Nov. 14, 2013: Comet ISON is now inside the orbit of Earth as it plunges
headlong toward the sun for a fiery close encounter on Nov. 28th.
Although the comet is not yet as bright as many forecasters predicted,
the comet is putting on a good show for observatories around the solar
system. NASA spacecraft and amateur astronomers alike are snapping crisp
pictures of the comet's gossamer green atmosphere and filamentary
double-tail.
Because ISON has never passed through the inner
solar system before (it is a first-time visitor from the distant Oort
cloud), experts aren't sure what will happen next. Can the comet survive
its Thanksgiving Day brush with the sun? Will it emerge as a bright
naked-eye object?
"I've grouped the possible outcomes into three
scenarios, discussed in chronological order," says Knight. "It is
important to note that no matter what happens, now that ISON has made it
inside Earth's orbit, any or all of these scenarios are scientifically
exciting. We're going to learn a lot no matter what."
#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.
#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.
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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," says Knight.
"Regardless
of what happens, we're going to be thrilled," he predicts. "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 says, "because this ride is just getting started."
NASA's Hubble Space Telescope (Planetary Science Institute), and the Hubble Comet ISON Imaging Science Team
These
wings, according to a post written on the International Astronomical
Union's Comets and Asteroids Facebook page, suggest the nucleus of ISON
is breaking apart.
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