Skip links

Facts about Comets for Kids

  • A comet is an icy small Solar System body (SSSB) that, when close enough to the Sun, displays a visible coma and sometimes also a tail.
  • Comet nuclei range from a few hundred meters to tens of kilometers across and are composed of loose collections of ice, dust, and small rocky particles.
  • Comets have a wide range of orbital periods, ranging from a few years to hundreds of thousands of years.
  • Short-period comets originate in the Kuiper belt, or its associated scattered disc, which lie beyond the orbit of Neptune.
  • Longer-period comets are thought to originate in the Oort cloud, a hypothesized spherical cloud of icy bodies in the outer Solar System.
  • Long-period comets plunge towards the Sun from the Oort cloud because of gravitational perturbations caused by either the massive outer planets of the Solar System (Jupiter, Saturn, Uranus, and Neptune), or passing stars.
  • However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids.
  • Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System.
  • However, this represents only a tiny fraction of the total potential comet population: the reservoir of comet-like bodies in the outer Solar System may number one trillion.
  • The Giotto space probe found that the nucleus of Halley’s Comet reflects about four percent of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly’s surface reflects just 2.4% to 3.0% of the light that falls on it; by comparison, asphalt reflects seven percent of the light that falls on it.
  • Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from the Hubble Space Telescope observations, but these detections have been questioned, and have not yet been independently confirmed.
  • As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them.
  • The streams of dust and gas thus released form a huge, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun’s radiation pressure and solar wind cause an enormous tail to form, which points away from the sun.
  • Comets are often classified according to the length of their orbital periods: the longer the period the more elongated the ellipse.
  • Unlike the return of periodic comets whose orbits have been established by previous observations, the appearance of new comets by this mechanism is unpredictable.
  • For example, Comet McNaught (C/2006 P1) had an heliocentric osculating eccentricity of 1.000019 near its perihelion passage epoch in January 2007, but is bound to the Sun with roughly a 92,600-year orbit since the eccentricity drops below 1 as it moves further from the Sun.
  • Similarly, the second and third known periodic comets, Encke’s Comet and Biela’s Comet, were named after the astronomers who calculated their orbits rather than their original discoverers.