The new comet goes by the mumbo-jumbo name of 1999 S4 LINEAR, an acronym which stands for Lincoln Laboratory Near-Earth Asteroid Research -- an automated-search program in New Mexico that images regions of the night sky looking for asteroids and comets. This one was found in digital images made on September 27 and was reported, at the time, as "an unusual moving object."
Once its cometary nature was confirmed, astronomers wasted no time in plotting its orbit and trying to predict how bright it would become. Determining the orbit was straightforward enough. Observations of the comet's motion and position among the stars were simply plugged into mathematical formulas that precisely established its path through the solar system. But predicting the brightness of a comet is more guesswork than science. Many factors come into play -- two of the biggest being that no one can possibly know where the comet is coming from or how the comet's structure and composition may affect its light output.
The truth behind what's bright and what isn't
The visual brightness of any celestial object, including a comet, is reckoned by its apparent "magnitude." In the magnitude system, the greater the value, the fainter the object (intuitively reversed, yes, but you get used to it). On a dark moonless night away from city lights, the faintest star you can barely see with your naked eye is about magnitude 6. The brightest stars are on the order of magnitude 1. (A handful of still brighter stars are assigned negative values.) Progressively fainter stars, then, range from magnitudes 2, 3, 4, 5, and so on.
Another thing: Even if the comet kindles to naked-eye levels, it still may not be visible to the human eye. Unlike a star's light, which is concentrated into a point, a comet's light is smeared like an ink drop on the sky. This diminishes the comet's overall brightness by lowering the contrast between it and the background. So the magnitude given is really an "integrated magnitude" -- how bright the comet would be if all of its light could be consolidated into a single point.
The future looked misleadingly bright
Astronomer Brian G. Marsden of the Harvard-Smithsonian Central Bureau of Astronomical Telegrams -- a clearinghouse of ongoing astronomical observations -- issued the initial magnitude estimates for Comet 1999 S4 LINEAR in early October. Based on the date when the comet would be closest to the sun (perihelion) and what its distance would be at that time, Marsden proposed that the comet might be bright enough to see with the naked eye in July 2000 -- perhaps as bright as magnitude 3 or 4.
But he also warned that LINEAR could fail to live up to predictions if it was a "new" comet heading in from the Oort Cloud, an immense "reservoir" of comets in the most outer reaches of the solar system.
New or "fresh" comets are first-time visitors to the inner solar system. Their outer mantle is composed of fluffy ices that have probably never felt the warmth of a star. As it departs the profound deep freeze of space and edges toward the sun, the frothy ices on the surface flash vaporize, even while still far from Earth. This outburst makes the comet look abnormally bright from a great distance.
Apparently, Comet LINEAR is one of these new comets that brightened when its fragile ices vaporized in the remote solar system. "The agreement of the early 'total' and 'nuclear' magnitudes suggests that the comet was very well condensed -- something that happens when a distant comet has an outburst," says Marsden.
Anita Cochran, a scientist who specializes in comets at the University of Texas at Austin, agrees. "We think the comet was in outburst when it was discovered," she said. "Currently, [Comet 1999 S4 LINEAR] is about 3 magnitudes fainter than prediction. If the 3-magnitude offset holds, it will be around 7 or 7.5 at brightest."
Astronomer and comet researcher David Schleicher of Lowell Observatory in Flagstaff, Arizona, has a slightly more optimistic prediction.
"Often a comet does not brighten significantly until it reaches 2 to 3 AU [Astronomical Units, each equivalent to the mean distance of Earth from the sun], when the new release of material overwhelms the old dust," says Schleicher. "My guesstimate is that LINEAR will reach 5th or 6th magnitude."
As of mid-June, observers' reports to the Central Bureau of Astronomical Telegrams have ranked the comet around magnitude 9. That's about 16 times fainter than the faintest star visible to the naked eye. Of course, no one can rule out a subsequent outburst that could dramatically increase its brightness, but astronomers don't think it's likely to happen.
So why should I look for this comet?
Although Comet LINEAR has so far not lived up to earlier brightness predictions, it will nonetheless be a fascinating object to observe. This is an object that has remained unchanged since the sun formed over 4.5 billion years ago. Moreover, it should be visible in binoculars and small telescopes as a "wooly" star in late June and early July. It may even sport a bantam tail.
Until the end of June, the comet will be located in the northeastern predawn sky moving from Andromeda into Perseus. On the morning of June 30, it passes within a degree of the rich open star cluster M 34. Its stay in Perseus is brief, and it enters Camelopardalis July 11. At this point, the comet becomes "circumpolar" for observers in the Northern Hemisphere, and is thus visible in the morning and evening sky. Its evening position, however, is just above the northern horizon, which comes into more favorable viewing range after midnight.
If the comet reaches naked-eye visibility it will do so sometime around July 20 or 21. At its brightest, which is expected on or about July 23, the comet skirts 10 degrees west of the cup of the Big Dipper. If you can find this famous star pattern, you should have no trouble locating the comet. In fact, you can use the Dipper's famous "pointer stars" -- Dubhe and Merak -- which point north toward the North Star, to point south toward the comet's position. It should emerge from the twilight sometime between 9:30 p.m. and 10:00 p.m., 25 degrees above the northwest horizon.
Comet LINEAR remains in the evening sky throughout July and into August. How long it remains visible, of course, will depend on how bright it stays. The plotted orbit shows it located in a twilight sky by August 15. Will it still be visible in binoculars then? We'll have to wait and see.
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