Pristine means pure and unchanged. Earth still boasts pristine forests, deserts and much untouched seafloor. When astronomers use the word in regard to comets they're talking about an object that's never passed near its host sun or any other star for that matter. Comets in our solar system formed at the same time as the planets 4.6 billion years. In the time since, self-gravity and solar heating have thoroughly processed the raw materials from which the planets formed. On Earth, heating from gravitational contraction and radioactivity literally melted the planet. Heavier materials like iron and nickel "drained" into its core, while lighter rocks floated to the top to form the crust.
Comets are small bodies just a few miles across composed of ice and dust that still preserve the original ingredients of the solar nebula, the cloud of gas and dust that went on to form the solar system. They're often described as leftovers because they were never incorporated into planets like 19th century mountain men holed up in the wilderness.
Numbering in the billions, most have been remained in a deep freeze since the dawn of the solar system. They orbit far, far away in the Oort Cloud, a thick, spherical shell of comets located between 2,000 and 100,000 times Earth's distance from the sun — a quarter to halfway to Alpha Centauri. They're so remote that at its current speed of 11 miles a second, NASA's Voyager 1 space probe won't enter the Cloud for another 300 years. Understandably, it's also very cold there with the temperature hovering just a few degrees above absolute zero (–459.67° F).
A pristine comet is one that has stayed put in the Oort Cloud since its birth, never having dropped into the inner solar system where it would be heated and altered by the sun's heat and radiation. One of the most familiar nearly-pristine comets ever seen is Hale-Bopp, which made a spectacular appearance in 1997 when it was easily visible with the naked eye. Prior to that visit it's thought to have passed the sun just once about 4,200 years ago. Jupiter's gravity shortened the comet's orbit during the recent passage, changing its next return date to 4385 A.D. Who has that kind of time?
Now it appears that astronomers have found a comet even more pristine than Hale-Bopp. In August 2019 amateur astronomer Gennady Borisov discovered a comet bearing his name. A few weeks later, astronomers determined that 2I/Borisov's orbit was hyperbolic, meaning it originated from another solar system. Happenstance directed it toward our own, where it made a brief appearance as the first-known interstellar comet before moving back into the great emptiness.
“2I/Borisov could represent the first truly pristine comet ever observed,” said Stefano Bagnulo of the Armagh Observatory and Planetarium, Northern Ireland. Bagnulo led a new study published recently in Nature Communications. He and colleagues used the European Southern Observatory's (ESO) Very Large Telescope (VLT) to study the comet's color — which provides clues to its composition — and how its dust polarizes light.
Brand new Chevy
Light vibrates in many planes, but polarizing filters, like those used in some sunglasses, only allow light that vibrates in a single plane to pass. This eliminates nasty things like glare and unwanted reflections from road surfaces, water and glass. By studying polarized light astronomers can learn about a comet's chemistry. Based on the data gleaned from their observations the team believes that the comet had never passed close to any star before it flew by the sun in 2019. Like a brand new Chevy it arrived untarnished.
Here's the cool part though. Comet Borisov is unlike any other comet we've seen except Hale-Bopp. Both appear to have remarkably similar compositions and colors which suggests that Borisov's birthplace in a distant, unknown star system isn't so different from the composition of our own early solar system. Amazing, Isn't it, that you can go so far away and still end up recognizing things from home? Perhaps our native-variety comets have distant relatives everywhere in space, from Alpha Centauri to Zubenelgenubi.
"Imagine how lucky we were that a comet from a solar system light-years away simply took a trip to our doorstep by chance, said Bin Yang, another ESO astronomer who studied 2I/Borisov during its flyby. Yang and her team used data from the Atacama Large Millimeter/submillimeter Array (ALMA) as well as the VLT to study 2I/Borisov’s dust grains for clues of its birth and home solar system conditions.
They found that the comet's coma, the envelope of dust and gases that surrounds the main body of the comet called the nucleus, contains compact, pebble-like grains a millimeter and larger as well as water and carbon monoxide ices. The relative amounts of those ices changed radically as Borisov passed near the sun, an indication that the comet is made up of materials that formed in different places in its home planetary system. Giant planets with their strong gravity may have mixed materials in that star's early solar system, the same process that occurred in our own.
Hopefully, we'll learn more about interstellar comets soon. The European Space Agency plans to launch the Comet Interceptor mission in 2029, which will be able to intercept either a pristine Oort Cloud comet on its way sunward or another interstellar interloper should it be on a suitable trajectory.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.