I planned to write about something entirely different this week, but interstellar comets have a way of shifting priorities. Comet Borisov (C/2019 Q4), the second interstellar minor body to enter the solar system, is now feeling the pull of the Sun and speeding toward a December 7th perihelion. It arrived from points unknown traveling at 33 km/s and will reach a peak velocity around 44 km/s at perihelion.
Comet
Borisov (C/2019 Q4) — at center — slowly slides across the sky in this
time-lapse made with a 17-inch telescope on September 12, 2019. José Chambó |
Astronomers obtained spectra of the object using the 4.2-meter William Herschel Telescope on La Palma in the Canary Islands and the 8.2-meter Gemini North telescope on Maunakea in Hawai‘i. The comet's slightly reddish surface color resembles that of our own solar system's long-period comets, two recent examples of which are comets Hale-Bopp (1997) and Hyakutake (1996).
Almost immediately after the announcement of Comet Borisov's discovery, amateurs got to work imaging the faint object. Although it isn't any more than a blip at present, some of us are hopeful we'll spot the comet visually closer to the time of perihelion.
The
spectrum of Comet Borisov (jagged curve) tracks well with D-class minor
bodies, like many outer main belt asteroids and long-period comets. D-type asteroids are reddish and may be enriched in organics and possibly water ice. Julia de León et. all / Roque de los Muchachos Observatory, La Palma |
How to View Comet Borisov
Diagram showing degrees of condensation of a comet's coma from 0 (none) to 9 (nearly stellar).
Bob King
Moving to a 10-inch instrument under similar viewing conditions, the limiting magnitude drops to 15.1, allowing for the tantalizing possibility of actually seeing the object. As comet observers know well, visibility depends on the coma's degree of condensation, or "DC." DC ranges from 0 for a completely diffuse object with no condensation at its center to 9, a comet so compact it appears stellar.
Size matters, too. Large, extended objects are generally more difficult to discern than small, compact ones. An 8th-magnitude comet can appear extremely faint if it's large and has a low DC because its light is spread across a large area. Likewise, a 9th-magnitude comet with a DC of 1 is trickier to see compared to another 9th-magnitude comet with a DC of 8.
Comet
Borisov, photographed on September 12, 2019, by amateur astronomer
Patrick Wiggins. He writes: "Not much to look at but considering the
wild fire smoke in the sky, the sky glow from a nearly full Moon, and
the time I took it being so close to sunrise I'm frankly surprised I got
anything at all." We're happy you did! Patrick Wiggins |
You can keep track of what others are seeing and judge whether the comet is in your viewing range by subscribing to the Comets Mailing List, a clearinghouse for amateur comet observations. I've been tipped off to many discoveries, outbursts, and late-breaking developments thanks to the zeal of its members.
My own personal limit with a 15-inch (38-cm) scope is about magnitude 14.5 for small, compact comets, placing Borisov's comet at the edge of visibility. If faintness isn't enough, weather and altitude also come into play. When brightest in December, clouds are often a problem (at least across the U.S.) and cold temperatures can give some observers second thoughts about heading outside at 5 in the morning. Yes, that's the time the comet stands highest in the southern sky before dawn, so stock up on hand warmers now.
Comet
Borisov hails from a distant point in Cassiopeia, not far from the
direction of the Double Cluster in neighboring Perseus. Stellarium |
Plan for December
From early December through early January 2020, when Comet Borisov is expected to be brightest, it will travel about 0.8° per day from central Crater (declination –19° on December 11 UT) south to Centaurus (declination –40° on January 9 UT). Suffice to say that northern visual observers should plan to look for the object in early December before the waxing moon enters the morning sky around December 11. Moon-free skies return again around December 22. Observers in the southern U.S. and farther south will have the best and most extended viewing opportunities.Comet
29P/Schwassmann-Wachmann is normally a 14th magnitude diffuse glow, but
when it undergoes an outburst, shown here on Sept. 23, 2018, it
transforms into a bright, strongly condensed object. Jean Francois Soulier |
Or they can crumble and fade the way Comet Elenin (C/2010 X1) did when it disintegrated shortly before perihelion. This is unlikely to happen with Comet Borisov because it will pass relatively far from the Sun, at a distance of 2 a.u., at perihelion.
This
animation shows the comet's dramatic plunge through the plane of the
solar system starting in March 2019 (top) through February 2020. Instituto de Astrofísica de Canarias – IAC |
Where Are All the Interstellar Comets?
You'll be interested to know that the subject of interstellar comets was investigated by Asoke K. Sen and N. C. Rana in a paper titled On the Missing Interstellar Comets summarized in the January 1994 issue of Sky & Telescope. Sen examined the frequency of interstellar comets passing through our solar system at a time when many astronomers had given up hope of ever finding them. It was assumed that stars in the solar neighborhood possessed their own Oort Clouds. Stellar encounters over time should have stripped away enough comets for astronomers to have detected as many as six interstellar visitors in the past 150 years. Yet not a single one had been found.Sen argued that stars in binary or multiple systems can't have Oort Clouds, leaving only single star systems as comet contributors. Based on this smaller subset of candidates, he revised the frequency to one comet every 200 years — a nice fit with current observations!
Bob King
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