Comets

See Historic Comet BA14 Up Close In These New Radar Images

These radar images of comet P/2016 BA14 were taken on March 23, 2016, by scientists using an antenna of NASA’s Deep Space Network at Goldstone, California. At the time, the comet was about 2.2 million miles (3.5 million kilometers) from Earth. Credit: NASA/JPL-Caltech/GSSR

On March 22, Comet P/2016 BA14 (Pan-STARRS) flew just 2.2 million miles (3.5 million kilometers) from Earth, making it the third closest comet ever recorded. The last time a comet appeared on our doorstep was in 1770, when Lexell’s Comet breezed by at about half that distance. Through a telescope, comet BA14 looked (and still looks) like a faint star, though time exposures reveal a short, weak tail. With an excellent map and large amateur telescope you might still find it making a bead across the Big Dipper and constellation Bootes tonight through the weekend.


Flyby Comet Imaged by Radar

While normal telescopes show few details, NASA’s Goldstone Solar System Radar in California’s Mojave Desert pinged P/2016 BA14 with radar over three nights during closest approach and created a series of crisp, detailed images from the returning echoes. They show a bigger comet than expected — about 3,000 feet (one kilometer) across —  and resolve features as small as 26 feet (8 meters) across.

“The radar images show that the comet has an irregular shape: looks like a brick on one side and a pear on the other,” said Shantanu Naidu, a researcher at NASA’s Jet Propulsion Laboratory. “We can see quite a few signatures related to topographic features such as large flat regions, small concavities and ridges on the surface of the nucleus.”

I honestly thought we’d see a more irregular shape assuming that astronomers were correct in thinking that BA14 broke off from its parent 252P/LINEAR though it’s possible it happened so long ago that the “damage” has been repaired by vaporizing ice softening its contours.

Comets are as dark as charcoal but appear light only because the sun illuminates them against the blackness of outer space. I shone a flashlight on a charcoal briquette (left) to simulate comet lighting. The same charcoal when viewed in normal light appears black. Credit: Bob King

Radar also shows that the comet is rotating on its axis once every 35 to 40 hours. While radar eyes focused on BA14, Vishnu Reddy, of the Planetary Science Institute, Tucson, Arizona, used the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii to examine the comet in infrared light. He discovered its dark surface reflects less than 3% of the sunlight that falls on it. The infrared data is expected to yield clues of the comet’s composition as well.

Illustration of Comet 67P/Churyumov-Gerasimenko brought down to Earth in the city of Los Angeles, Calif. Not only can we appreciate its size but also its truly dark surface.  Credit: ESA and anosmicovni

Comets are exceptionally dark objects often compared to the appearance of a fresh asphalt road or parking lot. They appear bright in photos because seen against the blackness of space, they’re still reflective enough to stand out. Comet 67P/Churyumov-Gerasimenko, still the apple of the orbiter Rosetta’s eye, is similarly dark, reflecting about 4% of sunlight.

What makes comets so dark even though they composed primarily of ice? Astronomers believe a comet grows a dark ‘skin’ both from accumulated dust and irradiation of its pristine ices by cosmic rays. Cosmic rays loosen oxygen atoms from water ice, freeing them to combine with simple carbon molecules present on comets to form larger, more complex and darker compounds resembling tars and crude oil. Dust settles on a comet’s surface after it’s set free from ice that vaporizes in sunlight.

What a view! Comet 67P/C-G photographed from a distance of just 7.5 miles (12 kilometers) on March 19, 2016 by Rosetta’s Navcam. The largest boulder to the right is Cheops, which stands about 82 feet (25 meters) high. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

I live in Minnesota, where our annual State Fair features every kind of deep-fried food you can imagine: deep-fried Twinkies, deep-fried fruit, deep-fried bacon and even deep-fried Smores. Just now, I can’t shake the thought that comets are just another deep-fried confection made of pristine, 4.5-billion-year-old ice toasted by eons of sunlight and cosmic bombardment.

Bob King

I'm a long-time amateur astronomer and member of the American Association of Variable Star Observers (AAVSO). My observing passions include everything from auroras to Z Cam stars. I also write a daily astronomy blog called Astro Bob. My new book, "Wonders of the Night Sky You Must See Before You Die", a bucket list of essential sky sights, will publish in April. It's currently available for pre-order at Amazon and BN.

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