Russian space scientists are to send a probe to an asteroid that continues to pose a small but real collision threat to Earth.

They want to plant a radio beacon on the space rock 99942 Apophis to help monitor its orbit and so calculate just how big the danger is.

Apophis, a near-Earth asteroid that is about 300 metres wide, was only discovered in June 2004 with a telescope at Kitt Peak, Arizona, but promptly lost again.

It was rediscovered in December the same year from Siding Spring Observatory in New South Wales, Australia. Calculations quickly showed it will come close to Earth twice in seven years.

The first will be in 2029. It could fly inside the orbits of geostationary satellites on Friday 13th of April 2029 when Apophis will pass within just 29,450 km of Earth (18,300 miles) and become bright enough to be seen with the unaided eye as it flies over the Atlantic Ocean, north of Brazil. 

Uncertainty over the effects of the 2029 near miss meant that scientists were uncertain how close it might come on its next approach in 2036. 

It means that, unusually, NASA's asteroid experts have still not been able to rule out an impact completely, though the odds of a catastrophic collision have been reduced from one-in-45,000 to a more reassuring one-in-250,000.

But the threat has focused minds on protecting the planet against cosmic impacts. In 2010, Rusty Schweickart, former Apollo astronaut and campaigner for action to defend the planet, wrote in the New York Times: "By preventing dangerous asteroid strikes, we can save millions of people, or even our entire species."

Russia's Academy of Sciences said on Saturday that they consider the threat from Apophis to be the most serious currently facing our planet.

The aim of their planned mission is to send a satellite powered by a radioisotope to the asteroid. The probe could be launched from 2015 onwards with a mission to discover the cosmic missile's exact trajectory.