Astronomy & Astrophysics 101: Comet

This ground image of Comet C/2020 F3 (NEOWISE) was taken on July 16, 2020 from the Northern Hemisphere. The inset image, taken on August 8, 2020, by the Hubble Space Telescope, reveals a close-up of the comet after passing through the Sun. The Hubble image focuses on the comet’s nucleus, which is too small to see. It is estimated to be no more than 3 miles (4.8 kilometers) in diameter. Instead, the image shows part of the comet’s coma, the hazy glow, which measures about 11,000 miles (18,000 kilometers) across in this image. Comet NEOWISE will not pass through the inner solar system for nearly 7,000 years. Credit: NASA, ESA, STScI, Q. Zhang (Caltech); ground image copyright © 2020 by Zoltan G. Levay, used with permission

A comet is a rocky, icy mass that passed close to the Sun in our solar system, and thus warmed up and began to release gases, resulting in the formation of a visible atmosphere – and sometimes a tail”.

Comets are small celestial bodies that have highly eccentric orbits around our Sun, meaning their orbital paths take them close to the Sun and then propel them deep into the Solar System beyond.[{” attribute=””>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 meters and a few tens of kilometers. For most of a comet’s orbit, it only comprises its core. However, after a comet has passed close by the Sun, the solar 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.


A comet is a rocky, icy mass that passed close to the Sun in our solar system, and thus warmed up and began to release gases, causing the formation of a visible atmosphere – and sometimes a “tail”. “. Credit: " data-gt-translate-attributes="[{" attribute="">Nasa & ESA

Unlike probes that have to travel great distances and require years of planning to visit planets, Hubble is also able to quickly turn its attention to sudden and dramatic events occurring in the solar system. This allowed him to witness the breathtaking plunge of Comet Shoemaker-Levy 9 into Jupiterin July 1994. Hubble tracked the comet fragments on their final journey and provided high-resolution images of the impact scars, from which important new information about conditions in the Jovian atmosphere was obtained. The consequences of the impact could be observed several days later, and by studying the Hubble data, astronomers were able to obtain fundamental information about the composition and density of the atmosphere of the giant planet.

Word Bank Comet

Comet. Credit: ESA/Hubble, NASA, ESA, Q. Zhang (California Institute of Technology), A. Pagan (STScI)

Hubble has also observed dramatic comet breakups. This includes the burst of comet 73P/Schwassmann-Wachmann 3 in 2006 as it visited the inner solar system. The Hubble images uncovered many more fragments than those reported by ground-based observers and provided an unprecedented opportunity to study the disappearance of a comet’s nucleus. Hubble also observed the burst of Comet ATLAS in 2020, giving astronomers the sharpest view yet of a comet burst. The telescope resolved about 30 fragments of the fragile comet. These Hubble images provided further evidence that comet fragmentation is likely common and may even be the dominant mechanism by which the solid, icy nuclei of comets die.

Also in 2020, Hubble captured the closest images to date of popular comet C/2020 F3 NEOWISE, after it passed through the Sun. The telescope images resolved the visitor’s coma, the thin shell that surrounds its core, and its dusty exit.

Comments are closed.