Stargazing

Comet Lines Up an Evening Show

July 24, 2000, Update This should be the best week for watching Comet LINEAR. It starts the week below the bowl of the Big Dipper, which is in the northwest as darkness falls. During the week, the comet moves slowly past the dipper and into the constellation Leo, the lion. On Sunday evening, it will appear almost directly above Denebola, the bright star that marks the lion's tail, which is very low in the west in early evening. The combination of the comet's brightness, its location near a prominent star pattern, and the lack of an interfering Moon in the evening sky should make it fairly easy to find LINEAR this week. It may be bright enough to see with the unaided eye. If not, look for it with binoculars. July 25 Finder chart

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Although it may not brighten enough to see with your eyes alone, 1999/S4 (LINEAR) should put on the best showing of any comet since Hale-Bopp in 1997. LINEAR will be visible with binoculars throughout July and into early August 2000.

Discovery
LINEAR was discovered on September 27, 1999, by the Lincoln Near Earth Asteroid Research (LINEAR) project, an automated telescope system at White Sands Missile Range, New Mexico, operated by MIT's Lincoln Laboratory.

Early calculations of the comet's orbit suggested it could grow as bright as magnitude 3.7 in late July -- slightly fainter than the star that connects the bowl and handle of the Big Dipper.

Predicting the behavior of a comet is almost as risky as predicting the behavior of the stock market, though: bullish predictions often turn into bearish results.

As LINEAR headed toward the Sun early this year, it failed to brighten as forecast. By the end of June, it remained one to two magnitudes fainter than predicted. (In the astronomical magnitude scale, a one-magnitude jump represents a 2.5-times increase in brightness.)

LINEAR's Heritage
The fainter-than-expected appearance suggests that LINEAR may be making its first pass through the inner solar system.

LINEAR probably came from the Oort cloud, a vast "shell" of a trillion or more icy cometary bodies up to one light-year or farther from the Sun. These objects are "dirty snowballs" of frozen water and gases mixed with rock, and most are no more than a few miles in diameter. In the perpetual deep-freeze of interstellar space, their surfaces freeze harder than granite.

As a comet approaches the Sun, its surface warms and some of its gas evaporates, freeing particles of rock called dust grains. The gas and dust grains form a cloud, called the coma, around the comet's nucleus. The coma can span millions of miles, and it makes the comet appear much brighter. Radiation pressure and charged particles from the Sun push some of the coma material into twin tails (one containing gas, the other particles of dust) that can stretch across tens of millions of miles or more.

But on its initial pass through the inner solar system, a comet's surface is frozen so hard that it doesn't lose much of its mass. It takes repeated trips to strip away the hard surface layers and expose fluffier material below.

Prime Time
Prime time for LINEAR should begin around July 15, when it might brighten to naked-eye visibility. It will grow brighter over the next 10 days or so, passing below the bowl of the Big Dipper, then above the tail of Leo, the lion (see accompanying charts).

The comet will fade rapidly in August as it enters the constellation Virgo. By the end of the month it probably will be visible only through a telescope.