• Lagrangian points in the Earth-Moon. The L4 and L5 Lagrangian points lie 60° ahead of and behind the Moon in the same orbit around the Earth and are two of five stable points in the Earth-Moon system where bodies can in principle remain in stable positions for long periods. It has been proposed that they would be possible sites for human space colonies, but this plan has the drawback that dust tends to gather there, which could damage spacecraft in the same location. Small ‘cloud satellites’ have been tentatively observed at L5.