Comet Borisov
A second interstellar interloper has been discovered within the Solar System, but it won’t be hanging around
A second insterstellar interloper has been discovered
Some mysteries have been perplexing astronomers for some time, while others have arrived on the scene only recently. In the case of the Solar System's newest guest, the latter is most certainly true. It was 30 August 2019 when amateur astronomer Gennadiy Borisov pointed his telescope skywards in Crimea, and what he saw that night not only amazed him, but sent fellow scientists scurrying for a look.
Borisov's observation was extraordinary. “It was the most exciting discovery of 2019,” exclaims
Paul Kalas, adjunct professor of astronomy at the University of California, Berkeley. What he discovered that night, it transpired, was an interstellar comet which has become known as Comet 2I/Borisov. But exactly where it came from and what it tells us about
our universe is now the subject of much study as astronomers around the world seek answers.
Interstellar visitors are rarely discovered. In fact, in the whole of space history we have only ever seen one before – the odd, cigar-shaped 'Oumuamua, which made its closest approach to Earth in October 2017. Comet Borisov's trajectory was such that astronomers could tell it had come from outside the Solar System. As you may expect, this led to some wild speculation.
“We can't rule out that this is an interstellar probe,” claimed Dr Seth Shostak, a senior astronomer at the SETI Institute in California, shortly after the discovery, hinting at an alien origin. And yet further analysis says we can. “It looks like a comet. It acts like a comet. It is clearly not orbiting the Sun,” says Max Mutchler, a research and instrument scientist from the Space Telescope Science Institute in Baltimore. “So until there is evidence of anything else, it is a comet.”
Not that this makes it any less interesting. The same speculations about alien craft were raised following the discovery of 'Oumuamua, and while they have since been dispelled, it remains a fascinating object. “An interstellar comet is plenty exciting without anyone heaping wild speculation on top of it,” Mutchler tells us. To that end, much of the work is focused on tracing Borisov's trajectory and discovering its composition as a comet. Time, however, is of the essence. Since the comet is now travelling at 177,028 kilometres (110,000 miles) per hour, studies have to be fast. One thing's for sure, Comet 2I/Borisov's visit to the Solar System is going to be fleeting.
So what do we know so far? Well, it would appear that the comet is more ordinary than first imagined. According to astronomers working across Europe, Borisov closely resembles comets that have originated within the Solar System – those which come from the Kuiper Belt in the region inhabited by Pluto or those from the Oort Cloud of icy objects situated a light year from our Sun.
Indeed, like home-grown comets Borisov has a reddish, carbon-rich surface, and it also produces some 170 grams of cyanide gas every second, caused by sunlight warming the ice that has formed across the comet. Further research suggests that the core beneath its halo of dust and gas could be anywhere between 1.4 to 6.6 kilometres (0.9 to 4.1 miles) across – again, not uncommon.
Even so, it is different to the previous interstellar interloper. “It has the features of a comet – a coma and a tail of dust – which require an ice-rich composition combined with strong heating as it approaches a star, whereas 'Oumuamua didn't show any comet-like features,” Kalas tells us of the fundamental difference.
“It's thought 'Oumuamua was probably like many other asteroids and Kuiper Belt objects in our Solar System, except that it was not gravitationally bound to our Sun.” Having two different types of object come in from outside the Solar System, however, makes the discovery of Comet 2I/Borisov extra exciting.
Astronomers are certainly fortunate to have spotted the interstellar comet on its approach to Earth rather than when it was already here, as with 'Oumuamua. Indeed, Borisov is due to pass about 306 million kilometres (190 million miles) from our planet in December 2019, according to NASA, and it's not due to leave until mid-2020. That's a huge amount of time in which astronomers can carry out their studies. Hubble is among the instruments being used for observations.
“The beautiful images we saw in October were the first observations made with the Hubble Space Telescope, and it delivers exquisite sharpness,” says Kalas. “With this data we can see that the coma [the dust surrounding the nucleus of the comet] has an hourglass shape, which has also been found in