Believe nothing

This has, undoubtedly, been one of the most eagerly-awaited comets in recent years. It was discovered back on December 10, 2010 by Leonid Elenin, an astronomer in Lyubertsy, Russia (affiliated with the Keldysh Institute of Applied Mathematics in Moscow), who is an accomplished observer of comets and who has in fact made several recoveries of expected periodic comets within the fairly recent past; this is his first comet discovery. Leonid was observing remotely with a 45-cm (18-inch) telescope (equipped with a CCD) at the International Scientific Optical Network observatory in New Mexico (not too far from where I live, incidentally), and at the time of his discovery the comet was a rather faint object between 19th and 20th magnitudes and was located 4.2 AU from the sun.

The first calculated orbits for Comet Elenin suggested it would remain a distant object, however it soon became clear that it was traveling on a remarkably low-inclination orbit (1.8 degrees) that would carry it quite close to both the sun and the earth. Furthermore, the geometry for forward scattering of sunlight — and thus an accompanying brightness enhancement — is quite favorable around that time. All this has led to a realization that Comet Elenin possesses at least the potential to become a relatively bright object — conceivably even a “Great Comet” — for a brief period of time around perihelion passage and its closest approach to Earth.

Following its discovery Comet Elenin slowly brightened as it approached the sun and Earth, and it went through opposition in mid-March of this year. Towards the end of that month I broke out the Earthrise telescope/CCD system (that has been dormant for a few years amongst all the transitions both Earthrise and I are undergoing) and managed to obtain some images of the comet as a faint object near magnitude 15 1/2 or 16. I also began visual searches for it around that time, however it remained elusive for quite some time thereafter. Finally, on the evening of May 23 I suspected it as a very faint, diffuse and “soft” object a little fainter than 14th magnitude, and was able to verify this as being the comet by observations on several additional nights over the subsequent week. (This confirmation process was hindered some by the fact that the comet was at its stationary point, and thus hardly moved at all from night to night.)

The comet is currently an evening-sky object, located in western Leo some six degrees southeast of the bright star Regulus and two degrees southwest of the star Rho Leonis. Over the coming several weeks it tracks to the east-southeast across Leo roughly parallel to the ecliptic, until finally it enters western Virgo in early August. Its elongation from the sun at that time will have shrunk to 40 degrees, and since it will be south of the sun observations may be difficult from the northern hemisphere but should still be fairly easy from the southern. Even though the elongation continues to decrease after that, since the sun is moving southward we northern hemisphere observers may be able to keep following it for a while, perhaps even up until the time of perihelion passage when the elongation will be 26 degrees. After that its motion on the sky reverses as it crosses almost directly between the earth and the sun, passing less than two degrees north of the sun on September 26.

By the end of the first week of October Comet Elenin will emerge into the morning sky, traveling rapidly west-northwestward through Leo and passing six degrees north of Regulus on October 11. It makes its closest approach to Earth (0.23 AU) on October 16 and travels rapidly through Cancer and Gemini around this time (passing just one degree north of the bright star Pollux on October 22), then enters Auriga shortly before the end of October and then northern Taurus at the end of the first week of November. The comet is at opposition shortly after the middle of that month, and then passes through the northern regions of the Pleiades star cluster on November 23. At the end of that month it crosses into eastern Aries, where it remains up through the end of January 2012.

Brightness predictions for any potentially bright comet like this one are always problematical. Unfortunately, the news thus far for Comet Elenin is not especially good; it is already running somewhat behind the original expectations, and the distinctly diffuse and uncondensed appearance it is currently exhibiting visually is also not a good sign. Furthermore, calculations now clearly show that Comet Elenin is making its first visit to the inner solar system from the Oort Cloud, and such comets quite often tend to appear relatively bright when far from the sun but under-perform as they approach and pass through perihelion. (The most famous, or infamous, example is Comet Kohoutek 1973f (no. 10), but compared to some Oort Cloud comets even it did not perform all that poorly.)

With all this in mind, a cautiously optimistic scenario would suggest that Comet Elenin might reach 10th magnitude by early August when it becomes a somewhat difficult object from the northern hemisphere, and may brighten further to around 6th magnitude by the time it passes through perihelion. It may still be near this brightness when it reappears in the morning sky in early October, and then might fade slowly by perhaps half a magnitude by the time it is nearest Earth. Fading may be more rapid thereafter, with the comet’s being perhaps 10th or 11th magnitude when near opposition in November, and no brighter than perhaps 13th magnitude by the end of the year. As I mentioned above, there is a potential for a forward scattering enhancement of brightness (perhaps by as much as a couple of magnitudes) in late September when the comet passes almost directly between the Earth and the sun, however this will probably be fairly short-lived and, more importantly, this will occur when the comet is at a very small elongation from the sun. (The LASCO coronagraphs aboard the SOHO spacecraft should nevertheless be able to provide a spectacular view.)

This is only a guide, of course; the comet could easily be one, or two, or more, magnitudes brighter, or fainter, than this. Conceivably, it could fragment as it passes through perihelion and come out distinctly brighter that it was earlier; on the other hand, it could break apart completely and disintegrate as it passes through perihelion. There is precedent for both scenarios, as well as precedent for “normal” behavior. We’ll just have to wait and see what the comet actually does.

Unfortunately, some elements in our society have latched onto the news about Comet Elenin and have been promulgating various apocalyptic scenarios regarding it. (I will not link to any of those, or give them any more publicity than they deserve.) Some of these have gone so far as to question Leonid Elenin’s existence as an actual person. (I can assure “Countdown” participants that he is a real person; while I have not met him, I correspond with him on a fairly regular basis.) Many of these scenarios are almost identical to the nonsense that I remember combatting when Comet Hale-Bopp (no. 199) was around 14 years ago — with some of these being put out by the same people! — and, needless to say, we’re all still here. There is, in fact, nothing all that unusual about Comet Elenin; it does have the potential to become somewhat bright, and it does come somewhat close to Earth (although several comets, including at least three earlier “Countdown” comets, have come closer to Earth during recent years), but there is nothing of any kind to fear. Let’s enjoy whatever show it gives us.

http://spaceobs.org/en/2011/07/15/countdown-to-500-comets-comet-elenin/

http://www.earthriseinstitute.org/coms48.html#487