Hubble Observes First Confirmed Interstellar Comet

October 16, 2019 10:00AM (EDT) Release ID: 2019-53 Hubble Observes First Confirmed Interstellar Comet

No one knows where it came from. No one knows how long it has been drifting through the empty, cold abyss of interstellar space. But this year an object called comet 2I/Borisov came in from the cold. It was detected falling past our Sun by a Crimean amateur astronomer. This emissary from the black unknown captured the attention of worldwide astronomers who aimed all kinds of telescopes at it to watch the comet sprout a dust tail. The far visitor is only the second known object to enter our solar system coming from elsewhere in the galaxy, based on its speed and trajectory. Like a racetrack photographer trying to capture a speeding derby horse, Hubble took a series of snapshots as the comet streaked along at 110,000 miles per hour. Hubble provided the sharpest image to date of the fleeting comet, revealing a central concentration of dust around an unseen nucleus. The comet was 260 million miles from Earth when Hubble took the photo.

In 2017, the first identified interstellar visitor, an object formally named 'Oumuamua, swung within 24 million miles of the Sun before racing out of the solar system. Unlike comet 2I/Borisov, 'Oumuamua still defies any simple categorization. It did not behave like a comet, and it has a variety of unusual characteristics. Comet 2I/Borisov looks a lot like the traditional comets found inside our solar system, which sublimate ices, and cast off dust as they are warmed by the Sun. The wandering comet provides invaluable clues to the chemical composition, structure, and dust characteristics of planetary building blocks presumably forged in an alien star system.

Comet 2I/Borisov

NASA's Hubble Space Telescope has given astronomers their best look yet at an interstellar visitor — comet 2I/Borisov — whose speed and trajectory indicate it has come from beyond our solar system.

This Hubble image, taken on October 12, 2019, is the sharpest view of the comet to date. Hubble reveals a central concentration of dust around the nucleus (which is too small to be seen by Hubble).

Comet 2I/Borisov is only the second such interstellar object known to have passed through the solar system. In 2017, the first identified interstellar visitor, an object officially named 'Oumuamua, swung within 24 million miles of the Sun before racing out of the solar system. "Whereas 'Oumuamua appeared to be a rock, Borisov is really active, more like a normal comet. It's a puzzle why these two are so different," said David Jewitt of the University of California, Los Angeles (UCLA), leader of the Hubble team who observed the comet.

As the second known interstellar object found to enter our solar system, the comet provides invaluable clues to the chemical composition, structure, and dust characteristics of planetary building blocks presumably forged in an alien star system a long time ago and far away.

"Though another star system could be quite different from our own, the fact that the comet's properties appear to be very similar to those of the solar system's building blocks is very remarkable," said Amaya Moro-Martin of the Space Telescope Science Institute, Baltimore, Maryland.

Hubble photographed the comet at a distance of 260 million miles from Earth. The comet is falling past the Sun and will make its closest approach to the Sun on December 7, 2019, when it will be twice as far from the Sun as Earth is.

The comet is following a hyperbolic path around the Sun, and currently is blazing along at an extraordinary speed of 110,000 miles per hour. "It's traveling so fast it almost doesn't care that the Sun is there," said Jewitt.

By the middle of 2020 the comet will streak past Jupiter's distance of 500 million miles on its way back into interstellar space where it will drift for untold millions of years before skirting close to another star system.

Crimean amateur astronomer Gennady Borisov discovered the comet on August 30, 2019. After a week of observations by amateur and professional astronomers all over the world, the International Astronomical Union's Minor Planet Center and JPL's Center for Near-Earth Object Studies computed a trajectory for the comet, which confirms that it came from interstellar space.

Until now, all cataloged comets have come from either a ring of icy debris at the periphery of our solar system, called the Kuiper belt, or the hypothetical Oort cloud, a shell of comets about a light-year from the Sun, defining the dynamical edge of our solar system.

Borisov and 'Oumuamua are only the beginning of the discoveries of interstellar objects paying a brief visit to our solar system, say researchers. According to one study there are thousands of such interlopers here at any given time, though most are too faint to be detected with current-day telescopes.

Observations by Hubble and other telescopes have shown that rings and shells of icy debris encircle young stars where planet formation is underway. A gravitational "pinball game" between these comet-like bodies or planets orbiting other stars can hurtle them deep into space where they go adrift among the stars.

Future Hubble observations of 2I/Borisov are planned through January 2020, with more being proposed.

"New comets are always unpredictable," said Max Mutchler, another member of the observing team. "They sometimes brighten suddenly or even begin to fragment as they are exposed to the intense heat of the Sun for the first time. Hubble is poised to monitor whatever happens next with its superior sensitivity and resolution."

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.