Does Hale-Bopp have a moon?

December 07, 1999

COMET Hale-Bopp has a moon, says a NASA physicist in California. If confirmed, "Baby Bopp" would be the first satellite ever discovered for a comet, and would allow astronomers to measure a comet's mass for the first time.

The claims are made by Zdenek Sekanina of NASA's Jet Propulsion Laboratory. Sekanina developed a mathematical model for the brightness of a comet's nucleus and its coma, the cloud of dust and gas that surrounds the nucleus. He then studied six Hubble Space Telescope digital images of Hale-Bopp's core taken between October 1995 and October 1996, and used his model to strip away the light from the nucleus and coma.

In five of the six images, a residual bright area emerged, Sekanina found. He concludes that the bright patch is a satellite about 33 kilometres wide orbiting about 200 kilometres from Hale-Bopp's nucleus. Sekanina thinks observers in Hawaii and Chile, who recorded a speck of light near Hale-Bopp's nucleus on three occasions between September 1996 and January 1998, also glimpsed the moon. "It is perfectly conceivable that this is the same object I was observing," he says.

But Harold Weaver, an astrophysicist at Johns Hopkins University who provided the Hubble images Sekanina analysed, is not convinced. Weaver and his colleague Philippe Lamy at the Space Research Laboratory in Marseilles, France, strongly doubt that "Baby Bopp" is real. "We are sceptical, to say the least," says Lamy.

Lamy and Weaver believe Sekanina is inferring something from the images that isn't there. "He's just trying to squeeze too much out of the data," says Weaver. He points out that Sekanina analysed the images pixel by pixel, and that a bright point of light falling on several pixels rather than just one can skew the results.

Weaver also questions Sekanina's model of the comet's dusty coma. "Hale-Bopp was an extremely dynamic comet, with jets and active regions," he says. "A piece along a jet can be interpreted as another nucleus."

Lamy and Weaver also discount the other observations Sekanina cites. Unlike Hubble images, these ground-based observations required mathematical processing to minimise atmospheric distortions. Weaver was using the Hubble telescope to observe Hale-Bopp around the same time. "If that double nucleus was really there, we would have seen it," he says.

Brian Marsden, a planetary astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, is more positive. Many comets have been known to split up, he says, and a comet as large as Hale-Bopp could, in theory, keep a fragment in orbit for some time.

Marsden has calculated that 4200 years ago, Hale-Bopp may have passed close enough to Jupiter to break up, as Comet Shoemaker-Levy 9 did before its spectacular crash into Jupiter in 1994. "I like Sekanina's idea, and I tend to believe it," says Marsden. "But it's not an easy type of observation to make, and I'm afraid Sekanina is in the minority."

In the minority or not, Sekanina is sure of what he saw. "If there were one image, I wouldn't believe it at all," he says. "With two, I'd still be sceptical. But I found it on five." If any new information confirms that the moon is real, observations of its orbit would reveal Hale-Bopp's mass.
Author: Robert Adler

Source: Earth, Moon and Planets (vol 77, p 155)


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