Thursday, January 30, 2020
The life of Earth has been evident in our atmosphere’s spectral lines for over a billion years. The oxygen, which is due to life, can be observed over interstellar distances, thousands of light years. Over this long time, many stars have swept near our solar system and Earth. If extraterrestrial intelligence (ETI) on such nearby stars sent probes to observe our ecosystem, they may have sent probes to study it. Where in the Solar System should we look? The Moon is the obvious, closest place. Another is the newly discovered class of co-orbital objects, a logical place to locate for observing Earth. These objects approach Earth very closely every year at distances much shorter than anything except our Moon. They are an ideal way for ETI to watch our world from a secure natural object that provides resources an ETI might need: materials, a firm anchor, concealment. They would likely be robotic probes, like our own Voyager and New Horizons probes. They would remain there after exhausting their energy supply. Studying the Moon and co-orbitals could be termed extraterrestrial archeology. For the Moon, we can use the photographic mapping of the Moon’s surface by the Lunar Reconnaissance Orbiter. Co-orbitals have been little studied by astronomy and not at all by SETI or planetary radar observations. This discussion describes the strategy of looking for ETI artifacts and proposes both passive and active observations by optical and radio listening, radar imaging and launching probes. We might even broadcast to them. But what if we find nothing there? That gives us a profound result: no one has come to look at the life of Earth, so the probability of ETI’s existence will be reduced. On that other hand, perhaps other civilizations are simply not as curious as we are.