s up with Comet ISON? I first saw it in a telescope on Jan. 7 as a barely visible bit of fluff. Guess what? It still is.
Remember, this is the comet that’s predicted to grow as bright or brighter than Venus come November when it will pass just 600,000 miles from sun. You’d think by now it would be brighter than back in January. But comets almost always have surprises up their sleeves, and ISON is no exception.
Three nights ago I struggled again to find it on a very dark night using the same telescope and magnification. Once again, I saw only smallest fuzzball shining at a very meek 15th magnitude. In a word, the comet had changed little in nearly 3 months (Jan. 7 – April 5). Amateur astronomers measuring ISON’s brightness from photographs confirm the lag.
Let’s look at the particulars using a distance measure called the astronomical unit or A.U. One A.U. is equal to the span between Earth and sun or 93 million miles.
On Jan. 7, the comet stood 4.21 A.U.s from Earth and 5.2 A.U.s miles from the sun. This week it’s nearly the same distance from Earth – 4.22 A.U.s – but significantly closer to the sun at 4.2 A.U.s. That’s 93 million miles closer to the sun now than in January, yet in spite of the increased heat it’s now receiving from the sun, ISON has remained at nearly the same brightness and size. Some amateur astronomers have reported that it’s even shrunk a bit. One encouraging sign however – the comet’s grown a short tail over the winter months.
Short animation of Comet ISON’s trek around the sun through May 2014
Comet ISON is likely making its first trip to the inner solar system ever, coming to us from the remote Oort Cloud, a refrigerated “storage locker” of trillions of comets gathered into a thick spherical halo at the fringes of the solar system. The near edge of the Cloud is some 5,000 A.U.s from the sun or 465 billion miles away (Pluto is “only” 3.6 billion miles away) and it extends outward perhaps as far as 9.3 trillion miles or more than a 1/4 the way to Alpha Centauri, the nearest star beyond the sun.
Far from the sun, an icy comet’s temperature hovers around absolute zero (-459 F). If you could see it up close, there would be no fuzzy coma or tail just an inert hunk of dark ice.
The sun turns a comet on when it’s somewhere between 280 to 465 million miles away. By that I mean that the coldest of its ices – nitrogen, carbon dioxide and carbon monoxide – begin to thaw and vaporize, forming a gassy shroud around the icy nucleus called a coma.
Because outer space has no atmospheric pressure, ice goes directly from solid to vapor with no liquid in between. This is what we see happening with Comet ISON right now.
Observations made on Jan. 30 by NASA’s Swift space probe measured how much material the comet was losing due to solar heating: 112,000 lbs. (51,000 kg) of dust and 130 lbs. (60 kg) of water per minute. The water amount is much lower because ISON is still so far from the sun. Based on these rates, scientists estimate the comet’s diameter at 3 miles (5 km), which is typical for these objects.
What’s curious however is that ISON has barely if at all brightened in 3 months. Maybe the comet’s unusually small or has already lost its coating of easily-vaporized exotic ices. Or maybe it’s just being a comet with all the unpredictability that implies.
Lest you be too concerned that ISON won’t perform to expectations, we’ll get a much better idea of where it’s headed brightness-wise when it moves to within 1.5-3 A.U.s or between Mars and the inner asteroid belt. Here solar radiation is intense enough to vaporize its abundant water ice which and give the comet a major “lift”. Expect that to happen sometime in July and continue through the fall.
Comet PANSTARRS served as the warm-up act to what we hope will be the year’s main venue. The grain of salt we tasted in hunting down that comet will not only temper expectations for ISON but help us appreciate the unique character of each. Like snowflakes, no two comets are alike.
As we drive off to work or school, Comet ISON continues its sunward trek across the constellation Auriga, now high in the northwestern sky at nightfall. Like the comet, we may start our day with certain expectations, but an unanticipated turn of events can sometimes lead to a different outcome at day’s end. Down here on Earth we call that life.