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139 neighborhood could give us information about distant civilizations more quickly than searching for signals or waiting for alien messages. The authors of Project Cyclops acknowledged that we might stumble onto something of this kind while engaged in archaeological or astronomi- cal research. They “felt” that the probability of this happening was extremely small. Physical artifacts may not be limited to spacecraft. MacVey suggested others: laboratories, radio relay stations, telemetry stations, marker beacons, monuments and edifices, implements, refuse, or environmental changes such as radioactive hot spots and paleomagnetic anomalies.** We might add graves. If visitors left traces of themselves on the surface of the Earth, those artifacts may have been destroyed or buried long ago by geological change and erosion by water and wind. On some other solar system bodies—our Moon, the moons of Mars, the asteroids—artifacts might survive undis- turbed over geological time. A Mirror Image. We ourselves have left artifacts on the Moon—uninten- tional cairns of human technology—that will remain visible for millions of years even if our species goes extinct. Other human artifacts on Mars, Venus, and Titan may have shorter detectable lifetimes due to erosive processes. Baxter proposed that traces might be found under Martian polar ice, including the trails of skis and sleds, artificial elements, or “unnatural” isotopic ratios. However, if alien visitors practiced the kind of planetary protection protocols that are to guide our own exploration of other planets, they may have left no trace at all. A human-made machine known as the Phoenix Lander, designed to settle on Mar’s North polar cap in 2008, will be buried gradually under several feet of frozen carbon dioxide after com- pleting its mission.*° What if aliens deliberately left evidence of their visits, as European explorers often did during their great age of exploration? Two obvious locations for a long-lasting monument in our solar system would be on one of the rocky bodies that lack an atmosphere (like Earth’s Moon) or in orbit somewhere. Unless the latter emitted signals, it would be very difficult to find.*’ In his science fiction story “The Sentinel” (one of the main inspirations for the film “2001: A Space Odyssey”), Clarke imagined an alien monu- ment deliberately placed under the surface of our Moon. Humans find it only after they achieve space travel. This intelligence detector reports to its home civilization when humans interfere with it.** Ground-penetrating radar mounted on orbiting spacecraft could open up new ways of looking for artifacts buried under the surfaces of solid bodies in our solar system. One device under development for a future Astroarchaeology