NASA's Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) spacecraft has watched a comet make closest approach (perihelion) with the sun in its orbit, causing the pristine ices in its nucleus to vaporize, blasting jets of dust and vapor into space. But when the asteroid-hunting mission first spotted the object, it was thought to be an asteroid.
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Comet C/2013 UQ4 (Catalina) takes more than 450 years to make one full orbit of the sun. It has a retrograde orbit, meaning it travels around the sun in the opposite direction to the planets. Each time the comet reaches perihelion, the sun's heat causes the pristine ices to sublimate (i.e. when a material is heated from an ice directly to a gaseous state without passing through a liquid phase), producing a beautifully long tail sweeping 62,000 miles (100,000 kilometers) away from the sun.
"The tail forms a faint fan as the smaller dust particles are more easily pushed away from the sun by the radiation pressure of the sunlight," said James Bauer, researcher at NASA's Jet Propulsion Laboratory in Pasadena, Calif., in a JPL news release.