In a galaxy teeming with potentially habitable planets, it’s reasonable to speculate that other spacefaring civilizations exist — and perhaps even that some, or their robotic probes, have already reached Earth. The distances between stars are daunting, yes, but if travel time doesn’t matter — either because the aliens are non-biological robots or because they go dormant for the trip (some cicadas, for example, sleep 17 years underground only to emerge for a few months) — the possibility of extraterrestrial visitors can’t be ruled out.
That raises an obvious question: As the physicist Enrico Fermi famously asked, “Where is everybody?” Despite thousands of UFO/UAP sightings in the past 80 years alone, there is still no generally accepted evidence of an alien spacecraft reaching Earth.
It’s not as though we haven’t looked. Any number of telescopes and spaceborne cameras might have spotted an incoming spacecraft that came close to us. Ground-based telescope networks can detect objects as small as 10 centimeters as far out as geosynchronous orbit (22,000 miles altitude). In recent decades, NASA-funded early-warning systems like the Catalina Sky Survey and the Asteroid Terrestrial-impact Last Alert System (ATLAS) have been on constant lookout for asteroids that might threaten Earth. ATLAS is able to spot a small (~20-meter) asteroid several days out, and a 100-meter asteroid several weeks out.
Every once in a while, these sentinels even mistake one of our own probes for an asteroid. In 2020, for example, Catalina dutifully reported a moving object in space, presumably yet another flying rock, that was quickly recognized as the BepiColumbo spacecraft making a close approach to Earth.
So far, though, nobody’s spotted any alien probes. Despite early speculation that an interloper called ’Oumuamua, discovered in 2017 — still one of only two large objects known to have arrived from outside our Solar System — might be artificial, the consensus is that it’s just another space rock.
Our ability to spot such interstellar visitors is about to take an enormous leap, however. This summer, the Vera C. Rubin Observatory, one of the most eagerly awaited telescopes of all time, is due to see its “first light” on a remote mountaintop in Chile. Conceived more than a quarter century ago as the Large Synoptic Survey Telescope or LSST, Rubin’s job, as the original name implies, is to take pictures of the entire southern sky with high sensitivity, using the largest digital camera ever built. More importantly, it will repeat its all-sky survey every three to four nights for at least a decade.
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