Spinning Comet Slows Down During Close Approach to Earth

University of Maryland astronomers have captured an unprecedented slowdown in the rotation of a comet. Observations made in May 2017 by NASA's Swift spacecraft, now renamed the Neil Gehrels Swift Observatory after the mission’s late principal investigator, reveal that comet 41P/Tuttle-Giacobini-Kresák was spinning more than twice as slowly as it was in March, when it was observed by the Discovery Channel Telescope at Lowell Observatory near Flagstaff, Arizona.

The abrupt slowdown of the comet, commonly referred to as 41P, is the most dramatic change in a comet's rotation ever seen. Published in the journal Nature on January 11, 2018, the researchers presented their findings at a press conference on January 10, 2018, at the 231st American Astronomical Society (AAS) Meeting in Washington, D.C.

"The previous record for a comet spindown went to 103P/Hartley 2, which slowed its rotation from 17 to 19 hours over 90 days," said Dennis Bodewits, an associate research scientist in the UMD Department of Astronomy and lead author of the study. "By contrast, 41P spun down by more than 10 times as much in just 60 days, so both the extent and the rate of this change is something we've never seen before."

Comet 41P orbits the sun every 5.4 years, traveling only about as far out as the planet Jupiter. Estimated to be less than a mile across, 41P is among the smallest of the “Jupiter family” comets, named as such because Jupiter’s gravitational influence controls their orbits. The small size of 41P helps explain how jets on the comet’s surface were able to produce such a dramatic spindown.

With a small and relatively inactive nucleus—the solid ball of dust and ice at the center of the comet—41P had proven difficult for astronomers to study in detail. That all changed in early 2017, when the comet passed within 13.2 million miles of Earth—the closest since its discovery.

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.

Water in the coma quickly breaks up into hydrogen atoms and hydroxyl molecules when exposed to ultraviolet sunlight. Because Swift's Ultraviolet/Optical Telescope (UVOT) is sensitive to UV light emitted by hydroxyl, it is ideally suited for measuring how comet jet activity evolves throughout the comet’s orbit.

Ground-based observations established the comet's initial rotational period at about 20 hours in early March 2017 and detected its slowdown later the same month. The comet came closest to Earth on April 1, making its closest approach to the sun eight days later.

Swift's UVOT imaged the comet May 7-9, 2017, revealing light variations associated with material recently ejected into the coma. These slow changes indicated that 41P's rotation period—or the time it takes for the comet to complete one full rotation on its axis—had more than doubled, from 20 hours 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 the comet’s surface area hosted sunlight-activated jets. In contrast, most active comets typically support jets over about 3 percent of their surface area.

"We suspect that the jets from the active areas are oriented in a favorable way to produce the torques that slowed 41P's spin," said Tony Farnham, a principal research scientist in astronomy at UMD and a co-author of the Nature paper. "If the torques continued acting after the May observations, 41P's rotation period could have slowed to 100 hours or more by now."

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. Bodewits and his colleagues note that 41P probably spun much faster in the past—possibly fast enough to induce landslides or partial fragmentation that would expose fresh ice. Strong outbursts of activity in 1973 and 2001 may be related to 41P's rotational changes, the researchers suggested.

A second team of astronomers from UMD, Lowell Observatory and the University of Sheffield independently confirmed the slowdown with a separate set of observations using the Discovery Channel, Hall and Robotic Telescopes operated by Lowell. The results suggest that the comet has an elongated shape and low density, with jets located near the end of its body. These jets provide the torque needed to slow the comet’s rotation.

“If future observations can accurately measure the dimensions of the nucleus, then the observed rotation period change would set limits on the comet’s density and internal strength,” said Matthew Knight, an assistant research scientist in the UMD Department of Astronomy. “Such detailed knowledge of a comet is usually only obtained by a dedicated spacecraft mission like the recently completed Rosetta mission to comet 67P/Churyumov-Gerasimenko.”

The Rosetta mission, which entered orbit around comet 67P/Churymov-Gerasimenko in 2014, documented a less extreme relationship between a comet's shape, activity and spin. The comet's spin sped up by two minutes as it approached the sun, and then slowed by 20 minutes as it moved farther away. As with 41P, scientists think these changes were produced by the interplay between the comet's shape and the location and activity of its jets.

Comets are believed to be remnants from the formation of the solar system, having changed little during the past 4.5 billion years. First discovered by Horace Tuttle in 1858, 41P was lost for years until it was rediscovered by Michel Giacobini in 1907. Lost again and rediscovered a third time in 1951 by Lubor Kresák, the comet now carries the names of all three independent discoverers.

This release was adapted from text provided by NASA’s Goddard Space Flight Center and Lowell Observatory.

This research published in the Nature research paper was supported by NASA’s Swift Guest Investigator Program (Award No. 1316125) and the National Science Foundation (Award No. AST-1005313). The research presented at the AAS Division for Planetary Sciences meeting in October 2017 was supported by NASA’s Planetary Astronomy Program and the Marcus Cometary Research Fund. The content of this article does not necessarily reflect the views of these organizations.