A comet is an icy, rocky mass that has passed close by the Sun within our Solar System, and has thus warmed up and begun to release gases, causing a visible atmosphere — and sometimes a ‘tail’ — to form.
Comets are small celestial bodies that have highly eccentric orbits around our Sun, meaning that their orbital paths take them close past the Sun, and then hurl them out deep into the Solar System, beyond Neptune’s orbit. This means that the amount of light that they receive from the Sun varies considerably. Comets have small cores or ‘nuclei’, made up of rock, dust and ice, with diameters between a few hundred metres and a few tens of kilometres. For most of a comet’s orbit, it only comprises its core. However, after a comet has passed close by the Sun, the Sun’s wind causes the ice to warm up and the comet begins to release gases, a process known as outgassing. The gases form a visible atmosphere around the comet which is not bound by gravity. This unbound atmosphere is called a ‘coma’. Sometimes, the coma trails behind the comet, forming a distinctive comet tail. Comets can orbit the Sun for millennia, but eventually the regular process of heating, outgassing and cooling may cause them to break up.
In contrast to probes that have to travel vast distances and require years of planning to visit the planets, Hubble is also able to turn its attention quickly to sudden dramatic events occurring in the Solar System. This allowed it to witness the stunning plunge of comet Shoemaker-Levy 9 into Jupiter’s atmosphere in July 1994. Hubble followed the comet fragments on their last journey and delivered high-resolution images of the impact scars, from which important new information on conditions in the Jovian atmosphere was obtained. The consequences of the impact could be seen for several days afterwards, and by studying the Hubble data, astronomers were able to gain fundamental information about the composition and density of the giant planet’s atmosphere.
Hubble has also observed spectacular comet breakups. This includes the breakup of comet 73P/Schwassmann-Wachmann 3 in 2006 as it visited the inner Solar System. The Hubble images uncovered many more fragments than were reported by ground-based observers and provided an unprecedented opportunity to study the demise of a comet nucleus. Hubble also observed the breakup of comet ATLAS in 2020, providing astronomers with the sharpest view yet of the breakup of a comet. The telescope resolved roughly 30 fragments of the fragile comet. These Hubble images provided further evidence that comet fragmentation is probably common and might even be the dominant mechanism by which the solid, icy nuclei of comets die.
Also in 2020 Hubble captured the closest images yet of the popular comet C/2020 F3 NEOWISE, after it passed by the Sun. The telescope’s images resolved the visitor’s coma, the fine shell that surrounds its nucleus, and its dusty output.