Like many other celestial bodies, many comets rotate around themselves. But this rotation does not always remain the same. Astronomers have discovered a small comet whose rotation has completely reversed – it is now orbiting the other way around than before its last solar passage, as images from the Hubble Space Telescope show. It is the first time that such a rotation reversal has been detected in a comet. The reason for this reversal is probably violent outgassing, which first slowed down the comet’s nucleus, which is only around 500 meters in size, and then turned it in the other direction, as the team reports.
Many comets and some ice-rich asteroids in the inner solar system originally come from the Kuiper Belt – a zone beyond the orbit of Neptune populated by dwarf planets and other ice-rich objects. As a result of collisions or the effects of gravity from nearby stars, chunks repeatedly fall out of their orbit and end up first in the outer solar system and then in the inner solar system. One of these chunks is Comet 41P/Tuttle-Giacobini-Kresák. This short-period comet was probably brought into its current orbit around 1,500 years ago. Over the course of 5.4 years, it moves between its closest point to the sun, just outside the Earth’s orbit, and its furthest point from the sun, just inside Jupiter’s orbit. 41P belongs to the so-called Jupiter family of comets and was discovered in 1858.
First slowly, then faster again
In 2017, comet 41P was particularly easy to observe when it passed its closest point to the sun on April 12, 2017 and clear outgassing became visible. Numerous telescopes focused on the comet during this time, including NASA’s Swift Space Telescope. Astronomers noticed that the comet’s rotation slowed dramatically. “The rotation period increased more than twice within the two months of the perihelion passage, from 20 hours to around 53 hours, reports David Jewitt from the University of California in Los Angeles. Why remained unclear at the time. That’s why Jewitt looked in the Hubble Telescope archive for further images of the 41P/Tuttle-Giacobini-Kresák from the time after its perihelion passage – and found what he was looking for.
“We have found unpublished observations that show a continued further development of the rotation period after perihelion,” reports the astronomer. These images revealed that comet 41P’s rotation initially slowed further after it passed the closest point to the Sun. In December 2017, however, this changed again: Now the comet nucleus suddenly rotated faster again – but in the opposite direction. It now rotated with a period of around 14 hours, as Jewitt determined. Such a rotation reversal has never been demonstrated before in a comet. But what triggered it?
Brought to reversal by outgassing
An answer to this question was provided by the increased outgassing from the comet’s surface around its perihelion: “Rays of escaping gas on the surface can act like small engines,” says Jewitt. “When these rays are unevenly distributed, they can dramatically alter the rotation of a comet, especially a small one.” When the already rather small comet approached its closest point to the Sun in spring 2017, the asymmetrical outgassing first slowed down its rotation, brought it almost to a standstill and then forced it in the other direction. “It’s like pushing a carousel,” explains Jewitt. “If it turns in one direction and then you push against it, you can slow it down and even reverse it.”
The Hubble data also revealed why the comet could be turned around so easily thanks to its outgassing: “Hubble photometry and non-gravitational acceleration measurements reveal a diminutive size of the comet nucleus with a radius of only around 500 meters,” reports Jewitt. This is very small for a comet and makes it vulnerable to the deflection effects of its strong outgassing. This could soon be the downfall of 41P/Tuttle-Giacobini-Kresák. Even if its orbit around the sun could remain stable for another thousand years, the comet will probably break up sooner. Modeling based on the measured torques and mass loss rates suggests that the centrifugal forces created by its increasing rotation could soon tear the comet nucleus apart.” “I expect this core to self-destruct very quickly,” says Jewitt. Specifically, he determined a remaining lifespan of around 25 years – that corresponds to around five orbits.
Source: David Jewitt (University of California, Los Angeles), The Astronomical Journal, doi: 10.3847/1538-3881/ae4355