Which sustainability practice has been most widely adopted by major astronomical observatories to reduce their environmental impact?

Sustainability has become increasingly important across all industries, including astronomy and space exploration. Major observatories and space agencies are implementing various green initiatives to reduce their environmental footprint while continuing their mission to explore the cosmos. Test your knowledge about cutting-edge sustainability practices in modern astronomical facilities and organizations.

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Solar power systems that provide at least 30% of operational energy needs
Biodegradable telescope mirror coatings that naturally decompose when replaced
Mandatory carbon offset purchases equal to 100% of facility emissions
Water recycling systems that convert atmospheric moisture into drinking water