page_title: Comet nucleus
section_title: Size
hierarchical_section_title: Comet nucleus / Size
caption_attribution_description: English: This image shows the nuclei of comets Tempel 1 and Hartley 2, as imaged by NASA's Deep Impact spacecraft, which continued as an extended mission known as EPOXI. Tempel 1 is five times larger than Hartley 2. Visible jets are easily seen in images of Hartley 2, but required extensive processing to be seen in images of Tempel 1. Tempel 1 is 7.6 kilometers (4.7 miles) in the longest dimension. Hartley 2 is 2.2 km (1.4 miles) long. The Tempel 1 image was built up from more than 25 images captured by the impactor targeting sensor on July 4, 2005. The Hartley 2 image was obtained by the Medium- Resolution Imager on Nov. 4, 2010. Polski: Porównanie rozmiarów jąder komet Tempel 1 i Hartley 2. Obie sfotografowany przez sondę Deep Impact/EPOXI. Temepl 1 ma 7,6 kilometra w najdłuższym miejscu. Hartley 2, 2,2 km.
context_page_description: The nucleus is the solid, central part of a comet, once termed a dirty snowball or an icy dirtball. A cometary nucleus is composed of rock, dust, and frozen gases. When heated by the Sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the coma. 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. A typical comet nucleus has an albedo of 0.04. This is blacker than coal, and may be caused by a covering of dust.
  Results from the Rosetta and Philae spacecraft show that the nucleus of 67P/Churyumov–Gerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals. Further, the ALICE spectrograph on Rosetta determined that electrons produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma.
context_section_description: Most cometary nuclei are thought to be no more than about 16 kilometers (10 miles) across. The largest comets that have come inside the orbit of Saturn are C/2002 VQ94 (≈100 km), Comet of 1729 (≈100 km), Hale–Bopp (≈60 km), 29P (≈60 km), 109P/Swift–Tuttle (≈26 km), and 28P/Neujmin (≈21 km).
  The potato-shaped nucleus of Halley's comet (15 × 8 × 8 km) contains equal amounts of ice and dust.
  During a flyby in September 2001, the Deep Space 1 spacecraft observed the nucleus of Comet Borrelly and found it to be about half the size (8×4×4 km) of the nucleus of Halley's Comet. Borrelly's nucleus was also potato-shaped and had a dark black surface. Like Halley's Comet, Comet Borrelly only released gas from small areas where holes in the crust exposed the ice to sunlight.
  The nucleus of comet Hale–Bopp was estimated to be 60 ± 20 km in diameter. Hale-Bopp appeared bright to the unaided eye because its unusually large nucleus gave off a great deal of dust and gas.
  The nucleus of P/2007 R5 is probably only 100–200 meters in diameter.
  The largest centaurs (unstable, planet crossing, icy asteroids) are estimated to be 250 km to 300 km in diameter.  Three of the largest would include 10199 Chariklo (258 km), 2060 Chiron (230 km), and the currently lost 1995 SN55 (≈300 km).
  Known comets have been estimated to have an average density of 0.6 g/cm³. Below is a list of comets that have had estimated sizes, densities, and masses.
caption: Tempel 1 and Hartley 2 compared
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