[meteorite-list] Astronomers Ready for Comet Encke's Return

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:27:48 2004
Message-ID: <200311141656.IAA21055_at_zagami.jpl.nasa.gov>

http://space.com/spacewatch/comet_encke_031114.html

Astronomers Ready for Comet Encke's Return
By Joe Rao
space.com
14 November 2003

One of the most studied comets in history will be favorably passing by the
Earth in the next few days. Aside from Halley's Comet, Comet Encke is the
most famous and richest in history of all of those mysterious icy wanderers
that wend their way among the planets.

While Encke is not expected to be visible to the unaided eye, it will be an
interesting target through binoculars and small telescopes, for those
experienced enough to find it.

Encke is the comet with the shortest orbital period known - taking about 3.3
years to complete one revolution around the Sun. It does not approach giant
Jupiter as closely as do some other periodic comets. So unlike other comets,
whose orbits get gravitationally adjusted by Jupiter, Encke's orbit has
remained more or less stable for hundreds of years.

This year, Encke's Comet will reach perihelion -- its closest point to the
Sun -- on Dec. 29. It will be closer to Earth, however, on Monday Nov. 17,
providing the best viewing opportunity in more than six decades.

Whenever perihelion falls in November, December or January, the comet
becomes very favorably placed for skywatchers in the Northern Hemisphere.
Conversely, when perihelion is in May, June or July, the comet is difficult
or impossible to see north of the equator, but can be well seen from the
Southern Hemisphere.

Lost and found

The history of Encke's comet stretches back more than two centuries. This
will be the 59th observed return of this object since it was first seen as a
fuzzy object on the edge of naked-eye visibility by the Parisian comet
hunter Pierre Méchain on Jan. 17, 1786.

Because three revolutions of this comet so nearly equal ten years, it
retraces almost the same path across the sky at such intervals. True to this
10-year interval, the comet was not seen again until Caroline Herschel
accidentally ran across it on Nov. 7, 1795. Comet Encke was then about 24
million miles from Earth, and her brother William reported that he could
even glimpse it without any optical aid.

Another observer compared the comet in brightness to the Andromeda Galaxy.
The comet was visible for three weeks before it disappeared into the evening
twilight, but unfortunately, astronomers were unable to calculate an
adequate orbit for it.

Yet another 10 years passed. The comet was discovered independently by not
one, but three observers: Pons (Marseilles), Huth (Frankfurt-on-Oder), and
Bouvard (Paris) within several hours of each other on the morning of Oct.
20, 1805.

The comet would pass unseen through the inner solar system three more times
before it was again recovered in 1818, when its unusually short period was
finally recognized.

Gaining a name

Jean Louis Pons at Marseilles discovered a comet on Nov. 26, 1818, but had
no way of knowing it was the same object that he had previously seen in
1805. Only when Johann Franz Encke, then just twenty-seven years old, worked
out the orbit, did it become clear to him that the comets observed in 1786,
1795, 1805, and 1818 were, in fact, one of the same.

Bringing his calculations forward, Encke predicted that the comet would next
come to perihelion on May 24, 1822, which it did.

So accurate was his forecast that astronomers universally attached the name
of Encke to the comet. And yet, to his dying day, Encke always refused to
accept credit for the comet that now bears his name. He always maintained
that he merely calculated its orbit and referred to it as "Pons' comet."

Since then it has been seen on every one of its returns with the sole
exception of August 1944, when its unfavorable position in the sky made
observations difficult at a time when most major observatories were hampered
by wartime conditions.

Encke's Comet is also the first comet that has been observed throughout its
orbit, for it has even been photographed at the far end of its orbit
(aphelion), first in September 1913 and again in August 1972.

Close call

Comet Encke was at aphelion in May 2002, at a distance of 381 million miles
from the Sun, and is now hurtling into our neighborhood where, on Dec. 29,
it will swing within the orbit of Mercury, 31.4 million miles from the Sun.

What makes the upcoming visit of Encke so favorable is the comet's
comparatively close pass by the Earth. According to Brian Marsden at the
Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, Encke
will come to within 24.3 million miles of the Earth on Nov. 17.

For the Northern Hemisphere, this will likely result in the chance to view
the comet since 1937.

In fact, over the past 200 years, Encke has come closer to Earth only twice
before: June 1832 (23.9 million miles) and July 1997 (17.7 million miles).
The 1997 pass was a good one for Southern Hemisphere skywatchers.

Despite its close approach, skywatchers should not expect an impressive
showing from Comet Encke. You will need a telescope or at the very least, a
good pair of binoculars to locate it, as well as a chart depicting its
projected path against background stars.

Rarely does Comet Encke develop much of a noticeable tail. With such a small
orbital period, and countless hundreds, if not thousands of visits to the
Sun's vicinity, this comet is probably worn out. By now, most of its ices
have been vaporized by the Sun, and it probably consists of a fairly compact
silicate residue, perhaps thinly mixed with the remnant of its original
ices.

Where is it?

Currently, the comet is in the constellation of Cygnus, the Swan, located in
the Swan's outspread right wing and shining at around ninth magnitude, too
dim to see with the naked eye.

On Friday evening, Nov. 14, it will be passing close to the second-magnitude
star Gienah. For the next couple of weeks, the comet will move swiftly south
and west against the background stars.

Interestingly, Encke will be passing through two unusual star patterns that
we've highlighted here on Night Sky over the past year. On Nov. 18 it will
passing across the upper part of the "Cowboy Boot" of Vulpecula, while on
Nov. 22 it will be very close to the "Coat Hanger" star cluster.

Although the comet will be moving away from the Earth after Nov. 17, its
continued approach to the Sun should offset its fading. In fact, Encke will
noticeably brighten, probably reaching magnitude 6.5 -- the threshold of
naked-eye visibility -- by Dec. 5.

"It might even be two magnitudes brighter, since it has often in the past
become unusually active two or three weeks before perihelion," notes veteran
comet observer Alan Hale in the Astronomical Calendar 2003. By this time,
however, Encke will have dropped low into the western sky as darkness falls
and will pretty much be gone from our evening sky

If you spot Encke's Comet, you will have seen it more times than Encke
himself.

As Robert S. Richardson (1902-1981), former Associate Director of the Los
Angeles' Griffith Observatory and Planetarium once noted: "Although he
devoted about 40 years of his life to keeping track of this comet, Encke
apparently never took the trouble to look at it through a telescope. A desk
man to the end!"
Received on Fri 14 Nov 2003 11:56:54 AM PST