Comet Holmes 17P - Brightening By 1 Million Times to Naked Eye Visibility!

The Dramatic appearance of Comet Holmes 17P
Comet 17P (Holmes) has brightened by over 6 orders of magnitude since October 24th, with another significant (10 fold) increase of brightness at 1500 UST on the 29th. Holmes was discovered and catalogued in 1892 by Edwin Holmes, who observed its eruption in that year. It's now naked eye visible in the constellation Perseus, as a brightly visible extra yellow star. At magnitude 2.8, it's brighter than most of the stars in Cassiopeia and the Big Dipper. Because it's shifting by nearly a 7th of a degree per evening, its very clearly a non-stellar object. 17P Holmes has an eccentric orbit that runs from roughly 2.054 AU at aphelion (closest point to the sun) and 5.02 AU at perihelion. One AU is an Astronomical Unit, and is an Earth-orbital radius. It's approximately 500 light seconds to the AU, or 150,000,000 km.
17P Holmes reached its aphelion last May, and was roughly 2.77 AU from Earth at the time, with the Earth trailing in its orbit relative to Holmes. To picture this, picture a board with a circle drawn on it that's 20 millimeters in diameter, that's Earth's orbit. In the center of that circle, put in a yellow push-pin for the sun. Now put an ellipse around that runs from 20.5 millimeters out to 50.2 millimeters. Earth travels counter clockwise around the sun in this view, and if you imagine that the circle has 12 clock facings, when Holmes was at its closest approach to the Sun, Earth was nearly at 6'oclock, while Holmes was at roughly 12 o'clock. Now, Holmes is sweeping out from the sun (and is at 2.77 AU, or 27.7 millimeters in the diagram we described above).
Both Holmes and Earth are at almost exactly 2 o'clock relative to the sun, with Holmes about 17 millimeters away from the Earth. It's this quirk of orbital positioning - the fact that the Earth is nearly on a direct line between the Sun and Holmes, that the tail is almost impossible to see. Comet tails are caused by the sun's light boiling off volatiles (mostly carbon dioxide, methane and other compounds) and taking dust with them that reflects the sunlight.
We are still uncertain what's causing Holmes to erupt the way it is - scientists at the LPL Observatory in Tucson, Arizona, are doing an extensive series of spectrographic tests on the coma of Holmes, which has nearly tripled in diameter with this change in brightness - or taken up nearly 27x the volume. Their aim is to see if they can determine what's caused the eruption from the outgassing that they're seeing. The current candidate is an ethane explosion deep in the comet's nucleus, though their Monday night (29 October) observation data hasn't been released yet.
You may be wondering why NASA hasn't pointed the Hubble Space Telescope at Holmes, or why there aren't more "big telescope" pictures of Holmes. In large part, this is because observation time on major observatory scopes is booked MONTHS in advance, and for the Hubble, it's booked over a year in advance. Trying to bring Holmes in for a beauty shot with one of these gigantic instruments, particularly as it's receding from Earth, would result in disrupting schedules for years (and the Hubble may not have many more years left, making the Goddard Space Flight center very Wary about opening that can of scheduling worms.)
To discover more about the night sky and how you can master it's hidden secrets for yourself, download a f.r.e.e copy of my Audio "Night sky secrets revealed" at http://www.nightskysecrets.com
Ian Maclean - Astronomy Author and PresenterHomepage: http://www.nightskysecrets.comBlogpage: http://www.nightskysecrets.com/blog
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