Chiron and Echeclus are therefore classified as both asteroids and comets, while Schwassmann-Wachmann 1 has always held a comet designation. Other centaurs, such as 52872 Okyrhoe, are suspected of having shown comas. Any centaur that is perturbed close enough to the Sun is expected to become a comet. The generic definition of a centaur is a small body that orbits the Sun between Jupiter and Neptune and crosses the orbits of one or more of the giant planets. Due to the inherent long-term instability of orbits in this region, even centaurs such as and , which do not currently cross the orbit of any planet, are in gradually changing orbits that will be perturbed until they start to cross the orbit of one or more of the giant planets. Some astronomers count only bodies with semimajor axes in the region of the outer planets to be centaurs; others accept any body with a perihelion in the region, as their orbits are similarly unstable. However, different institutions have different criteria for classifying borderline objects, based on particular values of their orbital elements:       The Gladman & Marsden (2008) criteria would make some objects Jupiter-family comets: Both Echeclus (, ) and Okyrhoe (; ) have traditionally been classified as centaurs. Traditionally considered an asteroid, but classified as a centaur by JPL, Hidalgo (; ) would also change category to a Jupiter-family comet. Schwassmann-Wachmann 1 (; ) has been categorized as both a centaur and a Jupiter-family comet depending on the definition used. Other objects caught between these differences in classification methods include 944 Hidalgo which was discovered in 1920 and is listed as a centaur in the JPL Small-Body Database. , which has a semi-major axis of 32 AU but crosses the orbits of both Uranus and Neptune is listed as an outer centaur by the "Deep Ecliptic Survey" (DES). Among the inner centaurs, (434620) 2005 VD, with a perihelion distance very near Jupiter, is listed as a centaur by both JPL and DES. A recent orbital simulation of the evolution of Kuiper Belt Objects through the centaur region has identified a short-lived ""orbital gateway"" between 5.4 and 7.8 AU through which 21% of all centaurs pass, including 72% of the centaurs that become Jupiter-family comets.