Comet on return journey to interstellar space after solar slingshot
Currently, there’s an interstellar visitor passing through our solar system. 2I/Borisov is the first interstellar comet ever seen, and Hubble has been watching its journey through our neighborhood. The space telescope has snapped new images of the comet, which has now swung past the Sun and is on its way back out of the solar system.
Borisov was discovered on August 30, and soon after that its trajectory was calculated – revealing that it wasn’t a local. The comet had come from another star system, making it only the second interstellar object to have ever been discovered. The first was the asteroid ‘Oumuamua, which dropped by in 2017.
As Borisov made its way towards the inner solar system, astronomers from around the world turned telescopes and observatories towards it to study this rare interstellar interloper. Among the things they learned was that it had a strangely similar composition and age to local comets.
Now Hubble has snapped two new images of Borisov, one as it was approaching the Sun, and the other just after it swung past our star and began its journey back out into the depths of interstellar space.
These shots allowed the team to calculate the size of its nucleus – the solid chunk of ice and dust in the very center. They found that it was actually much smaller than expected, measuring just 3,200 ft (975 m) wide.
"Hubble gives us the best upper limit of the size of comet Borisov's nucleus, which is the really important part of the comet," says David Jewitt, lead researcher on the team. "Surprisingly, our Hubble images show that its nucleus is more than 15 times smaller than earlier investigations suggested it might be. Knowing the size is potentially useful for beginning to estimate how common such objects may be in the solar system and our galaxy. Borisov is the first known interstellar comet, and we would like to learn how many others there are.”
In the two photos, Borisov has been artificially colored blue to make it stand out better. The image on the left was taken on November 16, when the comet was about 203 million miles from Earth. It can be seen streaking past a distant galaxy in the background.
The photo on the right was snapped on December 9, when the comet was around 185 million miles from Earth. At this point, it had already whizzed past the Sun and was on its way back out, approaching the asteroid belt. It had also gained a speed boost from this slingshot effect, shooting away at about 100,000 mph.
2I/Borisov is expected to make its closest pass to Earth in late December, at a distance of 180 million miles. After that it will zip away from us, never to return. Astronomers will no doubt continue to watch it go – by mid-2020 it will have reached Jupiter’s orbital distance, and eventually it will vanish into the inky blackness from whence it came.