Spectral type The visible and near-
infrared spectrum of Chiron is neutral, and is similar to that of
C-type asteroids and the nucleus of
Halley's Comet. The near-infrared spectrum of Chiron shows absence of water ice. The infrared spectrum, however, from 0.97 to 5.27 μm, as revealed by the James Webb Space Telescope reveals the presence of gas in the coma and a diverse inventory of ices, on the surface or in the coma. Absorption bands of
carbon dioxide,
carbon monoxide,
ethane,
propane, and
acetylene were detected, together with water ice in its amorphous state. In addition, irradiation byproducts of methane, carbon dioxide, and carbon monoxide were detected under both reducing (e.g., ethane) and oxidizing (e.g., CO3) conditions; however, complex carbon-bearing molecules resulting from the combined irradiation of –CH– and –CO– groups (such as H2CO, CH3CHO, or CH3COOH) were not observed, which may indicate a physical or temporal segregation of the methane and carbon dioxide reservoirs. A key finding was the detection of fluorescent methane emissions, providing the first evidence of a gas coma rich in this compound. Gaseous carbon dioxide emission was also identified in the fundamental stretching band at 4.27 μm. The authors argue that the presence of methane emission is the first proof of the desorption of methane due to a density phase transition of amorphous water ice at low temperature (61 K) previously studied in the laboratory.
Rotation period Four rotational
light curves of Chiron were taken from
photometric observations between 1989 and 1997. Lightcurve analysis gave a concurring, well-defined
rotational period of 5.918 hours with a small brightness variation of 0.05 to 0.09
magnitude, which indicates that the body has a rather spheroidal shape ().
Diameter The assumed size of an object depends on its
absolute magnitude (H) and the
albedo (the amount of light it reflects). In 1984 Lebofsky estimated Chiron to be about 180 km in diameter. Estimates in the 1990s were closer to 150 km in diameter.
Occultation data from 1993 suggests a diameter of about 180 km. Combined data from the
Spitzer Space Telescope in 2007 and the Herschel Space Observatory in 2011 suggests that Chiron is in diameter. Therefore, Chiron may be as large as
10199 Chariklo. The diameter of Chiron is difficult to estimate in part because the true absolute magnitude of its nucleus is uncertain due to its highly variable cometary activity.
Cometary behavior In February 1988, at 12 AU from the Sun, Chiron brightened by 75 percent. This is behavior typical of comets but not asteroids. Further observations in April 1989 showed that Chiron had developed a cometary
coma, A tail was detected in 1993. Chiron differs from other comets in that
water is not a major component of its coma, because it is too far from the
Sun for water to sublimate. In 1995
carbon monoxide was detected in Chiron in very small amounts, and the derived CO production rate was calculated to be sufficient to account for the observed coma.
Cyanide was also detected in the spectrum of Chiron in 1991. At the time of its discovery, Chiron was close to aphelion, whereas the observations showing a coma were done closer to perihelion, perhaps explaining why no cometary behavior had been seen earlier. The fact that Chiron is still active probably means it has not been in its current orbit very long. Chiron is officially designated as both a comet—95P/Chiron—and a minor planet, an indication of the sometimes fuzzy dividing line between the two classes of object. The term proto-comet has also been used. Being about 220 km in diameter, it is unusually large for a
comet nucleus. Chiron was the first member of a new family of
Chiron-type comets (CTC) with (
TJupiter > 3;
a > aJupiter). Other CTCs include:
39P/Oterma,
165P/LINEAR,
166P/NEAT, and
167P/CINEOS. There are also non-centaur asteroids that are simultaneously classified as comets, such as
4015 Wilson–Harrington,
7968 Elst–Pizarro, and
118401 LINEAR. Since the discovery of Chiron, other centaurs have been discovered, and nearly all are currently classified as minor planets, but are being observed for possible cometary behavior.
60558 Echeclus has displayed a cometary coma and now also has the cometary designation 174P/Echeclus. After passing perihelion in early 2008,
52872 Okyrhoe significantly brightened. == Rings ==