On Aug. 30, 2019, Gennady Borisov, an amateur astronomer in Crimea, spied the distant fuzzy object using a homemade 213-foot (0.65-meter) telescope, an incredible discovery that underscores the key role nonprofessional astronomers play in historic astronomical discoveries. After followup sightings and confirmation by other amateur and professional astronomers, the comet — which was initially designated C/2019 Q4 — was soon confirmed to be not from 'round these parts.
Calculations of its orbit around the sun proved that it had an extremely hyperbolic trajectory, a path that meant it could not be gravitationally bound to our star. The ancient object had originated far beyond our solar system's shores, ejected from another star system located elsewhere in the galaxy.
Typically, newly discovered comets have trajectories that betray their solar orbits and therefore make their origins clear. Long- and short-period comets all have elliptical orbits that can be nearly circular, or extremely elongated; their orbital speeds reveal that they originate from the ancient icy debris that is scattered in our solar system's hinterlands, maybe in the Kuiper belt (beyond the orbit of Neptune), or as far away as the hypothetical Oort Cloud (a vast swarm of gravitationally-bound objects extending up to 1.5 light-years from the sun). Many comets have regular periods, zipping through the inner solar system every few years, others may take tens of thousands of years to complete one orbit.
Observations of C/2019 Q4, however, reveal that it is simply moving too fast to be in orbit about the sun; it came from afar and will feel the gravity of our sun only very slightly, giving it a tiny course correction as it zooms back into interstellar space to continue its journey between the stars.
"The comet's current velocity is high, about 93,000 miles per hour (150,000 kilometers per hour), which is well above the typical velocities of objects orbiting the Sun at that distance," Davide Farnocchia, who works at NASA's Jet Propulsion Laboratory in Pasadena, California, said in a press release shortly after the object was discovered. "The high velocity indicates not only that the object likely originated from outside our solar system, but also that it will leave and head back to interstellar space."