Astronomers using NASA’s Hubble Space Telescope have uncovered the first-ever evidence of a comet reversing its direction of rotation, offering new insights into the physical evolution of small bodies in the solar system.
The object, Comet 41P/Tuttle-Giacobini-Kresák (41P), likely originated in the Kuiper Belt and now orbits the Sun every 5.4 years. Observations revealed that after its close approach to the Sun in 2017, the comet’s spin slowed dramatically before reversing direction entirely.
Initial data from Neil Gehrels Swift Observatory showed the comet’s rotation slowed significantly between March and May 2017. Follow-up observations from Hubble in December 2017 revealed a surprising acceleration in the opposite direction, with a rotation period of about 14 hours, compared to 46–60 hours earlier.
Scientists attribute this unusual behavior to outgassing jets streams of gas released as solar heat causes frozen materials on the comet’s surface to sublimate. These jets act like natural thrusters, gradually slowing the comet’s spin and eventually reversing it.
“Jets of gas streaming off the surface can act like small thrusters,” said David Jewitt of the University of California, Los Angeles. “If those jets are unevenly distributed, they can dramatically change how a comet rotates.”
Hubble also measured the comet’s nucleus at just 0.6 miles (about 1 km) across, making it particularly susceptible to such changes. The study indicates that 41P’s activity has significantly declined since earlier observations, suggesting rapid surface evolution as volatile materials are depleted or covered by dust.
These findings, published in The Astronomical Journal, provide a rare glimpse into real-time evolutionary processes of comets—phenomena that typically occur over centuries. Scientists warn that continued rotational changes could lead to structural instability, potentially causing the comet to fragment or disintegrate.
The discovery was made possible through archival data from the Mikulski Archive for Space Telescopes, highlighting the value of open-access space data in enabling new breakthroughs years after observations are made.
Operating for over three decades, the Hubble Space Telescope—an international collaboration between NASA and European Space Agency—continues to transform our understanding of the universe.
