As you read this, Comet Leonard is racing towards the Sun. It’s visible before sunrise with binoculars and small telescopes, and there’s a good chance it will become visible to the unaided eye in early December. That’s not a certainty because comet prediction is even less precise than weather prediction. But I agree with Winston Churchill that it’s not much use to be anything other than an optimist. I like to think that Leonard won’t disappoint.
Comets are leftover bits from the formation of our solar system. They are giant dirty snowballs, typically 6 to 10 miles in diameter, composed of ice, dust, and rock. When the solar system was young, about 4.5 billion years ago, comets were everywhere. They regularly struck Earth, doing great violence but also delivering huge quantities of water. As the planets settled into their orbits, comets largely disappeared from the inner solar system. Impact destroyed some; gravitational interactions with the planets flung many more into the Sun or to a distant region called the Kupier belt and the even more distant Oort cloud, where they remain.
Two trillion comets lurk out there along with hundreds of dwarf planets like Pluto and Eris. Even so, the Kupier belt and Oort cloud are dark and lonely places. The distances between these bodies defy human imagination. A year for an Oort cloud comet — one orbit around the Sun — can span tens of thousands of Earth-years.
If you could stand on the surface of one of these bodies, the Sun would look like any other star. At such distances the Sun’s gravitational grip is as tenuous as its light; it doesn’t take much to disturb the delicate balance of a comet’s orbit. A chance alignment with a giant planet like Jupiter or Neptune, or a star passing “close” to our solar system at a distance of three or four light years, or the faint-but-persistent tidal force of the Milky Way galaxy itself can shake loose a comet and send it tumbling into the inner solar system.
That trip can take a long time. Short-period comets have orbits of 200 years or less. Halley’s comet is the most famous of these; it passes by every 76 years. (I remember its last visit in 1986 and am looking forward to its return in 2061.)
Then there are long-period comets with orbits greater than 200 years. Leonard is one of these, with an orbit of 80,000 years. The last time Leonard swung through the inner solar system, humans were hunter-gatherers, living in scattered bands. We had no writing and no telescopes. Who knows if people were into stargazing back then?
Greg Leonard, a scientist at Mt. Lemmon Observatory in Arizona, discovered the comet on Jan. 3, 2021. It then received its official name: C/2021 A1 (Leonard). It appeared as a faint smudge in optical instruments, crossing the orbit of Jupiter, inward bound, and gaining speed every moment. Ten months later, Leonard has crossed the orbit of Mars and will pass Earth on Dec. 12 at a distance of 21 million miles. Perihelion, its closest approach to the Sun, will occur on Jan. 3, 2022.
When a big snowball like a comet gets closer to the Sun, its outer layers of ice vaporize, releasing trapped dust and rocks. The water vapor and other gases glow as the Sun’s radiation ionizes them, while the dust particles reflect sunlight; a trail of debris streams behind the comet. As the comet gets closer to the Sun, we can begin to see a bright glow called the coma surrounding the comet’s nucleus, with a long shimmering tail behind it.
If all goes well and Leonard continues to brighten, there will be a very short window of time to see it from now until Dec. 12. Bundle up and head outside well before sunrise to a location with a clear view to the east — 6 a.m. is a good time. You’ll see a bright orange star low in the sky; that’s Arcturus. On the morning of Monday, Dec. 6, the comet will be directly to the left of Arcturus. Before that date it will be to the left and above. After Dec. 6, it will be to the left and below. Look for a pale smudge, larger than a star; that’s the coma. If we’re lucky, we may see Leonard’s tail.
Check the internet for the latest news on Leonard’s predicted brightness, precise position, and the early morning weather forecasts, then seize the day. After Dec. 12, Leonard will become an early evening object instead of an early morning one, but it will be much more difficult for observers in the Northern Hemisphere to see. Then it will fade quickly.
If you’re thinking that it sounds like too much of a bother to see Leonard, and maybe you’ll catch it next time, 80,000 years from now, I have bad news for you: Leonard will never return. Its orbit is hyperbolic, which means that it has exceeded solar escape velocity, the speed necessary to escape the Sun’s gravity and soar off into interstellar space, leaving us behind forever.
Leonard’s story will not end, however, with its exit from the solar system. The comet will certainly spend millions of years alone. But sooner or later it will approach the vicinity of another star, whose gravity will gently tug on it, altering its trajectory and drawing it closer. Eventually, Leonard will plunge into that other solar system. It may pass planets like our own and delight observers like ourselves, intelligent beings able to contemplate the universe, before swooping past that other star and continuing on its long, lonely journey.
We know this will be Leonard’s story because of recent observations of extra-solar comets, that is, comets ejected from other solar systems, hurtling through our own at astonishing speeds, swinging around the Sun, and moving on. The first ever documented was ʻOumuamua in 2017. Its Hawaiian name roughly translates as “first distant messenger,” and was chosen by the discovery team at Haleakala Observatory in Hawaii. The second was 2I/Borisov, discovered in 2019 by Crimean amateur astronomer Gennadiy Borisov. Astronomers are confident that these were not the first interstellar visitors, nor the last.
What a story to be part of. I’ll be out looking for Leonard on any promising morning in the coming weeks; I hope you will be, too. Clear skies!
Leonard has its own Twitter feed, @comet2021a1, or follow its trajectory at skyandtelescope.org.
