One way that astronomers and astrobiologists search for life in the galaxy is observation of rocky planets orbiting other stars. Such planets may contain an atmosphere, liquid water, and other ingredients that are required for biological life on Earth. Once a number of these potentially inhabited planets have been identified, the next logical step in exploration is to send remote exploratory probes to make direct observations of these planets. Present-day study of other planetary systems is so far limited to remote observation with telescopes, but future plans for exploration include the design and deployment of small robotic exploratory spacecraft toward other star systems.
If intelligent, technological extraterrestrial life exists in the galaxy, then it is conceivable that such a civilization might embark on a similar exploration strategy. Extraterrestrial intelligent (ETI) civilizations may choose to pursue astronomy and search for planets orbiting other star systems and may also choose to follow-up on some of these targets by deploying their own remote exploratory spacecraft. If nearby ETI have observed the Solar System and decided to pursue further exploration, then evidence of ETI technology may be present in the form of such exploratory probes. We refer to this ETI technology as “non-terrestrial artifacts”, in part to distinguish these plausible exploratory spacecraft from the flying saucers of science fiction.
In a recent paper titled “On the likelihood of non-terrestrial artifacts in the Solar System”, published in the journal Acta Astronautica (and available on arXiv.org as a preprint), Jacob Haqq-Misra and Ravi Kopparapu discuss the likelihood that human exploration of the Solar System would have uncovered any non-terrestrial artifacts. Exploratory probes destined for another star system are likely to be relatively small (less than ten meters in diameter), so any non-terrestrial artifacts present in the Solar System have probably remained undetected. The surface and atmosphere of Earth are probably the most comprehensively searched volumes in the Solar System and can probably be considered absent of non-terrestrial artifacts. Likewise, the surface of the moon and portions of Mars have been searched at a sufficient resolution to have uncovered any non-terrestrial artifacts that could have been present. However, the deep oceans of Earth and the subsurface of the Moon are largely unexplored territory, while regions such as the asteroid belt, the Kuiper belt, and stable orbits around other Solar System planets could also contain non-terrestrial artifacts that have so far escaped human observation. Because of this plenitude of nearby unexplored territory, it would be premature to conclude that the Solar System is absent of non-terrestrial artifacts.
Although the chances of finding non-terrestrial artifacts might be low, the discovery of ETI technology, even if broken and non-functioning, would provide evidence that ETI exist elsewhere in the galaxy and have a profound impact on humankind. This is not to suggest that the search for non-terrestrial technology should be given priority over other astronomical missions; however, as human exploration into the Solar System continues, we may as well keep our eyes open for ETI technology, just in case.
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A challenge for ETI exploratory missions, just as for our own probes to the outer solar system, is stopping once you get to your destination. Travelling between stars in any sensible timeframe involves accelerating hardware to prodigious velocities; and that’s hard. The braking manoeuvres required to leave any detectable ETI artefacts in our solar system would be harder still.
The Haqq-Misra paper reminds us to keep our minds open about what qualifies as evidence of ETI. Artifacts may not be recent; old ones may be dead, non-emitting technology, very difficult to find.
Eves is right about the energy requirments of braking; our own first interstellar probes may be fly-throughs. Perhaps braking will be used only in follow-up missions, if the fly-through probes find something of great interest. Project Daedalus proposed launching much smaller sub-probes that would remain in the target system, but that implies a massive mother probe.