Comets from the solar system’s deep freezer often don’t survive their first encounter with the sun. Now a scientist thinks he knows why: Solar heat causes some of the cosmic snowballs to spin so fast they fall apart.
This suggestion could help solve a decades-old mystery about which destroys many “long period” comets, reports astronomer David Jewitt in a study posted August 8 to arXiv.org. long period comets originate in the Oort clouda sphere of icy objects on the fringes of the solar system (Serial number: 08/18/08). Those that survive their first trip around the sun tend to pass close to our star only once every 200 years.
“These things are stable in the Oort cloud, where nothing ever happens. As they get closer to the sun, they heat up, all hell breaks loose, and they fall apart,” says Jewitt.
Dutch astronomer Jan Oort first proposed the Oort cloud as a reservoir for comets in 1950. He realized that many of his comets approaching Earth were first-time visitors, not return travelers. Something was bringing out the comets, but no one knew what.
One possibility was that comets die by sublimating all of their water as they approach the sun’s heat until there is none left. But that didn’t square with observations of comets appearing to physically break apart into smaller pieces. The problem was that those breakouts are hard to see in real time.
“Breakups are really hard to watch because they’re unpredictable and happen quickly,” says Jewitt.
He ran into that difficulty when he tried to observe Comet Leonard, a bright comet that put on a spectacular show in the winter of 2021-2022. Jewitt had requested time to observe the comet with the Hubble Space Telescope in April and June 2022. But by February, the comet had already disintegrated. “That was a wake-up call,” says Jewitt.
So Jewitt turned to historical observations of long-period comets that had approached the sun since the year 2000. He selected those whose water vapor output had been measured indirectly through an instrument called SWAN on NASA’s SOHO spacecraft, to see how fast comets were losing mass. He also chose comets whose deflection motions from their orbits around the sun had been measured. Those motions are the result of jets of water vapor pushing the comet around, like a spray hose waving in a garden.
That left him with 27 comets, seven of which did not survive their closest approach to the sun.
Jewitt expected that the most active comets would disintegrate faster, expelling all of their water. But he found the opposite: It turns out that the least active comets with the smallest dirty snowball nuclei were the ones most at risk of falling apart.
“Basically, being a small nucleus near the sun makes you die,” says Jewitt. “The question is why?”
It’s not that comets were ripped apart by the sun’s gravity, they didn’t get close enough for that. And simply sublimating them to poof would have been too slow a death to match the observations. It’s also unlikely that comets would collide with anything else in the vastness of space and fly apart that way. And an earlier suggestion that pressure builds inside comets until they explode like a hand grenade doesn’t make sense to Jewitt. The top few centimeters of cometary material would absorb most of the sun’s heat, he says, so it would be difficult to heat the comet’s center enough for it to work.
The best remaining explanation, Jewitt says, is rotational breakdown. As the comet approaches the sun and its water heats up enough to sublimate, jets of water vapor form and cause the nucleus to start spinning like a pinwheel in fireworks. Smaller nuclei are easier to push than a larger one, so they spin more easily.
“It just spins faster and faster, until it doesn’t have enough pulling force to hold itself together,” says Jewitt. “I’m pretty sure that’s what’s going on.”
That deadly spin speed is actually pretty slow. He calculates that spinning at about half a meter per second could mean curtains for a kilometer-sized comet. “You can walk faster.”
But comets are fragile. If you held a fist-sized comet up to your face, a sneeze would destroy it, says planetary astronomer Nalin Samarasinha of the Planetary Science Institute in Tucson, who was not involved in the study.
Samarasinha thinks Jewitt’s proposal is compelling. “Even though the sample size is small, I think it is something that is really happening.” But other things could also be destroying these comets, she says, and Jewitt agrees.
Samarasinha awaits more observations of comets, which could come when the Vera Rubin Observatory begins surveying the sky in 2023. Jewitt’s idea “is something that can be tested observationally in a decade or two.”