An Interstellar Comet

This Saturday, Dec. 28, a comet from beyond our solar system will make its closest approach to Earth.

Comet Borisov on Dec. 9 NASA, ESA, D. Jewitt (UCLA)

The comet’s icy core is only a mile or so wide, but its surrounding cloud of gas and dust is many times larger than our planet.

Relative size of Earth Relative size of Earth Relative size of Earth Pieter van Dokkum, Cheng-Han Hsieh, Shany Danieli, Gregory Laughlin (Yale)

Just Passing Through

The comet, known as 2I/Borisov, is moving about 20 miles a second, fast enough to slip through the solar system and escape back into interstellar space.

Comet Borisov in October NASA, ESA, J. DePasquale (STScI)

Discovery

The comet was first spotted on Aug. 30 by Gennady Borisov, a comet hunter in Crimea who has discovered nine comets since 2013.

The comet moving across background stars Gennady Borisov

Comet 2I/Borisov is the second interstellar object ever observed inside our solar system, earning it the prefix “2I.”

The alien comet resembles other comets in our solar system, but likely formed around another star before being kicked out into interstellar space.

Comet Borisov in front of a distant spiral galaxy on Nov. 16 NASA, ESA, D. Jewitt (UCLA)

The sun’s gravity will bend the comet’s path, but not capture it. The comet made its closest approach to the sun on Dec. 8.

Comet Borisov on Nov. 12 National Optical-Infrared Astronomy Research Laboratory/Gemini Observatory/N.S.F./AURA

Oumuamua

The first interstellar object found passing through our solar system was 1I/Oumuamua, which was spotted in 2017 already moving away from the sun.

Oumuamua is circled in blue ESO/K. Meech et al.

Oumuamua was thought to be a reddish, cigar-shaped asteroid because it lacked a halo and reflected light in different ways as it turned.

A glimpse of Oumuamua Gemini Observatory/AURA/National Science Foundation

But recent observations suggest Oumuamua might actually be a comet with no tail, accelerating away from the sun with the slight kick of escaping gas.

Astronomers should have many future chances to observe alien comets. Dozens of interstellar objects are thought to pass through the solar system every year, unseen until now.







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Sources: NASA, Gennady Borisov and the Jet Propulsion Laboratory’s Small-Body Database Browser



