Images taken in May 2017 reveal that comet 41P/Tuttle-Giacobini-Kresák, 41P for short was spinning three times slower than it was in March, when it was observed by the Discovery Channel Telescope at Lowell Observatory in Arizona.
The abrupt slowdown is the most dramatic change in a comet's rotation ever seen.
Comet 41P orbits the Sun every 5.4 years. As a comet nears the Sun, increased heating causes its surface ice to change directly to a gas, producing jets that launch dust particles and icy grains into space.
This material forms an extended atmosphere, called a coma.
Ground-based observations established the 41P’s initial rotational period at about 20 hours in early March 2017 and detected its slowdown later the same month.
The comet passed 13.2 million miles (21.2 million km) from Earth on April 1, and eight days later made its closest approach to the Sun.
Swift's Ultraviolet/Optical Telescope imaged the comet from May 7 to 9, revealing brightness variations associated with material recently ejected into the coma.
These slow changes indicated 41P's rotation period had more than doubled, to between 46 and 60 hours.
UVOT-based estimates of 41P's water production, coupled with the body's small size, suggest that more than half of its surface area contains sunlight-activated jets.
That's a far greater fraction of active real estate than on most comets, which typically support jets over only about 3 percent of their surfaces.
Astronomers suspect these active areas are favorably oriented to produce torques that slowed 41P’s spin.
Such a slow spin could make the comet's rotation unstable, allowing it to begin tumbling with no fixed rotational axis. This would produce a dramatic change in the comet’s seasonal heating and may result in future outbursts of activity.
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