Mystery comet explodes into brightness

A once-faint comet has made a sudden leap from obscurity to
center stage.

Comet 17P Holmes, now visible to northern hemisphere
residents, increased its brightness by a factor of one million this week,
going from magnitude 17 to 2. This makes it visible to the unaided eye as
well as binoculars and telescopes, offering a unique viewing opportunity for
sky watchers.

“This is a terrific outburst,” said Brian Marsden, director emeritus of the
Minor Planet Center of the Harvard-Smithsonian Center for Astrophysics (CfA), which tracks known comets and asteroids. “And since it
doesn’t have a tail right now, some observers have confused it with a nova.
We’ve had at least two reports of a new star.”

Comet Holmes is located in the constellation Perseus and is visible for most
of the night. In fact, for observers at the latitude of Boston, the comet is
circumpolar, never setting below the horizon. In appearance, it resembles a
fuzzy, yellowish star.

The comet could fade in a matter of days or weeks, so astronomers recommend
that viewers take a look now. Sky charts showing where to look for the comet
are online at skyandtelescope.com and at space.com.

Amateur astronomer Edwin Holmes, who was looking at the Andromeda galaxy at
the time, discovered Comet Holmes in 1892. The comet has presented a mystery
to astronomers ever since. It likely was undergoing a similar outburst when
discovered, since it reached 4th magnitude and was faintly visible to the
unaided eye. After fading, it brightened again by a factor of 100 in January
1893 before fading again for good.

The comet orbits the Sun once every 7 years at a distance of about 200
million miles (compared to Earth’s 93-million-mile orbit). As a result, it
was re-observed in 1899 and 1906 before being lost for nearly six decades.
Based on a prediction by Marsden, the comet was recovered in 1964.

“Since then, it’s been behaving well – until now,” says Marsden.

On October 23rd, the comet was a dim 17th magnitude, 25,000 times too faint
to be seen with the unaided eye. One day later, it had brightened to 7th
magnitude, and the most recent observations peg it at magnitude 2 to 3: an
increase of a factor of one million. (The magnitude scale used by
astronomers is logarithmic.)

“When the Deep Impact probe hit Comet 9P Tempel, there was almost no change
in brightness,” says Marsden. “This outburst by Comet Holmes is extreme!”

Indeed, the outburst has left experts scratching their heads. How could a
tiny comet, whose nucleus is no more than two miles across, grow so bright
so fast? Perhaps a crack opened in the comet’s surface, exposing fresh ice
to the sun and causing an explosive eruption of dust and gas. No one knows
for sure. Undoubtedly, professional astronomers will be studying it closely
in the weeks to come.

Sky coordinates for Comet 17P Holmes can be found on the Minor Planet Center
website.