A Crash Course on Comet ISON

Hubble Space Telescope view of Comet ISON on April 30, 2013.  Click to enlarge.

Hubble Space Telescope view of Comet ISON on April 30, 2013.

Comet ISON is on the way.  It might be extraordinarily bright.  It might be a dud.  It’ll likely be somewhere in between.  This visitor from the outer solar system will brighten through October and November on its way to graze the Sun on November 28.  If the comet survives its close encounter with the Sun, it may grow spectacularly bright in the eastern sky before sunrise during December for northern-hemisphere observers.  Here’s a crash course on Comet ISON, along with a few tips of how and when to see it…

 

 

Where did Comet ISON come from?

Comet ISON likely came from the distant Oort Cloud, a spherical shell of comets far beyond the orbit of Neptune and Pluto.  Thousands of years ago, it was likely nudged towards the Sun when it interacted with another comet in the Oort Cloud.  It has been moving towards the inner solar system ever since.

What’s so special about Comet ISON?

Comet ISON is a “sungrazer”, which means its orbital path will take it very close to the visible surface of the Sun.  On November 28, 2013, the comet will pass within 1,100,000 km of the visible surface of the Sun.  That seems like a large distance, but it’s less than the Sun’s diameter and 50x closer than the average distance between the Sun and the planet Mercury.

Because it will get so close to the Sun, Comet ISON may grow very bright and feature a brilliant coma (head) and a long tail during the late days of November and through mid December.  It may grow as bright as Comet McNaught in 2007, or even as bright as Comet Ikeya-Seki in 1965, the brightest comet of the 20th century.

When was Comet ISON discovered?

The comet was first observed on September 21, 2012 on digital images taken with the Santel reflector telescope of the International Scientific Optical Network (ISON).

Who discovered Comet ISON?

The comet was first seen by astronomers Vitali Nevski and Artyom Novichonok in four 100-s CCD images made with the 0.4-m f/3 Santel reflector of the International Scientific Optical Network (ISON) near Kislovodsk, Russia.

Where did Comet ISON get its name?

Because its discoverers did not actually notice ISON was a comet when they first saw it, the comet is named after the telescope network used to find it.

The official name for Comet ISON according to the conventions of the International Astronomical Union is C/2012 S1.  The “C” means it is non-periodic (so it will not return to the inner solar system).  The date means it was discovered in 2012.  The “S1″ means it was the first comet discovered in the second half of September in that year.  The comet is properly called C/2012 S1 (ISON), which distinguishes it from other comets discovered with the ISON network.

Will Comet ISON hit Earth?

It will not.  Astronomers use very precise calculations using well-known physical laws to calculate the paths of comets and other celestial bodies.  They know very accurately the path the comet will follow.  The comet will not hit Earth or even come close.  At its closest approach to our planet, which happens on December 26, 2013, the comet will lie about 62.4 million kilometers from Earth.  That’s more than 160 times the distance from the Earth to the Moon.

Will Comet ISON hit the Sun?

The comet is not expected to hit the Sun.  But it will pass very close to the visible surface of the Sun.  On November 28, 2013 the comet will pass within 1,100,000 km of the visible surface of the Sun.  It will not hit the Sun, but the heat and radiation from the Sun will turn the outer layers of this “dirty snowball” into a boiling, steaming cauldron, causing the comet to grow very bright as it flies over the surface of the Sun at 684,000 km/h.  In these harsh conditions, it’s possible the comet will break up and boil away, never to be seen again.  Or it may hold together as swing around the far side of the Sun and remain immensely bright.  No one knows for sure.

The video below shows you the path of Comet ISON through the solar system.

Is Comet ISON a U.F.O?

Uh, no.  Some reports have surfaced that Comet ISON has actually split into three pieces, two of which are spacecraft escorting the comet to the inner solar system.  These reports, and others like them, are total rubbish.  Comet ISON is just a comet.  End of story.

Do I Need a Telescope to See Comet ISON?

You will need a telescope to see the comet in October and likely most of November.  It may grow bright enough to see with binoculars or possibly without optics in mid-to late November.  If it survives its encounter with the Sun, it may grow bright enough to see clearly without optical aid in December.

Where can I see Comet ISON in October?

At the beginning of the month, the comet is only visible with a large amateur telescope.  On October 16, the comet passes just 2º north of the bright star Regulus in the eastern early morning sky.  This will be a good chance to see the comet with a smaller telescope.   The comet will continue eastward moving south of the constellations Leo, Virgo, and Libra before its encounter with the Sun at the end of November (see maps below).

Path of Comet C/2012 S1 (ISON) in September and October 2013.

Path of Comet C/2012 S1 (ISON) in September and October 2013.

Comet C/2012 S1 (ISON) from Nov. 1, 2013 to Jan. 11, 2014.  Image credit: Virtual Telescope Project)

Comet C/2012 S1 (ISON) from Nov. 1, 2013 to Jan. 11, 2014. Image credit: Virtual Telescope Project)

When is the best time to see the comet?

Starting in mid-November, the comet may grow bright enough to see without optics.  On November 18, the comet passes very close (just 0.4º) north of the bright white star Spica in the early morning sky, presenting a great opportunity to see the comet with binoculars or, if it brightens enough, without optics.   When the comet passes around the Sun on November 28, it may grow very bright but will lie too close to the Sun to see from Earth.  As it moves away from the Sun (and if it survives the encounter), it may grow very bright and remain visible for most of December and much of January 2014.

Comet C/2012 S1 (ISON) in the pre-dawn sky in December.  Image credit: Sky and Telescope (www.skyandtelescope.com)

Comet C/2012 S1 (ISON) in the pre-dawn sky in December. Image credit: Sky and Telescope (www.skyandtelescope.com)

What will Comet ISON look like at its brightest?

Again, it all depends on whether the comet survives its encounter with the Sun.  It also depends on the composition of the comet itself, which is not completely understood.   As the great comet hunter David Levy once said, “Comets are like cats: they have tails, and they do precisely what they want.”

The image below shows Comet Hale-Bopp which visited the inner solar system in 1996-97.  It was the last bright comet visible from the northern hemisphere.   As you can see, this comet has a bright glowing head.  This is called a “coma”, and it’s usually the brightest part of a comet.  The head is mostly glowing gas boiled off as the comet gets within a few hundred million kilometers of the Sun.  The head spans a few hundred to a few thousand kilometers depending on the comet’s size and composition.  Buried within the coma is the “nucleus” of the comet, which is the solid “dirty snowball” that contains most of the comet’s mass.  A layer of invisible hydrogen gas called the “hydrogen envelope” surrounds the coma.  The nucleus itself… the only solid part of  Comet ISON… is about 7 km across.

Comet Hale-Bopp as seen in March 1997.

Comet Hale-Bopp as seen in March 1997.

Many comets develop a tail as they approach the Sun, and ISON already has a small tail.  There are two types of tail.  The “dust tail” is made of tiny bits of evaporated dust from the nucleus.  This dust is pushed away from the coma by light from the Sun.  The dust tail shines by reflecting the Sun’s light.  It’s sometimes curved in the same way water curves away from the nozzle of a moving hose.

The “ion tail” is made of glowing electrically charged particles pushed away by the steady wind of charged particles from the Sun.  This tail remains straight and often glows blue or blue-green.

Sometimes the dust and ion tails overlap.  Sometimes, as in the case of Comet Hale-Bopp (above), they are separated.  The shape and size of the tails of a comet often change from night to night.  If Comet ISON works out to be a respectably bright comet, its narrow tail may span 15-20º of sky.  That’s about the width of your outstretched palm held at arm’s length.