Twin iceberg. Radar observations of the nucleus of comet Tuttle reveals itto be a binary.

Arecibo Observatory

Comet Tuttle's Split Personality

ITHACA, NEW YORK--Astronomers have discovered their first grab-bag comet. Radar observations of the small, icy nucleus of a comet known as Tuttle suggest that it consists of two clumps that touch each other, like the two halves of the number eight. "It's almost certainly a contact binary," says John Harmon of the Arecibo Observatory in Puerto Rico, who presented his team's findings here this past weekend at the 40th meeting of the Division for Planetary Sciences of the American Astronomical Society. The unexpected find opens up a window on the early history of our solar system.

Comets are the fluffy, frozen remains of the solar system's birth. When their elongated orbits bring them close to the sun, they develop impressive tails of gas and dust. "They're the most spectacular sights in the night sky," says comet expert Michael A'Hearn of the University of Maryland, College Park. They're also astronomers' best primordial samples of the spinning disk of gas and dust that spawned the sun and the planets, 4.6 billion years ago.

Since the 1980s, a handful of cometary nuclei have been observed close-up by visiting spacecraft, most recently in July 2005 (ScienceNOW, 5 July 2005). Although these comets exhibit a wide variety of shapes and surface features, all of them are dark, irregular aggregates of ice, dust, and pebbles, measuring a few kilometers across. So far, every single comet has been observed to be, well, single.

But not Tuttle. This comet, discovered in 1858 by American astronomer Horace Tuttle orbits the sun every 13.6 years. On 2 January 2008, it flew within 40 million kilometers of Earth, enabling astronomers to study its nucleus in detail.

The Arecibo radar observations by Harmon and his team are best explained by a contact binary, with the individual components measuring 5.6 and 4.4 kilometers across. The pair rotates around each other once every 11.4 hours--slowly enough not to fly apart through centrifugal forces. Brightness variations of Tuttle's nucleus, spotted with the Hubble Space Telescope, also indicate a very complex shape, supporting the radar team's findings, says Philippe Lamy of the Astrophysics Laboratory in Marseille, France.

Tuttle's split personality lends credence to the recent idea that the early solar system went through a chaotic period some 3.8 billion years ago, when small, icy bodies were thrown around by the gravity of migrating planets. "In some cases, they may have collided, possibly creating the peanut-shaped cometary nuclei that have also been observed," says Michael Mumma, who heads NASA's Goddard Center for Astrobiology in Greenbelt, Maryland. In other cases, as Tuttle demonstrates, a gentle collision would lead to a contact binary.

Indeed, observations of Tuttle's outgassings, observed in January by Mumma and his colleagues with large ground-based telescopes, showed a surprising mix of volatiles, as though the comet has sampled different regions of the primordial nebula. It all fits together if the two components of the contact binary came from different locales, says Mumma.