The inbound comet C/2011 L4, discovered last June, has been brightening steadily the past few months. At this rate, it could become a pretty bauble in post-sunset skies next March.
The hearts of comet-watchers are beating a little faster these days. That's because Comet Pan-STARRS (C/2011 L4) continues its inward drive toward the Sun and, based on the most recent observations, remains on track for a fine showing in northern skies next March. All indications point to a peak magnitude of anywhere from +1 to -1 — bright enough to deliver the best northern-sky performance since Comet Holmes's unexpected splash in 2007.
Now, to quote all those stock-market come-ons, "Past performance is no guarantee of future results." As I mentioned last June, this interloper was only 19th magnitude when spotted by the 1.8-m robotic Panoramic Survey Telescope & Rapid Response System (Pan-STARRS) atop Haleakala in Hawaii. Here we are 10 months later, and it's barely inside Jupiter's orbit — still a whopping 5 astronomical units from the Sun and no more than a 14th-magnitude blip on the celestial radar screen.
But the comet brightened by a whole magnitude during March, exceeding expectations. From his site in Cloudcroft, New Mexico, veteran observer Alan Hale spotted it visually on March 30th with a 16-inch (41-cm) reflector. "It's small and quite condensed," he reports, "and despite its faintness is not especially difficult to see."
Based on the trend to date, both the IAU's Minor Planet Center and JPL's Horizons system predict that perihelion will occur next March 10th or 11th. By then the comet will be inside Mercury's orbit and close to magnitude 0 in brightness.
So how might that look from here? First, note that perihelion occurs on the far side of the Sun from both Mercury itself (thus no chance for Messenger to spot it) and from Earth. For much of 2012, Comet Pan-STARRS lollygags in Scorpius and Libra, then it dives a bit south at year's end before making a beeline into northern skies.
Throughout all of this, those of you below the equator will have the comet to yourselves. But it should finally burst on the scene for northern skygazers about March 10th — right at perihelion — again favoring those of you at low latitudes (Costa Rica, anyone?). Unfortunately, when brightest, C/2011 L4 will appear only 15° from the Sun.
So will you be able to spot it using just your eyeballs? Opinions vary. For sure you'll need a vantage with an unobstructed view toward west. According to comet chronicler Gary Kronk, "It could be a challenge to see with the naked eye, although observations using binoculars and telescopes will certainly be possible." Long-time observer John Bortle agrees: "At the moment I don't see Comet Pan-STARRS becoming strikingly evident in the evening twilight for the great majority of observers."
Hale is more optimistic: "On March 13th — three days after perihelion — the comet will be almost due east of the Sun and, from my latitude at least, sets over an hour after sunset, around the end of civil twilight. From my experience, a comet near magnitude 0.5 that sets an hour after sunset should be pretty easy to spot naked-eye. The background sky will still be fairly bright, of course, but if there is any significant tail development, we should be able to see a few degrees of it."
My take is: "Let's be cautious, with fingers crossed." Perhaps we'll see something akin to how Comet McNaught (C/2006 P1) appeared as it skirted the western horizon at dusk five years ago. I could see it if I knew where to look and, more importantly, when to look — not too soon after sunset.
In any case, you can count on us to keep you up to date about Comet Pan-STARRS.