Developments in Brief : Rotation of Halley’s Comet Nucleus Timed

A new analysis of observations of Halley’s comet confirms hints that its nucleus rotates every 2.2 days, a German scientist says.

Previous evidence had suggested that information, but some scientists proposed a rotation of 7.4 days to match observed fluctuations in the object’s brightness, said Klaus Wilhelm of the Max Planck Institute.

The two lines of evidence can be reconciled by proposing a 2.2-day rotation with a slow wobble in the axis, he wrote in Nature magazine.

The comet nucleus, which is the “dirty snowball” at the head of the comet’s tail, is a potato-shaped object measuring nearly 10 miles long and about five miles wide.

Wilhelm’s analysis suggests that the nucleus is perpendicular to its long axis. Thus, if the nucleus corresponds to the head of a hammer, its rotation is like the motion of the hammer head when the handle is spun.

But Wilhelm also says the axis wobbles slowly, as if the bottom of the hammer is being moved around in a circle as the handle spins. The comet’s axis takes 14.8 days to complete a circle, Wilhelm wrote.