10 Things You Need to Know About Comet ISON

Comet ISON is here! This new comet was in January 2013 just a dim speck in the constellation Gemini, between the stars Castor and Pollux. Astronomers could not agree if it was going to develop into the greatest astronomical spectacle in a decade or into a dim and disappointing smudge- and they’re not sure . Here is a quick look at why so much is expected of this visitor from deep space. If you are interested in observing it, see Where is Comet ISON in the Sky?


How was it found? The discovery of C/2012 S1 (ISON) was announced on 24 September 2012.It was found by Russian astronomers Vitali Nevski and Artyom Novichonok on CCD images made with a 0.4-m telescope of the International Scientific Optical Network (ISON) near Kislovodsk, Russia. The comet was further from the Sun than Jupiter, 6.6 AU distant (an AU is an astronomical unit, equal to 150 million km, the average distance the Earth is from the Sun), The comet was a very dim Magnitude 18.8 when discovered (objects fainter than Magnitude 6 cannot be seen with the unaided eye).

It was originally thought ISON would be more spectacular to see than Comet Lovejoy, visible near Earth’s horizon in this nighttime image photographed by NASA astronaut Dan Burbank, Expedition 30 commander, onboard the International Space Station on 22 December 2011. Unfortunately these predictions do not seem to be going to play out. (Image credit: NASA )


What is its real name? Strictly speaking this comet should be referred to as Comet C/2012 S1, but it is already universally called Comet ISON in the media. Oddly it has not been named after Nevski and Novichonok. For convenience I will call it ISON too.

Why all the excitement? Amateur and professional astronomers worldwide are eagerly anticipating comet ISON. Its orbit is nearly parabolic, which suggests that it may be a dynamically new comet coming freshly from the Oort Cloud, the little-known shell of literally trillions of comet nuclei lying tens of thousands of AU from the Sun. ISON has been on its way sunwards for a million years or so, such new comets are of great interest for what they can tell us about conditions at the dark and distant edge of the Solar System. Apart from the science, Comet ISON looked as though at its brightest it could be a very spectacular sight for astronomers to show off to their friends!

What is it? Like all comets, ISON’s nucleus is a sooty lump of rock and ice (mainly frozen water, but with carbon dioxide, methane, ammonia and other compounds too) a few kilometres across. In April 2013 Hubble Space Telescope images suggested that the nucleus of ISON is no larger than four miles (6.5 km) across which is surprisingly small in view of how active it was at the time. Comet nuclei are ancient, as old as the Solar System and unchanged for most of the past 4.5 billion years. However as it gets closer to the Sun, solar heat brings a comet to life, warming the frigid surface of the comet’s nucleus, melting and vaporising ancient ice. The once placid surface becomes a violent wasteland of erupting geysers. As the ice boils into space, the vapour forms a large but incredibly thin atmosphere called the coma around the comet’s nucleus. Eventually, when the comet is much closer to the Sun, the coma will probably shrink somewhat as the comet develops a couple of long tails, one of gas, the other of dust freed from the comet’s nucleus. The tails are blown directly away from the Sun by the solar wind. The tails and coma of a comet  are so thin as to be to all intents and purposes a vacuum. By the late summer of 2013 the tails had developed but had not yet made the comet more conspicuous. In June NASA’s Spitzer satellite observed a steady stream of carbon dioxide and dust flowing away from the comet in a tail over 186 000 miles (a light second) long. By September, the comet was noticeably green in colour indicating the presence of  cyanogen and diatomic carbon in the tail. You can examine the comet’s orbit with the Jet Propulsion Laboratory’s excellent Small-Body Database Visualizer.

Image of great comet of 1680

The Great Comet of 1680 over Rotterdam in a painting by Dutch artist Lieve Verschuier (Image credit: via wikimedia.org)


Has it been seen before? The answer is absolutely no. Comet ISON is making its one and only visit to the warmth of the inner Solar System.  However its orbit is similar to that of the Great Comet of 1680 (also known as Kirch’s or Newton’s Comet). In 1680, this comet passed extremely close to the Sun, brightening until it was plainly visible to the naked eye in daytime. This historic comet and ISON have such similar orbits that it seemed possible both comets may once have been one body which split apart in the distant past. This is no longer believed to be the case and it appears the two comets are unrelated. It is certain that ISON and the Great Comet of 1680 are completely different objects, anyone who tells you they are one body with an orbital period of 333 years (333 is of course half of 666, so this must be bad) is completely wrong.

Hubble’s view of Comet ISON on Oct. 9, 2013. Image Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

Hubble Space Telescope’s view of Comet ISON on Oct. 9, 2013. (Image Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA))



What is it going to do? ISON is classed as a sun-grazing comet. Like every other comet ever observed, Comet ISON  is speeding up as it gets nearer the Sun (its movement is not noticeable as you watch it, but is apparent from night to night). Comet ISON will make its closest approach to the Sun, or perihelion, on 28 November 2013 at a distance of  only 1.2 million km (about 750 000 miles) from the surface of the Sun. This orbit will actually take the comet through the Sun’s outer atmosphere and the comet’s surface temperature may exceed that of molten iron (1538 °C). If the comet survives this very close encounter, it may emerge as an easily spotted early morning object. Alternatively the comet’s nucleus, weakened by losing so much material and stressed by the mighty gravitation forces of the nearby Sun might have disintegrated into a cloud of rubble and ice. In this worst possible case, the comet would rapidly become a dim and fuzzy blob to observers. By early December, this worst case scenario had actually happened, the comet’s nucleus appeared to be crumbling as it approached perihelion, and at first it was not seen when it was expected to have cleared the Sun. However, a small part of the nucleus has survived its ordeal but will not be visible this will be to the average observers as it is just too dim.


