From omen to celestial body
People long saw comets as omens of war or the death of kings. East Asian astronomers called them "broom stars" and recorded them carefully for millennia; Korean and Chinese court chronicles preserve many sightings. Aristotle treated comets as atmospheric phenomena, but Tycho Brahe, observing the Great Comet of 1577, found almost no parallax and showed that comets lie far beyond the Moon. In 1705 Edmond Halley used Newton's theory of gravity to argue that the comets of 1531, 1607 and 1682 were one object and predicted its return around 1758. It reappeared on Christmas Day 1758, after his death, and became Halley's Comet.
Dirty snowballs and two tails
In 1950 Fred Whipple proposed that a comet's nucleus is a "dirty snowball" of ice and dust. Near the Sun the ice sublimates into a coma around the nucleus, and two tails stream out. The dust tail curves and glows yellowish, pushed by sunlight; the ion tail is dragged straight away from the Sun, glowing blue, by the solar wind — a flow of plasma — and its magnetic field. In 1951 Ludwig Biermann inferred the existence of the solar wind from ion-tail directions. Comets' homes were identified too: in 1950 Jan Oort proposed that long-period comets come from a vast cloud tens of thousands of astronomical units away, and in 1951 Gerard Kuiper and others speculated about a belt beyond Neptune. In 1992 David Jewitt and Jane Luu found the first Kuiper belt object, confirming a reservoir of short-period comets.
Comets seen by spacecraft, and impacts
In 1986 Europe's Giotto took the first close-up images of Halley's nucleus, revealing a charcoal-dark, potato-shaped body. In 2005 NASA's Deep Impact fired an impactor into comet Tempel 1 to study its interior, and in 2014 ESA's Rosetta orbited comet Churyumov–Gerasimenko and dropped the Philae lander. Rosetta found that the comet's water has a deuterium ratio unlike Earth's oceans, casting doubt on the idea that comets like it supplied most of Earth's water. The 1994 crash of comet Shoemaker–Levy 9 into Jupiter showed that collisions still happen in the Solar System. A large comet striking Earth is very unlikely, though researchers study whether such impacts contributed to past mass extinctions. Halley's Comet is due back in 2061.
FAQ
- How do comets differ from meteors (shooting stars)?
- A comet is an icy body orbiting the Sun; a meteor is the glow of a small grain burning up in Earth's atmosphere. Many meteor showers occur when Earth passes through dust left by a comet.
- Can you see comets with the naked eye?
- A bright naked-eye comet appears every few years. Dark skies away from city lights and binoculars help a lot; check observatory news for viewing tips.
Sources reviewed: 2026-10-26
References
- Brahe, T. (1588). De mundi aetherei recentioribus phaenomenis. Uraniborg.
- Halley, E. (1705). Astronomiae cometicae synopsis (A synopsis of the astronomy of comets). Philosophical Transactions of the Royal Society, 24, 1882–1899.
- Oort, J. H. (1950). The structure of the cloud of comets surrounding the Solar System and a hypothesis concerning its origin. Bulletin of the Astronomical Institutes of the Netherlands, 11, 91–110.
- Whipple, F. L. (1950). A comet model. I. The acceleration of Comet Encke. Astrophysical Journal, 111, 375–394.
- Biermann, L. (1951). Kometenschweife und solare Korpuskularstrahlung. Zeitschrift für Astrophysik, 29, 274–286.
- Kuiper, G. P. (1951). On the origin of the solar system. In J. A. Hynek (Ed.), Astrophysics: A Topical Symposium (pp. 357–424). New York: McGraw-Hill.
- Keller, H. U., et al. (1986). First Halley Multicolour Camera imaging results from Giotto. Nature, 321, 320–326.
- Jewitt, D., & Luu, J. (1993). Discovery of the candidate Kuiper belt object 1992 QB1. Nature, 362(6422), 730–732.
- A'Hearn, M. F., et al. (2005). Deep Impact: Excavating comet Tempel 1. Science, 310(5746), 258–264.
- Altwegg, K., et al. (2015). 67P/Churyumov-Gerasimenko, a Jupiter family comet with a high D/H ratio. Science, 347(6220), 1261952.