By ALISON HORWOOD

Declassified government files have shed new light on the discovery of mysterious space rubble on Canterbury farmland 30 years ago.

On March 31, 1972, the Russian space vessel Soviet Cosmos 482 broke into four parts, two of which remained in low orbit and eventually rained down on Earth.

At 1am on April 3 that year, four red-hot 13.6kg titanium alloy balls landed within a 16km radius of each other, just outside Ashburton.

The 38cm-diameter spheres scorched holes in crops and made deep indentations in the soil, but no one was injured.

A similarly shaped object was discovered near Eiffelton, 20km from Ashburton, six years later.

The findings were reported at the time, but declassified Ministry of Foreign Affairs reports filed with Archives New Zealand contain further information, including correspondence on the matter between the then Soviet and United States authorities.

The documents say the balls were thoroughly analysed by New Zealand scientists.

They announced that they were Soviet in origin because of manufacturing marks and the high-tech welding of the titanium - a science on which the Russians had expended a good deal of resources.

Correspondence from Sir Keith Holyoake, then Foreign Affairs Minister, concludes that they were probably gas pressure vessels of a kind used in the launching rocket for a satellite or space vehicle and had decayed in the atmosphere.

The only two space vehicles at the time containing the objects concerned were American and Soviet, and the US craft's re-entry path would not have carried it over New Zealand.

Article 5 of the Agreement on the Rescue of Astronauts, the Return of Astronauts and the Return of Objects Launched into Outer Space - an international treaty - required that any recovery of space rubble be reported to the United Nations Secretary-General and the launching authority.

But Sir Keith said the Soviet authorities had formally disclaimed ownership.

A report from the US Air Forces Systems Command foreign technology division traced the spheres to a Soviet attempt to fire its Venus 9 unmanned spacecraft out of a parking orbit.

It failed on March 31, 1972, and the craft was re-designated Soviet Cosmos 482.

The US authority also supplied a background report on space debris found worldwide between 1960 and 1972.

According to the list of 44 strange findings, the titanium balls were the first space rubble found in New Zealand.