Astronomers have sighted a comet that could be the second interstellar object ever detected. First seen by Gennady Borisov at the MARGO observatory in Nauchnij, Crimea, on August 30, 2019, C/2019 Q4 (Borisov) is exhibiting a speed and trajectory that suggests that it came from outside the solar system.
When the first observed interstellar object, 'Oumuamua, was confirmed in October 2017, it was something of a nine-day wonder as scientists tried to figure out what it was. An asteroid? A comet? A probe from another civilization?
It turned out to be most likely a comet, but if C/2019 Q4 is confirmed it also illustrates the principle that finding the first example of a thing is the hardest.
In C/2019 Q4's case, the initial observations by Borisov were analyzed by NASA's Scout system at the Jet Propulsion Laboratory in Pasadena, California. Scout is designed to flag any asteroids that may pose a hazard to the Earth, but it can also flag potential interstellar objects, like C/2019 Q4. The object's trajectory was then looked at in more detail by Davide Farnocchia of NASA's Center for Near-Earth Object Studies, and ESA's Near-Earth Object Coordination Center in Frascati, Italy.
They found that the object is well outside the ecliptic (the plane of the Earth's orbit around the Sun) and is headed for the inner solar system at an angle of 40° and a present speed of 93,000 mph (150,000 km/h). According to Farnocchia, this high speed indicates that it could be on a hyperbolic trajectory that originated outside our system and that it will later return to interstellar space.
At the moment, C/2019 Q4 is 260 million miles (420 million km) from the Sun and is expected to cross the ecliptic on October 26. It and will reach its closest point, or perihelion, on December 8, 2019, when it will come within about 190 million mi (300 million km) of the Sun, which is about the distance Mars is from the Sun. The closest the object will come to Earth is roughly the same distance, 190 million mi.
While scientists are still working to see if the object's trajectory is really interstellar, they have been able to get a rough idea about C/2019 Q4 itself. According to Karen Meech and her team at the University of Hawaii, it's about 1.2 and 10 mi (2 and 16 km) in diameter with a fuzzy appearance indicating that it's an icy nucleus surrounded by a coma of gas and dust that grows thicker as it nears and is warmed by the Sun – typical of a comet.
However, because its current position is in line with the Sun, observations are difficult for the next couple of months.
"The object will peak in brightness in mid-December and continue to be observable with moderate-size telescopes until April 2020," says Farnocchia. "After that, it will only be observable with larger professional telescopes through October 2020."
Source: NASA
