PLANET Earth will get a close look at a wandering demon when asteroid Apophis passes within a few million kilometres on Thursday.

99942 Apophis - named after the Egyptian demonic serpent-spirit of destruction - blasted itself into public awareness back in 2004 when astronomers calculated a 1 in 45 chance of it hitting the Earth on 13 April 2029.

New data and fresh calculations have all but ruled out the risk for 2029 - but exposed a small chance that the asteroid could get alarmingly close in 2036.

The 390m lump of space rock will be close enough to be seen in the night sky after it has passed its closest point at 14.5 million kilometres later this week.

Astronomers are not sure about the exact risk Apophis poses in 2036.

This is why a forest of telescopes will be pointed towards the asteroid as it comes into view on Thursday.

NASA will use deep-space radars at Goldstone, located in the Mojave desert in California, and Arecibo in Puerto Rico to scan the asteroid.

"Using new measurements of the asteroid's distance and line-of-sight velocity, we hope to reduce the orbital uncertainties and extend the interval over which we can compute the motion into the future," NASA Jet Propulsion Laboratory's Lance Benner said.

"It's possible that the new measurements improve the orbit to the point that we can completely rule out an impact."

The asteroid's mass, shape and spin need to be discovered to create the most precise possible calculations of its orbit to determine just how much of a risk it will be in 2036.

Russia has gone so far as to announce plans to land a homing beacon on Apophis in 2020 to guarantee pinpoint accuracy.

That's not to say it won't get close in 2029: It is calculated to skim past at 30,000km. The Moon orbits at 385,000km, and communication satellites at 36,000km.

Apophis is now on top of the Potentially Hazardous Asteroids list maintained by the International Astronomical Union.

If the asteroid were to hit Earth in 2036, it would explode with the power of 100,000 Hiroshima nuclear bombs. Thousands of kilometres will be affected directly, and the cloud of smoke and dust will envelop the planet.

And we need to calculate Apophis' orbit fast: A meeting of near-Earth object experts in London last years said it could take decades to invent, design and build the equipment necessary to deflect the space rock.