Comet Should Have Torn Itself Apart. It Didn't.

Comet 41P/Tuttle-Giacobini-Kresak should have destroyed itself. It didn't. That's the real story.

In March 2017, the small comet was spinning once every 20 hours. By May, the period had stretched to 46 hours — a dramatic spin-down documented in a 2018 Nature paper led by Dennis Bodewits at the University of Maryland. Then Hubble looked again in December, after perihelion, and found the period collapsed to 14.4 hours, spinning in what appears to be the opposite direction. The torques involved were the most extreme example of jet-driven rotation change ever documented in a comet.

The NASA press release called it 'the first-ever spin reversal.' But the peer-reviewed paper in The Astronomical Journal says the direction change is 'likely' — not confirmed. Lightcurve analysis measures period, not direction; inferring the sign of rotation requires assumptions about nucleus shape and viewing geometry. The period shift is solid. The reversal claim is an interpretation, and the paper is careful about it. NASA found a headline. The researchers found a puzzle. The puzzle is more interesting.

Comet 41P/Tuttle-Giacobini-Kresak orbits the Sun every 5.4 years. Jupiter threw it into this orbit roughly 1,500 years ago, according to NASA. At 500 meters in effective radius, it's one of the smallest cometary nuclei ever characterized. More than half its surface was actively jetting during perihelion, producing water at a rate of roughly 90 kilograms per second, according to a preprint posted to arXiv.

When jets fire off-center from the spin axis, they act like a brake — transferring momentum until the nucleus slows. Off-axis jets in the opposite direction act like a reverse thrust. The previous record for comet spin change — 103P/Hartley 2 slowing from 17 to 19 hours over 90 days — doesn't come close. 41P's rotation slowed by more than ten times as much, in roughly a third of the time.

The question is why 41P survived the encounter. The torques should have spun it up to destruction decades ago. 'I expect this nucleus will very quickly self-destruct,' said David Jewitt of the University of California, Los Angeles, who wasn't involved in the recent study, in NASA's press release. 'It's like pushing a merry-go-round. If it's turning in one direction, and then you push against that, you can slow it and reverse it.'

The new paper examined the survival paradox directly. The researchers found a dimensionless moment arm — essentially how far off-center the jet torque acts — of about 0.013, roughly twice the median for short-period comets. More centrally located jets mean more distributed torque, which might delay catastrophic spin-up. They also tracked how the active fraction — the fraction of the surface producing jets — declined from roughly 2.4 in 2001 to 0.14 in 2017, indicating long-term surface modification that reduced jet activity before the 2017 spin event.

The water production rate at perihelion was a modest 3x10^27 molecules per second, about 90 kilograms per second, well within the range where off-axis jets can reshape rotation without tearing the nucleus apart. The jets were strong enough to flip the spin state in a single orbit, but not strong enough — yet — to reach the spin-up limit that would fragment the nucleus.

The paper doesn't pretend to have a full answer. 'We don't really understand that,' Bodewits told The New York Times regarding how jets change spin. The authors note that the physical lifetime of this small nucleus to spin-up is short compared to the roughly 1,500 years it has spent in this orbit. The math says it should have come apart long ago. Something is extending its life — whether a favorable nucleus structure, a particular distribution of jet activity, or a stretch of lucky orbital geometry.

The next perihelion is predicted for February 2028. The viewing geometry will be less favorable than in 2017, but the researchers expect new observations to be valuable in tracking whether 41P's spin is continuing to evolve. After 1,500 years of defying the odds, there's no reason to assume the show is over.