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Narrator: Every 

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5.4 years, comet 41P swings around the Sun

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and puts on a show for observers on Earth.

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Between March and April 2017, as the comet made its closest approach

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to Earth, astronomers caught it doing something never before seen.

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41P is an icy body from the Kuiper Belt,

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the cold storage zone beyond Neptune.

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Neptune's gravity first sent it hurtling toward the sun, and Jupiter trapped

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it in its current orbit. As it nears the Sun,

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the comet's icy areas warm up and turn to gas.

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This forms jets that blast gas and dust into space.

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This material becomes an extended atmosphere around the comet and makes up its

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tail. To better understand how comets work,

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astronomers study how these jets change as a comet approaches and

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departs the inner solar system. From this, astronomers can measure

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how fast the comet rotates. 

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When 41P approached, in March 2017, astronomers found

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it to be rotating about once every 20 hours. But when

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NASA's Swift studied the comet in May, 41P's rotation

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period had more than doubled. This is the largest, most

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abrupt rotational change ever seen on a comet.

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Comet 41P is a small object, smaller than most of the

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so-called Jupiter Family Comets, and very active.

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Astronomers think a particularly strong active area produced jets that

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lined up in just the right way to suddenly put the brakes on the comet's spin.

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Extrapolating from the Swift measurements,

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41P could have continued to slow in the following months,

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spinning less than once every four days by summer.

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This spin is too slow to keep the comet stable, so even

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small jets can set it wobbling like a top, or tumbling,

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and ultimately rotating around a different axis.

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Such changes affect which parts of the comet are exposed to sunlight.

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Perhaps, this will drive new levels of activity that will further change

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the comet's spin. Scientists have never before

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observed this phase of comet evolution, and they look forward to 

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41P's next visit in 2022.

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[Beeping]

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[Beeping]