Published on November 15th, 2013 | by James Ayre0
Comet ISON Now Visible To The Naked Eye
Comet ISON may finally be starting to live up to the hype — the much ballyhooed comet is apparently now visible to the naked eye, according to recent reports. With two weeks still left until ISON reaches perihelion (closest approach to the Sun), this is a good sign.
Perhaps Comet ISON will end up rivaling Comet McNaught — which was only visible from the Southern Hemisphere, but was spectacular — or perhaps Comet Hyakutate? Guess that we will just have to wait and see.
Of course, “visible to the naked-eye” can mean many things — in this case it is no doubt referring to visibility from a very dark rural setting, far from any significant light pollution. According to amateur reports so far, Comet ISON is probably currently somewhere around magnitude 5.7 to 6.1 — notably brighter than before, but still a bit below most predictions.
For more information on Comet ISON, see: Comet ISON Observation Guide, Dates, Times, Path, Updates, and Pictures.
The recent change represents an increase in brightness of around 16-fold — if ISON continues to brighten at such a significant rate it could still end up being quite a bright object in the night’s sky. The next few weeks should be interesting…
With regard to what happens next — as per NASA, the possibilities are:
#1 Spontaneous Disintegration before Thanksgiving
The first scenario, which could happen at any time, is that ISON spontaneously disintegrates. A small fraction (less than 1%) of comets have disintegrated for no apparent reason. Recent examples include Comet LINEAR (C/1999 S4) in 2000 and Comet Elenin (C/2010 X1) in 2011. ISON is now reaching the region of space, within ~0.8 AU of the Sun where comets like these have disintegrated.
Comet ISON is being observed by a tremendous variety of telescopes on Earth and beyond. If ISON does disintegrate, it would be the best-observed case of cometary disruption in history and would likely contribute vast new information about how comets die.
#2 Death by Sunburn around Thanksgiving Day
Assuming ISON survives the next few weeks intact, it faces an even more daunting challenge: making it around the Sun. At closest approach to the sun, the comet’s equilibrium temperature will approach 5000 degrees Fahrenheit, hot enough to cause much of the dust and rock on ISON’s surface to vaporize.
While it may seem incredible that anything can survive this inferno, the rate at which ISON will likely lose mass is relatively small compared to the actual size of the comet’s nucleus. ISON needs to be 200 m wide to survive; current estimates are in the range 500 m to 2 km. It helps that the comet is moving very fast so it will not remain long at such extreme temperatures. Unfortunately for ISON, it faces a double whammy from its proximity to the Sun: even if it survives the rapid vaporization of its exterior, it gets so close to the sun that the suns gravity might actually pull it apart.
Destroyed comets can still be spectacular, though. Sungrazing Comet Lovejoy, for instance, passed within 100,000 miles of the sun’s surface in December 2011. It disintegrated, forming a long tail of dust that wowed observers on Earth.
The final case is the most straightforward: ISON survives its brush with the sun and emerges with enough nuclear material to continue as an active comet. If ISON survives in tact, it would likely lose enough dust near the Sun to produce a nice tail. In a realistic best-case scenario, the tail would stretch for tens of degrees and light up the early morning sky like Comet McNaught (C/2006 P1) did in 2007.
The best of all possible worlds would be if ISON broke up just a bit, say, into a few large pieces. This would throw out enough extra material to make the comet really bright from the ground, while giving astronomers pieces of a comet to study for months to come.
“I’m clearly rooting for #3,” states researcher Matthew Knight. “Regardless of what happens, we’re going to be thrilled. Astronomers are getting the chance to study a unique comet traveling straight from 4.5 billion years of deep freeze into a near miss with the solar furnace using the largest array of telescopes in history.”
“Hang on,” he continues, “because this ride is just getting started.”
Image Credit: NASA/MSFC/Aaron Kingery