This fall's big astronomical mystery is Comet ISON (C/2012 S1), the new comet discovered in Russia Sept. 21, 2012. While the initial enthusiastic predictions have been walked back, observers generally remain optimistic that the comet will be well worth observing as it rounds the sun later this month.
In his Astronomical Calendar for 2013, Guy Ottewell declared ISON could "become a daylight-brilliant immense-tailed Sungrazer, to rival the few great ones of the past few centuries." By June, comet expert John Bortle was a bit more subdued, saying that "from January through May Comet ISON brightened hardly at all, remaining stuck at magnitude 16 or 15 and falling nearly two magnitudes behind the early predictions." In August, Sky and Telescope magazine's Alan MacRobert seemed to be preparing observers for the wost, saying "the comet could still turn out to be fairly good, or it might never reach naked-eye visibility at all."
The comet is getting a little brighter. By late October, it had reached magnitude 9. That is still about 16 times fainter than the theoretical unaided-eye limit of magnitude 6.
In October, my friend Tom saw the comet from his mountaintop West Virginia house (elevation 4,440 feet) using a very large telescope equipped with a 24-inch diameter mirror. One morning in late October, I attempted to observe ISON using only a 12-inch telescope, but the interfering light from the third quarter moon washed out the faint comet.
A recent email message announced a new "comet outburst." I thought ISON had finally decided to get with it by pouring on the coal and living up to all of our expectations. Unfortunately, it turned out to be a totally different comet. Comet C/2012 X1 (LINEAR) had suddenly burst from 14th to 8th magnitude - a 251-fold increase in brightness (but still six times too faint to see with the unaided eye).
This is reminiscent of the 2007 outburst of Comet Holmes (see my 2007 photos from Oct. 28, Nov. 17, Nov. 28 and Dec. 9), during which it experienced a half-million times increase in brightness, making it plainly visible to the unaided eye even in very bright urban environments. The comet had undergone a previous brightness outburst that was ongoing at the time of its 1892 discovery and, according to the Harvard-Smithsonian Center for Astrophysics, "has presented a mystery to astronomers ever since."
One possibility expressed in the prestigious Astrophysical Journal involved a "fragmentation event" or splitting of the comet nucleus. However, the article concluded that "if this had happened, there should be big fragments trailing the comet nucleus ... images, however, do not show any large fragments near the nucleus of Comet 17P/Holmes. Therefore, this hypothesis must be ruled out."
A commonly expressed theory states that Holmes' extreme increase in brightness was impact-related, involving an unfortunate encounter with a rogue asteroid. The energy involved in a collision would release quite a cloud. However, the chances of such a needle-in-a-haystack collision were discounted. New Scientist paraphrased the late Brian Marsden, former director of the International Astronomical Union's Minor Planet Center, as saying, "it's hard to believe that this comet, among all those that pass through the asteroid belt, has been struck twice by objects in the belt - once in 1892 and again [in 2007]."
According to the late Washington-based astronomer Tom Van Flandern, the impact theory was on the right track, but it wasn't asteroids that hit Holmes. He first introduced his "satellite model for comets" in a 1981 paper published in the journal Icarus. According to this model, comets as well as planets can be surrounded by satellites or moons within the parent body's gravitational "sphere of influence."
Under Van Flandern's model, when a comet has an encounter with another body, particularly the sun or Jupiter, its orbit can be altered. Also, tidal forces from the interacting body can compromise the comet's sphere of influence disrupting the orbits of its satellites. According to astronomer Gary Kronk, Comet Holmes had encounters with Jupiter in 1908 and 2004. Respectively, these served to increase and decrease the comet's distance from the sun.
An October 2007 article by Baltimore Sun reporter Frank D. Roylance reported that "the late comet expert Fred Whipple theorized [in 1984] that Holmes might have had a satellite that crashed into it in 1892, causing that year's odd flare-up." In the same article Marsden objected, saying "even if he were right then, that satellite is long gone. And yet the same thing has happened again." Thus, Marsden implies that the satellite impact can only be a one-time event.
Van Flandern believed that a "debris cloud" of satellites surrounds Comet Holmes. Following the 2004 encounter with Jupiter that brought Holmes nearer to the sun, the next time the comet rounded the sun, tidal forces were sufficient enough to disturb any loosely bound satellite into an unstable orbit, eventually causing it to crash into the comet. This might also have happened in 1892. Indeed, in the satellite model for comets, the supply of available satellites need not be limited to just one. "Such major outburst events are a natural occurrence in the satellite model for comets because there is a large supply of candidate satellites vulnerable to tidal decay or gravitational disruption," wrote Van Flandern.
Besides the two comets mentioned above, there have been other comet outbursts as well. Comet P/Halley experienced a sudden 6 magnitude (200 fold) outburst in 1991. Comet 29P/Schwassmann-Wachmann 1 has experienced multiple outbursts in brightness, sometimes as great as 7-8 magnitudes.
In a private email sent to this writer following Comet Holmes' 2007 outburst, Dr. Van Flandern noted how all the elements for his explanation of cometary outbursts were already in place in his satellite model for comets. Nothing new had to be added or modified for this explanation unlike "mainstream" theories that could not account for it without patching together new hypotheses.
It is time for us to get back on subject regarding what to expect from Comet ISON.
The comet is in the morning sky, where it will remain until the end of November when it rounds the sun on Thanksgiving Day, Nov. 28. Until then, it will be getting lower in the sky but should also be brightening and possibly developing a tail. By early December, it will be in the evening sky, getting higher and fainter each night.
How bright will it get? The comet's brightening is behind schedule. It's hard to find firm estimates on the web. Most just say it could reach naked eye brightness, a far cry from earlier predictions of its being visible in broad daylight. Even so, it will probably be visible in binoculars and tripod-mounted time exposures made with digital cameras.
Where to see it? Comet ISON will hang in the evening sky in early December, setting early in the evening. Your best bet is to search the web for finder charts or ask any friends who happen to be amateur astronomers. You may also wish to check the calendar on the Westminster Astronomical Society web site (
) for public viewing opportunities at the organization's regularly scheduled programs.