Gifts From Heaven

Heaven Sent, Comet Tsuchinshan–ATLAS Above the Sierra Crest, Alabama Hills
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As much for its (apparently) random arrival as its ethereal beauty, the appearance of a comet has always felt to me like a gift from the heaven. Once a harbinger of great portent, scientific knowledge has eased those comet fears, allowing Earthlings to simply appreciate the breathtaking display.

Unfortunately, scientific knowledge does not equal perfect knowledge. So, while a great comet gives us weeks, months, or even years advance notice of its approach, we can never be certain of how the show will manifest until the comet actually arrives. For every Comet Hale-Bopp, that gave us nearly two years warning before becoming one of the most widely viewed comets in human history, we get many Comet ISONs, which ignited a media frenzy more than a year before its arrival, then completely fizzled just as the promised showtime arrived. ISON’s demise, as well as many highly anticipated comets before and after, taught me not to temper my comet hopes until I actually put eyes on the next proclaimed “comet of the century.” Nevertheless, great show or not, the things we do know about comets—their composition, journey, arrival, and (sometimes) demise—provide a fascinating backstory.

In the simplest possible terms, a comet is a ball of ice and dust that’s (more or less) a few miles across. After languishing for eons in the coldest, darkest reaches of the Solar System, perhaps since the Solar System’s formation, a gravitational nudge from a passing star sends the comet hurtling sunward, following an eccentric elliptical orbit—imagine a stretched rubber band. Looking down on the entire orbit, you’d see the sun tucked just inside one extreme end of the ellipse.

The farther a comet is from the sun, the slower it moves. Some comets take thousands, or even millions, of years to complete a single orbit, but as it approaches the sun, the comet’s frozen nucleus begins to melt. Initially, this just-released material expands only enough to create a mini-atmosphere that remains gravitationally bound to the nucleus, called a coma. At this point the tail-less comet looks like a fuzzy ball when viewed from Earth.

This fuzzy phase is usually the state a comet is in when it’s discovered. Comets are named after their discoverers—once upon a time this was always an astronomer, or astronomers (if discovered at the same time by different astronomers), but in recent years, most new comets are discovered by automated telescopes, or arrays of telescopes, that monitor the sky, like ISON, NEOWISE, PANSTARRS, and ATLAS. Because many comets can have the same common name, astronomers use a more specific code assembled from the year and order of discovery.

As the comet continues toward the sun, the heat increases further and more melting occurs, until some of the material set free is swept back by the rapidly moving charged particles of the solar wind, forming a tail. Pushed by the solar wind, not the comet’s forward motion, the tail always fans out on the side opposite the sun—behind the nucleus as the comet approaches the sun, in front of the comet as it recedes.

Despite accelerating throughout its entire inbound journey, a comet will never move so fast that we’re able to perceive its motion at any given moment. Rather, just like planets and our moon, a comet’s motion relative to the background stars will only be noticeable when viewed from one night to another. And like virtually every other object orbiting the sun, a comet doesn’t create its own light. Rather, the glow we see from the coma and tail is reflected sunlight. The brilliance of its display is determined by the volume and composition of the material freed and swept back by the sun, as well as the comet’s proximity to Earth. The color reflected by a comet’s tail varies somewhat depending on its molecular makeup, but usually appears as some shade of yellow-white.

In addition to the dust tail, some comets exhibit an ion tail that forms when molecules shed by the comet’s nucleus are stripped of electrons by the sun’s ultraviolet radiation. Being lighter than dust molecules, these ions are whisked straight back by the solar wind. Instead of fanning out like the dust tail, these gas ions form a narrow tail that points directly away from the sun. Also unlike the dust tail that shines by reflected light, the ion tail shines by fluorescence, taking on a blue color courtesy of the predominant CO (carbon monoxide) ion.

One significant unknown upon discovery of a new comet is whether it will survive its encounter with the sun at all. While comets that pass close to the sun are more likely to shed large volumes of ice and dust, many sun-grazing comets approach so close that they’re overwhelmed by the sun’s heat and completely disintegrate.

With millions of comets in our Solar System, it would be easy to wonder why they’re not a regular part of our night sky. Actually, Earth is visited by many comets each year, though most are so small, and/or have made so many trips around the sun that they no longer have enough material to put on much of a show. And many comets never get close enough to the sun to be profoundly affected by its heat, or close enough to Earth to shine brightly here.

