Seven weeks ago, an amateur astronomer scanning the skies from his personal observatory on the Crimean Peninsula discovered a comet that quickly caught the attention of scientists around the world, including here in northern Arizona.
Comet discoveries aren’t that unusual but this particular one stood out because astronomers realized it doesn’t belong to our solar system family, but instead was a visitor from another system. This marked only the second “interstellar” comet (a comet originating outside of our solar system) discovered to date.
Borisov discovered the comet on Aug. 30 and scientists soon realized it was moving much faster than typical comets orbiting our sun. Even more arresting was its eccentricity — essentially a measure of how much its orbit is squished.
The eccentricities of solar system objects can range from 0 (this would be a perfect circle; no objects actually exhibit this) to 1. Anything greater than 1 means a so-called hyperbolic orbit, and by definition that object can’t be orbiting the sun. Earth’s eccentricity is 0.1067, Pluto’s is 0.2488, and that of perhaps the best-known comet, Halley, is 0.967.
This new object exhibits a staggering eccentricity of 3.3, meaning it is interstellar. Astronomers thus christened the object 2I/Borisov, with the “2I” indicating this was the second (2) interstellar (I) comet discovered.
The first interstellar comet-like body was discovered in 2017. Dubbed ‘Oumuamua, it has an eccentricity of 1.2. Unfortunately, it was already on its way out of our solar system when discovered and astronomers didn’t get much of a chance to study it before it disappeared from view.
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2I/Borisov, on the other hand, is on its way into our solar system and will make its closest approach to the sun on Dec. 7, when it will come within twice the distance between the sun and Earth. Unless the comet breaks up, astronomers will be able to view it for a year or so, giving them ample opportunity to characterize it.
This effort is already well underway in Flagstaff. Lowell Observatory comet expert Dave Schleicher said that early observations with the observatory’s 31-inch robotic telescope “showed that it indeed has the appearance of a comet, with a head or "coma" mostly composed of dust coming from the surface of the dirt/ice nucleus.” A
stronomers have also observed it using Lowell’s Discovery Channel Telescope: Lowell post-doctoral fellow Maxime Devogale has obtained spectra of the comet, James Bauer of the University of Maryland has imaged it, and his colleague Matthew Knight has analyzed some of the resulting data and found that cyanogen gas — commonly found in the fingerprint of comets — is present in 2I/Borisov. These and other scientists are now looking for other elements typical of comets.
With further study, astronomers will be able to characterize 2I/Borisov and see how similar its chemistry and other properties are to comets within our solar system. Perhaps even more exciting is the story that 2I/Borisov can tell about its parent solar system.
Comets are celestial time capsules. They are some of the oldest bodies found in a solar system and thus can reveal a lot about the chemical composition and structure of that system when it first formed. This comet may thus allow scientists to compare our solar system to that of 2I/Borisov and begin to see just how unique our system is.