Shown here are the 50 objects, mainly rocket stages, that an international panel of experts view as the most dangerous pieces of orbital debris. Credit: Centauri

PARIS — An international group of space experts, mainly scientists using their own algorithms, assembled a list of the 50 most dangerous pieces of orbital debris and concluded that the top 20 are all Soviet/Russian Zenit 2 SL-16 rocket stages.

The criteria adopted by the 19-member group, which included Russian, Chinese, European, Japanese, U.S. and private-sector representatives, began with an assessment of the . . .

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