A huge asteroid flew past the Earth today, and is expected to be visible from Canada with strong binoculars or a small telescope  tonight.

Based on its brightness, asteroid 2004 BL86 was estimated to be about 500 metres across. But NASA scientists later revised their estimate to about 300 metres, based on radar measurements. If it were sitting on Earth beside Toronto's CN tower, its top would be a little lower than the revolving restaurant.

Asteroid 2004 BL86 compared to CN Tower and a cruise ship

The asteroid is about 300 metres across - here's how its size compares to the CN Tower and a cruise ship. (Earl Cabuhat/CBC)

They also revealed Monday morning that the space rock has a very short "day" — it rotates once every 2.6 hours — and is orbited by its own tiny moon.

The asteroid's moon, along with features that may be boulders on the rotating asteroid, could be seen in a video of the radar images shown by NASA research scientist Lance Benner on a live webcast during the space rock's close approach Monday morning. The event was broadcast by Slooh, a group that streams celestial events online from telescopes around the world.

At its closest approach at 11:19 a.m., the huge space rock was about about 1.2-million kilometres from the Earth, or about three times further away than the Earth's moon. 

Asteroid orbit0:06

That's a safe distance, but closer than any other asteroid this big will come until 2027 — the year when we can expect a visit from another chunky rock called 1999 AN10.

Viewing tips

You may be able to see the asteroid this evening using strong binoculars or a small telescope, NASA says.

It will probably be easiest to find around 11 p.m. ET when it will be to the right of the planet Jupiter, between the constellations Leo and Gemini, high in the eastern sky.

"Jupiter is the brightest star-like object in the sky these days. You can't miss it," said J. Randy Attwood, executive director of the Royal Astronomical Society of Canada. "And the asteroid will be really close to Jupiter."

He recommends that you:

  • Find a dark back yard or park where you can block out as much light as possible. 
  • Point your telescope toward the east with Jupiter within view, taking care that the field of view is wide enough for you to see both Jupiter and the asteroid.
  • Make a diagram of what you see and check back later to see if anything moved.

Attwood said this is a rare opportunity because it's not often that celestial objects are moving fast enough for us to notice their motion — planets plod along so slowly that their motion against the stars is only visible on the scale of days or weeks.

In this case, the asteroid will move the width of four full moons in an hour, not that quickly.

"You may not see it moving," Attwood said.

You may not recognize the asteroid the first time you look, but it should be obvious the second time, when it will be in a different place.

He says there's great satisfaction that comes with spotting an object like this yourself.

"To think this is something half a kilometre in size, 1.2 million kilometres away and you're seeing it from your backyard — that's exciting."

NASA will also be watching the asteroid, capturing radar images and data using its Deep Space Network antenna at Goldstone, Calif. NASA Arecibo Observatory in Puerto Rico will also be following it over the next few days.

Benner said this will be the best asteroid of the year to observe by radar.

"The big night is tonight," he added.

The asteroid was detected in 2004 and will not come this close to Earth for at least 200 years, NASA says.

On mobile and can't see the video replay below? Watch it here.