It's a comet - it's an asteroid - it's both

It used to be an asteroid was an asteroid and a comet was a comet. Now, astronomers realize, the line's getting blurry.

In 1999, the Heliospheric Observatory (SOHO) spacecraft spotted an object (later dubbed 'P/2007 R5') passing close to the Sun. The object was assumed to be a comet, because it was passing the sun, and that's what comets are known to do. What puzzled some scientists was that P/2007 R5 did not have classic comet features such as a tail and a coma – the cloud of gas and dust that surrounds a comet's body, or nucleus. The tail and coma are created when the Sun's heat vaporizes frozen water and carbon dioxide on the comet, and blasts a brightly illuminated cloud of dust off the comet's surface. Without these features, P/2007 R5 would be better classified as an asteroid. Asteroids traditionally do not contain high enough quantities of solid carbon dioxide and water ice to make such a display.

When SOHO spotted P/2007 R5 swinging by the Sun again in 2003, scientists officially designated it as a short-period comet.

Pesky little P/2007 R5 appeared near the Sun again in September 2007. It still didn't have the classic comet features, but on this passage near the Sun, when it was just 15% of Mercury's distance from the star, it brightened by a factor of a million and faded again. These brilliant flashes are common for comets, but not for asteroids.

Based on its orbit, P/2007 R5 is likely a comet from the Kuiper belt, a reservoir of icy comets and asteroids that orbit the Sun way out beyond the orbit of Neptune. Most of P/2007 R5's surface ice was probably baked off during previous passes near the Sun, so that it now shows little to no tail or comma when it feels the Sun's heat.

So, is it a comet or is it an asteroid? Scott Sheppard of the Carnegie Institution of Washington in DC says, "There's probably not a sharp cutoff. Comets probably have a lot of rock in them and asteroids probably have a lot of ice in them as well." link