Last June astronomers at the University of Hawaii announced they’d discovered a comet with the 1.8 meter (70.7 inch) telescope atop Mount Haleakala as part of the Panoramic Survey Telescope & Rapid Response System or Pan-STARRS. The survey’s goal is to photograph the entire sky several times a month in search of Earth-approaching comets and asteroids that could pose a danger to our planet.
At the time, Comet C/2011 L4 PANSTARRS (or PANSTARRS for short) was extremely faint and nearly as far away as the planet Saturn.
After more observations pinned down the comet’s orbit, predictions showed it would pass perihelion – its closest point to the sun – at a distance of 28 million miles on the evening of March 9, 2013. That’s close enough to vaporize a lot of cometary ice, releasing the dust needed to form a bright coma and tail.
Just how bright, no one can be certain. We all know how unpredictable comets can be; the break up and fading of Comet Elenin is just one recent example. But estimates based on the PANSTARR’s distance from the sun and Earth at the time of perihelion put it at magnitude 0 or as brilliant as Vega or Arcturus.
Circumstances for viewing a bright comet couldn’t be better. PANSTARRS pops into the evening sky only a few days after closest approach to the sun. Moving rapidly northward, it soon becomes visible all night long from mid-northern latitudes in April.
You might be wondering why I’d bother writing a blog about something happening 10 months down the road. Let’s just say I want as many amateur astronomers as possible to have the opportunity to see the comet early.
Die-hard comet observers have been photographing and observing the comet since late this winter, more than a year before perihelion. I’ll take that as a good sign that PANSTARRS is on schedule.
I sought the comet a week ago using a 15-inch reflecting telescope and was surprised at how easy it was to see. Located near the bright star Antares in Scorpius the Scorpion, I estimated the comet’s brightness at magnitude 12.5 (at discovery it was 19 — faint!). PANSTARRS was a very small but dense knot of light about 20 arc seconds in diameter with a faint star-like center. Its compact appearance is a good indicator of lots of dust activity in the comet’s nucleus – another positive sign for the coming apparition. A second look this past Saturday morning showed it smidge brighter yet.
If you start observing now, you’ll have the pleasure of watching Comet PANSTARRS brighten and develop on its journey to perihelion and beyond. Following a comet night by night can be very rewarding, comparable to studying a species of bird to better understand and appreciate its behavior. For the moment, you’ll need a 10-inch or larger scope and dark skies but as the weeks and months advance, it will gradually brighten.
Skywatchers in mid-northern latitudes will be able to follow PANSTARRS through early August before it’s too low to view and lost in the glow of evening twilight. Our next opportunity won’t be until next March post-perihelion. Southern hemisphere observers will fare much better with the comet high in the sky and well-placed for viewing for months to come.
To assist you in your quest, either download comet orbital elements for your favorite star charting program at the IAU Minor Planet Center site or use the map above which shows the comet’s position around 11:30 p.m. CDT every five nights. PANSTARRS is still 325 million miles from Earth or more than halfway to Jupiter.
Returning to the question of the comet’s brightness, that may depend on whether it’s making its first or hundredth trip around the sun. On a first swingby, exotic ices of nitrogen and carbon dioxide, long preserved in the deep freeze of the outer solar system, vaporize at great distances from the sun, making the comet appear unusually bright. If we’re not careful, we might extrapolate that behavior to the time of closest approach and predict a very bright passage. Unfortunately, once those ices are gone, the comet may have only a modest amount of water ice remaining for the sun to vaporize and not brighten as expected when closer to the sun.
Comets that return time and again all have elliptical orbits around the sun like the planets but more stretched out or elongated. Comet PANSTARRS’ orbit appears for the moment to be nearly parabolic. A parabola is a sort of open-ended ellipse with one end near the sun and the other a return trip to infinity. Most comets on parabolic orbits come from the far edge of the solar system and have their orbits reworked by giant planets Jupiter and Saturn into very long but closed ellipses with orbital periods of hundreds of thousands to millions of years. PANSTARRS might be one of those “fresh” comets and putting on a good show now despite its distance. We’ll have to just wait and see.