Visitors from other planetary systems don’t come around every day. Here’s what you should know.

On August 30, 2019, an amateur astronomer from Crimea named Gennady Borisov discovered the very first interstellar comet. That comet is now called 2I/Borisov. The name “Borisov” comes from the name of its Crimean discoverer. The “2I” in the name refers to it being the “second interstellar” object that has been identified entering into the solar system.

Not the first interstellar visitor

The first interstellar object that entered our solar system was named Oumuamua, which is a Hawaiian word that means “messenger from afar arriving first” of “scout.” Oumuamua entered our solar system in 2017. It was discovered by Robert Weryk at Haleakala Observatory in Hawaii. When Weryk discovered it, it was approximately 22,000,000 miles from Earth and heading away.

Oumuamua was an object between 100 and 1,000 meters long, and between 35 and 156 meters thick. It did not display the fuzzy atmosphere typical of a comet when seen in a telescope and was considered a possible remnant of a disintegrated rogue comet. Oumuamua wasn’t considered a proper comet, but “‘the husk of a comet’ with all its ices baked out.” Turns out those theories that Oumuamua was an alien probe or alien sail weren’t correct.

NASA Goddard Space Flight Center

2I/Borisov is unique

Yale University astronomers at Hawaii’s Keck Observatory captured a photograph of 2I/Borisov on November 24, 2019. The photograph was posted on the observatory’s Instagram page with an image of Earth superimposed on it for scale. Observers calculated that Borisov’s solid nucleus is about a mile wide, and that it has a tail of gas and dust that is nearly 100,000 miles long. That tail of gas and dust is fourteen times the diameter of the planet Earth.

Borisov is no “husk” of a comet, and has the fuzzy atmosphere characteristic of comets on film that is called the “coma.” Planetary systems have discs of gas and dust that tend to accumulate. If they accumulate in really rocky clusters, they’re asteroids. If they accumulate in really icy clumps, they’re comets. The frozen gases trapped in comets warm up and boil off into space as the comet gets closer to the Sun. That, combined with dust, makes the “coma” around the comet.

How close will it get to us?

On December 28, 2019, Borisov is going to get as close to Earth as it’s going to get. It’ll be moving at a rate of about 33 kilometers per second or nearly 74,000 miles per hour. Start packing the kids! Go get batteries!

Just kidding. Seventy-four thousand miles per hour is fast, but even at its closes point, Borisov is going to be approximately 300,000,000 kilometers (186,411,358 miles) away. At that distance, it’ll still be beyond the orbit of the planet Mars. Check out for information about how and where to find Borisov in the sky.

There could be water on board

Borisov has a tail of gas and dust that is nearly 100,000 miles long. That tail of gas and dust is fourteen times the diameter of the planet Earth.

Scientists at Apache Point Observatory in New Mexico have studied light reflected by Borisov. Their analysis of that reflected light revealed large amounts of oxygen. They theorize that the oxygen may be being released by water changing from solid to gas as it warms up nearer the Sun. How much water? Estimates are that Borisov produces 19 kilograms (42 pounds) of water every second.

The discovery that there is water outside our solar system is not new, but this is the closest that water from another planetary system has ever gotten this close. As Alan Fitzsimmons of Queen’s University in Belfast, UK says: “This would be the first package of water from another planetary system.” There are other possible explanations for the presence of oxygen, but the possibility of water aboard or within Borisov is an exciting one. “That has implications for the origin of life, and how common life is throughout the universe.”

A deeper dive — Related reading from the 101:

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