I’m starting to get the chills about Comet ISON. I can’t help it. With practically every telescope turned the comet’s way fewer than three short weeks before perihelion, every week brings new images and developments. The latest pictures show a brand new tail feature emerging from the comet’s bulbous coma. For months, amateur and professional astronomers alike have watched ISON’s slowly growing dust tail that now stretches nearly half a degree or a full moon’s diameter. In the past two days, photos taken by amateur astronomers reveal what appears to be a nascent ion or gas tail. Damian Peach’s Nov. 6 image clearly shows two spindly streamers.
Ion tails are composed of gases like carbon monoxide and carbon dioxide blown into a narrow straight tail by the solar wind and electrified to fluorescence by the sun’s ultraviolet light. Being made of ions (charged particles), they interact with the sun’s wind of charged particles. Changes in the intensity and direction of the magnetic field associated with sun’s exhalations kink and twist ion tails into strange shapes. Strong particle blasts can even snap off an ion tail. Not that a comet could care. Like a lizard, it grows a new one back a day or three later.
A fresh forked tail isn’t ISON’s only new adornment. Its inner coma, location of the bright “false nucleus”, is becoming more compact, and the overall magnitude of the comet has been slowly but steadily rising. Two mornings ago I pointed a pair of 10×50 binoculars ISON’s way and was surprised to see it glowing at magnitude 8.5. Things happen quickly now that the comet is picking up speed While it appeared as little more than a small smudge, any comet crossing into binocular territory is cause for excitement. Other observers are reporting magnitudes as bright as 8.0. Estimates may vary among observers, but the trend is up. Seiichi Yoshida’s excellent Weekly Information about Bright Comets site predicts another half magnitude brightening over the next few days. You can use the map here to spot it in your own glass before the moon returns to the morning sky.
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But wait, there’s more. Emmanuel Jehin, a member of the TRAPPIST ( TRAnsiting Planets and PlanetesImals in Small Telescopes) team, a group of astronomers dedicated to the detection of exoplanets and the study of comets and other small solar system bodies, reports a rapid rise in ISON’s gas production rate in the past several days. They’ve increased by a factor of two since Nov. 3. Could the spike be connected to the development of an ion tail? Jehin and team have also recorded two active jets coming from the comet’s nucleus using specialized filters. Dust production rates however have remained flat.
Casey Lisse of the Comet ISON Observing campaign (CIOC) reports that the Chandra X-ray Observatory just became the 9th spacecraft to image the comet . More details and photos should be available soon. The campaign predicts the comet will peak in brightness between -3 to -5 magnitude when it zips closest to the sun on Nov. 28. Want to ride alongside the comet during its passage through the inner solar system? Click on this awesome, interactive simulator.
Because ISON is a fresh-faced visitor from the distant Oort Cloud that will soon face the full fury of the sun, speculation of its fate has ranged across the spectrum. Everything from breakup and dissolution before perihelion to surviving intact trailing a spectacular dust tail. The comet is currently approaching the 0.8 A.U. mark (74.4 million miles / 120 million km) when previous comets C/1999 S4 LINEAR in 2000 and C/2010 X1 Elenin in 2011 crumbled to pieces and vaporized away. Will ISON have the internal strength to pass the test and venture further into the solar boil? Should it survive, it faces a formidable foe – the sun. Both the intense solar heat and gravitational stress on the comet’s nucleus could easily tear it apart. If this happens a few days before perihelion we’ll be left with little to see, but if ISON busts up a day or two after perihelion, watch out baby. When the comet reappears in the morning sky, it may be missing its head but make it up for the loss with a spectacular tail of fresh dust and ice many degrees in length. This is exactly what happened to Comet C/2011 W3 (Lovejoy) in December 2011. After its close graze with the home star, the nucleus disintegrated, producing a striking tail seen by skywatchers in the southern hemisphere.
The final scenario sees Comet ISON pushing past all barriers intact and ready to put on a splendid show. Whatever happens, I suspect we’re in for surprises ahead. For a more detailed analysis of these possibilities I invite you check out Matthew Knight’s blog on the CIOC website.