Fermi Paradox, posed (if only, perhaps, apocryphally) by nuclear physicist Enrico Fermi. More expansively: if life arises naturally on suitable planets, then why -- with so MANY stars around us -- hasn't intelligent life contacted or visited Earth? If aliens haven't, maybe the premise about life arising naturally and then intelligent life following naturally is suspect.
(Regular readers of this blog know that as an SF author I'm interested in the search for extraterrestrial intelligence (SETI) and in First Contact scenarios. This post is about one small aspect of the science of SETI. If you're curious about my fictional uses of SETI, check out my InterstellarNet series and Moonstruck posts.)
Traditional SETI, based on listening for radio signals, has been noticeable for its lack of success. It turns out listening isn't the only SETI option. There's a SETI offshoot called search for extraterrestrial artifacts (SETA). A subset of SETA is search for extraterrestrial vehicles (SETV). The dressed-up term for it is xenoarcheology. Like astrobiology, xenoarcheology is a science for which, to date, there is no proof of the existence of its subject matter.
Finding (or thinking one has found) physical evidence of aliens is an SF staple. Consider, for example, the black monoliths in the acclaimed 2001: A Space Odyssey. And I've always been partial to the cover at left, from James P. Hogan's Inherit the Stars.
The SETV argument goes like this. Signaling across interstellar distances takes a lot of energy. Sending that (expensive) signal yields a benefit only if someone hears it. Sending a space probe is much more energy efficient -- albeit slower -- than beaming across the light-years. The visiting probe can gather data independent of the tech level of any creatures in the visited solar system. The probe can also choose to announce itself with a locally transmitted signal. Proponents of SETV advise us to look around our solar system for alien spacecraft.
IMO, for the SETV conjecture to merit consideration, one must assume either that:
- the aliens have far better propulsion tech than humans (probes that can speedily cross light-years, then signal home or return with findings), or
- that the aliens have a very patient civilization that's expected to be much longer-lived (and static?) than anything we humans have accomplished.
To amplify briefly on that second bullet point: I can't see humans sending probes across light-years at chemical rocket speeds because they might report back something interesting in thousands of years. In thirty-three years, Voyager 1, the most distant man-made object, has reached a point almost 18 billion kilometers from the sun. That sounds impressive --but Voyager 1 has traveled less than 0.1% of the roughly 40 trillion kilometer distance to our sun's nearest neighbor, Proxima Centauri. (No, Voyager isn't headed that direction.)
Where would one look for an alien artifact? Robert Freitag, a chief proponent of the artifact hypothesis, suggests some criteria in THE SEARCH FOR EXTRATERRESTRIAL ARTIFACTS (SETA). If a probe is smart enough to do its job, Freitag suggests, it will first figure out what planet to watch. Presuming (which Freitag did) that the aliens are something like us, they'd spot Earth. Then they'd want locations that could watch Earth:
- for long periods of time,
- without interruption, and
- in conditions safe for the probe.
We should be looking for alien spacecraft, Frietag says, in:
- Geocentric orbits located between two Earth-centred concentric spheres of radii 70,000 km
and 326,400 km;
- Selenocentric orbits between 3000-58,100 km lunar altitude;
- Stable synodic libration orbits around Earth-Moon Lagrangian points L4 and L5;
- Earth-Moon halo orbits near collinear Lagrangian points LI and L2; and
- Sun-Earth L4/L5 Lagrangian orbits.
There's not a lot of weather or seismic activity on the moon. There's plenty of sunlight for energy (any probe that can cross light-years can surely store power for two-week lunar nights) and material for many kinds of self-repairs. Viewing is uninterrupted and is from a comparatively close distance -- certainly closer than the sun/Earth L4/L5 points.
NASA's Lunar Reconnaissance Orbiter has captured images of Apollo landing sites. Maybe someday soon the LRO will spot something even more interesting.
It will take a concerted search, though. The moon, even "just" the Earth-facing side, is a big place. And nothing says the hypothetical probe will be anywhere near as large as an Apollo landing stage.
Maybe squinting will help :-)