Comet Corner

By Don Pearce

This is a periodic update on observational aspects of visible comets.

April 4, 2008

Comet 17/P Holmes. Discovered on Nov. 7, 1892 by Edwin Holmes from England, this short period comet will be known as one of the most astounding comets, ever. To understand why, let us go back to its discovery in 1892. At that time it was in outburst, because we know that normally, this comet ranges from about 15-19^th magnitude as it makes its 7-year (average) trip around the Sun. But, then in November of 1892 it literally exploded, brightened to about 4^th magnitude, and was discovered by Holmes. After about 3 weeks it, then, dimmed again, only to, suddenly brighten again in January of 1893. After that it started declining, slowly, dimming to about 12^th

mag. by March, and disappearing, altogether, by April 6^th of that year (1893). It was seen again at the next two apparitions, but only as a dim comet. However, a close encounter with Jupiter in 1908 (around Dec. 8^th at about .543 AU) increased the size of its orbital period, and although hunted down, was lost for most of the 20^th century.

This has to be the second most fascinating quality of Holmes. For not only has it had these unpredictable outbursts, after its discovery astronomers had a very difficult time in even locating this comet, as evidenced that between 1906 and 1964 no one knew where it was. Finally, in 1963 Brian Marsden recalculated its orbit, and it was recovered at 19^th magnitude in 1964. With improved instrumentation and understanding of orbital mechanics, it has been observed at every return since then.

The third most astounding feature of this comet is why it was so quiet for the 115 years since its initial out bursts and discovery in 1892, and then really turned on October 23^rd of last year. It went from about 16.5 to about 2.5 magnitude in the span of about 2-3 days, which is about a 400,000 increase in brightness. Nothing like that has ever been observed in cometary science.

The next astounding aspect of this comet is its appearance. It developed a huge circular, distinctly yellow coma, and within days it expanded from about 1-2 arc minutes to 7 or 8 arc minutes in diameter. It then sprouted an outer halo surrounding the coma, and within the coma developed a bright feature, which was described as a fan-like structure, a blob, etc., which, incidentally, gave the appearance in low power that the pseudo-nucleus, which had been mostly stellar, as being off-center. But the pseudo-nucleus at that time remained centered; it was this bright feature that had been consistently southwest of the central condensation in the anti-solar direction. I, and most other observers, believed that this bright feature was the dust tail. Because of the geometrical perspective, it appeared severely foreshortened. Later, a faint tail, with “streamers,” also had been imaged in the same direction (anti-solar). It was a little uncertain whether this was a gas or dust feature. As November progressed, the size of the coma continued to increase, as it became larger and larger. At the same time the surface brightness decreased while the overall integrated magnitude remained between 3^rd and 3.5 magnitude. This continued throughout December and January.

More amazing has been the continued increase in the size of the coma and its shape changing from near circular to parabolic, so that it became longer than its width. For a while the coma resembled an umbrella or jellyfish, but later had become even more elongated, and even from a moderately dark sky had lost its sharp edge, visually. From a very dark sky, it remains naked eye, although it has lost naked eye visibility from any area with even the slightest light pollution. I had observed it from our familiar “Area 51” site on New Year’s night, estimated its magnitude at 3.1, the size of the coma at about a full degree by about 35 arc minutes. It was very obvious naked eye. There has been much discussion as to how this comet could continue to shine at close to 3^rd magnitude, while it continues to recede from the Earth. In fact, there is still no definitive resolution as to the cause of the original outburst and its subsequent behavior.

For myself, I am leaning towards the idea that at some time in the past, 17/P had a slightly closer perihelion distance enabling it develop a thick crust, then was shifted into an orbit further out where the ices went into “deep freeze”. It has, since 1968, been migrating again towards a closer perihelion distance. At some point after its last perihelion passage (May 4, 2007), the internal pressure built up (due to the thick crust), and with its unique combination of ices (gases) and particulates, exploded into what we have witnessed and measured.

