The Solar System has a new visitor: comet C/2017 K2 (PanSTARRS), discovered between the orbits of Saturn and Uranus in May 2017. If the calculations are correct, C/2017 K2 will make its closest approach to Earth on July 14, and then 'sunbathe' with our parent star on December 19. What does that mean? If you are in a good location and have a small telescope or fairly sturdy binoculars, you'll be able to see it.
Experts detected C/2017 K2 with the help of the Pan-STARRS system installed in Hawaii, and from the start it caused quite a stir. Why? Because never before had an active comet been seen so far from the Sun (it has since lost that title to Bernardinelli–Bernstein), where light is only a fraction of what we receive on Earth.
This gentleman has also given headaches when it comes to confirming its size. Observations made by the CFH telescope suggest its nucleus has a diameter between 30 and 160 kilometers; however, data from Hubble reduce those values to 18 kilometers. On the other hand, its coma (or 'cometary atmosphere') extends to a diameter of 130,000 kilometers, and new observations in September 2020 have established that its tail measures an impressive 800,000 kilometers.
How to see comet C/2017 K2 in the coming weeks
C/2017 K2 is a fairly large comet, but if you want to study its journey, you'll need a small telescope or quality binoculars (for example, the Skywatcher 130/650 PDS used in the video costs less than 300 euros), with an estimated magnitude between 8 and 7. Observing it will favor those in the Northern Hemisphere until mid-September (although that 'doesn't' mean it will be exclusive), and then it will move to the Southern Hemisphere.
In September 2017, the Hubble Site shared the orbit of C/2017 K2.
The easiest way to locate C/2017 K2 right now is by visiting Stellarium Web, which reports the comet's location immediately. At the time of writing, it is near the center of the constellation Ophiuchus. On July 14, when it approaches 1.8 astronomical units from Earth, C/2017 K2 will be visible almost next to Messier 10, but there's a catch: on the 13th we have a supermoon.
If you prefer to wait, another interesting date to consider will be August 21. That day, C/2017 K2 will be very close to Acrab or Beta Scorpii, in the constellation Scorpius. To finish, C/2017 K2 will reach perihelion on December 19.
Sources: EarthSky, Hubble Site
