Imagine this: one hundred million years ago, an advanced civilization detected strange life forms on a greenish blue planet not far from home in the Milky Way. They try to send signals, but whatever is happening in that unknown world does not respond. Then, the curious galactic explorers try something different. They send a robotic probe to a small, quiet space rock that orbits near the planet rich in life, just to watch.
If a story like this develops at any time in the 4.5 billion year history of the Earth, it could have left an archaeological record. At least, that is the hope behind a new proposal to verify the so-called Earth's coorbitals for signs of advanced extraterrestrial technology.
Coorbitals are space objects that orbit around the Sun at approximately the same distance as Earth. "They are basically spinning around the Sun at the same speed as Earth, and they are very close," said James Benford, a physicist and independent researcher at SETI (Search for Extraterrestrial Intelligence) who dreamed of the idea that aliens might have eyes positions on Earth through these coorbitals while I was at a conference in Houston last year. If you're right, coorbitals could be a way of detecting alien activity that happened before humans evolved, much less directed their attention to the stars.RELATED
To be clear, even SETI researchers who like the idea of reviewing Earth's coorbitals recognize that it is a remote possibility. "How likely it is that the alien probe is in one of these coorbitals, obviously extremely unlikely," said Paul Davies, a physicist and astrobiologist at Arizona State University who did not participate in Benford's new article on the idea, published on 20 September in The Astronomical Journal. "But if it costs very little to take a look, why not? Even if we don't find E.T., we might find something of interest," the scientist argues.
When humans began to seriously contemplate how to find extraterrestrial intelligence in the 1950s, they began simply by listening, Davies said. Unfortunately, half a decade of exploring the heavens in search of radio or other transmissions of extraterrestrial life has produced only what Davies called "the mysterious silence" in his book of the same name (Houghton Mifflin Harcourt, 2010). Recently, Davies told the Live Science portal that the SETI field has been interested in "technology signatures," or any sign of technology in the universe that has not been created by humans.
Coorbital probes would be a good example. Little is known about the coorbitals themselves, Benford said. The first was discovered in 1997, and most of the other 15 known coorbitals near Earth were found after 2010. They move around the Earth in strange configurations, some of which look like horseshoes or tadpoles, while they travel.
The most immediate, known as Earth's closest companion, is approximately 38 times farther from Earth than the moon, and appears to be locked in a stable configuration with Earth that will last centuries, according to NASA. If the coorbitals remain on Earth for long periods, Benford said, they would be a perfect place for extraterrestrial surveillance devices.
Find extraterrestrial technology
The closest star to Earth other than the Sun right now is Alpha Centauri, 4.37 light years away. But every half a million years or so, a star is about a light year from Earth, said Benford, which means that hundreds or thousands of stars (and their possible accompanying planets) have been close enough to our planet during the long history of the Earth to make contact.
A long time ago, aliens may have observed photosynthetic bacteria or dinosaurs if they were lucky. But their probes could still be sitting on the coorbital surface. "I am essentially talking about extraterrestrial archeology," Benford said.
The moon may seem a better candidate for alien spyware than some small space rocks; but any given point on the moon is in the dark for two weeks in a row, Benford said. A probe would have to be able to store energy until it could charge the Sun again. Still, he and Davies have advocated looking closely at the high-resolution images of the Moon sent by NASA's Lunar Recognition Orbiter, in case that there is something there.
Benford suggests observing Earth's coorbitals with optical and radio telescopes, as well as pinging the planetary radar, which could send a signal to any extraterrestrial civilization that, perhaps, could still be listening. Sending small spaceships to the coorbitals would also be relatively cheap and easy, he said. In fact, China's space agency announced plans in April to send a probe to the closest to Earth.
The search for signs of intelligent aliens near Earth is informative even if the search is a failure, Benford said. That no one has heard or seen extraterrestrial signals in 50 years or so, does not mean much, given the amazing time span of Earth's history. The lack of evidence covering hundreds, millions or even billions of years would be much more convincing.
"If we don't find anything, that means no one has come to look at the life of the Earth for billions of years. That would be a big surprise, something surprising," said Benford.
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