Rubber Ducky II — Radar Reveals Spinning, Twin-Lobed Comet

This animation of comet 45P/H-M-P was made by bouncing radio waves off the comet. Credit: USRA

When the first close range photos of Rosetta’s Comet started coming in, a few of us joked it looked just like one of those rubber ducks we played with as kids in the bathtub. Only this duck was made of porous ice and dust and 2.5 miles (4 km) wide, though I suspect it would float if you had a tub big enough. Now, new radar images have bagged yet another duck.

The recent close flyby of another mouthful of a comet, 45P/Honda-Mrkos-Pajdusakova, provided a not-to-be-missed opportunity to bounce radio waves off its nucleus and study the returning echoes to create close up if shadowy images. While asteroids routinely pass near Earth — there are millions of them in the neighborhood — close approaches of comets are relatively rare.

This rubber duck appears has a certain air of malevolence about it. It’s a photo of comet 67/Churyumov-Gerasimenko taken by the Rosetta probe that went into orbit about the comet. Sunlight vaporizing comet ices creates jets or streamers of dust and gas which form a temporary, tenuous atmosphere around the comet called a coma. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

That’s why astronomers fired up the giant, 1,000-wide radio dish at the Arecibo Observatory in Puerto Rico and began pinging it with radio waves starting on the Feb. 12 and continuing right through to today, the 17th. This is only the 7th comet to have its portrait taken with radar.

The 1000-foot dish at Arecibo in Puerto Rico was used to bounce radio waves off the comet. The returning echoes contain information that can be used to reconstruct an image as well as position, size and rotation information. Credit: USRA

After sending their Valentine’s Day card, astronomers were very happy with the comet’s response. In the animation, comprised of 13 images made during 2 hours of observation on Feb. 12, we see a peanut-shape or twin-lobed body that resembles the rubber ducky comet 67P. We also learned that the nucleus or heart of the comet measures 0.8 mile (1.3 km) across and rotates about once every 7.6 hours. Go to work at 8 and about the time you wrap up for the day, 45P  has made one complete spin. If you watch and re-watch the animation, you can make out bright spots and rugged textures in places that indicate erosion by vaporization.

Comet 45P glows aqua in this photo taken this morning from Arizona. The comet is very diffuse with a central brightening. Hidden by bright, sunlight-reflecting dust at its very center is the actual comet nucleus, 0.8 mile across. The coma measures hundreds of thousands of miles in diameter. Credit: Chris Schur

A comet loses material everything they approach the sun as water and carbon dioxide (dry ice) vaporizes directly into space both from its surface and from pits that release heated and pent up gases from below the surface. The gas carries along dust and the dust creates the big fuzzy glow around the nucleus called the coma. Sunlight both energizes gas and physically pushes the dust behind the comet to create the two basic types of comet tushes: the gas or ion tail and the familiar dust tail.

A selection of high-resolution photos taken by Rosetta that were used to identify patterns in comet 67P/C-G’s layers revealing that each lobe was a separate comet long ago. Copyright: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; M. Massironi et al. (2015)

Astronomers have shown through close-up study of 67P/C-G that twin-lobed comets form when two smaller comets collide at low speed and become welded together to form a single larger one. They found that rock terraces and layers of exposed material in cliffs on each lobe of 67P were inclined in opposite directions, indicating two separate objects that long ago fused into one.

I selected separate frames from the Arecibo animation for closer study. Credit: USRA

For more about 45P and how to see with your telescope, please see my earlier write-up.

** If you’d like to know more about comets and the meteors and meteor showers that come from them, pick up a copy of my recently published book, Night Sky with the Naked Eye, at Amazon or Barnes and Noble. It covers lots of different night sights — no equipment needed!


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