Comet ISON (Hubble Space Telescope's View)

Advanced amateur astronomers are already getting good images of Comet ISON, which currently shines at around +12th magnitude in the constellation Cancer. And although NASA’s Deep Impact/EPOXI mission is down for the count, plans are afoot for the Curiosity rover and the Mars Reconnaissance Orbiter to attempt imaging the comet when it makes its closest approach to the Red Planet on October 1st at 0.0724 Astronomical Units (A.U.) or 10,830,000 kilometres distant. If MSL is successful, it would be the first time that a comet has been observed from the surface of another world.

Currently, ISON sits about a magnitude below the projected light curve, (see below) but that isn’t all that unusual for a comet. Already, there’s been increasing talk of “ISON being a dud,� but as Universe Today’s Nancy Atkinson pointed out in a recent post, these assertions are still premature. The big question is what ISON will do leading up to perihelion, and if it will survive its passage 1.1 million kilometres above the surface of the Sun on November 28th to become a fine comet in the dawn skies in the weeks leading up to Christmas.

ISON is already starting to show a short, spikey tail in amateur images. Tsutomu Seki estimated it to be shining at about magnitude +11.1 on September 16th. Keep in mind, a caveat is in order when talking about the magnitudes of comets. Unlike stars, which are essentially a point source, the brightness of a comet is spread out over a large surface area. Thus, a comet may appear visually fainter than the quoted magnitude, much like a diffuse nebula. Although +6th magnitude is usually the limit for naked eye visibility, I’ll bet that most folks won’t pick up ISON with the unaided eye from typical suburban sites until it breaks +4th magnitude or so.

The forward scattering of light also plays a key role in the predicted brightness of a comet. The November issue of Astronomy Magazine has a great article on this phenomenon. It’s interesting to note that ISON stacks up as a “9� on their accumulated point scale, right at the lower threshold of comet “greatness,� versus a 15 for sungrazing Comet C/1965 S1 Ikeya-Seki. Another famous “9� was Comet C/1996 B2 Hyakutake, which passed 0.1018 A.U. or 15.8 million kilometres from Earth on March 25, 1996.

ISON will pass 0.429 A.U. or 64.2 million kilometres from Earth the day after Christmas. Bruce Willis can stay home for this one.

Here is a blow-by-blow breakdown of some key dates to watch for as ISON makes its plunge into the inner solar system:

-September 25th: ISON crosses the border from the astronomical constellation of Cancer into Leo.

-September 27th: ISON passes 2 degrees north of the planet Mars.

-October 1st: The 12% illuminated waning crescent Moon passes 10 degrees south of Mars & ISON.

-Early October: ISON may break +10th magnitude and become visible with binoculars or a small telescope.

-October 4th: New Moon occurs. The Moon then exits the dawn sky, making for two weeks of prime viewing.

-October 10th: ISON enters view of NASA’s STEREO/SECCHI HI-2A CAMERA:

-October 16th: ISON passes just 2 degrees NNE of the bright star Regulus, making a great “guidepost� to pin it down with binoculars.

-October 18th: The Full Moon occurs, after which the Moon enters the morning sky.

-October 26th: A great photo-op for astro-imagers occurs, as ISON passes within three degrees the Leo galaxy trio of M95, M96, & M105.

-October 30th: The 17% illuminated Moon passes 6 degrees south of ISON.

-Early November: Comet ISON may make its naked eye debut for observers based at dark sky sites.

-November 3rd: A hybrid (annular-total) solar eclipse occurs, spanning the Atlantic and Central Africa. It may just be possible for well placed observers to catch sight of ISON in the daytime during totality, depending on how quickly it brightens up. The Moon reaching New phase also means that the next two weeks will be prime view time for ISON at dawn.

-November 5th: ISON crosses the border from the astronomical constellation of Leo into Virgo.

-November 7th: ISON passes less than a degree from the +3.6 magnitude star Zavijava (Beta Virginis).

-November 8th: ISON passes through the equinoctial point in Virgo around 16:00 EDT/20:00 UT, passing into the southern celestial hemisphere and south of the ecliptic.

-November 14th: ISON passes less than a degree from the 10th magnitude galaxy NGC 4697.

-November 17th: The Moon reaches Full, passing into the morning sky.

-November 18th: ISON passes just 0.38 degrees north of the bright star Spica.

-November 22nd: ISON crosses into the astronomical constellation of Libra.

-November 23rd: ISON sits 4.7 degrees SSW of the planet Mercury and 4.9 SSW of Saturn, respectively.

-November 25th: ISON pays a visit to another famous comet, passing just 1.2 degrees south of short period comet 2P/Encke which may shine at +8th magnitude.

-November 27th: ISON enters the field of view of SOHO’s LASCO C3 coronagraph.

-November 28th: ISON reaches perihelion at ~18:00 PM EST/ 23:00 UT.

After that, all bets are off. The days leading up to perihelion will be tense ones, as ISON then rounds the Sun on a date with astronomical destiny. Will it join the ranks of the great comets of the past? Will it stay intact, or shatter in a spectacular fashion? Watch this space for ISON updates… we’ll be back in late November with our post-perihelion guide!

Be sure to also enjoy recently discovered Comet C/2013 R1 Lovejoy later the year.

(source: http://www.universetoday.com/104818/comet-ison-a-viewing-guide-from-now-to-perihelion/#ixzz2fpLTKJwD)


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