Astronomers Pedro Bernardinelli and Gary Bernstein made a singular discovery while searching through images from the Dark Energy Survey. They came upon a comet, now called C/2014 UN271, that turned out to be something special. When first photographed in October 2014, it was 2.7 billion miles (4.3 billion kilometers) from the sun — beyond the orbit of Neptune.
Since then it's edged inward to the orbit of Uranus and will come closest to the sun in January 2031, just outside of Saturn's orbit. On a side note, the Dark Energy Survey uses a large telescope in Chile to map remote galaxies and galaxy clusters with the goal of determining the large-scale structure of the universe and the motions of the galaxies that inhabit it. All the images are scanned in search of moving objects like comets and asteroids.
Several things make 2014 UN271 exceptional. Even at its tremendous distance, where temperatures dip to 350 degrees below zero Fahrenheit (212 degrees below Celsius), it's fuzzy. And "fuzzy" means that ice on the comet's surface is vaporizing to create a temporary atmosphere called a coma. Water ice, the primary ingredient of many comets, will only fizz away in the relative warmth of the inner solar system. Ices made of carbon monoxide and carbon dioxide, which vaporize more readily in cold temperatures, are responsible for its current appearance. Still, it's unusual to see an active comet at such a great distance.
Even more amazing, this one's prodigious! Based on its current brightness of 20th magnitude, paired with its known distance, astronomers estimate that 2014 UN271 is around 100 miles (160 kilometers) across, making it the largest comet known. It more resembles a moderately-large asteroid than a typical comet, most of which are just a couple kilometers across. On a level path, you could walk completely around a mile-wide comet in just three hours.
2014 UN271 dwarfs Comet Hale-Bopp, one of the best known and largest comets, with an icy core about 37 miles (~60 kilometers) across. It also eclipses the previous record-holders: Comet Sarabat of 1729 (62 miles / 100 kilometers) and C/2002 VQ94 (LINEAR) (60 miles / 96 kilometers).
Long time coming — and going
But its orbit is what I find most mind bending. Prior to looping back toward the sun, 2014 UN271 resided in the Oort Cloud approximately 40,000 times the Earth-sun distance from the sun or 0.6 light-years away. When an object is closest to the sun astronomers say it's at perihelion. The farthest point in its orbit is called aphelion. It took this comet 1.4 million years to travel the long road from aphelion to its present location.
After swinging near Saturn in 2031, gravitational interaction with the planet will modify its orbit, increasing aphelion to 55,000 times the Earth-sun distance or nearly one light-year. This will swell 2014 UN271's travel time to the Oort Cloud to 2.2 million years! That's insane.
To compare, consider Halley's Comet. It returns about every 76 years on an orbit that takes it just outside that of Uranus. Even Pluto, the coldest place most of us can think of, takes only 248 years to circle the sun.
The only potentially bad news about the new discovery concerns what you'll need to see it. While large, it's not expected to become a naked-eye object because it will still be relatively far away even when closest. Most likely, 2014 UN271 will glow dimly around magnitude 13 and require an 8-inch or larger telescope to view. Maybe the hardest part is staying alive long enough to behold it in the first place. 10 years. Hmm. That's a long time from now. I'll hold out hope, the same way I did in the early 1980s when I anticipated seeing Halley's Comet.
Care for a strawberry moon on that ice cream?
Moving closer to home, tonight (June 24), the Full Strawberry Moon will rise a little after sunset. My forecast is clear, so I'll be down on the shore of Lake Superior to watch it rise. Whether I'm having a bad day or a good one a moonrise always improves my outlook.
The name refers to the strawberry harvest which traditionally takes place in June. The moon will shine forth from Sagittarius the Archer and pass VERY close to 3rd magnitude Phi (Φ) Sagittarii around 12:45 a.m. CDT. Here in Duluth, Minnesota, they'll be less than 0.2 degrees apart. From farther south, their separation increases slightly, while farther north in Canada, it decreases. In the Eastern Time Zone, closest approach occurs around 1:30-2 a.m., 11:30 p.m. Mountain and 10:30 p.m. Pacific.
You won't see the near-miss with the naked eye because of the moon's brilliance. Use binoculars or a small telescope. To make sure you don't miss the moment the moon edges over the horizon, click here for your local moonrise time.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.