Astronomers have detected a half-kilometre wide asteroid that appears to be on a collision course with the Earth — in 172 years time.

An artist's image depicts an asteroid hitting Earth. Scientists say the asteroid named 1999-RQ36 has a one-in-a-thousand chance of hitting the Earth in 2182. (NASA)

In a study reported in the journal Icarus, scientists say the asteroid named 1999-RQ36 has a one-in-1,000 chance of hitting the Earth in 2182.

Known as a PHA, or potentially hazardous asteroid, 1999-RQ36 was first discovered about 10 years ago and since then astronomers have been plotting its orbital trajectory.

Co-author of the study, Dr. Maria Eugenia Sansaturio from Spain's Universidad de Valladolid, says the total impact probability for 1999-RQ36 is estimated to be 0.092 per cent, and that 2182 is the most likely year for an impact to occur.

The research also involved scientists from Italy's University of Pisa, the INAF-IASF Research Institute in Rome and NASA's Jet Propulsion Laboratory in Pasadena, Calif.

The scientists used two mathematical models to estimate the potential of an impact between the 560-metre wide asteroid and the Earth through to the year 2200.

They also made some 290 optical observations and 13 radar measurements to help define the asteroid's orbit.

The Yarkovsky effect

Although the orbit of 1999-RQ36 is now well determined, there remains a significant orbital uncertainty because, besides gravity, its path is influenced by the Yarkovsky effect.

Associate Prof. Dr. Charley Lineweaver from the Australian National University's Mount Stromlo Observatory says the Yarkovsky effect is caused by the heating of rotating bodies.

"The principle is best demonstrated on Earth by the fact that it's usually hotter in the afternoon, than when the sun is directly overhead at lunchtime," says Lineweaver.

"This means that the hottest part of the surface isn't facing the sun, but has rotated 20, 30 or whatever degrees away from perpendicular.

"It's not much, but on a body the size of a small asteroid, photons emitted from the hottest part of the surface act like a sort of rocket exhaust pushing the object in the opposite direction. Because the object is rotating the opposite direction is not away from the sun but at an angle to it, which over time can provide enough pressure to slightly modify the orbit."

According to Lineweaver, this provides a degree of uncertainty in orbital projections for the asteroid.

"And it's that level of uncertainty that's the real issue for now," he says.