[meteorite-list] How to Spot a Crumbling Comet (Schwassmann-Wachmann-3)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri Apr 14 14:33:19 2006
Message-ID: <200604141756.KAA08945_at_zagami.jpl.nasa.gov>

http://space.com/spacewatch/060414_night_sky.html

How to Spot a Crumbling Comet
By Joe Rao
space.com
14 April 2006

Coming close on the heels of the recent passage of Comet Pojmanski in
March, sky watchers will have an unusually favorable opportunity during
the next several weeks to view the passage of another comet in small
telescopes, binoculars and even the unaided eye.

The comet bears the names of two German astronomers - Arnold Schwassmann
and Arno Arthur Wachmann - who together discovered three periodic comets
during the early 20th century. This was their third and final discovery
hence it was christened comet Schwassmann-Wachmann-3.

Discovered 76-years ago, in May 1930, orbital calculations quickly
revealed that this comet was moving in a small elliptical orbit around
the Sun. In fact, it's actually a member of Jupiter's family of comets,
since at aphelion (its farthest point from the Sun) the comet lies near
Jupiter's orbit and sometimes its orbit can be perturbed by that giant
planet's gravitational pull. Shortly after its 1930 discovery, Comet
SW-3 passed a mere 5.7 million miles from the Earth at the beginning of
June, one of the closest comet approaches ever recorded.

If SW-3 were a reasonably sized comet, it would have created quite an
amazing sight in the sky back then. But careful observations made at
the Meudon Observatory in France suggested that its nucleus was probably
no more than about two-tenths of a mile in diameter. By cometary
standards, tiny to say the least! It is for this reason that the comet
never got very bright in 1930, and was best seen in telescopes or good
binoculars.

Even though SW-3 orbits the Sun in only 5.3 years, 1930 was the last
time anyone saw it for quite a long time. In some ways, it's like a
near-Earth asteroid in that very occasionally it can come tantalizingly
close to us, but when it doesn't (more often than not) it's usually much
too faint to be seen. In fact, between 1935 and 1974, SW-3 came and
went by Earth eight times without being recovered. It finally was caught
on photographs taken in Australia in 1979, was missed in 1985 and
recovered again in 1989-90.

Going to pieces

The comet was supposed to make another uneventful return in the fall of
1995. During October of that year, word spread rapidly of a new comet
that numerous observers had sighted, low in the twilight evening sky.
Some reported the object as bright as magnitude 5?, and plainly visible
in binoculars as a slightly diffused star.

But this wasn't a "new" comet at all. It was comet
Schwassmann-Wachmann-3!

This was a huge surprise, because the comet didn't get any closer to
Earth than 122 million miles and was not expected to get much brighter
than twelfth magnitude. Yet, here it was, somehow having increased its
brightness by many hundreds of times!

As for what caused this tremendous brightness outburst, the answer came
a couple of months later, when astronomers at the European Southern
Observatory in Chile reported that SW-3's tiny nucleus had fractured
into several parts. No one can say for sure what caused the comet to
break apart, though the most likely reason is thermal stress, similar
to what would happen if you poured hot tea into a cold glass: the
comet cracked apart after as it approached the Sun after spending a
long time in the frigid depths of space some half a billion miles
from the Sun.

During the fall of 2000, although poorly placed for observation, the
comet was widely seen because it was again brighter than expected. And
while two of the nuclei seen in 1995 (known as B and C) were back, a new
fragment (E) was also discovered. Astronomers now count at least eight
and according to Donald Yeomans at NASA's Jet Propulsion Laboratory in
Pasadena, California, "Some of the fragments are themselves forming
their own sub-fragments, which means the number could multiply further
as the comet approaches."

And indeed, the well-known comet observer, John Bortle, recently noted
that on the night of April 2-3, Fragment B looked " . . . like a bright
little star instead of the faint diffuse patch it had been just a few
days before and implying that it has fragmented further (the much
fainter fragment G has also reportedly split recently)."

What to expect

Comet Schwassmann-Wachmann-3 is in our sky once again. And the
circumstances for viewing it will be the most favorable since the year
of its discovery back in 1930.

How bright will the comet get?

Nobody can quite say for sure. While it was much brighter than normal
at its return in 2000, it was only about 1/6 as bright as it appeared
after its 1995 breakup. This fading trend is likely to continue for
2006, but counteracting this is the fact that the comet will approach
Earth very closely during mid May. So it still should get at least as
bright as fifth magnitude, making it an easy binocular object, as well
as making it dimly visible to the unaided eye.

