Astronomers using NASA’s Hubble Space Telescope have found evidence that the spinning of a small comet slowed and then reversed its direction of rotation, offering a dramatic example of how volatile activity can affect the spin and physical evolution of small bodies in the solar system. This is the first time researchers have observed evidence of a comet reversing its spin. The astronomers’ findings were published in The Astronomical Journal.
This artist’s concept depicts comet 41P, a tiny Jupiter-family comet, as it approached the Sun and frozen gases began to sublimate and shoot material off into space.
Illustration: NASA, ESA, CSA, Ralf Crawford (STScI)
The object, comet 41P/Tuttle-Giacobini-Kresák, or 41P for short, likely originated in the Kuiper Belt and was flung into its current trajectory by Jupiter’s gravity, now visiting the inner solar system every 5.4 years.
After its 2017 close passage around the Sun, scientists found that comet 41P experienced a dramatic slowdown in its rotation. Data from NASA’s Neil Gehrels Swift Observatory in May 2017 showed the object was spinning three times more slowly than it had in March 2017, when it was observed by the Discovery Channel Telescope at Lowell Observatory in Arizona.
Hubble images from December 2017 detected the comet spinning much faster again, with a period of approximately 14 hours, compared to the 46 to 60 hours measured by Swift. The simplest explanation, researchers say, is that the comet continued slowing until it almost stopped, and was then forced to spin in the near-opposite direction by outgassing jets on its surface.
Outgassing jets act as variable thrust mechanisms
Hubble also constrained the size of the comet’s nucleus, measuring it at around 0.6 miles across (about 1 kilometer). This is especially small for a comet, making it easy to torque, or twist. As a comet approaches the Sun, heat causes frozen ices to sublimate, venting material into space.
“Jets of gas streaming off the surface can act like small thrusters,” said study author David Jewitt of the University of California at Los Angeles. “If those jets are unevenly distributed, they can dramatically change how a comet, especially a small one, rotates.”
The comet was originally spinning in one direction, but gas jets pushing against that motion gradually slowed it down. Because the jets kept pushing, they ultimately caused the comet to start rotating in the opposite direction.
“It’s like pushing a merry-go-round,” said Jewitt. “If it’s turning in one direction, and then you push against that, you can slow it and reverse it.”
Multi-instrument approach captures rotational dynamics
The Hubble Space Telescope provided the high-resolution data needed to constrain the nucleus size and measure the post-reversal spin. The team used the F350LP ultra-wide filter, which spans from 3500Å to 8000Å. This filter was chosen to maximize the signal-to-noise ratio of the faint nucleus against the background coma.
Because the nucleus is too small to resolve as a disc, Hubble measured the light curve, periodic variations in brightness caused by the rotation of an asymmetrical, elongated body.
The Neil Gehrels Swift Observatory’s ultraviolet/optical telescope (UVOT) was used to monitor the emission of hydroxyl molecules, a byproduct of water ice sublimation. By correlating water production rates with changes in brightness, the team estimated the active fraction of the surface, effectively calculating the “fuel consumption” of the comet’s natural thrusters.
Structural instability a risk as spin rate changes
The study also shows that the comet’s overall activity has declined significantly since earlier returns. During its 2001 perihelion passage, 41P was unusually active for its size. By 2017, its gas production had decreased by roughly an order of magnitude. This change suggests that the comet’s surface may be evolving quickly, possibly as near-surface volatile materials become depleted or covered by insulating dust layers.
The reversal suggests a highly asymmetric distribution of active vents. 41P acts as a variable-thrust system where the ‘fuel’ (sublimating ice) is unevenly depleted, creating a dynamic feedback loop between thermal input and mechanical output.
Most changes in comet structure occur over centuries or longer. The rapid rotational shifts observed in comet 41P provide a rare opportunity to witness evolutionary processes unfolding on a human timescale.
Modeling based on the measured torques and mass loss rates suggests that continued rotational changes could eventually lead to structural instability for comet 41P. If a comet spins too rapidly, centrifugal forces can overcome its weak gravity and strength, potentially causing fragmentation or even disintegration.
