The International Astronomical Union (IAU) said this week (September 24) that the orbit of the second suspected interstellar visitor – originally designated C/2019 Q4 – is now sufficiently well known they are are confident the object is:
… unambiguously interstellar in origin.
And thus they’ve now given this object – which is believed to be a comet – a new name. Its name is 2I/Borisov. The “I” stands for interstellar. The “2” means it’s the second such object known to astronomers. And, following a long-standing naming convention for comets, Borisov is the name of its discoverer.
The IAU said in a statement:
On August 30, 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. Mr. Borisov made this discovery with a 0.65-meter telescope he built himself.
After a week of observations by amateur and professional astronomers all over the world, the IAU Minor Planet Center was able to compute a preliminary orbit, which suggested this object was interstellar — only the second such object known to have passed through the solar system …
2I/Borisov will make its closest approach to the sun (reach its perihelion) on December 7, 2019, when it will be 2 astronomical units [AU, or Earth-sun distances] from the sun and also 2 AU from Earth. By December and January, it is expected that [the object] will be at its brightest in the southern sky. It will then begin its outbound journey, eventually [probably] leaving the solar system forever …
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 10.4-meter 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.
Why are astronomers convinced this object is interstellar? The reason is its orbit, which they’ve now tracked long enough and well enough to know that – after it rounds the sun in December – 2I/Borisov will head outward again and (most likely) won’t be returning to our solar system.
Astronomers describe orbits like that of 2I/Borisov as hyperbolic. Astronomers have been discovering weakly hyperbolic comets that were perturbed out of the Oort Cloud since the mid-1800s. They’ve discovered thousands of hyperbolic comets so far. But, of all of these comets, the IAU said:
…none has an orbit as hyperbolic as that of 2I/Borisov. This conclusion is independently supported by the NASA JPL Solar System Dynamics Group. 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.
The IAU also said that astronomers are eagerly observing this object, which will be continuously observable for many months, a period longer than that of its predecessor, 1I/‘Oumuamua. They said:
Astronomers are optimistic about their chances of studying this rare guest in great detail.
Bottom line: The first known interstellar visitor received the official name ‘Oumuamua, meaning ‘scout.’ This one has a less romantic name and one that sets a standard for future discoveries: 2I/Borisov.