This week, AOP PhD Student Zuri Grey tells us all about Comet 2I/Borisov!

1) What is the difference between an interstellar comet and a regular comet?

Solar System comets are believed to have come from two sources. Short period comets, which take less than 200 years to complete an orbit around the Sun, originate from the Kuiper Belt. The Kuiper Belt is a disk-shaped region past the orbit of Neptune, sitting roughly between 30 to 100 AU from the Sun. It contains millions of icy bodies including cometary nuclei. The orbits of these objects can be drastically altered by close encounters with planets – so much so that they may be nudged into orbits with much shorter periods, even to those of just a few years. 

On the other hand, long-period comets are believed to originate from the Oort Cloud and take over 200 years to complete an orbit around the Sun. The Oort Cloud marks the very outer fringes of the Sun’s gravitational domain and is believed to be home to trillions of cometary and planetesimal bodies, chunks of leftover primeval material that formed the Sun and the planets. The inner limits of the Oort Cloud begin at approximately 2,000 AU from the Sun and is thought to occupy the vast space up to as far as 50,000 AU. Unlike the Kuiper Belt, the bodies within this neighbourhood are too far away to feel the gravitational effects of the planets. It is therefore believed that long-period comets are comets which have been disturbed and knocked out of the Oort Cloud by the passage of a nearby star.

Animation Credit: Zuri Gray

An interstellar comet is a rogue comet that is not gravitationally bound to a star. In other words, it isn’t orbiting continuously around a single star – rather it is travelling freely through space and, by sheer chance, has zipped through our Solar System. By looking at its orbit, there are two ways in which we can tell that comet 2I/Borisov is interstellar: its trajectory and its velocity. Unlike the highly elliptical orbits of short- and long-period comets, 2I/Borisov has a strong hyperbolic trajectory and is travelling too fast to become trapped in the Sun’s gravitational well. In fact, 2I/Borisov has used the Sun’s pull as a gravitational slingshot to fling itself faster and further out of our Solar System. We will most likely never see it again. 

Animation Credit: Zuri Gray

2) What is the chemical make-up of 2I/Borisov?

Results suggest that 2I/Borisov is, in fact, not largely dissimilar to ‘normal’ Solar System comets except for a few subtle differences. The gas/dust and the relative amounts of cyanide (CN) and diatomic carbon (C2) found in 2I/Borisov are consistent with the bulk population of Solar System comets previously measured. However, as 2I/Borisov approached the Sun, a high carbon-monoxide (CO) to water ratio was observed in its coma, a cloud of dust that forms around the comet nucleus as surface ices sublimate. There is a huge variation in the concentration of CO in comets and it is thought to be related to the place in which the comet was formed and/or how many times a comet has orbited the Sun (or it’s relative star). This clue suggests that the comet was formed in a place very rich in CO ice, which is only present at the lowest of temperatures found in space. Additionally, studies of the comet’s coma conducted by researchers at AOP indicate that the dust particles are predominantly compact “pebbles” ~1mm in size. These characteristics tell us that 2I/Borisov’s birthplace is different to that of the comets in our own Solar System. 

3) Will we be able to see 2I/Borisov in our night sky?

No, unfortunately 2I/Borisov has already passed its perihelion (closest approach to the Sun) and we will most likely never see it again. 

4) Do we know where 2I/Borisov came from?

2I/Borisov entered the Solar System from the direction of the Cassiopeia constellation, telling us that it came from the direction of the galactic plane (the plane on which most of the stars in the Milky Way lie, the “galactic Kuiper Belt”, one could say), rather than from the galactic halo (the “galactic Oort Cloud”). Since it’s likely that its past trajectory was affected by the gravitational pull of other massive bodies, it’s difficult to determine the exact origin of our interstellar visitor.

5) Where is the comet going?

Calculations of its current trajectory indicate that 2I/Borisov will leave the Solar System in the direction of the Telescopium constellation. Again, it’s difficult to determine the comet’s destiny since it’s likely the comet’s trajectory will be perturbed again and again in its path through the galaxy. 

6) How did 2I/Borisov get its name?

There are a number of ways in which comets are named, but they are generally named according to their discoverer, their orbit and the number of comets that are already in their orbital family. 2I/Borisov gets its name from its discoverer, Gennady Borisov, the “I” indicates it is an interstellar object and the “2” tells us that it is only the second interstellar object ever discovered. The first interstellar object discovered is 1I/’Oumuamua.

7) Will we ever see an interstellar comet in our solar system again?

Yes! There are several powerful telescopes currently in the making, including the Vera Rubin Observatory that will photograph the entire night sky every few nights. It is expected to detect about one interstellar object per year! 


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