ST. PETERSBURG -- The asteroid and comet threat is a real challenge to mankind in the 21st century, and it is described as "space terrorism", Russia's Itar-Tass news agency reported Saturday quoting the Director of the Institute of Applied Astronomy Andrei Fenkelshtein, as saying.

Speaking at the regional Itar-Tass centre, Fenkelshtein said: "This phenomenon is well studied from the scientific point of view even though speculations continue to revolve around the event that occurred one hundred years ago."

In his opinion, what happened in Southern Siberia is "a serious warning to the earthlings.

Filkenshtein, who is also the correspondent member of the Russian Academy of Sciences, said the risk of a new event like the Tunguska explosion is assessed as quite plausible now that scientists have detected potential killer Asteroid Apophis just half a million kilometers from Earth, which is an infinitesimally small distance by astronomical standards.

Most experts agree that Apophis requires closer scrutiny, and to that end, in February 2008 the Planetary Society awarded US$50,000 in prize money to companies and students who submitted designs for space probes that would put a tracking device on or near the asteroid.

According to mathematical modeling data, if an asteroid several kilometres wide or a comet nucleus collides with the planet, "mankind may be wiped out of the face of Earth instantly or may be thrown back several centuries in its development".

If a space body falls into the oceans, the effect will be "much more devastating than, for example, seismic tsunami", Filkenshtein said.

He said: "The impact of a rock asteroid some 500 metres in diameter on the surface of the ocean at a speed of 10-20 meters per second will create a cavern causing circular waves that may come as high as four kilometers in the centre and 300-400 metres when they hit the continents."

He believes it is quite likely that similar events "caused 90 percent of all animals on Earth to die millions of years ago".