Comet NEOWISE has a sodium tail. Here's what it looks like.

These false-color images of Comet NEOWISE show the concentration of sodium atoms in the comet's dusty ion tail. Astronomers created the images using the Planetary Science Institute's Input/Output facility near Tucson, Arizona. The image on the left shows light reflected off of cometary dust, while the image on the right shows light emitted by sodium atoms. (Image credit: Jeffrey Morgenthaler/Carl Schmidt/Planetary Science Institute)

New images of the bright Comet NEOWISE show signs of a sodium tail, giving scientists fresh insight on what's happening on the surface.

Comet brightness is notoriously hard to predict due to the complexity of surface outgassing, so any insight into these surface processes is useful to scientists. NEOWISE, formally known as comet C/2020 F3, is relatively bright in the sky for Northern Hemisphere observers, but nobody knows how the comet's brightness will fare as it races away from the sun the sun and back toward the outer solar system.

Images of the comet were obtained on July 8 using the Planetary Science Institute's (PSI) Input/Output facility, which has also spotted sodium in Mercury's comet-like tail as well as Jupiter's atmosphere during past observations.

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NEOWISE and comets like it are made of dust, gas and plasma (ionized gas). As comets rocket toward the sun from interstellar space or the outer solar system, sunlight causes ice in the comet to turn directly into gas (or sublime). As the ice gasses away, it pulls material from the comet's surface with it.

Scientists can already get a sense of cometary activity by looking at the dust coming from a comet, which can move faster if the particles are tiny and more easily pushed by sunlight. Slower particles tend to be larger and harder to move around. These individual particles affect how the dust tail is shaped. 

Like dust, atomic sodium also is affected by sunlight, although it has more specific changes.

"Its [atomic sodium's] momentum kick comes from a very particular wavelength of yellow light — the same color seen in sodium vapor street lamps,"  research team member Jeffrey Morgenthaler, a PSI senior scientist, said in a statement.

"Thanks to acceleration by intense sunlight," co-author and Boston University research scientist Carl Schmidt added in the same statement, "the sodium tail takes on a different shape than the tail seen in off-band filtered images, which are dominated by reflected light from dust. In comparison, the sodium tail is narrower, longer and points directly away from the sun."

Sunlight's push on sodium atoms tends to be stronger than its effect on dust and other gases that come off comets. It is difficult to see sodium tails, however, due to the sun's emissions. Notable examples of comets with sodium tails include Hale-Bopp (the famed 1997 naked eye-comet) and the notorious Comet ISON, which fell apart shortly after rounding the sun in 2013.

Morgenthaler and Schmidt will continue to observe NEOWISE as it rounds the sun. They are also using Monte Carlo computer models to simulate the sodium tail and estimate outgassing rates and speeds.

It was not revealed in the statement exactly when the team plans to submit their results to a journal for peer review, but it is common when observing quickly changing astronomical phenomena like NEOWISE to release interim information to keep the community informed.

Editor's note: This story has been updated to included a corrected photo of Comet NEOWISE supplied by the research team.

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Elizabeth Howell
Staff Writer, Spaceflight

Elizabeth Howell (she/her), Ph.D., is a staff writer in the spaceflight channel since 2022 covering diversity, education and gaming as well. She was contributing writer for for 10 years before joining full-time. Elizabeth's reporting includes multiple exclusives with the White House and Office of the Vice-President of the United States, an exclusive conversation with aspiring space tourist (and NSYNC bassist) Lance Bass, speaking several times with the International Space Station, witnessing five human spaceflight launches on two continents, flying parabolic, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, "Why Am I Taller?", is co-written with astronaut Dave Williams. Elizabeth holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, a Bachelor of Journalism from Canada's Carleton University and a Bachelor of History from Canada's Athabasca University. Elizabeth is also a post-secondary instructor in communications and science at several institutions since 2015; her experience includes developing and teaching an astronomy course at Canada's Algonquin College (with Indigenous content as well) to more than 1,000 students since 2020. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday. Mastodon: