spaceSpace and Physics

Welcome 2I/Borisov, Officially The Second Interstellar Visitor Of The Solar System


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

clockSep 24 2019, 18:37 UTC

2I/Borisov in all its glory. Gemini Observatory/NSF/AURA

The International Astronomical Union has confirmed that the comet spotted a few weeks ago by amateur astronomer Gennady Borisov from MARGO Observatory, Crimea, is indeed coming from interstellar space. This is only the second known interstellar object to pass through our Solar System (the first was ‘Oumuamua).

Since it was first observed on August 30, astronomers have been tracking the comet's orbit in detail. It is now clear that the comet is a visitor from another star. The Union has also officially given it a name: 2I/Borisov. 2I stands for second interstellar object and, given that it is clearly a comet, they kept the tradition of naming it after the discoverer.


2I/Borisov was discovered when it was over 400 million kilometers (250 million miles) from the Sun – and it is only getting closer. It will reach its aphelion on December 8, when it will be roughly 300 million kilometers (190 million miles) away. That’s about twice the Earth-Sun distance. It is moving with a speed of about 150,000 kilometers (93,000 miles) per hour and coming toward the inner Solar System at a 40-degree angle with respect to the orbital plane of the Earth. Its orbit, orientation, and the fact that the comet is much faster than any known Solar System object at that distance were the initial clues that led researchers to suspect the comet was interstellar.

The Gran Telescopio Canarias has obtained the light spectrum of the object, from which they were able to establish its composition. It turns out that it isn’t much different from comets that formed in the Oort Cloud, the region far beyond the orbit of Pluto. Formations of comets in other star systems are likely to be similar to our own.

Astronomers have not yet been able to measure its size precisely but current estimates from NASA put the comet's nucleus at between 2 and 16 kilometers (1.2 and 10 miles) across. While this is only the second known interstellar object to pass by, it likely won’t be the last. Estimates suggest that at any given point there is at least one within the orbit of Mars. Unfortunately, most of them are too small and faint to be discovered.

This illustration depicts Comet C/2019 Q4's trajectory. NASA/JPL-Caltech

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