The unknown visitor came from deep space. It passed nearly as close to the Earth as the moon on May 21st. Its spectrum didn't match any known asteroid. At a feeble absolute magnitude of +28.9, the traveler must have only been about the size of a truck.
Object 2010 KQ, what are you?
Is this a scouting ship for Stephen Hawking’s hypothesized evil aliens planning a mass invasion of Earth?
No, more likely it is a discarded interplanetary rocket booster abandoned in solar orbit.
The detection is tantalizing, nevertheless, because we can identify space objects down to a few feet across. That's the scale of what you might expect any alien probe might be, assuming their technology is comparable to ours. (Forget about those huge motherships in the 1996 film "Independence Day," the fuel costs are astronomical.)
On the hypothesis that we might have been visited long ago, could there be alien artifacts left behind, perhaps abandoned in solar orbit alongside our own space junk?
The fact that we haven't found anything yet makes it clear that any visiting aliens didn't do anything obvious to say they came by. But I can make a few cautious extrapolations from how they might have conducted the exploration of our solar system – that is, if they think like we do!
First, we have to assume there is a nearby extraterrestrial civilization that is inquisitive enough to invest the resources into building interstellar probes.
They first identified Earth as inhabited in telescopic surveys, then they wanted to know what lives here. Given the extreme physics of interstellar travel, answering that question is no small expense.
Secondly, these probes are built purely for data collection and beaming findings to their home planet. They are the mechanical equivalent of Lewis & Clark, with the intelligence to self-reprogram their mission depending on what they discover.
A probe's mission is not to plant the flag of Zork on top of the U.S. Capitol, or make direct contact such as: "Greetings from the Zeta Reticulans. how are you?"
In a paper published in the 1960s, Carl Sagan, using the Drake Equation, statistically estimated that Earth might be visited every few tens of thousands of years by an extraterrestrial civilization.
If this is a reasonable estimate, then what evidence might have been left behind? (And pull-eeze — it's not the Great Pyramids, not the Nazca lines and not Stonehenge!)
For starters, the probe might self-destruct after reconnoitering the solar system and transmitting its findings back to its builders. It would be “retired” simply for the sake of not polluting our system with evidence of alien technology. For biological quarantine, NASA crashed its Galileo orbiter into Jupiter, and that is the ultimate fate of the Cassini probe at the end of its mission to Saturn.
But could you program an artificially intelligent machine to commit suicide? It simply might change its mind.
Given all the expense to get the probe here, its builders would strive to make it immortal through self-repair, if not replication. This is embodied in the popular idea of a Von Neumann machine.
The probe would have to find a place to park and collect resources for self repair (but not a stop at Radio Shack). It may be instructed to do a re-reconnoiter of our solar system periodically and go into hibernation for long intervals. (No, it wouldn't need to tap into our power grid as alleged in several goofy UFO stories. Who needs energy from a fossil fuel-dependent medieval planet?)
The visitor could park in a solar orbit. If the probe detected artificial electromagnetic transmissions from Earth, it might go into stealth mode to avoid detection and capture. After all, it couldn't collect much data if it were snagged and put on display at the National Air and Space Museum in Washington D.C.
Even if visitors to our solar system were sloppy, with one or more probes leaving debris on asteroids or even planets, the artifacts would be pretty difficult for us to detect.
NASA's Lunar Reconnaissance Orbiter (LRO) has photographed our lunar artifacts, such as the Surveyor 6, shown here. But we knew what we were looking at. Even the shadow from a black alien monolith on the moon might get overlooked as just another tall boulder.
To get attention, a piece of space junk from an alien probe would have to be big and morphologically peculiar, or have an odd spectral signature or albedo.
One example is an LRO image of the Soviet Lunokhod 2 rover that landed on the moon in 1973. It stands out brightly against the regolith and has parallel wheel tracks.
I can’t help but wonder if aliens would want to just leave a calling card that one of their robotic explorers passed through our solar system. It could be as simple as a passive artifact with etchings or some indelible feature that would survive micrometeors and radiation exposure over hundreds of millions of years. This is similar in concept to the Pioneer plaque and Voyager record placed on our first probes to escape the solar system.
But where would aliens place such an artifact so that it is easily found? The fact that we haven’t stumbled across anything obvious yet reduces the likelihood of this hypothesis.
If you were an alien visiting the solar system, where would you leave a message? And, what would it say? Maybe something like: "We Came in Peace for All Klattukind."
Image Credits: Lucasfilm, MGM