​New York, April 19 (IANS) Space missions are likely to encounter a growing threat of more debris, scientists warned on Tuesday at the 7th European Conference on Space Debris.

The four-day meeting was held in the southern German city of Darmstadt, where the European Space Operations Centre (ESOC) is located.

Since 1957, over 4,900 space launches have led to an on-orbit population of more than 18,000 tracked objects, Xinhua news agency reported.

Of those, only 1,100 are functional spacecraft and the remaining are space debris, according to European Space Agency (ESA), an intergovernmental organisation consisting of 22 European member states.

As regards tiny objects larger than one millimetre, which are hard to be tracked but able to harm spacecraft in a collision, the amount of those objects has risen to ca. 150 million.

In addition, around 20,000 orbiting fragments with sizes over 10 centimetres have been found nowadays, 12,000 more than the total amount in 1993.

"We are very much concerned," said Rolf Densing, director of operations at the ESA.

In the coming days, experts will further discuss different aspects of space debris research including measurement techniques, environment modelling theories, risk analysis techniques, and protection designs.