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When Comet 8P/Tuttle passed close to Earth early this year, astronomers took its portrait with the Arecibo radio telescope in Puerto Rico. To their surprise, the radar images have revealed that the comet consists of two chunks that appear to be held together by a narrow neck of material.

The portrait suggests that the body is the first known example of a comet that is a contact binary. Researchers aren’t sure how the structure formed.

John Harmon of Arecibo Observatory reported the findings on October 11 in Ithaca, N.Y., at the annual meeting of the American Astronomical Society’s Division for Planetary Sciences.

Collisions that might form a binary are much more common among the rocky bodies in the asteroid belt than in the much more remote and sparsely populated regions of the solar system where comets originate.

“To make a [comet] contact binary implies a formation mechanism that we don’t understand, but we’re guessing would be different than that in the asteroid belt,” says study collaborator Mike Nolan of Arecibo.

It’s possible, he says, that 8P/Tuttle broke into pieces sometime in the past when it neared the sun — the comet make its closest approach every 13.5 years — and its surface warmed.

Such fragmenting is well known among comets, but in this case some of the pieces, which travel on similar but not identical orbits, would have had to reassemble. “Our understanding of how you make comets must now include the possibility of making an object” like 8P/Tuttle, says Nolan.

Observations using the Hubble Space Telescope and the Spitzer Space Telescope show that one the comet’s chunks is 5.6 kilometers in diameter and the other is 2.4 kilometers in diameter, Philippe Lamy and Olivier Groussin of the Astrophysics Laboratory in Marseilles, France, reported.