For only the second time, astronomers believe they have detected a space rock that formed in some distant system before making the interstellar journey to fly through our own solar system. The object, a comet named C/2019 Q4 (Borisov), was recently verified by the Minor Planet Center. According to available observations of the comet, C/2019 Q4 is moving too fast, some 30.7 kilometers per second (68,700 miles per hour), to have origininated in our solar system.
The likely interstellar comet was first observed by Gennady Borisov, a Ukrainian amateur astronomer working at the Crimean Astrophysical Observatory, on August 30. The object is still inbound, and it will make its closest approach to the sun on December 7, and its closest approach to Earth—within 180 million miles—on December 29, as reported by Michael Greshko at National Geographic. Further observations by astronomers have determined that C/2019 Q4 is a comet stretching a couple miles wide, with a coma of gas and dust enveloping the object that forms when icy material is heated by the sun. (Asteroids have less icy material and do not develop comas.)
In addition to C/2019 Q4’s great speed, the object is on a hyperbolic trajectory through the solar system, meaning rather than circling the sun, it will fly in close—almost as close to the sun as Mars—and then sling back out into interstellar space. Astronomers use a measurement called eccentricity to determine how circular an object’s orbit is: An eccentricity of 0 is perfectly circular, while an eccentricity of 1 is highly elliptical, and anything greater than is hyperbolic. C/2019 Q4’s eccentricity is greater than 3.5, according to NASA JPL’s Small-Body Database.