8P/Tuttle Has Peanut-shaped Nucleus

The comet responsible for the Ursid meteor shower taking place every late December, 8P/Tuttle, made its closest approach to Earth on January 2nd this year. However, its swing through the inner regions of the solar system brought it only about 37 million kilometers away from Earth, too great a distance to conduct optical observations on its main body. To fix this little problem, astronomers chose to compensate using the Arecibo Radio Telescope to view the comet's nucleus.

What they found revealed a surprising fact. The comet's main body has a peanut-like shape and is actually composed of two large rocks bound together by each other's weak gravitational pull. This spawned two theories about how the nucleus of the periodic comet might have formed. 8P/Tuttle comes in the inner regions of the solar system every 13.6 years and could have formed as a result of a low speed collision between two separate objects.

The Arecibo Radio Telescope revealed that the nucleus is composed of two sphere-like objects, 3, respectively 4 kilometers across, with a rotation period around its axis of about 7.7 hours. However, the low speed impact theory resulting in a so-called 'contact binary' would require incredibly small velocities for the objects to stick together. An extremely high speed would result in either an impact that would not allow the two to bond together, or a catastrophic collision that would destroy both objects.

A more valid second theory suggests that actually the comet's shape might be a result of the gravitational interactions with the Sun or one of the gas giants, during an orbit through the solar system that tore apart the nucleus. Similar outcomes had been previously observed, such as the famous Shoemaker-Levy 9 comet, which was broken into multiple fragments during its passing by Jupiter. The comet eventually collided with the planet more than fourteen years ago.

It is mostly believed that cometary nuclei have weak internal structure, and might be severely deformed even during low gravitational interactions. Derek Richardson from the University of Maryland argues that the gravitational pull of an object could stretch the main body of the comet so much that it would eventually separate into two distinct objects bonded together by weak forces. A secondary reason for the splitting effect might have been the centrifugal force resulting from the comet's fast rotation around its axis.

Although held together only by weak interactions, such 'contact binaries' can in fact be stable over very long periods of time. A third explanation for the odd shape of the coma could involve an asymmetric melting of ice, during the comet's passing through the inner regions of the solar system.