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If a previous industrial civilization put satellites into geo-stationary orbit, they'd still be up there. If there are unknown objects in geo-stationary orbit,

• droithomme 7y ago
  Not all industrial civilizations launch geo-stationary satellites.
  • hermitdev 7y ago
    Only the ones we know of.
    • droithomme 7y ago
      Not necessarily. It is true that the industrial civilizations we know of, considered on a consolidated planetary short-recent-history level, have launched satellites in geostationary orbits. But we can not logically infer from this that only the ones we know of have launched satellites in geostationary orbits. There may be ones that have launched geostationary satellites which we do not know of.
• dredmorbius 7y ago
  "Geostationary" might have been rather lower in elevation than today, depending on when in time that civilisation might have occurred. The rate of decrease has not been constant with time, and may have been roughly constant from ~2.5 gya to 600 mya. During the Precambrian era, the Earth's day was ~21 hours long, and a geostationary orbit would have been about 32,265 km rather than 35,787 km today. So the satellites might not be precisely where we're putting them, but: 1. We'd all but certainly spot them. 2. The elevation / orbital period itself would give a very strong indication as to when the civilisation had existed.