The comet put on the brakes on its own

The Hubble Space Telescope captured footage of a rotating comet slowing down on its own and eventually starting to rotate in the opposite direction. This is the first time this phenomenon has been observed, showing that comets are much more dynamic celestial bodies than we thought.

The main character of this observation is ‘Comet 41P/Tuttle-Jacobini-Cresac’. This comet is classified as a Jupiter-family comet. After entering the inner solar system from the Kuiper Belt, it is captured by Jupiter's gravity and orbits the sun with a cycle of approximately 5.4 years.

The rotation speed slowed down three times and then sped up again.

Comet 41P's most recent passage of perihelion (the point of closest approach to the Sun) was in September 2022, but what the researchers analyzed was observation data from 2017. At the time, several instruments, including the Hubble Space Telescope, NASA's Neil Gerrells Swift Observatory, and the Lowell Discovery Telescope in Arizona, tracked the comet.

Professor David Jewitt, a planetary scientist at the University of California, Los Angeles (UCLA), discovered and analyzed Hubble data from the Mikulsky Space Telescope archive, revealing surprising facts. At the time of the Lowell telescope observation in March 2017, the comet's rotation period was a certain value, but when Swift observed it in May of the same year, it had slowed down by about three times to 46 to 60 hours. However, in Hubble observations in December, the rotation period was accelerated again to about 14 hours. What caused the comet's rotation to change so dramatically?

The gas eruption served as a ‘miniature propellant’

Professor Jewitt analyzed that the cause was the phenomenon of volatile gases on the surface of the comet expanding and erupting due to solar heat when passing the perihelion. These gas jets acted like tiny propellants, spewing out cometary dust with them.

“The gas jets emitted from the surface act as miniature propellants,” Professor Jewitt explained. “If these jets are unevenly distributed, they can cause dramatic changes in rotation, especially for small comets.”

The nucleus of comet 41P is only about 1 kilometer in diameter, so it is so small that even Hubble cannot directly resolve its shape. However, the rotation speed was able to be measured by analyzing the change in brightness (light curve) shown as the elongated nucleus rotates. Because the core was small, it was more vulnerable to the torque (torsional force) generated by the gas jet.

The direction of rotation is completely reversed

Professor Jewitt synthesized the observation data and concluded that the comet's rotation direction itself had been reversed. The gas jet first acted in the opposite direction to the comet's existing rotation, slowing it down (a deceleration between the Lowell and Swift observations), and then continued to exert force in the same direction, eventually stopping the comet's rotation and then starting to spin in the opposite direction.

“It’s like pushing a spinning amusement ride to make it stop and then turning it in the opposite direction,” Professor Jewitt likened it.

Is the comet’s lifespan coming to an end?

Comets that show such drastic changes are rare. Compared to Hubble's observation data from 2001, comet 41P's perihelion activity has decreased by about one-tenth since then. It is estimated that about 1,500 years have passed since the comet entered its current orbit, and its volatile ice may have begun to deplete due to repeated perihelion passes. Alternatively, the dust emitted by the gas jets may accumulate on the comet's surface, forming an insulating layer, which may be slowing the rate at which the ice sublimates under solar heat.

Future outlook [AI analysis]

This observation provides a new perspective on comet research. It has been proven that a comet is not simply a 'dirty snowball' orbiting the sun, but a dynamic celestial body that can control its rotation through its own power, which is gas ejection.

If additional observations are made at the next perihelion of comet 41P (expected around 2028), it is expected that the comet's rotation change pattern and long-term trends in surface activity will be identified more precisely. In addition, it is highly likely that research will be followed to determine whether this rotation reversal phenomenon also occurs in other small comets.

Whether a comet is depleted of volatile substances can be an important clue to understanding the conservation state of early materials in the solar system, which may also affect target selection for future space exploration missions.