Observed for millennia, comets are pristine remnants of our Solar System's birth. With nuclei spanning a few kilometers, they orbit from the distant Kuiper Belt. As astronomers explain, what defines these wanderers, and what fuels their iconic brilliance?
Comet C/2020 F3 (NEOWISE) is currently visible to the naked eye in French skies. Discovered March 27, 2020, it survived its solar closest approach and will pass Earth at 103 million km on July 23.
These icy bodies feature nuclei 1 to 20 km in diameter, spinning every 4 to 70 hours. Composed of rock, ice, gas, and dust, comets divide into two categories.
Short-period comets (orbits of 3–15 years) circle the Sun in the Kuiper Belt, 30–55 AU out (1 AU = Earth-Sun distance). Long-period comets (50–5,000 years) hail from the Oort Cloud, a vast shell 20,000–100,000+ AU away.
Solar System fossils, comets venture inward due to gravitational perturbations, tracing elongated elliptical orbits that swing close to the Sun.
Distant from the Sun, comets stay faint. Solar proximity vaporizes ices via intense radiation, primarily shining from sunlight reflection, much like the Moon.
Sublimating ices release gas that sweeps up dust, forming the expansive coma (or "hair"), 10,000–100,000 km wide.
A whitish dust tail trails 1–10 million km opposite the Sun's direction, curved by radiation pressure in the orbital plane.
A bluish plasma tail stretches 10–100 million km, aligned precisely along the comet-Sun axis.