Comet ISON appears as a peculiar bright smudge with two tails, above the Sun in this image from SOHO’s LASCO C3 camera at 09.22 UT today. Credit: NASA/ESA

Comet ISON appears as a blob with two tails above the Sun (obscured by an occulting disc) in this image from the SOHO spacecraft at 09.22 UT on 29 November 2013. (image credit: NASA/ESA)


How can we see it? Assuming ISON is a typical comet, it will gradually get brighter as it nears the Sun, then dim again as it recedes from the Sun. During August 2013, it became bright enough to be visible through large amateur telescopes (it was invisible to the unaided eye), appearing dimmer and harder to see than hoped. Observers saw it as a morning object in the constellation of Cancer, near M44, the Beehive Cluster. It was thought that by late October/early November it could be visible to binocular observers as it began to speed through the constellations, passing through Leo and Virgo, but these predictions have been proven to be over optimistic. However by mid-November its brightness had surged to naked eye visibility. At the end of November its proximity to the Sun made it impossible to see and the comet’s disintegration at perihelion means most people will never get a chance to observe it.



HiRISE saw a small spot at the position of ISON that is relatively bright, like a star, but moving relative to actual stars. These are the initial raw images, more and better images are expected. (Image credit: NASA/JPL/University of Arizona)

HiRISE saw a small spot at the position of ISON that is relatively bright, like a star, but moving relative to actual stars.  (Image credit: NASA/JPL/University of Arizona)


Was it visible from other planets? Everyone thought that ISON would be the first comet in human history to be observed from more than one planet. On 1 October 2013, the comet passed about 0.072 AU (10.8 million km) from Mars. Mission controllers at JPL planned to observe it with the cameras of the Curiosity and Opportunity Mars rovers but the dimmer than expected comet could not be seen.  The European Mars Express orbiter was also used to try to make images of the comet but it seems this was unsuccessful. The Mars Reconnaissance Orbiter made some images with its HiRISE instrument starting on 29 September 2013 but they were less than spectacular. Despite its close pass there was no chance at all that this comet would collide with Mars (and, despite what some You Tube videos predicted, it didn’t), however there is a very small possibility that another comet, Comet C/2013 A1 (Siding Spring) could impact the Red Planet when it makes an extremely close approach on 19 October 2014.  Despite the predictions of some,  ISON and Mars were not linked by alarming electrical discharges during the comet’s pass.


The Ultraviolet/Optical Telescope aboard NASA's Swift imaged comet ISON (center) on Jan. 30, when it was located about 3.3 degrees from the bright star Castor in the constellation Gemini. At the time of this 5.5-minute optical exposure, ISON was about 5,000 times fainter than the limit of human vision. Image released March 29, 2013. CREDIT: NASA/Swift/D. Bodewits, UMCP

The Ultraviolet/Optical Telescope aboard NASA’s Swift imaged comet ISON (centre) on 30 January 2013, when it was located about 3.3 degrees from the bright star Castor in the constellation Gemini. At the time of this 5.5-minute optical exposure, ISON was about 5000 times fainter than the limit of human vision. (Image credit: NASA/Swift/D. Bodewits, UMCP)


Is it dangerous? Comet ISON presents no danger at all to life on Earth. Absolutely none. There is not even the most minute chance that it could collide with our planet (anyone who says otherwise is completely misinformed- note I have toned my language here down after some justified criticism). It will pass about 0.42 AU (63 million km) from Earth on 26 December 2013. I cannot believe anyone really thinks this is a near miss. Earth will pass close to the comet’s dust trail on 16 January 2014 and some super-optimists have said we may just be lucky to observe a meteor shower that night.  However this is fantastically unlikely and I am certain it will not happen.

Comet ISON is NOT accompanied by other objects (asteroids or spaceships), this is a (deliberate?) misinterpretation of one of the images of the comet. According to Pete Lawrence who made the image the ‘companions’ are hot pixels, flaws, on the camera’s CCD chip (see Comments for his direct input). The comet is far, far smaller than any planet, there is no possibility that its gravitational pull will cause earthquakes or other environmental disasters on our planet. The comet is also not causing potentially dangerous effects on the Sun, nor is it going to at any future time. Science has never, ever recorded a comet influencing the Sun. If you are still not reassured, all I can do is advise you to read the comment sections on the posts on this blog about Comet Elenin (see also here.) Many people were worried about Elenin causing mayhem to our planet but it passed around the Sun without  any effect on our lives at all. Comet ISON will be just the same. You can listen to me discussing Comet ISON with Howard Hughes on his Unexplained podcast (link).





Is it the only bright comet in 2013? ISON is not the only  comet visible in 2013. Another comet, called 2011 L4 (PanSTARRS), was also to be seen in the evening sky in March and April 2013. In September 2013, another comet was announced, this is C/2013 R1 (Lovejoy) which will share the sky with ISON although it is too dim to see without a telescope.

If we are very lucky, by this time next year we could be celebrating a truly memorable comet. Let’s hope!

(Last update 25 February 2014.)

(Article by Colin Johnston, Science Communicator)