Two Big Dirty Snowballs in 2013?

Forecasting how bright a comet will become months in advance a lot of times is like forecasting hurricane intensity 5 days out: good luck!

However, we have two particularly noteworthy comets with real potential in 2013: Comets PanSTARRS (C/2011 L4) and ISON (C/2012 S1).

First up is PanSTARRS, which should be at its nearest point to the sun, perihelion, on 10 March 2013.  The week following perihelion is usually when a bright comet is at its best, and is also when PanSTARRS shoots to higher declination, making an appearance low in the western sky after sunset for those in the Northern Hemisphere.  Current estimates put PanSTARRS somewhere around magnitude 0 to 1 in mid-March.  Its perihelion distance is 0.30 AU (1.0 AU = mean earth sun-distance), coincidentally the same distance from Mercury to the sun when Mercury is at perihelion.  This means that the comet will have some distance from the sun in the sky when seen from earth, as long as it is not in line with the earth and sun.  That indeed is the case this return of the comet, and there is a fairly comfortable 15 degree angular separation from the sun in the sky.  Sufficient angular separation from the sun in the sky, along a higher declination than the sun in the sky soon after perihelion will mean that PanSTARRS should be favorably placed for Northern Hemisphere observers when it may have a beautiful tail the week or two immediately after perihelion.

PanSTARRS will hopefully be a treat, as the following picture would suggest (http://waitingforison.wordpress.com/comet-panstarrs/).

Later in the year, we have Comet ISON.  ISON was discovered less than 2 months ago, and has a tremendous amount of brightening to still undergo.  Stellar brightness is measured by a concept called magnitude, in which a more negative value means an object is brighter. For example, the sun is about magnitude -27.  The full moon comes in at about -13.  Venus, the brightest planet, is usually around -4.  Sirius, the brightest star in the nighttime sky is about -1.5, and the faintest stars seen by the naked eye in a very dark country sky are about magnitude 6.   A small telescope that many people own will often show you object down to around magnitude 10.

If an object becomes brighter, from magnitude 5.0 to 4.0, then it will become about 2.52 times brighter.  If it brightens more, from 4.0 to 3.0, it will brighten by another factor of about 2.52- it’s a logarithmic scale.  If an object brightens 5 magnitudes, it will become about 100 times brighter.  Comet PanSTARRS is currently too close to the sun to be observed but is estimated around magnitude 10 right now.  If it brightens to magnitude 0, it will brighten by a factor of 10,000.  ISON is further from the sun and fainter right now, around magnitude 17.  If it brightens to magnitude -13, then it will have to brighten by 30 magnitudes- yes, a factor of 10^12- a trillion!  If it doesn’t get that bright but still a very respectable magnitude -8 at its brightest, it would still brighten by a factor of 10^10, or 10 billion.  So, you can see, ISON still has a LOT of brightening to do.

Comet ISON is what we call a sungrazer.  It will get very close to the sun at perihelion on November 28, 2013- actually, in the solar atmosphere at a mere 0.0125 AU.   Being that close to the sun is very dangerous for a comet, and it could well disintegrate.  If ISON brightens as forecast, it would be visible in broad daylight but very close to the sun in the sky.  It would likely have a long, skinny, very well defined tail.  As it passes perihelion, it will also gain declination and be well suited in the Northern Hemisphere the days thereafter.  It may be a splendid object the first week of December, visible in both the evening and morning sky.  The maximum brightness of Comet ISON is much more uncertain than PanSTARRS at this point.  At this time, I’d say that PanSTARRS has a greater likelihood of being a great comet as its situation has less extreme uncertainty.  Great comets appear in the sky every 5-10 years, on average.  ISON probably has a greater chance of fizzling… it also probably also has a greater chance of being an absolutely tremendous comet, a once-in-a-lifetime type of opportunity.  So, the “error bars” on ISON are much higher at this point… greater uncertainty.  If either of the two comets becomes a great comet, then that will be a wonderful thing to behold, and should be most appreciated!

ISON’s tail will be sticking up out of the WSW after sunset and ESE before sunrise in early December, if all goes well.  Shown is what ISON may look like from northern midlatitudes, if it brightens as hoped (image courtesy http://waitingforison.files.wordpress.com).

Time will tell!