On August 30th, 2019, Gennady Borisov, an optician and astronomer at the Crimean Astrophysical Observatory, using equipment he built himself, discovered a dim (18th magnitude) object moving with respect to the distant stars. Further observations indicated it was cometary in appearance, with a coma around its brightest spot and apparent short tail. Orbital computations from the limited number of observations indicate that it was discovered at a distance of around 3 astronomical units (AU) (the mean radius of the Earth’s orbit) from the Sun, inbound toward a perihelion on December 10th near 2 AU.
As with ’Oumuamua (1I/2017 U1) in 2017, attempts to fit a typical elliptical or parabolic orbit to the observations failed, and the best fit was found to be a hyperbolic orbit with an eccentricity in excess of 3. Such an object is not gravitationally bound to the solar system and must be of interstellar origin; after rounding the Sun, it will depart into interstellar space never to be seen again. This is only the second such object to be observed. From observations so far (and with less than two weeks of data, these figures will be revised as further observations are made), its inbound velocity to the solar system before it began to be accelerated by the Sun’s gravity was around 30 km/sec, which rules out a hyperbolic orbit due to interactions with solar system objects, as such perturbations cannot create a velocity greater than 3 km/sec. Here is the Minor Planet Center Circular, MPEC 2019-R106, announcing the discovery, its apparent interstellar nature, and preliminary orbital elements based on the news that’s come to Harvard.
This is a chart of the object’s orbit prepared by the Jet Propulsion Laboratory based upon their estimate of its orbit. The grey lines lead from the orbit to the plane of the ecliptic (plane defined by the Earth’s orbit, near which all of the major planets orbit), showing how the comet is arriving from above the plane and will depart below it.
Unlike ’Oumuamua, which was discovered after passing perihelion (its closest approach to the Sun), and could only be observed departing the solar system, this object was discovered while still inbound and should provide ample opportunities for observation.
Here are links for further reading about this intriguing discovery.
- “Interstellar comet 2I/Borisov” at arXiv
- “Another Interstellar Visitor Is Headed Our Way” at Sky & Telescope
- “C/2019 Q4 (Borisov): A Likely Interstellar Comet” at Centauri Dreams
- “C/2019 Q4 (Borisov)” at Wikipedia
- “Newly Discovered Comet Is Likely Interstellar Visitor” from the NASA Jet Propulsion Laboratory
- “FAQ for gb00234 = C/2019 Q4 (Borisov)” by Bill Gray