Matthew KnightJan 23, 2015

Addressing some common questions about Comet Lovejoy

Comet Lovejoy (C/2014 Q2) has been a naked eye comet for dark sky observers for several weeks and has already yielded some stunning images. I’ve seen several points of confusion come up repeatedly in coverage of this comet so I thought I’d address them here. 

1. Which Comet Lovejoy is this?

This might seem like a silly question, but it isn’t. Terry Lovejoy, the very talented amateur comet hunter who discovered Comet Lovejoy (C/2014 Q2) has actually discovered five “Comet Lovejoy”s:

  • The first two, C/2007 E2 and C/2007 K5, were both relatively non-descript comets that were observed for a few months in mid-2007 and never got bright enough to attract much attention.
  • The third Comet Lovejoy, C/2011 W3, was a pretty big deal. It was a member of the famous “Kreutz group” of sungrazing comets and was the first comet from the group discovered from the ground since 1970. Just three weeks after its discovery, this Comet Lovejoy put on a show in the fields of view of various solar observatories as it passed just ~130,000 km above the Sun’s surface (frequent Planetary Society contributor Karl Battams chronicled C/2011 W3’s harrowing journey here). While its nucleus evidently disintegrated shortly after perihelion, this disruption produced a spectacular tail that briefly made C/2011 W3 a stunning naked eye object for southern hemisphere observers before it gradually faded away.
  • The fourth, C/2013 R1, was not nearly as impressive as C/2011 W3, but it too briefly reached naked eye status. Discovered in September 2013, this comet Lovejoy was observed for about a year by amateur and professional astronomers, but didn’t attract as much media attention as the preceding or subsequent Comet Lovejoys.
  • The fifth Comet Lovejoy, C/2014 Q2, was discovered in August 2014 and is the one you can go out and see right now. It is likely to remain a naked eye comet for a few more weeks, and to be observed by amateurs and professionals throughout 2015.
Three different “Comet Lovejoy”s
Three different “Comet Lovejoy”s Top left is C/2011 W3 on December 23, 2011 (image credit Alex Cherney), top right is C/2013 R1 on November 7, 2013 (image credit Damian Peach), and bottom is C/2013 R1 on December 23, 2014 (image credit Gerald Rhemann).Image: Alex Cherney / Damian Peach / Gerald Rhemann

The easiest way to tell the various Comet Lovejoys apart is to look at the date, assuming the image is properly credited. If the image was from 2007, it was either C/2007 E2 (more likely) or C/2007 K5. If from 2011 or 2012, it was probably C/2011 W3. If from 2013 or the first half of 2014, it was probably C/2013 R1. If it was August 2014 or later, it was almost certainly the current one, C/2014 Q2.

Whew, that was a lot of letters and numbers. On to other topics! 

2. Why does it look green?

I addressed this question in detail for Comet ISON (C/2012 S1) a couple of years ago, and the reason is the same for Comet Lovejoy – emission from diatomic carbon (C2) in the comet’s coma. A very sensible follow up question is “Why is (this) Comet Lovejoy green, but many comets are not?” The predominance of the green color is because Comet Lovejoy has a relatively large amount of C2 gas compared to dust. This high “gas-to-dust” ratio means that there is more light reaching your eye (or the camera’s CCD) in the green portion of the spectrum. Most comets have lower gas-to-dust ratios than Lovejoy, so proportionally more of the light that we see is simply sunlight reflected off the comet’s dust. Since dust is relatively neutral in color, the light that reflects off of it basically looks like sunlight, which is white to your eye (technically, this “reflected solar continuum” is slightly reddened, but the level of reddening is imperceptible to your eye and can only be detected by analysis of images), so low gas-to-dust comets comets look white.

3. Why aren’t professional telescopes observing it?

You’ve undoubtedly seen many gorgeous images of Comet Lovejoy taken by amateur astronomers, but chances are you haven’t seen many images taken using professional telescopes. There are two main reasons for this.

Timing. Observations at most professional observatories are scheduled many months in advance. By the time Comet Lovejoy was discovered in August, most observatories had already set their schedules through early 2015. For example, the next “semester” for Keck Observatory begins February 1, and immediately features observations of Comet Lovejoy by two groups. Keck won’t be alone in observing Comet Lovejoy: observations are already underway at places with dedicated comet observing programs such as TRAPPIST and my institution, Lowell Observatory; a “Target of Opportunity” program to use the Hubble Space Telescope is hoping to observe Comet Lovejoy in early February; and a recently begun Spitzer program to study comet compositions is likely to observe Comet Lovejoy after it enters Spitzer’s observing window on February 14 (Spitzer is in orbit around the Sun away from the Earth so it has a different visibility window than do Earth-based telescopes). Given how bright Comet Lovejoy currently is, many other IR, radio, and optical telescopes are likely to observe it if they haven’t started already.

Aesthetics. Comet Lovejoy’s tail currently spans many degrees making the comet an ideal target for astrophotographers who are set up to make gorgeous images of large areas of sky. However, most professional observations produce spectra rather than images, and even when images are obtained, they typically have much smaller fields of view and are designed to study one particular aspect in great detail. This approach is crucial to understanding the link between today’s comets and the formation and evolution of our solar system, but generally can’t compete with the eye-candy being produced by the many talented astrophotographers out there. Thus, even if top professional observatories have already been collecting data on Comet Lovejoy, they likely won’t produce an “ooh aah” image or land on the Astronomy Picture of the Day Website.

Wrapping up

No matter your involvement in nighttime observing, the 2014 version of Comet Lovejoy has probably been a good comet for you. It has been naked eye brightness for several weeks and should continue to be naked eye for a few more weeks. If you’re not in a dark site, it can easily be seen with binoculars or a small telescope soon after sunset, and is yielding some stunning wide field photographs. Finally, it is beginning to be observed by some of the most powerful telescopes in the world, and eventually should yield new insights into how our solar system came to be.

Not bad for a fuzzy green dot!
Not bad for a fuzzy green dot! Image: Terry Lovejoy

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