Comet ISON Dallies On Path To Greatness

Comet C/2012 S1 ISON with its tadpole-like tale photographed through a 14-inch telescope on April 1. The comet has brightened little in the past three months. Credit: William Wiethoff

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.

If it holds together during its extremely close swing around the sun on Nov. 28 this year, Comet ISON should become much brighter and display a longer tail than Comet PANSTARRS did this spring. The illustration shows the comet’s visibility in two of the Solar and Heliospheric Observatory’s cameras (red is close up) at that time. Credit: NASA

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.

A tailless Comet ISON on Jan. 4 this year. Credit: Artyom Novichonok

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.

Artist’s conception of the Oort Cloud and Kuiper Belt, the two places from which comets originate. PANSTARRS and ISON hail from the Oort, a huge spherical shell extending to the edge of the solar system. Credit: NASA

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.

Comet ISON passes through Gemini, Cancer and Leo as it falls toward the sun. Credit: NASA’s Goddard Space Flight Center/Axel Mellinger

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.

If it survives its close pass to the sun, ISON should put on a great show in late Nov. and December in both evening and morning skies. It passes closest to Earth on Dec. 26. Created with Christ Marriott’s SkyMap program

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.

13 Responses

  1. Edward M. Boll

    If the comet starts to brighten now at regular rate, it still could be brighter than the Full Moon for a few hours.

  2. Tim Hutton

    Hi Bob,
    According to the animation it looks like Ison will pass close to Earths orbital plane. Any chance it creates a new meteor shower?

    1. astrobob

      Hi Tim,
      I’ve heard from one amateur comet observer that it’s a possibility. Nothing definite though.

    1. astrobob

      It will be too faint for naked eye viewing until probably mid-late October and then it will brighten rapidly.

  3. Hi Bob – I believe this comet is periodic. That it returns every 333 yrs. and that, in fact it is the comet that Sir Isaac Newton viewed in 1680 – which he used to postulate his theory of gravitational effects on celestial bodies [Laws of Planetary Motion]. Later proved by Sir Edmund Hally. In 1346 the comet was called Negra.

    1. astrobob

      Comet ISON may possibly be a fragment of the same parent comet that created the one in 1680. It’s not a fact or certainty … at least not at this point.

  4. mark halsall

    Does the comets closest point to the Earth happen on the way to the Sun, or only if it survives its stargraze and is on the way out again?, what are the dates of the closest point to Earth on the way in and on the way out and what will be the comets distance from us on these dates?

    1. astrobob

      Hi Mark,
      Comet ISON is closest to Earth on Dec. 26 at a distance of about 40 million miles. Comets can pass closest to Earth on the way in, the way out or even at the same time they’re closest to the sun. It all depends on where the Earth is when the comet swings by. It’s best when the comet flys near the sun first – and gets all cooked up – and then passes by Earth. That’s the situation with ISON and why many think it will be a spectacular comet. I’ll answer your question about distance from Earth this way – ISON is getting incrementally closer every day until closest approach on Dec. 26, so there isn’t one day closer than another on its way in.

Comments are closed.