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186 Other technological civilizations capable of undertaking interstellar travel with probes or inhabited vehicles do not pursue it. One or more civilizations have undertaken interstellar travel in the past, but not throughout the Galaxy. Interstellar exploration and colonization may be episodic, limited in time and space rather than continuous; motiva- tions for exploration and colonization may change over time. If other civi- lizations expand more continuously, the rate may be slow. Colonies may not engage in interstellar exploration or colonization themselves. Isolation Other civilizations are exploring or colonizing on an interstellar scale, but their explorations or expansions have not entered our part of the Galaxy. The gap between their location and our solar system may seem, to them, not worth crossing, even if they know we exist. We may be in an uninterest- ing neighborhood, or our system may be unattractive to them. They are exploring or expanding toward our solar system, but they are not here yet. They may be coming our way for reasons unrelated to us; they may not know of our existence. Even if they do know, we may not be the primary reason for their journey. Or they may be coming because they detected our presence. Extraterrestrials or their machines have reached our solar system in the past, without leaving evidence of being here. If they did leave evidence, we have not found it or do not recognize it. Within this context, a probe or inhabited vehicle may have visited the Earth. It may or may not have been detected by humans. A probe or inhabited vehicle is in our solar system, but we do not see it. If it is a machine, it may have ceased operating long ago. If it is a habitable spacecraft, its passengers may no longer be alive or may have transferred to another vessel and left. Reformulating the Problem Possible, But Not Worth Doing Limited Ambitions On Their Way They Visited a Long Time Ago Here But Undetected