A Tail of Two Comets

On 30 August 2019 the amateur astronomer Gennady Borisov, from MARGO observatory, Crimea, discovered an object with a comet-like appearance. The object has a condensed coma, and more recently a short tail has been observed.

Its orbit is now sufficiently well-known, and of the thousands of comets discovered so far, none has an orbit as hyperbolic as that of this one. This suggests that this object is interstellar — only the second such object known to have passed through the Solar System.

Mr. Borisov made this discovery with a 0.65-metre telescope he built himself. In this case, the IAU (International Astronomical Union) has decided to follow the tradition of naming cometary objects after their discoverers, so the object has been named 2I/Borisov. The indication "2I" in its name stands for "2nd Interstellar Object."


Coming just two years after the discovery of the first interstellar object 1I/‘Oumuamua, this new finding suggests that such objects may be sufficiently numerous to provide a new way of investigating processes in planetary systems beyond our own. There are, however, huge differences between these two objects. In fact, the only similarity between the two is that they are both interstellar.

It has not been possible to show conclusively that 1I/‘Oumuamua is a comet and studies suggest that it is neither an asteroid nor a comet. This has led some astronomers, such as professor Avi Loeb, to believe that a non-natural origin could not be excluded. They claim that 1I/‘Oumuamua has some characteristics of a device manufactured by intelligent creatures, such as an alien light sail.

The big question is what we can learn from this new discovery about its predecessor 1I/‘Oumuamua. One thing it tells us is that a comet that visits us from another solar system can be exactly the same as our own well-known comets. This is an observation that only seems to deepen the mystery surrounding the origin of 1I/‘Oumuamua.

2I/Borisov will make its closest approach to the Sun on 7 December 2019, when it will be 2 astronomical units (AU) from the Sun and also 2 AU from Earth. By December and January it is expected to begin its outbound journey, eventually leaving the Solar System forever.

Astronomers are optimistic about their chances of studying this rare guest in great detail as it will be continuously observable for many months, a period much longer than that of 1I/‘Oumuamua. Estimates of the sizes of comets are difficult because the small cometary nucleus is embedded in the coma, but, from the observed brightness, 2I/Borisov appears to be around a few kilometers in diameter. One of the largest telescopes in the world, the 10.4m Gran Telescopio Canarias in the Canary Islands, has already obtained a spectrum of 2I/Borisov and has found it to resemble those of typical cometary nuclei.

This new interstellar visitor raises intriguing questions: Why have interstellar objects not been discovered before? What is the expected rate of their appearance in the inner Solar System? How do such objects compare to similar bodies within the Solar System?