C/2017 K2 (PanSTARRS)

Lore & Background

The comet was discovered in images taken on 21 May 2017 by the PanSTARRS survey using its 1.8-m Ritchey-Chretien telescope at Haleakala, Hawaii. At discovery it had an apparent magnitude of about 21, later refined to 19.5. Precovery images from 2013 were located by July 2017. The comet remained in the constellation of Draco from July 2007 until August 2020. It brightened quickly between 4.3 AU and 2.7 AU from the Sun as water-ice sublimation began, increasing dust production. Between 130 and 50 days before perihelion, activity plateaued and the comet ended up dimmer than expected. Its closest approach to Earth occurred on 15 July 2022 at 1.81 AU, when it appeared near the globular cluster Messier 10 with a dust tail about one degree long. Perihelion occurred on 19 December 2022 in the constellation of Ara, near the border with Pavo, at an apparent magnitude of 8.5. Post-perihelion the comet faded fast and consistently.

Reader's Guide

C/2017 K2 is significant as a record-breaking comet because it was already active at 16 AU from the Sun, where sunlight is only 1/225th as bright as seen from Earth and temperatures reach −262.2 °C. Only Comet Hale–Bopp had shown such activity at that distance, though C/2017 K2 did not become as visible because it does not come as close to the Sun. The comet's activity at such a great distance was driven by sublimation of ancient ices containing oxygen, nitrogen, carbon dioxide, and carbon monoxide, which created a vast coma 130,000 km wide. Outgassing of carbon monoxide was detected when the comet was 6.72 AU from the Sun. Observations with the Canada–France–Hawaii Telescope in 2017 inferred a nucleus radius between 14 and 80 km, while Hubble Space Telescope data estimated a circular equivalent diameter of less than 18 km. Near-infrared observations by the James Webb Space Telescope in 2023 revealed a much smaller nucleus, less than 8.4 km in diameter. The comet's orbit suggests it is not dynamically new, with a 29% chance it is of interstellar origin captured within the last 3 million years. Its outbound orbital period will be about 25,800 years with an aphelion near 1,750 AU.

Did You Know?

• The comet was discovered at a distance of 16 AU, beyond the orbit of Saturn.
• Its coma of dust reached 130,000 km in diameter.
• The rotational period of the nucleus is estimated at 14.24 hours.
• There is a 29% chance the comet is of interstellar origin captured within the last 3 million years.