Andromeda galaxy image Andromeda, our galaxy’s next-door twin with 100 billion stars. Both galaxies should harbor spacefaring civilizations. Where are they? . ASTROPHOTOGRAPH BY FRANK SANDERS[/caption]

Recently, we’ve checked out a UFO that turned out to be a high-altitude research balloon. That’s a typical outcome for UFOs; most, maybe all, UFOs would be resolved if observers had binoculars.

There is no evidence that any extraterrestrial spacecraft have visited earth, either during the modern era or at any other time in the last three billion years. The lack of any apparent alien visitation across this vast stretch of geological time is called the Fermi (FAIR-mee) Paradox, after the nuclear physicist Enrico Fermi (1901-1954).

Here’s the paradox’s reasoning.

Our home Milky Way galaxy has 100 billion stars (closely resembling our neighboring galaxy, Andromeda, 2.5 million light years away). Working through probabilities for formation of planets, the likelihood of some planets having life, the chances of developing intelligent life, and the possibility of some ultimately becoming spacefaring, the Milky Way ought to have evolved a dozen interstellarcapable civilizations over the last few billion years.

A spacefaring civilization only slightly more advanced than us could send robots to visit every star system in the galaxy. Their tools would be automated, self-replicating probes called Von Neumann Machines after mathematician John Von Neumann (1903-1957).

In this scheme, the home planet builds two VNMs and sends them to nearby stars at one per cent of the speed of light. In 500 years, these two initial VNMs arrive at neighboring stars. They use artificial intelligence to locate planets with resources, land, configure themselves as factories, and each build two more VNMs. That second generation of four VNMs goes out to other nearby stars, in a viral reproduction process.

In 37 probe-replication generations, the number of contacted stars exceeds 100 billion, all the stars in the galaxy. The elapsed time might be a few million years, an eye blink in the life of our galaxy (which has had life-friendly star systems for over 5,000 million years).

Along the way, the probes leave cosmic calling cards for any intelligent beings that might evolve and find them later. In our solar system, logical places for these would be our moon and Mars; metal or stone leave-behinds will survive there for billions of years.

Finding no such monuments indicates that none of the dozen or more spacefaring civilizations that should have evolved by now in the Milky Way has ever been here. (Or they have visited, but have inexplicably left no notes behind. Which would be strange, considering that it costs nothing and that we ourselves were bolting calling cards into our own interstellar Pioneer and Voyager probes as far back as the 1970s.)

That’s the Fermi Paradox. (Fermi having exclaimed in 1950, during a lunch discussion about extraterrestrials, “So where is everybody?”) Why, if spacefaring civilizations commonly evolve, has nobody swung through our solar system in the last several billion years?

Apparently, spacefaring civilizations are either much rarer than we think they should be, or else they tend to obliterate themselves when they get to, or beyond, our own stage of development. Something for us to contemplate as we face global existential challenges?

Backyard Astronomy Talk Series: This correspondent’s next talk, “Decoding the Star of Bethlehem” will be at the Nederland Library at 6 p.m. on December 6. Details and free registration at https://nederland. colibraries.org/backyard-astronomy/.

In November Skies: The sun begins the month in Libra, entering Scorpius on November 23. At mid-month, days and nights are 9.5 and 14.5 hours long, respectively.

The moon’s dates are: Last Quarter November 5; New November 13; First Quarter November 20; Full (Beaver Moon) November 27. Standard Time begins on November 5; we gain time as we move our clocks backward one hour at 2 a.m.

November Meteors: The North Taurid shower (parent comet Encke) peaks November 11-12. The Leonid shower (parent comet Tempel-Tuttle), one of the year’s best, peaks November 18. See it while stretched out in a lawn chair with a warm drink.

Best Sky Viewing Nights (Minimal Moon): November 5-20.

Sunset (Mid-Month): The Milky Way glistens, southwest to northeast. Fomalhaut rises with the Pleiades and Capella. The Andromeda galaxy (most-distant object visible to the naked eye at 2.5 million light years) is high in the northeast; it presents a nice view in binoculars. Deneb is directly overhead, with Vega a bit westward and Altair southward. Arcturus sets in the west.

Midnight (Mid-Month): Gemini twins Castor (above) and Pollux (below) are high in the east. Below them and to the right is Procyon. Orion and Sirius are in the southeast. M42, the Great Nebula that births stars in Orion’s sword, is naked-eye visible; it presents a terrific view in binoculars. Andromeda is high in the northwest. Brilliant Capella is nearly overhead; Cassiopeia is high in the northwest.

Sunrise (Mid-Month): Spica is low in the southeast; Arcturus is high in the east; the Big Dipper is high overhead. Capella is high in the northwest as Orion sets in the west.

Mercury is low in the west at sunset, leftward of the just-set sun, at the end of November. Look for it at maximum elongation on December 4, from Barker Reservoir.

Venus is the Morning Star, in the southeast before sunrise.

Mars is lost in the sun.

Jupiter, in the Ram, is at opposition on November 2. Directly opposite the sun, this month is the best time to see it with its moons all year.

Saturn, in the Water Carrier, is in the south-southeast at sunset, setting at midnight.

Notable Space Missions: NASA’s Lucy probe will fly by Asteroid Dinkinesh on November 1. The close approach, at 270 miles distance, is an engineering test for a later rendezvous with Trojan asteroids near Jupiter in 2027.

Frank Sanders, a spectrum scientist at the U.S. Department of Commerce in Boulder, takes astronomy-related inquiries at backyardastronomy1@gmail.com.