new planets this week, or something else, but the subject of interstellar exploration has been bouncing around more than usual. A discussion that sometimes crops up when talking to others engaged in exoplanetary science is firmly in the speculative, but intriguing, category. It goes like this; let's suppose we find a terrestrial-type planet around a relatively nearby star (read less than 30 light years away), perhaps even around one of the Alpha Centauri members. Let's further suppose that - possibly with the James Webb Space Telescope, or a next-gen ground-based super 'scope - we garner evidence for an atmosphere and several big chemical clues that there could readily be a biosphere on this world. What do we do next?
There are somewhat mundane answers - build better instruments, get better statistics - that may be the most realistic, but there's also that nagging idea that the next thing to do would be to find a way to study such a planet up close. If enough coffee has been consumed then it's a matter of finding a handy Tony Stark, willing to sink hundreds of billions into a robotic interstellar probe, on a long-shot for glory. There's a problem though, unless you intend a very long round trip, how do you get the information back? While we are now pretty good at picking up signals from distant spacecraft - even from Voyager 2 at over 100 AU from the Earth - getting data back from a few light years is going to be hugely difficult. The required transmitter power, as well as interstellar scintillation, is a major hurdle.
A solution, that has cropped up in various guises, even in the idea of von Neumann probes, and the interplanetary internet, is that you don't just send one probe. Rather, you send a chain of probes - pearls on a string - capable of communicating between themselves even if not individually directly back to Earth. It would take a long time, but as the furthest end of the chain crept towards a target stellar system we'd have ongoing feedback, the continuous relay of data as we crept through interstellar space. It might be optimal to build the biggest receiver and transmitter at the outermost practical limits of our solar system - the equivalent of an internet 'backbone' - with a clear line back to Earth. So how many probes would you need to get to somewhere like Alpha Centauri?
This system is about 278,000 astronomical units (AU) away. If we optimistically think we could build probes capable of to-and-fro communication over a few hundred AU then we're talking about a thousand or more devices. This sounds awfully challenging, but remember that we (as some hypothetical sublimely patient species) don't expect probe-1 to reach Alpha Centauri for a few tens of thousands of years. We only have to launch every ten years or so. Even if each probe cost 10 billion dollars (allowing for lowered cost after the first few models) that's peanuts over this timescale. In the meantime we have an ever extending tendril out into interstellar space. Being an innovative species we would undoubtedly think of ever more wonderful things to add to the probes, increasing the scientific return.
Powering transmitters and receivers, as well as sizing their antennae or dishes, is still a problem. Given the timescale to reach the target star then even radioisotopes are going to peter out (fission reactors are a no-go, the fuel burns out too fast). Chemical energy might actually be the best option; a store of redox components, mix them periodically and recharge the batteries, the ultimate fuel-cell.
All over-caffeinated speculation. But if we ever get serious about stepping beyond, then making sure we don't drop the signal is going to be a very real issue.