Spinning Comet Slams its Brakes as It Makes Earth Flyby

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Images of Comet 41P/Tuttle-Giacobini-Kresak’s jets as observed by the Discovery Channel Telescope on March 19, 2017 (Schleicher/Lowell Observatory)

Although comets are static lumps of ancient ice for most of their lives, their personalities can rapidly change with a little heat from the sun. Now, astronomers have witnessed just how dynamic comets can be, seeing one dramatically slow its rate of rotation to the point where it may even reverse its spin.

Comets are the leftover detritus of planetary formation that were sprinkled around our sun 4.6 billion years ago. These primordial icy remains collected in the outermost reaches of the solar system and that’s where they stay until they get knocked off their gravitational perches to begin an interplanetary roller coaster ride. Some are unlucky and end up diving straight to a fiery, solar death. But others set up in stable orbits, making regular passes through the inner solar system, dazzling observers with their beautiful tails formed through heating by the sun.

One mile-wide short-period comet is called 41P/Tuttle-Giacobini-Kresak and it’s a slippery celestial object. First discovered in 1858 by U.S. astronomer Horace Parnell Tuttle, it disappeared soon after. But in 1907, French astronomer Michael Giacobini “rediscovered” the comet, only for it to disappear once again. Then, in 1951, Slovak astronomer Ľubor Kresák made the final “discovery” and now astronomers know exactly where to find it and when it will turn up in our night skies.

Its name, Tuttle-Giacobini-Kresak, reflects the wonderful 100-year discovery and rediscovery history of astronomy’s quest to keep tabs on the comet’s whereabouts.

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Comet 41P/Tuttle-Giacobini-Kresak as observed on March 22, 2017 (Kees Scherer/Knight Observatory, Tomar)

Now, 41P is the focus of an interesting cometary discovery. Taking 5.4 years to complete an orbit around the sun, 41P came within 13-million miles to Earth earlier this year, the closest it has come to our planet since it was first discovered by Tuttle. So, astronomers at Lowell Observatory, near Flagstaff, Ariz., used the 4.3-meter Discovery Channel Telescope near Happy Jack, the 1.1-meter Hall telescope and the 0.9-meter Robotic telescope on Anderson Mesa, to zoom-in on the interplanetary vagabond to measure its rotational speed.

Comets can be unpredictable beasts. Composed of rock and icy volatiles, when they are slowly heated by the sun as they approach perihelion (the closest point in their orbit to the sun), these ices sublimate (i.e. turn from ice to vapor without melting into a liquid), blasting gas and dust into space.

Over time, these jets are known to have a gradual effect the comet’s trajectory and rotation, but, over an astonishing observation run, Lowell astronomers saw a dramatic change in this comet’s spin. Over a short six-week period, the comet’s rate of rotation slowed from one rotation every 24 hours to once every 48 hours — its rate of rotation had halved. This is the most dramatic change in comet rotation speed ever recorded — and erupting jets from the comet’s surface are what slammed on the brakes.

This was confirmed by observing cyanogen gas, a common molecule found on comets that is composed of one carbon atom and one nitrogen atom, being ejected into space as the comet was being heated by sunlight.

“While we expected to observe cyanogen jets and be able to determine the rotation period, we did not anticipate detecting a change in the rotation period in such a short time interval,” said Lowell astronomer David Schleicher, who led the project, in a statement. “It turned out to be the largest change in the rotational period ever measured, more than a factor of ten greater than found in any other comet.”

For this rapid slowdown to occur, the researchers think that 41P must have a very elongated shape and be of very low density. In this scenario, if the jets are located near the end of its length, enough torque could be applied to cause the slowdown. If this continues, the researchers predict that the direction of rotation may even reverse.

“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,” added collaborator Matthew Knight. “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.”

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