An interstellar web? (Original image by B. Torrissen)
A speculative but intriguing discussion that sometimes crops up when talking to people engaged in exoplanetary science goes like this; let’s suppose that we find an unmistakably terrestrial style planet around a relatively nearby star (less than about 30 light years away), perhaps even around one of the Alpha Centauri members, a touch over four light-years distant. Let’s further suppose that – possibly with the James Webb Space Telescope, or a next-generation ground-based super ‘scope – we gather evidence for an atmosphere and even find big chemical clues that there could plausibly 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 thought 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 (or perhaps call up his real life role model, Elon Musk of SpaceX). 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 conceivably 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 high-bandwidth 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) might not 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 more and 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 – but perhaps carrying along enriched uranium is an option, as one of the commentators on an earlier Life, Unbounded post discusses). I personally think that supplementing fissile material with chemical energy might actually be the best option; carry a store of naturally chilled redox components, mix them periodically and recharge the batteries when a power-boost is needed, the ultimate fuel-cell.
One way to increase the efficiency of communication is to use lasers instead of more conventional radio frequency transmitters. In 1994 Lesh, Ruggier, and Cessarone of the Jet Propulsion Laboratory wrote up an interesting study of this in which they concluded that conventional radio communication from the vicinity of Alpha Centauri 4 light years away, with mega-watt power requirements, could perhaps be replaced by modulated lasers with a 20 watt output power (see also this excellent piece by Paul Gilster at Centauri Dreams). The bandwidth is nothing exciting, about 10 bits per second (no Netflix streaming then), but hey folks, it’s from around another star. Clearly if one instead used the pearls-on-a-string spacecraft configuration, the average comm-link distance could be drastically reduced, and the bandwidth and ease of signal-lock could be greatly increased.
All over-caffeinated speculation? Perhaps, 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, and building the outermost limbs of our information-obsessed species’ internet may be the easiest way to bring the universe back to us. This post is a reworking of an old Life, Unbounded piece from back in 2010. It seems like an appropriate followup to the previous post on Mass Effect and the Fermi Paradox, perhaps a glimmer of where we go next…
About the Author: Caleb Scharf is the director of Columbia University's multidisciplinary Astrobiology Center. He has worked in the fields of observational cosmology, X-ray astronomy, and more recently exoplanetary science. His book 'Gravity's Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos' will be available Aug. 7th 2012, and he is working on 'The Copernicus Complex' (both from Scientific American / Farrar, Straus and Giroux.) Follow on Twitter @caleb_scharf.