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Comet NEOWISE captivates observers

Astronomy has been abuzz with observations of a bright unaided-eye comet in the early morning hours. Comet C/2020 F3 NEOWISE is the brightest comet that’s shown up in the skies in years. Over the past several days, Comet NEOWISE has been visible pre-dawn, but the good news for those of us who aren’t early risers is the comet is now visible just after sunset. That’s because it’s moving quickly in its trajectory around the Sun, and so the geometry between Earth and the comet is also changing rapidly. In the evening, it's visible in the Northwestern sky, just above the horizon. I plan on looking for it the next several evenings. As you’re also eagerly scanning the sky for Comet NEOWISE, here are some other interesting facts and trivia about this visitor from cold storage.

  • Like other comets, Comet NEOWISE is a dirty snowball, meaning it’s comprised of ice and dust. It spends most of its time in the frigid, outer regions of the solar system. As it nears the Sun and heats up, the ice sublimates and turns directly to gas. That gas then drags some of the dust off the comet’s body. Both the gas and dust trail the comet as tails — yes, two tails. Really bright comets have two tails as they’re near the Sun. The brighter one is the dust tail, and the other is what’s known as an ion tail. This one forms by a few-step process. First, the Sun’s ultraviolet radiation kicks electrons off some gas particles, ionizing them. The Sun’s constant solar wind of radiation and magnetic material then affect those ionized particles; this shows up as a second tail flowing directly away from the Sun.
  • Comet NEOWISE made its closest approach to the Sun July 3, when it passed just inside Mercury’s orbit. The comet’s period is some 6,800 years, so it’ll be awhile until it passes near us again.
  • From observations of Comet NEOWISE using infrared light, scientists say the comet’s nucleus — the actual dusty, icy body of the comet — is about 3 miles (5 km) wide.
  • Comet NEOWISE’s dual tails currently stretch about 6 degrees on the sky, which is about half the width of your fist held at arm’s length.
  • The Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) space mission discovered the comet March 27. NEOWISE is actually an interesting mission in itself: The space telescope had a different project and mission after it launched in 2009. It operated as the Wide-field Infrared Survey Explore (WISE) and scanned the sky in infrared light for nearly two years. It was turned off in 2011, but was reactivated in 2013, as NEOWISE, to look for asteroids and other near-Earth objects.

NASA is also hosting a public broadcast and a media teleconference tomorrow afternoon about Comet NEOWISE, if you’d like to find out more about this visitor.

Image: NASA’s Parker Solar Probe captured this image of Comet NEOWISE July 5. [NASA/Johns Hopkins APL/Naval Research Lab/Parker Solar Probe/Brendan Gallagher]

Liz Kruesi is the editor of Mercury magazine and Mercury Online. She has shared the stories of astronomy since 2005. Read more articles by Liz.