These two images, taken by NASA’s Hubble Space Telescope, capture comet 2I/Borisov streaking though our solar system and on its way back to interstellar space. It is only the second interstellar object known to have passed through the solar system. “Hubble gives us the best upper limit of the size of comet Borisov’s nucleus, which is the really important part of the comet,” said David Jewitt, a UCLA professor of planetary science and astronomy, whose team has captured the best and sharpest look at this first confirmed interstellar comet. “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 it 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. Borisov is the first known interstellar comet, and we would like to learn how many others there are.” Crimean amateur astronomer Gennady Borisov discovered the comet on August 30, 2019, and reported the position measurements to the International Astronomical Union’s Minor Planet Center in Cambridge, Massachusetts. The Center for Near-Earth Object Studies at the Jet Propulsion Laboratory, working with the Minor Planet Center, computed an orbit for the comet which shows that it came from elsewhere in our Milky Way galaxy, point of origin unknown. Nevertheless, observations by numerous telescopes show that the comet’s chemical composition is similar to the comets found inside our solar system, providing evidence that comets also form around other stars. By the middle of 2020 the comet will have already zoomed past Jupiter’s distance of 500 million miles on its way back into the frozen abyss of interstellar space. [left] November 16, 2019 photo The comet appears in front of a distant background spiral galaxy (2MASX J10500165-0152029). The galaxy’s bright central core is smeared in the image because Hubble was tracking the comet. Comet Borisov was approximately 203 million miles from Earth in this exposure. Its tail of ejected dust streaks off to the upper right. [right] December 9, 2019 photo Hubble revisited the comet shortly after its closest approach to the Sun where it received maximum heating after spending most of its life in frigid interstellar space. The comet is 185 million miles from Earth in this photo, near the inner edge of the asteroid belt but below it. The nucleus, an agglomeration of ices and dust, is still too small to be resolved. The bright central portion is a coma made up of dust leaving the surface. The comet will make its closest approach to Earth in late December at a distance of 180 million miles. The Hubble Space Telescope is a project of international cooperation between the European Space Agency (ESA) and NASA. NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C. The Minor Planet Center and the Center for Near-Earth Orbit Studies are projects of NASA’s Near-Earth Object Observations Program of the Planetary Defense Coordination Office at NASA Headquarters.

From somewhere else in the Milky Way galaxy, Comet 2I/Borisov was just visiting the Solar System. Discovered by amateur astronomer Gennady Borisov on August 30, 2019, the first known interstellar comet is seen in these two Hubble Space Telescope images from November and December 2019. On the left, a distant background galaxy near the line-of-sight to Borisov is blurred as Hubble tracked the speeding comet and dust tail about 327 million kilometers from Earth. At right, 2I/Borisov appears shortly after perihelion, its closest approach to Sun. European Southern Observatory observations indicate that this comet may never have passed close to any star before its 2019 perihelion passage. Borisov’s closest approach to our fair planet, a distance of about 290 million kilometers, came on December 28, 2019. Even though Hubble’s sharp images don’t resolve the comet’s nucleus, they did lead to estimates of less than 1 kilometer for its diameter.