Comet Corner

By Don Pearce

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

August 1, 2008

Comet 17P/ 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 has gradually been fading, and I cannot find any observations later than April 26^th, when Alan Hale reported he could barely see it in 8-inch aperture from a very dark location at high altitude. He estimated its magnitude at 7.5.

As Comet Holmes has not been observed since late April it is has been obscured by poor solar elongation. It may, however, become visible again by late August as a morning object, on the other hand it may not be seen again until it returns towards perihelion in 2014.

Holmes is currently in Gemini and will end August in Cancer.

Comet C/2007 W1 (Boattini) reached both closest approach to the Earth (.21 AU) in mid-June (11^th) and perihelion (.85 AU) later that month, and due to its proximity with the Earth, had reached about 5.2 magnitude as a southern hemispheric object. It has moved north now, but has faded to about 7.5 mag. and will continue to fade as it recedes from the Earth and Sun. Boattini is currently in Aries where it will reside all during the month of August as it slowly moves in a northeasterly direction.

Comet C/2008 A1 (McNaught) is currently a southern hemispheric object residing in Vela, but as it reaches both closest approach to the Earth and perihelion with the Sun it late September, it will begin turning north and be visible for us as perhaps a 7^th-8^th magnitude comet by October.

Comet C/2008 J1 (Boattini) is very close to the celestial pole as it crosses from Cepheus into Ursa Minor during August. On August 18^th it passes less than about 3 degrees of Polaris. The comet is currently about 10^th mag. and fading as it recedes from the Earth and Sun.

Comet 6P/ d’Arrest is a periodic comet, an old favorite, with about a 6.5-year orbital period, that has again been recovered. This comet, to me, somewhat resembles 9P/Temple, leading me to believe that it is very dusty, and is in its last throes, (astronomically speaking) of its life as an active comet. 6P is currently about 12^th magnitude and will only brighten slightly as it reaches closest approach to Earth on August 8^th at about .35 AU and perihelion on Aug. 15^th at about 1.35 AU. This comet is currently in Aquila, trudges through Capricornus as it moves in a southerly direction, and ends August in Microscopium. After that it will begin to fade.

Current comet magnitudes (August 1) and observable region (August 1)

· Comet Magnitude Trend Observable When visible
· 17P/Holmes 6 fade 45 N to 25 N early morning
· Boattini (2007 W1) 7.5 fade 55 N to 60 S best morning
· McNaught (2008 A1) 9.5 bright 25 S to 90 S evening & morning
· Boattini (2008 J1) 10 fade 55 N to 25 N all night
· 15P/Finlay 10.5 fade 20 N to 5 N early morning
· Lulin (2007 N3) 11 bright 50 N to 90 S best evening
· 19P/Borrelly 11 ? steady Poor elongation
· Broughton (2006 OF2) 12 bright 55 N to 15 S best morning
· 6P/d'Arrest 12 bright 55 N to 80 S all night
· Christensen (2006 W3) 12.5 bright 55 N to 10 N best morning
· McNaught (2006 Q1) 12.5 fade 10 N to 60 S early evening
· McNaught (2005 L3) 13 fade 55 N to 35 S evening
· LINEAR (2007 G1)  13 ? steady 30 N to 90 S best evening
· 29P/Schwassmann-Wachmann 13 ? varies Poor elongation
· Skiff (2007 B2) 13.5 fade 15 N to 70 S evening
· 85P/Boethin [14 bright 35 N to 85 S best evening