An asteroid the size of Vancouver's Stanley Park, and which has its own moon, zipped past the Earth today, NASA says.

At 4:59 p.m. ET Friday, the pair of space rocks, known together as a binary asteroid, made their closest approach to Earth for the next two centuries at 5.8 million kilometres away, or about 15 times the distance between the Earth and the moon. That is considered a safe distance and the asteroid is not at risk of hitting the Earth.

The 2.7-kilometre-wide asteroid, 1998 QE2, was discovered 15 years ago, but no one knew about its travelling companion until the smaller rock showed up in radar images captured Wednesday evening. The satellite or moon is about 600 metres wide or about three times the width of Toronto's Rogers Centre.

The asteroid's moon (small white dot) was discovered when it appeared in a sequence of radar images of the asteroid, including these two, taken Wednesday evening. (NASA)

According to NASA, about 16 per cent of near-Earth asteroids larger than 200 metres wide are binary or triple systems, with one or two moons.

The radar images of 1998 QE2 were captured by NASA's Deep Space Network antenna at Goldstone, Calif., which is working with the Arecibo Observatory in Puerto Rico to study its size, shape, rotation and surface features, as well as measure its distance and speed to get more accurate information about its orbit.

Both asteroids are much larger than DA14, the 45-metre wide asteroid that caused a buzz when it flew by Earth in February.

However, DA14 passed just 27,700 kilometres from the planet's surface — a mere tenth of the distance between the Earth and the moon — far closer than this week's visitors.