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 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.
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.”
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.
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.