Asteroid with its own moon flies by Earth this weekend

An asteroid zipping by the Earth this weekend is so big, it has its own moon.

Asteroid (66391) 1999 KW4 is a binary system - two asteroids orbiting each other. The larger is 1.5km across and resembles a spinning top or a muffin, and the smaller measures 500m.

Because of its size, 1999 KW4 is classified as potentially hazardous by the Minor Planet Center. But it poses no threat to Earth this time around, and will fly by just over 5 million kilometres away - close on an interplanetary scale, but far enough away we don't need to worry.

"The asteroid will approach from the south, and the first day of visibility also coincides with the closest approach," which is Saturday (US time), NASA said.

It'll be travelling at about 21.5km a second through the constellations of Puppis and Antlia. EarthSky reports 1999 KW4 might be visible to skywatchers with telescopes, particularly in the southern hemisphere.

It takes 188 days to complete a full elliptical orbit around the sun, crossing the orbits of Earth and Venus in doing so.

In 17 years' time the duo will fly by again, this time much closer.