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Comet ISON is now fizzing with activity

Paul Sutherland, News Editor
25 July 2013, 23:00 UTC

Sen— One of the most eagerly awaited comets in history is livening up as it heads for its rendezvous with the Sun later this year.

Comet ISON has already been monitored by the Hubble Space Telescope and the Swift satellite as it races in between the orbits of Jupiter and Mars. Now another orbiting observatory, NASA’s Spitzer Space Telescope, has taken its own close-ups.

Images newly released show the comet, officially labelled C/2012 S1, as it appeared with Spitzer’s infrared array camera on 13 June when it lay about 500 million km from the Sun. It was clearly already fizzing with activity.

The picture on the left, taken at a wavelength of 3.6 microns, shows a tail of fine rocky dust being ejected from the comet’s head and being blown away by the pressure of the solar wind.

The right-hand picture, at 4.5-micron wavelength, reveals a neutral gas atmosphere around the comet that is probably caused by carbon dioxide bubbling off its surface at a rate of around one million kg a day.

Comet ISON is still too faint to be seen with standard amateur equipment but that will change over the next few weeks and it may be within range of backyard telescopes towards the end of August in the morning sky.

Its discovery on September 21 last year by the Russian International Scientific Optical Network (ISON) excited astronomers when they had calculated its orbit through the Solar System. It showed that the cosmic visitor will pass less than two million km from the Sun when it reaches its closest point, called perihelion, on November 28 this year.

Optimists hope to see the comet reach an unrivalled brightness of between magnitude -11 and -16, which means that at best it could shine brighter than the Full Moon and become the comet of a lifetime. Bear in mind that it will be in the daytime sky when this close - even so it would be visible in broad daylight if it performs at best.

Comet ISON from Hubble

How the Hubble Space Telescope saw Comet ISON on April 10. Credit: NASA, ESA, J.-Y. Li (Planetary Science Institute), and the Hubble Comet ISON Imaging Science Team 

The comet may also already be extremely bright as it approaches the Sun and as it heads back away from it, hopefully sporting a fine tail, providing it survives its close encounter. And there lies the rub, because the behaviour of comets is notoriously difficult to predict at the best of times.

UK astronomy populariser Stuart Atkinson has setup a blog, Waiting for ISON, with observing advice and star maps to help people view the comet. He told Sen: “These are fascinating observations from Spitzer. We’re all crossing our fingers that this is a good sign and that ISON will become very active as it approaches and rounds the Sun.

“Hopefully it means ISON will captivate us all in early December, but of course everyone should just try and stay calm and not get too carried away. ISON might dazzle and delight us, or disappoint and depress us, it’s too early to say yet, no matter what anyone tells you.

“It’s often said that comets are like cats but I think they’re more like politicians: sometimes they promise us the world at first, to get our attention, and then let us down! But let’s hope for the best!”

The comet has an orbit that is close to a parabola, which suggests it may be on its first journey into the inner Solar System from the Oort cloud of icy bodies that is thought to surround it. It has the consistency of a dirty snowball, being made up of dust and gases such as water, ammonia, methane and carbon dioxide left over from the formation of the planets 4.5 billion years ago.

A previous sungrazing comet, Ikeya-Seki (C/1965 S1) reached magnitude -10 in 1965 to the delight of observers. However, they were left disappointed in 1973 by another similar visitor, Comet Kohoutek, (C/1969 O1), which proved to be a bit of a damp squib.

Comet ISON was discovered by Vitali Nevski, of Vitebsk, Belarus, and Artyom Novichonok, of Kondopoga, Russia, on CCD images made with a 0.4-metre reflecting telescope near Kislovodsk, Russia. That forms part of ISON, a network of observatories across ten countries that operate to detect and track objects in space.