Hubble catches interstellar visitor speeding past the Sun

The comet 2l/Borisov has been observed by the Hubble Space Telescope as it speeds past the Sun on its way out of the solar system.

The Hubble Space Telescope has captured stunning images of the interstellar comet 2I/Borisov as it hurtles past the Sun on its journey out of the solar system and back to interstellar space. Not only is Borisov just the second interstellar visitor to be spotted in the solar system, but travelling at

175,000 km/h it is the fastest comet ever observed.

Hubble first spotted the comet in October 2019 at a distance of around

420 million km from Earth. The new observations from November and December snap Borisov at a closer distance and thus exposes the comet’s characteristics in finer detail.

Comet 2l/Borisov against the galaxy 2MASX J10500165–0152029 In this image taken by the NASA/ESA Hubble Space Telescope in November 2019, the comet appears in front of a distant background spiral galaxy. The galaxy’s bright central core is smeared in the image because Hubble was tracking the comet. Borisov was approximately 326 million kilometres from Earth in this exposure. Its tail of ejected dust streaks off to the upper right. NASA, ESA, and D. Jewitt (UCLA)

The first image (above) shows Borisov framed against the distant spiral galaxy 2MASX J10500165–0152029 — its central core blurred because Hubble was tracking the comet. In this particular image, Borisov is approximately 326 million km from Earth — its trail of ejected dust can just about be made out streaking towards the upper right-hand corner.

In December 2019, Hubble revisited the comet shortly after its closest approach to the Sun. The comet is 298 million kilometres from Earth in this photo, near the inner edge of the asteroid belt. 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. NASA, ESA, and D. Jewitt (UCLA)

In this second image, the comet — now 298 million km from Earth, the inner edge of the asteroid belt — is captured by Hubble upon its closest approach to the Sun. At this near approach, it experiences a greater degree of heating than it has been exposed to before in the extreme cold of interstellar space. The nucleus — made up of ice and dust — is still too small to be resolved by the telescope. 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 when it will be at a distance of 290 million kilometres.

“Hubble gives us the best measure of the size of comet Borisov’s nucleus, which is the really important part of the comet,” explains David Jewitt, a professor of planetary science and astronomy at the University of California Los Angeles, whose team has captured the best and sharpest images of this first interstellar comet. “Surprisingly, our Hubble images show that its nucleus is more than 15 times smaller than earlier investigations suggested it might be. The radius is smaller than half a kilometre.

“This is important because knowing the size helps us to determine the total number, and mass, of such objects in the Solar System, and in the Milky Way. Borisov is the first known interstellar comet, and we would like to know how many others there are.”

Side By Side of Hubble’s New 2I/Borisov Observations. NASA, ESA, and D. Jewitt (UCLA)

2l/Borisov is named for its discover, Crimean amateur astronomer Gennady Borisov who discovered the comet on 30 August 2019. This initial discovery was followed by a week of observations by amateur and professional astronomers all over the world. The International Astronomical Union’s Minor Planet Center was able to calculate an orbit for the comet demonstrating its origins from interstellar outside the solar system (below).

This animation presents the orbit of comet 2I/Borisov — only the second interstellar object known to have passed through our Solar System. ESA/spaceengine.org/L. Calçada

Pre-Borisov, all the comets we have spotted have either come from the Kupier belt — a ring of icy debris at the edge of the solar system, or from the Oort cloud — a shell of icy objects believed to lurk at the outermost regions of our Solar System, its innermost edge at about 2000 times the distance between the Earth and the Sun.

Astronomers suspect that 2I/Borisov may represent the first of a series of discoveries of interstellar objects paying a flying visit to our Solar System. Theory suggests that there may be thousands of such interstellar objects in the solar system at any given time. Despite this abundance, spotting these interstellar visitors is extremely difficult as they are too faint to be detected with present-day telescopes.

Observations by Hubble and other telescopes have shed some light on the possible origins of such interstellar objects. In rings and shells of icy debris spotted encircling young stars where planet formation is underway, a gravitational interaction between these comet-like objects and other massive bodies could possibly cause these objects to be hurled deep into space — leaving them adrift amongst the stars.