Anatomy of A Comet: NEOWISE


By Dr. Bryony Richards
On Friday March 27, 2020, astronomers on the NEOWISE (Near-Earth Object Wide-field Infrared Survey Explorer) mission discovered a comet that would become the brightest in the northern hemisphere in almost a quarter century. Over the following weeks, comet NEOWISE, full name C/2020 F3 (NEOWISE), would not only become a spectacle for the thousands of scientists and photographers following it with specialized equipment, but it would become so bright that it would be visible to the naked eye and amaze millions across the northern hemisphere. Comet NEOWISE made its closest approach to the sun (its perihelion) on Friday July 3rd, within 29.4 million miles of the sun, inside the orbit of mercury.

I had been lucky enough to previously ‘photograph’ (even though it turned out as a green speck) a comet, the memorably named 252P Linear, but had never before purposefully set out to document a comet. I certainly had never seen a comet with the naked eye*.

During the few weeks I was able to photograph and observe NEOWISE its evolution from a simple icy nucleus and dust tail to a carbon-cyanide spewing comet with a vibrant-blue secondary ionizing carbon monoxide tail (as it got closer to its perihelion) stunned me. I had not been aware, from my previous, rather sparse, comet-hunting that comets could become so complex. So, I set out to take a photo where I could see the various features of NEOWISE. This meant overcoming certain challenges, from the easy, finding a dark sky location, to using a refractor on a tracking mount to track the comet for long exposure photographs, and perhaps the most difficult, waiting patiently for clear skies. In the end a trip to the Sawtooth Mountains of Idaho, US, (Photo 1) provided the dark skies (with minimal light pollution and a new moon), I setup a rather simple tracking mount with a 250 mm refractor lens attached to my regular camera, and then waited for the clear skies to emerge.

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Photo 1. Panoramic photo(s) of NEOWISE with the Milky Way over the hills of central Idaho. Note that even at a focal length of 24 mm the two tails of NEOWISE are visible.

As you can see from the photos and Figure 1, I ‘lucked out’ (scientific term) and was able to capture NEOWISE in all its two tailed glory. That begged the question of why the tails evolved, up until the perihelion, into two very different structures. The first being the large, wavy, (main) dust tail of NEOWISE. Starting simply, lighter particles in the dust tail are pushed out by sunlight, with the curve in the tail made up of heavier particles that are able to resist pressures and continue along a solar orbit. The striations (visible best in Photo 2) are more complex, and still not well understood, but likely relate to rotating streams of sun-reflecting grit liberated by the icy nucleus melting.

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Figure 1. Anatomy of NEOWISE. Figure shows a breakdown of the main features of the comet using Photo 3.
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Photo 2. NEOWISE shot with an 85 mm standard camera lens. Note that the striations of the main dust tail are now visible!

The secondary, show-stopping, blue ion tail of NEOWISE (Photo 3) is noted to point directly away from the sun, pushed out by the charged, flowing solar wind. The incredible, wave-like structure of the blue-ion tail comes from differing rates of expelled CO ion from the comets nucleus coupled with continually shifting structure of the solar wind. Some of the other novel features of NEOWISE (and there are far too many to mention here!) include the green, glowing, coma surrounding its nucleus, caused by the stimulation of carbon monoxide, cyanide, and diatomic carbon by ultraviolet light, and the hydrogen gas cloud surrounding the comet estimated to be 2 million miles in diameter.

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Photo 3. NEOWISE taken with a 250 mm refractor lens. Note that the coma, nucleus, and waves in the ion tail are now visible!

If you weren’t lucky enough to spot NEOWISE on this go-around, calculations by orbital experts indicate that its far-end orbit (its aphelion) to be about 63 billion miles from the sun, reachable by around the year 5420. That would mean that it would return to our part of the inner solar system somewhere around the year 8863… Time to look for a different comet to document!

*As a side issue, you may wonder why I missed seeing Hale-Bop in 1997; well, unfortunately growing-up in the UK, clear skies were not something that were common!

Dr. Bryony Richards is Senior Petrologist at EGI. Her professional expertise is mineralogy, geochemistry, radiochemistry and evaluation and interpretation of complex mineralogical and geochemical datasets across a variety of geological settings using multiple, integrated techniques in order to solve complex geological problems. In addition to her formidable professional and academic background, Bryony is (clearly!) a highly accomplished astrophotographer. She offers specialized workshops through the University of Utah as well as on-demand.
Her photography merges art and science with spectacular results. She has won numerous awards and been featured in exhibitions both locally in Utah and across the US. Most recently two of her photos were short listed for Astronomy Photographer of the Year 2020. You can read more about the competition, Bryony’s photos and the other amazing shortlisted entries below.

Forbes: https://www.forbes.com/sites/jamiecartereurope/2020/07/22/worlds-best-nightscape-photos-15-shortlisted-images-for-astronomy-photographer-of-the-year-2020/#b15fa6d3695e

PetaPixel: https://petapixel.com/2020/07/15/shortlist-for-astronomy-photographer-of-the-year-2020-revealed/