Oumuamua was discovered on 19th October 2017. Now there may be another interstellar visitor, this time by the recently discovered comet known as C/2019 Q4 (Borisov), formerly gb00234.

Gennady Borisov captured the object on 30th August 2019, at the MARGO Observatory near Nauchnij, Crimea when it was approximately 3 Astronomical Units (AU’s) from the Sun.

Unlike 'Oumuamua, which was not seen until after perihelion, the new comet is approaching the plane of the solar system and will reach perihelion on 10th December 2019 at a distance of 1.94 AU’s. These dates may alter as data from observations are updated. What sets C/2019 Q4 apart from nearly every other comet is the eccentricity of its orbit.

Eccentricity measures how much an orbit deviates from a perfect circle, which has an eccentricity of 0. Elliptical orbits, typical of planets, asteroids and comets, have eccentricities between 0 and 1. Parabolas are equal to 1 and an eccentricity greater than 1 indicates a hyperbolic orbit. A few days after its discovery it bore the temporary designation gb00234, observers are cautious of an interstellar origin. Possibility of finding it in pre discovery images is not high, because for some months the comet has been too close to the Sun to have been swept up by wide field surveys. However, it should remain visible for many months before and after it comes closest to Earth during December and more positional measurements will strengthen the orbital fit and make a stronger case for an interstellar origin.

Based on current observations, C/2019 Q4's eccentricity is approximately 3.2, certainly hyperbolic. Objects on hyperbolic orbits are unbound to the Sun. They are most likely to come from beyond the solar system, travelling in from vast distances to pay our solar system a short visit before heading off to the unknown.

If this result is confirmed, astronomers have an unprecedented opportunity to study a potentially interstellar object in great detail over a long time span. Based on the comet's current magnitude (~18) and distance from the Sun (2.7 AU’s), it appears to be a fairly large object, possibly 10 km or greater across, depending on the reflectivity of its surface. Images show the incoming object has a faint but distinct coma and the hint of a tail, something Oumuamua did not have and so appears to be a comet. Astronomers want to obtain a spectra of the new find to determine what compounds may be escaping from the surface.

Assuming the comet's hyperbolic orbital solution holds, the comet appears to be coming from the direction of the galactic plane in Cassiopeia. There will be updates following additional observations and more photographic evidence. C/2019 Q4 (Borisov) designation will remain provisional until the orbit has been established. The name of the first interstellar object discovered, 1I Oumuamua, evolved from C/2017 U1 to A/2017 until finally receiving the, I designation for "interstellar." Now it is known it is a prowler from another star system, it may become 2I.

The comet is presently moving at 41 kilometres per second, (Gray, B. Guide Astrometry Software) which will increase to 44 km/sec at perihelion. In the long term, approaching us from "infinity" and departing for the same C/2019 Q4 speeds through space at approximately 33 km/s. The high velocities, along with the object's hyperbolic orbit, preclude an origin in either the Oort, Cloud or due to a close planetary encounter within our own Solar System.

For reference, the average speed of stars relative to the Sun in the solar neighbourhood is approximately 20 km/s. The fact that the comet's relative speed is considerably higher indicates that it is unbound and is moving as a solo object. Upon discovery, C/2019 Q4 possessed a moderately condensed coma approximately 7″ across with a possible 15″ tail to the northwest (P.A. 310°). Additional images from other observers revealed a star like nucleus, strongly condensed ~15″ coma and confirmed a tail up to 50″ long.

By mid-October it is expected to break 16th magnitude and peak at 14.7 around the time of its December 10th perihelion. When at its brightest, the interstellar visitor will be in central Crater around declination –20°. The first 10 days in December will be best for amateur imaging and also visual attempts before full Moon on December 12th interferes. C/2019 Q4 remains a morning object throughout the current apparition, moving from the constellation Cancer through Leo, Sextans, Crater and points south.


Borisov, G. Sternberg Astronomical Institute, Moscow University, Dunn, T. Gray, B. (creator of Guide astrometry software). Harvard and Smithsonian Observatory, USA.

International Astronomical Union, Minor Planet Center for Astrophysics, MARGO Observatory near Nauchnij, Crimea. Stamcose et al