What future behavior will Holmes exhibit? It now appears that it will not follow its 1892 second outburst pattern. On the night of Jan. 29^th I, along with my wife Ann Nette, Kenneth Drake and his wife Joy, observed Holmes naked eye from a very dark location just a few miles south of Ft. Griffin, Texas. I was astounded that it was still visible naked eye, and I estimated its magnitude at 3.9 with a total coma size of 1.5 degrees (in binoculars). That translates into a real size of almost 6 million miles, which makes the coma of Holmes the largest body in the solar system. In binoculars the coma was very bright and discernible, and while there was a gradual brightening toward the center, there wasn’t any visual hint of a pseudo-nucleus. Holmes continues, in April, to be the brightest comet, although as it continues receding from both the Earth and Sun it is slowly fading. Current magnitude estimates are about 5.2 magnitude, but naked eye sightings are now almost nonexistent and only from very dark locations. Its elongated coma size is still about one degree in diameter. Holmes starts out April still in Perseus, the same constellation it has been in since its outburst in late October of last year, but finally exits that constellation and enters Auriga, where it will spend the remainder of the month.

8P/Tuttle is a returning periodic comet with about a 13-year orbital period. This return was particularly favorable, as it passed within about .025 AU (about 23 million miles) of the Earth on New Year’s Day, 2008. In addition, it reached perihelion on Jan. 27^th at slightly over 1 AU. 8P also passed about 1.5 arc minutes from Polaris on Oct. 5^th, indicating that it had passed almost directly above the Earth at that point. Because of its proximity in early January, 8P covered a lot of territory the last few months. It starts April in Dorado and ends the month in Carina, thus it is strictly a southern hemispheric object. In addition, this comet is receding from both the Earth and the Sun. 8P is, currently, about 8.5 magnitude, and slowly fading.

C/2007 W1 (Boattini) reaches both closest approach to the Earth (.21 AU) in mid-June and perihelion (.85 AU) later that month, and due to the coming proximity with the Earth, could be reasonably bright, even to naked eye status, if it weren’t for the fact that it will be lost in the solar glare. Currently W1 is in Corvus, moving westward, and is best observed around midnight in northern latitudes. It will move into Crater and end April in Hydra while it continues to brighten. It is, currently, about 9.7 magnitude.

Comet C/2008 C1 (Chen-Gao) is a fairly recently discovered comet (February) moving towards the southeast in Auriga. C/2008 C1 is, currently about 9.8 magnitude and remains steady as it is passed the Earth (closest approach on March 12^th at 1.3 AU) and will reach perihelion, April 16^th at 1.26 AU. C1 moves through Auriga, then into Taurus, and ends April in Orion.

Current comet magnitudes (April 3) and observable region (April 1)

· Comet Magnitude Trend Observable When visible
· 17P/Holmes 5 fade 70 N to 35 S best evening
· 8P/Tuttle 8.5 fade 5 N to 80 S best evening
· Chen-Gao (2008 C1) 10 steady 70 N to 30 S evening
· Boattini (2007 W1) 10 bright 50 N to 80 S best midnight
· 46P/Wirtanen 11 fade 70 N to 30 S evening
· McNaught (2006 Q1) 11 steady 30 N to 80 S best evening
· McNaught (2007 T1) 13 fade 45 N to 65 S evening
· 29P/Schwassmann-Wachmann 13 ? varies 70 N to 25 S evening
· Broughton (2006 OF2) 13 ? bright Poor elongation
· McNaught (2008 A1) 13 ? bright 10 N to 80 S evening
· McNaught (2005 L3)  14 ? fade 70 N to 45 S morning
· LINEAR (2007 G1) 14 ? bright 50 N to 65 S early morning
· Skiff (2007 B2) 14 ? bright 65 N to 55 S best evening
· 26P/Grigg-Skjellerup [14  bright 40 N to 70 S morning
· Christensen (2006 W3) [14 bright 70 N to 25 N evening