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Bright comet to visit Northern skies

March 7, 2013
Updated March 8
Courtesy of the Roy­al As­tro­nom­i­cal So­ci­e­ty, JPL
and World Science staff

Sky­watch­ers in the north­ern hem­i­sphere should en­joy a rare treat in the weeks ahead, as Com­et C/2011 L4 PANSTARRS vis­its the eve­ning sky, ac­cord­ing to the U.K.-based Roy­al As­tro­nom­i­cal So­ci­e­ty.

Al­though com­et bright­ness is no­to­ri­ously hard to pre­dict, it looks as though this one may be vis­i­ble to the un­aided eye in the sec­ond half of March, as­tro­no­mers said.

The path of Com­et C/2011 L4 (PANSTARRS) as seen through March. The com­et will ap­pear to move through the con­stel­la­tions of Pi­sces, Peg­a­sus and An­drom­e­da. From the UK it will be in the west­ern and north-west­ern eve­ning sky. (Cred­it: Al­an Fitz­sim­mons us­ing with The Sky© Soft­ware Bisque 2010)

“Bright com­ets are fairly rare and we usu­ally don’t know when the next one is on the way. Wheth­er you’re an ex­pe­ri­enced am­a­teur as­tron­o­mer or just have an in­ter­est, PANSTARRS is well worth a look,” said Mark Bai­ley, di­rec­tor of the Ar­magh Ob­serv­a­to­ry in North­ern Ire­land.

It’s named af­ter the Pan-STARRS tel­e­scope in Ha­waii, which was used to dis­cov­er it in June 2011. It ap­peared then as an ex­tremely faint ob­ject 1.2 bil­lion km (750 mil­lion miles) from the Sun. Look­ing at its path, as­tro­no­mers real­ised that it could be­come very bright at its clos­est ap­proach to the Sun, or per­i­he­li­on, on March 10 this year.

Like oth­er com­ets of its type, it’s thought to have orig­i­nat­ed in the Oort Cloud, a vast re­gion con­tain­ing mil­lions of com­ets more than two light years from the Sun (a light year is the dis­tance light trav­els in a year). 

The ob­ject trav­elled in to­wards the in­ner So­lar sys­tem for mil­lions of years, dor­mant for most of this time as a small nu­cle­us or co­re made up of rock and ices, sci­en­tists said. When com­ets ap­proach the Sun, these ices heat up, even­tu­ally turn­ing to gas­es that je­t out in­to space along with dusty ma­te­ri­al to form a head or “co­ma” around the com­etary nu­cle­us. Par­t­i­cles from the Sun—the so-called so­lar wind—blow the gas­es back in a straight tail, whilst sun­light ex­erts a pres­sure on the dust par­t­i­cles to cre­ate a curved tail. The two tails and co­ma make up the clas­sic com­et fa­mil­iar in so many as­tro­nom­i­cal im­ages but are not al­ways easy to pick out with the eye.

An im­age of Com­et C/2011 L4 (PAN­STARRS) made by Aus­tral­ian as­tron­o­mer Ter­ry Love­joy. The curved dust tail can be seen sweep­ing up from the co­ma on the left hand side of the im­age, while the straight gas tail runs up­wards from left to right. (Cred­it: Ter­ry Love­joy)

En­cour­ag­ing­ly, PANSTARRS has al­ready been seen by ob­servers in the south­ern hem­i­sphere be­fore per­i­he­li­on, with re­ports that it’s vis­i­ble to the un­aided eye, as­tro­no­mers said. By March 8 it is ex­pected to start to be vis­i­ble from the north­ern hem­i­sphere, al­though per­haps only through bin­oc­u­lars or tel­e­scopes at first.

By March 12 and 13, it will be fur­ther from the Sun and should be eas­i­er to spot, they added. To find it, sky­watch­ers will need a clear sky, ide­ally be away from the lights of towns and ­ci­ties and have a good west­ern ho­ri­zon. Af­ter sun­set on those dates the com­et will be low down in the west and ap­pear as a misty patch not far from the cres­cent Moon. Us­ing bin­oc­u­lars will make it eas­i­er to find and will cer­tainly help iden­ti­fy the tails which should point up from the ho­ri­zon, sci­en­tists ex­plained.