Despite all the things that can go wrong, every once in a while, all the stars align (so to speak), and the heavens assuage the disappointment of prior underachievers with a brilliant comet. Early one morning in 1970, my dad woke me and we went out in our yard to see Comet Bennett. This was my first comet, a sight I’ll never forget. I was disappointed by the faint smudges of Comet Kohoutek in 1973 (a complete flop compared to its advance billing), and Halley’s Comet in 1986 (just bad orbital luck for Earthlings). Comet Hale-Bopp in 1996 and 1997 was wonderful, while Comet ISON in 2012 disintegrated before it could deliver on its hype.

In 2013 Comet PANSTARRS didn’t put on much of a naked-eye display, but on its day of perihelion, I had the extreme good fortune to be atop Haleakala on Maui, virtually in the shadow of the telescope that discovered it. Even though I couldn’t see the it, using a thin crescent moon I knew to be just 3 degrees from the comet to guide me, I was able to photograph PANSTARRS and the moon together. Then, in the dismal pandemic summer of 2020, Comet NEOWISE surprised us all to put on a beautiful show. I made two trips to Yosemite to photograph it, then was able to photograph it one last time at the Grand Canyon shortly before it faded from sight.

October 2024 promised the potential for two spectacular comets, Comet Tsuchinshan–ATLAS (C/2023 A3) in the first half of the month, and Comet ATLAS (C/2024 S1) at the end of the month. Alas, though this second comet had the potential to be much brighter, it pulled an Icarus and flew too close to the sun (RIP). But Comet Tsuchinshan–ATLAS was another story, brightening beyond expectations.

I shared the story of my trip to photograph Tsuchinshan–ATLAS in my October 16 I’m Not Crazy, I Swear… blog post, but have a couple of things to add about this image. First is how important it is to not get so locked into one great composition that you neglect capturing variety. I captured this wider composition before the image I shared a couple of weeks ago, and was pretty thrilled with it—thrilled enough to consider the night a great success. But I’m so glad that I changed lenses and got the tighter vertical composition shortly before the comet’s head dropped out of sight.

And second is the clearly visible anti-tail that was lost in thin haze near the peaks in my other image. An anti-tail is a faint, post-perihelion spike pointing toward the sun in some comets, caused when larger particles from the coma, too big to be pushed by the solar wind, are left behind. It’s only visible from Earth when we pass through the comet’s orbital plane. Pretty cool.

When will the next great comet arrive? No one knows, but whenever that is, I hope I’ve kindled enough interest that you make an effort to view it. But if you plan to chase comets, either to photograph or simply view, don’t forget the wisdom of astronomer and comet expert, David Levy: “Comets are like cats: they have tails, and do precisely what they want.”

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More Gifts From Heaven

Click any image to scroll through the gallery LARGE

Yosemite Dawn, Comet Neowise and Venus from Glacier Point, Yosemite

Solar Return, Total Solar Eclipse, Central Idaho (August 21, 2017)

Heaven Sent, Comet Tsuchinshan–ATLAS Above the Sierra Crest, Alabama Hills

Northern Lights Reflection, Aurora and Glacier Lagoon, Iceland

Comet McNaught, California

Aurora Ribbons, Dyrhólaey Coastline, Iceland

Comet NEOWISE With Ion Tail, Taft Point, Yosemite

Wings of Angels, Aurora Above Dyrhólaey, Iceland

Comet NEOWISE in the Clouds, Navajo Point, Grand Canyon

Northern Lights, Kirkjufell, Iceland

Comet PanSTARRS and New Moon, Haleakala, Maui

Totality, Total Solar Eclipse, Central Idaho (August 21, 2017)

Neon Night, Aurora and Glacier Lagoon, Iceland

Comet NEOWISE and the Big Dipper, Grandview Point, Grand Canyon

Aurora Streaks, Glacier Lagoon, Iceland

Neowise and the Big Dipper Above El Capitan, Taft Point, Yosemite

Green Streak, Aurora and Glacier Lagoon, Iceland

Comet PanSTARRS and the Grand Canyon by Moonlight, Yavapai Point

Swoosh, Northern Lights Over Kirkjufell, Iceland

Comet NEOWISE, El Capitan from Taft Point, Yosemite

Green Curtain, Kirkjufell Aurora, Iceland

Let There Be Light, Planet Earth, Solar System, Milky Way (August 21, 2017)

Nature’s Light Show, Aurora Over Dyrhólaey Coast, Iceland

Comet Tsuchinshan–ATLAS and Mt. Whitney, Alabama Hills, California