And it could possibly even become as bright as fourth magnitude around
the time when its largest fragment (C) passes closest to Earth: 7.3
million miles on the morning of May 12. Not since Comet
IRAS-Araki-Alcock passed just 2.9 million miles from Earth in May 1983
has a comet passed so close to our planet. In fact, this will be the
twelfth closest approach of any comet in known history.

As you can see on our map
<http://www.space.com/php/multimedia/imagedisplay/img_display.php?pic=060414_comet_map_02.jpg&cap=Sky+Map%3A+The+positions+of+comet+SW-3+at+one-week+intervals+as+of+1+a.m.+local+time+from+mid-northern+latitudes.>,
SW-3 will spend much of the rest of April slowly cruising through the
constellation of Corona Borealis, the Northern Crown and visible
throughout the night, passing almost directly overhead between 1 and 3
a.m. local daylight time. On the nights of April 20 and 21, it will be
passing just to the north of the second-magnitude star Gemma (the "gem"
of the Crown). The comet will be about 19 million miles from Earth, yet
perhaps no brighter than seventh magnitude.

At the start of May, SW-3 will be picking up speed as it continues to
approach the Earth. From May 1 through 3 it will be passing across the
lower half of the famous "Keystone" of Hercules.

Hurrying eastward, it will pass south of the brilliant blue-white star,
Vega in the constellation of Lyra on the morning of May 8 and probably
shining as bright as magnitude 4?. In the nights that follow, the comet
will turn southeast and will race across the Milky Way in Cygnus (May
10-12), then gradually slow its pace as it begins to move into western
Pegasus (May 15), and graze the "Circlet" of Pisces (May 21-22).

If you plan to look for the comet, you should also be aware during early
May of the presence of the waxing Moon, whose increasingly bright light
will make dim, fuzzy objects like the comet harder to find. Certainly,
the best time to look for SW-3 will be after the Moon has set and before
the onset of morning twilight.

Generally speaking, from May 1 through 9, the comet will appear highest
in the sky (almost overhead) between 3 and 4 a.m. local daylight time.
The Moon will have set earlier in the night, leaving the sky nice and
dark. However, the interval between moonset and the first light of dawn
will be shrinking each day. On the morning of May 1, for instance, the
Moon will set around midnight and twilight won't begin until after 4
a.m., so you'll have at least four hours of a dark sky.

The very best view of SW-3 might come for just a scant several minutes
around 4 a.m. (local daylight time) on May 9, with the comet standing
nearly overhead in a dark sky. The bright Moon will have just set, but
morning twilight will just about be ready to begin.

Thereafter, unfortunately, the Moon will always be above the horizon,
right on through the growing light of dawn, becoming full on May 13,
less than a day after SW-3's closest approach. (Talk about bad timing!).

The comet will arrive at perihelion (its closest approach to the Sun) on
June 7 at a distance of 87.3 million miles. By then, low altitude and
closeness to the bright dawn twilight will make it difficult to see in
the Northern Hemisphere, but those in the Southern Hemisphere will have
it well placed in the east-southeastern predawn sky as it passes through
the constellation Cetus, the Whale. It will still be only about 21
million miles from Earth and might still be glowing at around magnitude 6.

Comet crumbs

Comet Schwassmann-Wachmann-3 is crumbling before our very eyes. Bortle
comments that if big fragment C were to undergo another big disruption
around the time of its closest approach to Earth (only a very small
possibility), it could suddenly appear very bright in our sky and really
put on a show for us!

"Enough new fragments of 73P have been discovered inbound during this
return of the comet to use up the entire alphabet (!). There are now up
to 40 reported fragments and conjecture is that as the procession passes
by us perhaps 100 or more will be briefly in sight of very large
telescopes! Right now it looks like half a dozen will come within visual
range of amateur telescopes."

As for those other tiny comet pieces, they're all lagging well behind
fragment C and most will come even closer to Earth than C will. If, for
example, the tiny Fragment E hasn't already completely disintegrated, it
is (or was) predicted to pass a mere 4.8 million miles from Earth on May
17.

No need to be alarmed, however. That's still twenty times farther than
the Moon!
Received on Fri 14 Apr 2006 01:56:35 PM PDT