

Jan. 22, 2004
by Seth Shostak - Senior Astronomer

Have aliens sent mechanical emissaries to our solar system -- robotic probes on a snoopy mission to reconnoiter Earth?

Its certainly an intriguing idea: sophisticated spy satellites from light-years away monitoring our planet, watching the slow evolution of life, and reporting back to their alien masters. Such a scenario has frequently appeared in the SETI literature, and Allen Tough, at the University of Toronto, has urged that we take the idea seriously enough to make a search for these alien "bugs."

You might question whether interstellar spying makes sense. After all, there are several hundred billion star systems in the Galaxy, spread across a disk 100,000 light-years in diameter. Sending billions of probes over such daunting distances sounds like a project that no alien congress would ever approve.

But the idea cant be dismissed that easily. Advanced societies -- even those that are only modestly beyond our own -- will have catalogs of planets known to support life. This inventory can be assembled by using large telescopes to collect and analyze the light reflected from the atmospheres of other worlds; looking for "biomarkers" such as oxygen and methane. Finding these gases on someone elses planet would be a clue that biology is present. Surprising as it may seem, microbes can be detected at light-years distance using this technique.

If worlds with life are plentiful, then some of them will be relatively close to the aliens home planet, encouraging a close-up look via a probe. If bio-worlds are rare, then these would be so singular as to make the greater travel times required to reach them worth the wait. Either way, there could be some stimulus to post a probe. And the total number of probes need not be extraordinarily large.

How big do the probes have to be? Allen Tough has written that these alien bugs "could be smarter and more knowledgeable than any human being, yet be smaller than a basketball or baseball." Smaller is better for two reasons: less energy is required to hurl them to other worlds, and they would be harder to find and confiscate by any intelligent beings on the spied-upon planet.

On the other hand, the probes are pretty useless if they dont have the oomph required to send back data, either via a radio signal or a tightly focused infrared laser (the latter seems more sensible to me). So the aliens might opt for a "master-slave" setup somewhat akin to the scheme used to retrieve data from the Mars rovers. A relatively small probe could orbit near to Earth, making high-resolution photos and collecting information, while a larger, more distant "mother ship" could be hanging out in, say, the asteroid belt, where it would process the data from the smaller probe and relay it back home. Another approach would be to have only one probe, but on a highly elliptical orbit (like a short-period comet). This scheme would keep the probe largely out of sight, but bring it close to our planet for detailed looks every few dozen years.

What about the costs? For a truly advanced society, the bill for the hardware might be negligible. But to send a probe to the stars at even a leisurely one-tenth light speed requires a substantial dollop of energy. If the probe weighs 10 pounds, the minimum energy necessary to rocket it to target and then slow it down on arrival is roughly 5,000 trillion joules. If you buy that much energy from your local electric company, it will cost you $120 million. Frankly, although a bill that size would probably stupefy your spouse, its not an unthinkable amount (a Mars Exploration Rover costs three times as much).

Still, there are some probe-o-phobes who ask, "why would the aliens bother? After all, the probes can only telemeter their data at the speed of light. Why wouldnt the snoopy extraterrestrials simply await our television signals? These would reach them at the same time any report from a probe would."

Indeed. And one could argue that the content of our broadcasts would tell alien viewers all they might want to know about our society (I wont lapse into the obvious)

But theres another angle. Life on Earth has been detectable (via atmospheric biomarkers) for two billion years. Intelligent life is only a recent, and so far, brief, phenomenon. Very few (if any) of the biologically active worlds investigated by probes are likely to have brainy life. The aliens will surely know this. They will send probes, not to find us after all, our signals, in the end, will find them but to investigate, up close and personal, life that isnt ever going to broadcast. For 500 million years, theres been a rich pantheon of plants and creatures on this planet, making it a natural history exhibit of enough interest to warrant a close-up look.

So probes are a possibility. And maybe one was launched our way, sometime in the last two billion years. We might still be able to find it, if we knew where to look. Should we make a careful investigation of the asteroid belt? The Earth-Moon Lagrange points? Perhaps parked in a nearby orbit, or on the Earth itself?

We just dont know, and that makes the search space dreadfully large. Probes are not crazy, but how to find one is hazy.