The detection of Oumuamua, a mysterious cigarette-shaped object, revolutionized the scientific community around its possible origin. It is believed that this object that is considered the first interstellar object detected in the vicinity of our Solar System is, in fact, an asteroid and not an alien spacecraft.
On September 2018, NASA announced the discovery of what would become the second interstellar object detected. It was discovered on August 30 and was named C/2019 Q4 otherwise known as Borisov. The object is most probably an asteroid and is still on its way to the Sun but is believed to remain beyond Mars' orbit.
On December 8, 2019 this object reached its closest point to the Sun, the perihelion. It did not approach the Earth more than 300 million kilometres. After this quick visit, Borisov leaves again at full speed towards the immense distance of the cosmos, never to return.
Observations made by Karen Meech and her team at the University of Hawaii indicate that the comet's core has a diameter between 2 and 16 kilometres.
This is an image captured by NASA/ESA's Hubble Space Telescope in mid-December when it reached its peak brightness.
How do we know where it came from?
One of the clues that have led to the conclusion that this object is indeed a visitor to deep space is its speed. It moves at about 150,000 kilometres per hour, much faster than the typical speed of objects orbiting the Sun at that distance.
High speed indicates not only that the object probably originated outside our Solar System, but that it will not be able to stay. Just as it came, it will leave and return to interstellar space.
Borisov is one of the fastest asteroids ever seen; it travels at an impressive speed of over 175,000 kilometres per hour / NASA/JPL-Caltech.
Why do we know it's a comet, not an alien object?
As published by NASA, C / 2019 Q4 was designated as a comet because of its blurred appearance, indicating that the object has a central frozen body that produces a cloud of dust and surrounding particles as it approaches the Sun and heats up.
Its location in the sky (as seen from Earth) is close to the Sun, an area of the sky that is generally not scanned by NASA's asteroid-hunting spacecraft.
After the detection of C/2019 Q4, the Scout system, located at NASA's Jet Propulsion Laboratory in Pasadena, California, automatically qualified it as a possible interstellar object.
Davide Farnocchia of NASA's Center for Near-Earth Object Studies at JPL worked with astronomers and the European Space Agency's Near-Earth Object Coordination Center at Frascati, Italy, to obtain additional observations. He then worked with the NASA-sponsored Minor Planet Center in Cambridge, Massachusetts, to estimate the precise trajectory of the comet and determine whether it originated within our Solar System or came from another part of the galaxy.
When will it leave?
Visitor C / 2019 Q4 will be observable by professional telescopes in the coming months. The object was expected to reach its peak brightness in mid-December and will remain observable with moderate-size telescopes until April 2020. After that, it will only be observable with larger professional telescopes until October 2020.
Astronomers will continue to collect observations to further characterize the physical properties of the comet (exact size, rotation, etc.) and will also continue to better identify its trajectory.
Image: The illustration shows the trajectory of comet C / 2019 Q4, considered a possible interstellar object. Image via NASA / JPL-Caltech.