Comet makes its celestial entrance
Will Comet Ikeya-Zhang brighten or fade away? Stay tuned
By Joe Rao
Feb. 19 — A newly discovered comet, now approaching the sun and Earth, could develop into a relatively bright naked-eye object in coming weeks, researchers say. The best views of the comet may be reserved for those under dark skies far from bright lights, but even city dwellers should be able to spot it.
KAORU IKEYA of Japan and Daqing Zhang from China first sighted the comet in the constellation Cetus, the Whale, on Feb. 1. Both described it as a weak, condensed glow in their telescopes with no mention of a tail.
The comet is called Ikeya-Zhang. The latest orbit calculation indicates it will pass closest to the sun, a point called perihelion, on March 18 at a distance of 47.1 million miles (75.8 million kilometers). After rounding the sun, the comet will continue moving toward Earth, making its closest approach to our planet, called perigee, on April 28, when it will be 37.6 million miles (60.5 million kilometers) away.
Ikeya-Zhang’s expected path across the sky in the coming weeks will greatly favor Northern Hemisphere observers. During most of March on into early April, the comet will be visible near to the north-northwest horizon about an hour after sundown. Bright moonlight may hinder observations during the last week of March.
After the first week of April, with the moon no longer a factor, the comet will also be visible in the morning sky, rising earlier and getting progressively higher above the northeast horizon each night.
Initially, it appeared that this comet would not get brighter than fourth magnitude, which is similar to the brightness of a relatively dim star. Magnitude is a measure of a celestial object’s apparent brightness.
But John Bortle, a longtime comet consultant for Sky & Telescope magazine, said it could get brighter.
Soon after Ikeya-Zhang’s orbit was calculated, some orbital specialists noticed a similarity to a pair of much earlier comets that appeared in 1532 and 1661, Bortle explained in an e-mail interview last week. The 1532 comet, in particular, was apparently a bright comet, according to Asian records.
Brian Marsden of the Smithsonian Astrophysical Observatory in Cambridge, Mass., an orbital specialist, said last Thursday that “a revolution period of 400-500 years (for Ikeya-Zhang) is likely,” keeping alive speculations that this may be a return of the 1532 comet.
The key to figuring out if the comets are the same may lie in Ikeya-Zhang’s orbital period — how long it takes to go around the sun.
“In recent days, several observers have made their own independent calculations suggesting that Ikeya-Zhang might have an orbital period of roughly 500 years, making for a strong argument that there may indeed be a direct connection with the comet of 1532,” Bortle said.
WHAT TO EXPECT
How Comet Ikeya-Zhang ultimately performs is anyone’s guess. So far it is brightening more rapidly than originally expected. As of late last week, it had nearly doubled in brightness in just one week, and was at magnitude 6.8 as of Feb. 15.
But Terry Lovejoy, an assiduous comet watcher from Australia, says it’s a bit early to get excited.
“We’ve seen this situation before,” Lovejoy cautions. “At first a new comet appears to brighten at a much faster than normal rate, but then as it gets closer to the sun it seems to run out of puff. My best guess is that this comet will be no different and will peak somewhere at around magnitude 3.5.”
Such a brightness is just slightly fainter than Megrez, the star in the Big Dipper that joins the handle with the bowl.
“A peak of magnitude 3.0 would not be at all surprising to me,” says Bortle, adding that he expects Ikeya-Zhang to unfurl an impressive tail perhaps up to 15 degrees in length as it sweeps by Earth. For comparison, your fist held at arm’s length measures roughly 10 degrees.
However, because this tail is likely to be chiefly composed of gas and not dust, it will appear faint and bluish and likely only be visible to those with access to dark skies free of light pollution.
So how might Ikeya-Zhang stack up against other popular comets?
The 1986 appearance of Halley’s Comet, considered disappointing by many, also peaked at around magnitude 3.0. In contrast, Comet Hale-Bopp, which put on a memorable show in April 1997, attained a brightness close to magnitude -1, or about 60 times brighter than Halley.
Binoculars or a small telescope should allow most skywatchers a view of Ikeya-Zhang’s fuzzy head, called a coma, and of the tail.