Mesmerizing. The recent video of Comet 67P/Churyumov-Gerasimenko tumbling end over end looks like a boot booted into space. 36 images were used to create the brief time lapse which makes it look like as if the comet’s spinning rapidly. Its actual rotation period is 12.4 hours. Still, the extremely irregular shape of the 67P/C-G poses new challenges for the Rosetta team as they contemplate how to safely set down the Philae lander on such an irregularly shaped body come November 11.
Questions abound on how 67P/C-G got its peculiar shape. Most familiar solar system bodies like the planets and many of their moons are spherical or nearly so. Gravity’s the sculptor here. If an object’s about 240 miles (385 km) or larger in diameter, self-gravity will pull everything to the center and collapse the body into a sphere. Small objects like comets and most asteroids just don’t have enough material to ‘go spherical’. Comet 67P/C-G is only a few miles across, so it’s free to assume a variety of shapes.
This all reminds me of a famous anecdote about Fritz Zwicky, a brilliant but prickly Swiss astronomer who worked most of his life at Caltech. He pioneered the use of supernovae as ‘yardsticks’ to measure distances to faraway galaxies and was the first to propose the existence of dark matter. Zwicky didn’t get along with everyone at Mt. Wilson Observatory, calling the astronomers there “Spherical bastards”. Why? “Because they’re bastards no matter how you look at them.”
ANYWAY … comets, being small icy bodies, come in a great variety of shapes from round to bowling pins to rubber duckies. Many ideas have been tossed around as to how 67P looks the way it does. I haven’t taken a poll but would suspect many astronomers would consider the comet a contact binary, two separate comets on convergent paths moving slowly enough that they melded together into one larger object.
We also see contact binaries among the asteroids, but ice makes comets unique. Heat from the sun vaporizes that ice and carves away at the comet’s surface. Could eons of solar heating have shaped Churyumov-Gerasimenko? Comets are also fragile compared to most asteroids; some even crumble apart as they near the sun. It’s possible that vaporization of subsurface ices left parts of 67P in a weakened state which then crumbled away to sculpt its peculiar outline. Other possibilities include a near-catastrophic impact or gravitational stretching during close encounters with Jupiter or Saturn.
Starting August 6 when Rosetta enters orbit around 67P, scientists will have more than a year to study it up close. Perhaps then we’ll get some more answers on its shapely origins. For instance, if we discover that each lobe of 67P has a different density or composition, the contact binary explanation would make a good fit. For now, let’s just say that comets’ weird shapes make them even more lovable.