Gas escaping from Comet 41P likely slowed it to a stop and started spinning it in the other direction.
Observations from NASA’s Hubble Space Telescope suggest that comet 41P/Tuttle-Giacobini-Kresák (shown) reversed the direction of its rotation in 2017.
NASA, ESA, ASC, Ralf Crawford/STScI
For the first time, a comet may have been caught spinning its rotation.
Between April and December 2017, comet 41P/Tuttle-Giacobini-Kresák apparently started twirling in the opposite directionreports astronomer David Jewitt in the April issue Astronomical Journal. The simplest explanation, according to the study, is that gases escaping from the small comet forced its rotation to slow, stop and reverse.
The roughly kilometer-wide comet could continue spinning faster in the new direction until it tears apart, says Jewitt, of the University of California, Los Angeles. The fatal spin demonstrates why small comets – those less than a kilometer wide – are relatively rare, he says. “They spin so fast that they disappear in a relatively short time.”
41P is believed to have assumed its current orbit around the sun around 1,500 years ago, after a close encounter with Jupiter. Its trajectory takes it into the inner solar system every 5.4 years.
In May 2017, NASA observations Neil Gehrels Swift Observatory showed that the rotation of 41P decelerated rapidly. At the time, 41P rotated every 46 to 60 hours, taking more than double the time it had in March, when scientists at the Lowell Observatory in Flagstaff, Arizona, observed it. The change was the fastest change in the rotation of a comet never before observed, the researchers reported the following year in Nature.
For the new study, Jewitt analyzed images taken by NASA Hubble Space Telescope in December 2017. He found that 41P was spinning about once every 14 hours, about a third of the time observed in May of that year – its rotation had gone from slowing down to speeding up.
The sun’s heat likely sublimated some icy parts of 41P, generating gases that would have acted like propellants on a spacecraft, Jewitt says. The torque generated by these gases would have first slowed the 41P’s rotation to a stop, then caused it to begin rotating in almost the opposite direction. This interpretation would explain the observed changes, says Jewitt.
Jewitt’s calculations suggest that, as The rotation of 41P speeds up, centrifugal forces will eventually overcome the comet’s gravity, causing it to separate. It’s difficult to predict exactly when this will happen, because the comet’s outgassing can fluctuate, but it won’t take long, Jewitt says – perhaps only a few decades.
There are objects in the sky that may seem eternal, Jewitt says, but it reminds us that some won’t be around much longer.
