[meteorite-list] Comet Cracks Under Pressure After Rapid Brightening

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:46:22 2004
Message-ID: <200105031723.KAA01164_at_zagami.jpl.nasa.gov>

http://www.space.com/scienceastronomy/solarsystem/comet_breakup_010503.html

Comet Cracks Under Pressure After Rapid Brightening
By Robert Roy Britt
space.com
03 May 2001

What at first was a mundane comet zooming into the inner solar system
suddenly brightened unexpectedly this spring and was on the verge of putting
on a minor show by becoming visible to the naked eye.

Then it cracked under the pressure.

Carl Hergenrother, an astronomer at the University of Arizona's Lunar and
Planetary Institute, was conducting a routine sky survey on Monday with his
colleagues when he spotted a change in the comet, named C/2001 A2.

Like most comets, C/2001 A2 showed up in a telescope as just a bright spot,
which represents the core, or nucleus, of the object. When comets near the
sun, a fuzzy halo grows around them as charged particles streaming out from
the Sun burn off the frozen gases and dust in the core. Nearer the Sun,
tails form.

"Usually in the middle you just see one little fuzzy point source,"
Hergenrother said in a telephone interview on Wednesday. "And that's how it
looked when I shot it last week. But then on April 30th ... it looked a
little funny."

At first, the fuzzy core of the comet looked less like a point and more like
a bar.

"And then upon getting some better images, I could break up that bar into
two separate little bright spots."

Hergenrother and his colleagues, graduate student Matt Chamberlain and his
wife Yen Chamberlain, reported their finding May 1 in a circular of the
International Astronomical Union.

Cracking under pressure

Because they are surrounded by a cloud of their own gas and dust, the nuclei
of comets never reveal their exact size. Nuclei are frequently estimated to
be a few miles (or kilometers) in diameter, but only after lengthy
observations.

So how big is C/2001 A2?

"We have no clue," Hergenrother said. "It might be small. It might be big."

And there's also no way to know how big each of the two new pieces are.
Sometimes the smaller parts produce more light depending on the composition
of each piece, Hergenrother said. The bit that broke off could be a big
boulder, or a small chunk of crust.

And though researchers aren't sure why a comet breaks up, it has to do with
the way they blow off steam. As a comet heats up, its ice "sublimates"
directly into gas, bypassing the liquid stage. This generates outward
pressure.

"Kind of like a geyser like Old Faithful," Hergenrother explains, "the
pressure builds up and finally [a piece] pops off."

Naked-eye visible by June?

It's not unusual for comets to break apart. A spectacular example occurred
last summer when another comet, popularly called LINEAR, broke into several
pieces.

(MIT's Lincoln Near Earth Asteroid Research telescope, or LINEAR, discovers
dozens of comets each year, generating some confusion in press reports over
which comet is which. C/2001 A2, the one that just broke apart, was also
discovered by LINEAR, in January of this year.)

Hergenrother says that while comet C/2001 A2 could also further break apart
any day, he does not expect that to happen, though the two pieces will tend
to drift apart as they continue to orbit the Sun.

The comet is currently visible through binoculars, though it takes a
high-powered telescope to see the double nucleus.

And first you have to find it.

It has been visible low on the horizon near the feet of the constellation
Orion, the Hunter. But the comet is heading into the Southern Hemisphere
skies and may no longer be visible from the United States, Hergenrother
said.

It will come back into our Northern Hemisphere skies at the end of June
after it swings under the Sun. By then, it might be visible to the naked
eye, though that is not certain. And no one expects it to put on any kind of
show like Hale-Bopp or other popular comets past.

Sudden brightening

The brightness of stars and other celestial objects is measured on a scale
of apparent brightness. Smaller numbers are brighter, and negative numbers
are the brightest. Magnitude 6.0 is the faintest object visible to the naked
eye under dark skies. The brightest star is -1.4 on the scale, and Venus is
-4.4 at its brightest.

Hergenrother and his colleagues have been watching this newly found comet
for months using ASU's Catalina Telescope. It was a typical, rather mundane
target that was not expected to brighten much.

"Then one day, all of a sudden whammo the thing got a hundred times
brighter."

The jump in visual brightness occurred March 25-26, when it rose from an
indistinct magnitude 12 to 10.7. A few days later, it was near magnitude 8.
Then it faded slightly, but began brightening again over the past two weeks
and ultimately approached magnitude 6.
Received on Thu 03 May 2001 01:23:43 PM PDT