Cologne, Germany - German scientists said Thursday they are planning a satellite especially to hunt space rocks that might slam into the planet Earth and wipe out complete cities or countries.

The modest satellite, costing 15 million euros (24 million dollars), would scan space for near earth objects. An NEO is a celestial body which orbits the Sun on a track that crosses Earth's orbit.

The German space agency DLR said modelling showed there must be more than 1,000 such objects more than 100 metres wide, but only nine have been discovered so far.

It spoke as the centenary approaches of the June 30, 1908 Tunguska Event.

The DLR said its interpretation of that event was that a rock 30 to 50 metres wide entered the atmosphere over Siberia and exploded 8 to 12 kilometres above the surface, destroying 2,000 square kilometres of forest.

"It's vital to identify NEOs before they get onto a collision course with the Earth," said Ekkehard Kührt of the DLR planetary research institute.

Development work on the satellite is probably to begin this autumn. The satellite would also track space junk, which consists of satellite debris orbiting the Earth.

DLR says that with sufficient warning time, such as 10 years, an NEO could be nudged away into an orbit that did not threaten the Earth. A spacecraft could crash into the NEO and knock off a chunk, leading to a change in the NEO's track. (dpa)