“You'll need a rel­a­tively un­ob­struct­ed view to the south­west at twi­light and, of course, some good com­et-watching weath­er,” said said Amy Mainzer, the prin­ci­pal in­ves­ti­ga­tor of NA­SA's NE­O­WISE mis­sion at the Je­t Pro­pul­sion Lab­o­ra­to­ry in Pas­a­de­na, Ca­lif.

The best view­ing time will be just af­ter twi­light dur­ing a brief pe­ri­od that may be easy to miss, sci­en­tists added. “Look too ear­ly and the sky will be too bright,” said Ra­chel Ste­ven­son, a NA­SA Post­doc­tor­al Fel­low at JPL. “Look too late, the com­et will be too low and ob­structed by the ho­ri­zon. ” 

As the days pass, the com­et will move away from the Sun and fade and light from the Moon will in­ter­fere more. But the comet will also be high­er up, will be vis­i­ble lat­er in the night and so be seen in a darker sky. Af­ter its brief pe­ri­od of vis­i­bil­ity, the com­et will trav­el back out to­wards the depths of space where it will be only be view­able by large tel­e­scopes.

Astronomers say viewers may also get the rare treat of seeing two naked-eye comets in one year, as comet ISON may come into view later this fall.

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Correction: The original version of this story contained the wrong conversion from kilometers to miles for the distance to PANSTARRS.

Skywatchers in the northern hemisphere should enjoy a rare treat in the weeks ahead, as Comet C/2011 L4 PANSTARRS visits the evening sky, according to the U.K.-based Royal Astronomical Society. Although comet brightness is notoriously hard to predict, it looks as though this one may be visible to the unaided eye in the second half of March, astronomers said. “Bright comets are fairly rare and we usually don’t know when the next one is on the way. Whether you’re an experienced amateur astronomer or just have an interest, PANSTARRS is well worth a look,” said Mark Bailey, director of the Armagh Observatory in Northern Ireland. It’s named after the Pan-STARRS telescope in Hawaii, which was used to discover it in June 2011. It appeared then as an extremely faint object 1.2 billion km (7.5 billion miles) from the Sun. Looking at its path, astronomers realised that it could become very bright at its closest approach to the Sun, or perihelion, on March 10 this year. Like other comets of its type, it’s thought to have originated in the Oort Cloud, a vast region containing millions of comets more than two light years from the Sun (a light year is the distance light travels in a year). The object travelled in towards the inner Solar system for millions of years, dormant for most of this time as a small nucleus or core made up of rock and ices, scientists said. When comets approach the Sun, these ices heat up, eventually turning to gases that jet out into space along with dusty material to form a head or “coma” around the cometary nucleus. Particles from the Sun—the so-called solar wind—blow the gases back in a straight tail, whilst sunlight exerts a pressure on the dust particles to create a curved tail. The two tails and coma make up the classic comet familiar in so many astronomical images but are not always easy to pick out with the eye. Encouragingly, PANSTARRS has already been seen by observers in the southern hemisphere before perihelion, with reports that it’s roughly as bright as the stars in the Big Dipper and thus visible to the unaided eye, astronomers said. By March 8 it is expected to start to be visible from the northern hemisphere, although perhaps only through binoculars or telescopes at first. By March 12 and 13, it will be further from the Sun and should be easier to spot, they added. To find it, skywatchers will need a clear sky, ideally be away from the lights of towns and cities and have a good western horizon. After sunset on those dates the comet will be low down in the west and appear as a misty patch not far from the crescent Moon. Using binoculars will make it easier to find and will certainly help identify the tails which should point up from the horizon, scientists explained. As the days pass, the comet will move away from the Sun and fade and light from the Moon will interfere more. At the same time however, PANSTARRS will be higher up, will be visible later in the night and so be seen in a darker sky. After its brief period of visibility, the comet will travel back out towards the depths of space where it will be only be detected by large telescopes.