A comet the size of a small mountain has whizzed past Mars, wowing space enthusiasts with the once-in-a-million-years encounter.

The comet, known as Siding Spring (C/2013 A1), made its closest encounter with Mars on Sunday at 1827 GMT (0527 AEDT), racing past the Red Planet at a dazzling 203,000 kilometres per hour.

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At its closet, Siding Spring was 139,500 kilometres from Mars – less than half the distance between Earth and our moon.

“Signal confirming closest approach has just been received,” the European Space Agency said on Twitter.

Before the comet passed, it could be seen in space racing toward the bright planet, trailed by a tail of debris.

The ball of ice, dust and rocks is believed to have originated billions of years ago in the Oort Cloud, a distant region of space at the outskirts of the solar system.

The comet is around a mile wide and is only about as solid as a pile of talcum powder.

NASA’s fleet of Mars-orbiting satellites and robots on the planet’s surface were primed for the flyby of the comet, hoping to photograph the rare event.