NASA’s Hubble Detects First-Ever Spin Reversal of Tiny Comet - UNIVERSE

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 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.

A new analysis of follow-up Hubble observations has shown the spin of this comet took an even more unusual turn.

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.

The science paper detailing this finding published Thursday 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)

Small, temperamental nucleus

Hubble also constrains the size of the comet’s nucleus, measuring it at around 0.6 miles across (about a kilometer), or about three times the height of the Eiffel Tower. 

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 paper 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.”

Evidence of rapid evolution

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.

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 suggest 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.

“I expect this nucleus will very quickly self-destruct,” said Jewitt.

Yet, comet 41P has likely occupied its present orbit for roughly 1,500 years. 

This artist’s concept depicts comet 41P as it approached the Sun and frozen gases began to sublimate off the comet’s surface. This animation only depicts one jet, but this comet may have multiple streams of material ejecting into space. This jet is pushing against the comet’s spin, then forcing it in the opposite direction. Small fragments of the comet are also shown spewing into space.

Animation: NASA, ESA, CSA, Ralf Crawford (STScI)

Archival find

Hubble has been collecting imaging and spectroscopic data from across the cosmos for over 35 years, and all of those observations are available in the Mikulski Archive for Space Telescopes, a central repository for data from more than a dozen astronomical missions, including Hubble.

Jewitt found these observations while browsing the archive, and realized they were yet-to-be analyzed. 

By making NASA’s science data open to all, observations made years, or even decades ago, can be revisited to answer new scientific questions. In many cases, scientists continue to make discoveries not just with new observations, but by mining the archive built over decades of space exploration.  

Source: NASA's Hubble Detects First-Ever Spin Reversal of Tiny Comet - NASA Science