Comet 2I/Borisov, the second interstellar object ever discovered, made its closest pass to the Sun on December 8, about 300 million kilometers (186 million miles) from our star. Given Earth's position currently around the Sun, the comet is about 185 million kilometers (115 million miles) away and this proximity allowed Hubble to take some new and exciting observations.
The Hubble Space Telescope is one of several observatories both in space and on Earth that have been keeping track of this fascinating object, and it has been offering up some fantastic views of the comet that let us calculate information about it.
The latest data suggests that the previous estimates of the comet's nucleus, between one and several kilometers, may have to be corrected.
"Hubble gives us the best upper limit of the size of comet Borisov's nucleus, which is the really important part of the comet," David Jewitt, a UCLA professor of planetary science and astronomer who took the images with Hubble, said in a statement.
"Surprisingly, our Hubble images show that its nucleus is more than 15 times smaller than earlier investigations suggested it might be. Our Hubble images show that the radius is smaller than half a kilometer."
"Knowing the size is potentially useful for beginning to estimate how common such objects may be in the Solar System and our galaxy," Jewitt added. "Borisov is the first known interstellar comet, and we would like to learn how many others there are."
The findings are published in a yet-to-be-peer-reviewed article posted on the ArXiv site. Other studies yet to be reviewed have also been released on Arxiv, including one study suggesting that the tail of the comet is surprisingly water ice poor, at least before getting very close to the Sun. Another one suggests a slightly wider size for the nucleus at 1.5 kilometers (0.93 miles|). This new data may help us narrow it down further.
Hubble also caught 2I/Borisov on November 16, as it appeared in front of spiral galaxy 2MASX J10500165-0152029. The galaxy's core looks smeared as Hubble was tracking the moving comet, which was around 203 million miles from Earth at the time.
The first known interstellar visitor, 'Oumuamua, was discovered passing through our Solar System in October 2017. However, we are still not sure what kind of object it is, as it's unlike anything we've seen before. It is most likely an asteroid as it doesn't have any of the classic traits of a comet, such as a tail. Borisov, on the other hand, is remarkably similar to comets that have originated from within our Solar System, suggesting that the formation mechanism of comets is likely to assemble in similar ways around different stars.
The comet was discovered on August 30, 2019, by Crimean amateur astronomer Gennady Borisov. Its orbit and speed was a clear hallmark that it did not form around the Sun.
The comet will get closest to Earth in the next couple of weeks – just 180 million kilometers (112 million miles) from us – before continuing on its orbit, which will take it back out of the Solar System.
ESA/spaceengine.org/L. Calçada (music: Johan B. Monell)