Radar Reveals A Rugged Comet Wirtanen / Asteroid 2016 AZ8 Gets A Moon

This sequence of radar images of Comet 46P/Wirtanen from Dec. 15 shows apparent rotation of the nucleus in a counter-clockwise direction. The nucleus spins once every 8.9 hours. By combining radar echoes taken over several days, scientists were able to build an image of the comet. NASA / NSF

When the naked-eye comet 46P/Wirtanen zipped just 7.2 million miles (11.2 million km) from Earth last month, lots of people were looking including a team of scientists led by Ellen Howell from the University of Arizona’s Lunar and Planetary Laboratory. Her team used Arecibo Observatory’s 1,000-foot-wide radio telescope to ping the comet with radio waves. By studying the return “echoes,” they were able to create an image of the solid part of the comet called the nucleus.

Comet 46P/Wirtanen is a fuzzy aquamarine ball of light in this photo taken Jan. 1, 2019. The comet’s nucleus is hidden within the small bright spot at center. The spot itself is many thousands of miles across. Rolando Ligustri

Comet nuclei are generally tiny, around a kilometer or two across, and hidden inside a cloud of icy vapors and dust that make up the comet’s head or coma. Even the most powerful optical telescopes can’t clearly distinguish a comet’s core, but a radio telescope can sort out the different frequencies of the returning radio waves to build up an image showing the object’s shape and details like craters, hills and moons.

The 1000-foot dish at Arecibo in Puerto Rico bounced radio waves off the comet and captured the returning echoes to reconstruct an image as well as position, size and rotation information. USRA

Likewise, those returning radio waves give us the comet’s precise distance which astronomers determine an accurate orbit for the comet. You send out a ping, wait for the echo to return to the antenna and then multiply the time by the speed of light. For a great explanation on how astronomers make radar images, check out Emily Lakdawalla’s blog on the topic.

Howell and team observed 46P/Wirtanen for 9 days from Dec. 10-18 to make the images that show the comet as an elongated object about 0.9 miles (1.4 km) across that has a much rougher and bumpier surface that most comets we’ve seen up close. They were also able to detect an extensive “skirt” of larger particles — from just under an inch (2 cm) and bigger —  that accompany the comet, probably broken off from the nucleus as it was heated by the sun.

“Radar observations give us images of the comet nucleus we can’t get any other way. This comet has a really rugged looking surface, which might be related to the large population of grains in its coma,” said Howell. “Every comet we study is unique. Radar images are important pieces of the puzzle.”

Radar video of comet 45P/Honda-Mrkos-Pajdusakova made with the Arecibo Observatory on Feb. 12, 2017. USRA

Howell’s team was also able to find some surprising differences between this and other comets of the same family. 46P/Wirtanen belongs to the Jupiter-family group, so-called because the comets’ orbits are controlled by Jupiter’s gravity. Two other kindred comets, 45P/Honda-Mrkos-Pajdusakova and 41P/Tuttle-Giacobini-Kresak, were also studied by radar in 2017 and found to have very different amounts of debris hovering around their nuclei. Comet 46P/Wirtanen has lots of larger pieces, 45P had a smaller amount and 41P had none. 

Comet Wirtanen is only the eighth imaged using radar in the last 30 years. Unlike the small, Earth-approaching asteroids you always hear about in the news,  comets rarely come close enough to the Earth to get detailed images. The next close approach by Comet 46P/Wirtanen will be in 2029 but at 10 times the distance of the current flyby, making this the best known opportunity to image a comet with radar for the next 30 years.

You can still spot 46P in the evening sky before the moon gets too bright. It’s now too faint to see with the naked eye, but I easily saw it as a misty glow in 10×50 binoculars two nights ago from my home on the city’s fringe. The comet is traveling from Lynx the lynx into the Big Dipper this week. To track it, click here for a map and more information.

2016 AZ8 and its little moon imaged with the Arecibo dish. NASA / NSF

Since we touched on asteroids, another team of scientists used the Arecibo dish to make a new discovery at an old asteroid. On Jan. 4, while pinging the quarter-mile-wide 2016 AZ8, the team discovered it has a moon! The companion orbits roughly 1,300 feet (400 meters) away from its mama.

Earth also has this thing called a moon, and it orbits about 239,000 miles away. You can see it tonight if you step outside during evening twilight and face southwest. The 2½-day-old crescent shines from the dim constellation of Capricornus the sea-goat.


6 Responses

  1. Edward N Boll

    Sometime between 11-3, tonight Ibplan on getting a view of Wirtanen. This may be the last easy, bright view. The rest of the week predicted cloudy till Sunday, then the moon becomes a menace, we’ll not really. But after Full Moon, Wirtanen May be only mag 8 or 9.

  2. Edward M. Boll

    I finally got a view of Wirtanen, at 11:59 last night after scanning with my binoculars for a few minutes. I did it the hard way, calculating between Gemini and the pointer stars of the Big Dipper. Now notably fainter, around mag 6.5. If the weather stays clear, I may try again tonight, but going to wait until after midnight till it is a little higher on the sky.It would take an eagle eyed observer now under almost perfect conditions to see it now with their own eyes.

  3. Hiram Berry

    Wow! I stumbled upon your wonderful site, Astrobob, while looking for images of directly imaged stars… and you had them! Then saw your in depth discussion of the utterly intriguing object Oumuamua. And now this: radar images at far greater resolution than I thought possible for the distance involved. Thank you!

    So I’ll ask, were any radar observations made of Oumuamua as it did its close approach to Earth?

    And concerning the resolution, from looking at your direct images of star disks it seems like the resolution limit is something on the order of the ratio of the wavelength to the mirror diameter of the telescope. A quantum limitation? So my rough estimations say that the Wirtanen images are sharper than 1 milliarcsecond while the wavelength to diameter ratio for Arecibo is WAY bigger. Is this because it’s monochromatic?

    If so, is there a way for amateurs to “scale down Arecibo” to get images of this quality? I’m thinking along the lines scaling down the wavelength into IR, using a large pulsed CO2 laser to illuminate the asteroid ganged with a gated filter on the receiving scope to be in sync with the return signal? Would it be possible to apply Emily Lakdawalla’s signal processing techniques?

    1. astrobob

      Hi Hiram,
      Glad you found the site — thank you! No radar images were made of ‘Oumuamua. Though I don’t know the exact reason why, I suspect it was because when discovered it was already 21 million miles away (much further than 46P) and on its way out. It’s also much smaller, only a few hundred feet across. Resolution limits for a telescope are defined not by quantum theory but by something called the diffraction limit which varies according to the size of the mirror or lens and how well it’s figured. Atmospheric turbulence is also a major limitation, but much of that can be removed by adaptive optics techniques used by major observatories. Arecibo can resolve details down to about 3 arc minutes. When pinging and assembling the echoes to build an image, it can obviously do much better. I don’t know enough to answer your “monochromatic” and “scaling” questions, but I do know that while amateurs build their own radio telescopes, I’ve not heard of a single one that does radio imaging.

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