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 ...