C/2017 K2 (PanSTARRS)

C/2017 K2 (PanSTARRS) is an Oort cloud comet with an inbound hyperbolic orbit, discovered in May 2017 at a distance beyond the orbit of Saturn when it was 16 AU from the Sun. Precovery images from 2013 were located by July. It had been in the constellation of Draco from July 2007 until August 2020. , the 3-sigma uncertainty in the current distance of the comet from the Sun is $±6,000 km$.

The comet is record breaking because it was already becoming active at such a distance. Only Comet Hale–Bopp produced such a show from that distance with a similar nucleus. However, this comet will not be as visible as Hale–Bopp was in 1997 in part because it does not come nearly as close to the Sun. Astronomers had never seen an active inbound comet this far out, where sunlight is 1/225th its brightness as seen from Earth. Temperatures, correspondingly, are at -440 °F (-262 °C) in the Oort cloud. However, as it was approaching the Sun at a distance of 16 AU at discovery, a mix of ancient ices on the surface containing oxygen, nitrogen, carbon dioxide and carbon monoxide began to sublimate and shed the dust frozen into it. This material expands into a vast 130,000 km wide halo of dust, called a coma, enveloping the solid nucleus. Outgasing of carbon monoxide was detected when the comet was 6.72 AU from the Sun.

Research with the Canada–France–Hawaii Telescope (CFHT) infers the comet nucleus to have a radius between 14-80 km, so there is a chance the nucleus could be as large as C/1995 O1 (Hale-Bopp). However, research with the Hubble Space Telescope (HST) estimates the nucleus to have a circular equivalent diameter of less than 18 km. On 17 September 2020, morphological studies of the inner coma, observed on 12 September 2020, were reported, noting that two jet-streamed structure were emitted from the nucleus and, as well, that the length of the tail was about 800,000 km long.

The comet was within 5 AU of Earth by 11 January 2022. Around 6 July 2022, the comet crossed the celestial equator, and then on 14 July 2022, it passed 1.8 AU from Earth and shone around 9.0 magnitude making it a decent binoculars object. It reached perihelion on 19 December 2022, close to the orbit of Mars, and was not visible to naked eye at 8.0 magnitude.

On 27 July 2021, further detailed observations of the comet were reported on The Astronomer's Telegram. JPL Horizons models that C/2017 K2 took millions of years to come from the Oort cloud at a distance of roughly 50000 AU. The heliocentric orbital eccentricity drops below 1 in December 2023. The outbound orbital period will be around $19,000$ years with aphelion being around 1400 AU. There was a dispute whether that was the first time the comet entered the inner solar system, but its orbit suggests that the comet isn't dynamically new and there is a 29% chance that the comet is of interstellar origin and was captured by the solar system within the last 3 million years.