See a "Dirty Snowball" in April

As the title of this column may hint, it is not uncommon to get a late snowstorm in early April here in southern New England. I, for one, hope we have seen the last of the frozen precipitation for this season. However, a keen-eyed observer can catch a glimpse of a cosmic dirty snowball of gas, dust and ice 138,570,000 miles away from us and closing during the second week of April.

What I'm referring to is an icy visitor from the outer reaches of our solar system. Back in January I mentioned some of the astronomical highlights for 2004. Included among those highlights were two comets. After spending almost five billion years within a cloud of comets (the Oort Cloud) beyond Pluto, two dirty snowballs got nudged out of their orbits and began a journey sunward. One is called Comet C/2000 Q4 NEAT and the other one is called Comet C/2002 T7 LINEAR. These comets are named for the automated observing programs that discovered them while looking for near-Earth asteroids.

For more information, including sky charts, see Sky & Telescope's "The Double Comet Show of 2004"

Though these comets will be better viewed from the southern hemisphere, we still can get some good looks here as well. Since Comet NEAT won't be visible here until May, I will concentrate on Comet LINEAR in this column.

Orbital calculations indicate this is Comet LINEAR's first visit to the inner solar system and a rendezvous with the sun. Being a "new" comet, it is thought the surface is coated with a thin "frost" that will quickly react to the Sun's heating. This heating, combined with the solar wind "sandblasting" dust particles off the surface will cause the comet to develop a halo called the coma. The solar wind also "blows" a stream of particles away from the comet, forming a dust tail.

If the amount of material on the surface is limited, the comet will quickly brighten, then diminish after the material has been depleted. If there is a more abundant frost layer, the comet will maintain its brightness. Also, if the material below the frost is loose, and not hard-packed, it too will contribute to the brightness of the comet. Gas may erupt and escape from below the surface in the form of geysers (remember the scene in the movie Armageddon?), adding more dust to the coma and the tail. A keen observer with a telescope may be able to detect sudden increases in brightness due to these outgassings.

How bright will Comet LINEAR become? Recent observations indicate the comet's brightening has stalled. Unfortunately this has often been the case with a comet's first trip to the inner solar system. It can even be worse. Another Comet LINEAR back in 2000 disintegrated after its encounter with the Sun. Anything is possible.

During the second week in April Comet LINEAR will be seen rising above the eastern horizon during bright dawn. Sunrise on April 11 is approximately 6:09 am EDT, so plan accordingly. You'll probably need binoculars to view it at this time due to the brightness of the pre-sunrise sky. LINEAR will make its closest approach to the Sun on April 23. It could have a five degree long tail at this time. As the comet rises higher above the horizon and continues to move closer to the Earth, it should appear a little brighter. After the 26th of April Comet Linear will turn back toward the horizon. You'll be able to observe it on consecutive mornings until about the second to fourth day in May. By then it will be too low in a brightening sky to be visible.

Its closest approach to the Earth occurs on May 19 (24,738,000 miles), but Comet LINEAR will not be visible here. Then at its brightest, it will be most prominent from the southern hemisphere. But we northerners do get another chance beginning in late May to observe this comet as it speeds back out into the icy depths of the outer solar system. That is, of course, if it survives its close encounter with the Sun!!

I do want to caution everyone about the challenge that Comet LINEAR will present us during its early morning apparition. The comet will appear no more than five or so degrees above the eastern horizon from our latitude. You'll need an unobstructed view to maximize your chances of observing it. My suggestion: pick a coastal observing site with a good view to the east. Even then we'll need some darn good weather conditions to help us with this event. Thick clouds will obviously ruin the view, and even thin clouds may spoil the show if the comet doesn't live up to expectations.

Depending upon how bright LINEAR becomes, it may only present itself in binoculars or telescopes. It could be as bright as the brightest stars in the Big Dipper, but that's very deceiving in a bright dawn sky. Just be careful when observing so close to sunrise. Once the Sun rises put your instruments away. Don't risk your eyesight by accidentally looking at the Sun. LINEAR will be lost in the bright morning twilight long before the Sun peeks above the horizon anyway.

About three weeks after we lose Comet LINEAR during its early May morning appearance, the comet will reappear in the west-southwest sky during bright dusk. Start looking around May 27 or 28. Each night the comet will climb higher and higher into a darker twilight sky. The increased contrast should allow the comet to be more readily seen with the naked-eye. Use any optical aid you have to enhance the view. Binoculars work great, but if you have a telescope, use it! Observe Comet LINEAR as long as you can keep it in view.

In conclusion, we think the local meteorologists have a difficult task predicting weather for southern New England. Comet performance predictions can be exceedingly tricky. The only thing predictable about comets is that they are unpredictable. Like the weather, you can't change it! So simply enjoy the beauty our universe provides. The evening apparition may prove to be a better observing opportunity. Again, anything is possible.

Don't forget that Seagrave Memorial Observatory on Peeptoad Road in North Scituate is open to the public on any clear Saturday night. Come out and visit us once the sky darkens enough to see the stars. Our members will be happy to share their love of the sky with you. More information, including directions and membership, can be found at our website: (www.theskyscrapers.org). Our website can also link you to an online Sky and Telescope article about Comet LINEAR, including graphics to help you locate the comet.

As always, keep your eyes to the skies.