Just after the full moon on May 5, astronomers will be checking the early evening sky for a comet that is just coming into view in the Northern hemisphere.
C/2001 Q4 was discovered by the Near Earth Asteroid Tracking (NEAT) program
at Palomar Observatory. Recently, Q4 has been visible in the Southern Hemisphere,
but during the first week in May, it will be coming into view in the United
States. This will also be its time of peak brightness.
Comets originate in the farthest reaches of our solar system, the Oort Cloud.
This layer of dust and ice surrounds our solar system like a snowball, only
several trillions of miles away, about halfway between our sun and its nearest
star neighbors. Occasionally, one of the dirty chunks of ice is knocked loose
of the cloud and begins traveling towards the sun - slowly at first (which
can mean a few thousand years), then picking up speed as it nears the sun.
The ice ball, usually several miles wide, warms up as it approaches the sun.
This causes cracks in its surface, which in turn allow dirt and gases to
escape from its core. The gasses form an atmosphere, or coma, around the
ice ball. As the comet nears the sun, the solar winds blow the coma away
from the sun, creating the comet's distinctive tail. Some comets even have
two tails, one made of gases and one made of dust.
One of the exciting things about comets is that one can't predict what they
will look like. They can be bright enough to see with the naked eye, even
in daylight, or so dim that they can barely be picked out through a telescope.
Sometimes the pressure even causes them to crack up and disappear.
Q4 will be visible in the early evening, so any of the days following the
full moon on May 5 will be fine for observing. Begin looking for it in the
southwest at dusk. On May 5, it will be just above the horizon, to the left
of Sirius, the bright star in Canis Major. By May 9 and 10, it will be to
the left of Procyon in Canis Minor. Castor and Pollux, in Gemini, will point
toward the comet on the May 14 and 15, and it will be tantalizingly close
to M44, the Beehive Cluster in Cancer. (Even if you can't see the comet,
look directly west and admire the nice grouping of Venus, Mars, and Saturn
at the feet of Castor.)
If you have never seen a comet before, you may be initially disappointed
in what you see. First, while you will be able to observe the comet's motion
over a period of nights, you will not see it "streak across the sky" before
your eyes. Also, just as with the deep sky objects, what you see with binoculars
or even a telescope is considerably different than the fabulous comet photographs
you have seen. Don't be dismayed if you can only see a fuzzy spot or a slight
elongation. That is perfectly normal.
The current prediction is that Q4 will reach peak brightness, magnitude 2.5, on May 6, and will lose about one degree of magnitude every 5 to 7 days thereafter.
Magnitude 2.5 should be visible to the naked eye from a darkish urban location. That is approximately the same brightness as the two stars at the bottom of the Big Dipper's bowl.
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