Mystery As Comet 41P/Tuttle-Giacobini-Kresak Appears To Have Changed Its Direction Of Rotation

Comet 41P/Tuttle-Giacobini-Kresak, as you might be able to guess from the name, has been discovered no less than three times in the last few centuries. Horace Parnell Tuttle, an American Civil War veteran and astronomer working at the Harvard College Observatory, was the first to spot the comet on May 3, 1858. Next came Michael Giacobini, who discovered the comet in 1907. On its third discovery, by Ľubor Kresák in 1951, astronomers finally had enough information about the comet to predict its orbit, and link the several sightings to the same astronomical body.

41P/Tuttle-Giacobini-Kresak is now known to be in the Jupiter family of comets, going from a little inside Jupiter's orbit to almost as far as Earth on its 5.4-year orbit. It is notable for its large outbursts and highly variable brightness. In 1973, the comet was particularly bright following one outburst, reaching magnitude 4.

In May 2017, the comet got a little weirder. Observations by NASA’s Swift spacecraft showed that its rotation had quite abruptly slowed, and was spinning three times slower than it had been when it was observed by the Discovery Channel Telescope at Lowell Observatory in Arizona.

"The previous record for a comet spindown went to 103P/Hartley 2, which slowed its rotation from 17 to 19 hours over 90 days," Dennis Bodewits, at the time an associate research scientist at the University of Maryland, said in a 2018 NASA statement. "By contrast, 41P spun down by more than 10 times as much in just 60 days, so both the extent and the rate of this change is something we’ve never seen before."

Over that time, the comet went from rotating once every 20 hours to once every 53 hours. In the new paper, David Jewitt, an astronomer in the Department of Earth, Planetary and Space Sciences at UCLA, used archived data from the Hubble Space Telescope to study what happened to the comet following these observations.

Hubble observed the comet from December 11-14, as part of the telescope's General Observer program. Finding 24 useful observations of the comet, Jewitt determined that the rotation of its nucleus continued to change after its closest approach to the Sun, likely leading to a reversal of its spin.

"The simplest explanation of the changing period is that the nucleus was torqued by recoil forces from anisotropic outgassing, as has been widely demonstrated in other comets," Jewitt explains in his paper. "Indeed, ground based observations set an upper limit to the nucleus radius rn ≲ 0.7 km, a size which renders the nucleus susceptible to rapid spin evolution through outgassing torques."

While outgassing is to be expected in a comet, the outgassing seen here is particularly jet-like. What is particularly puzzling about the comet is that it has been determined to be in a stable orbit.

"The lifetime of the nucleus to rotational instability (a few decades) is short compared to the dynamical lifetime (∼ 103 years) in its current orbit," Jewitt adds. "The continued existence of 41P therefore suggests that either the current level of outgassing activity is substantially larger than on average, and/or that the nucleus is a remnant of a once much larger body."

Further observations could tell us more about cometary rotation, and perhaps how comets are destroyed by their own spin.

“The evidence is that comets just don’t live that long,” Jewitt told Jonathan O’Callaghan, writing for The New York Times. “There’s some other process that destroys the comets, and I think it’s rotation.”

The comet will dip back into the inner Solar System in 2028, giving us further opportunities to observe this cosmic oddity. Or maybe discover it for a fourth time, for old times' sake.

The study is accepted for publication in The Astronomical Journal and is available as a preprint on arXiv.

[H/T: The New York Times]