NASA has downgraded the threat from a near-Earth asteroid.

Asteroid Apophis was discovered on June 19, 2004. (UH/IA)

Researchers at NASA's Near-Earth Object (NEO) program in Pasadena, Calif., said Wednesday that a recalculated orbit of the asteroid Apophis puts the odds of it hitting our planet on April 13, 2036, at one-in-250,000.

The asteroid Apophis, discovered in June 2004, is about 270 metres in diameter and weighs 27 million tonnes.

The calculation is based on new observations of the asteroid from large optical telescopes in Hawaii, Arizona and Puerto Rico. The asteroid's quirky orbit is now expected to bring it close to the Earth in 2029, 2036 and 2068.

Shortly after it was found, it was given a 2.7 per cent chance of striking Earth in 2029. Later study dropped the likelihood of a collision in 2036 to one in 45,000.

Apophis is expected to come no closer than 29,500 kilometres above the surface of the Earth on April 13, 2029. By comparison, the moon is more than 375,000 kilometres away from the Earth.

The closest approach on record was a small asteroid called 2004 RU162, which passed within 6,500 kilometres, or about one Earth radius.

In 1908, an asteroid exploded in the atmosphere at an altitude of six to 10 kilometres above the Tunguska region of Siberia in 1908.

That rock, estimated to be about 50 metres in length, sent a blast of heat and a shockwave with a likely energy equivalent of about 10 to 15 megatons of TNT, or about 1,000 times more powerful than the blast from the bomb dropped on Hiroshima during the Second World War.

An estimated 80 million trees covering more than 2,150 square kilometres were flattened, but only two deaths were reported in the sparsely populated region.

Apophis was once rated a record-breaking four out of 10 on the Torino Impact Hazard Scale. The Torino scale starts at zero, given to events of "no likely consequences." Phrases such as "regional devastation" start creeping in at about four on the scale.

The scale ends at 10, with what the NEO office describes as a certain collision "capable of causing global climatic catastrophe that may threaten the future of civilization as we know it."

Currently, the Torino rating for Apophis is zero.