Discussion and news about the modern effort to understand the nature of life on Earth, finding planets around other stars, and the search for life elsewhere in the universe

Saturday, August 28, 2010

Stepping stones

Maybe it's the flurry of 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.


Anonymous said...

what do you think about some project daedalus/longshot - style space probe?

Eniac said...

This reminds me of Kare's sailbeam idea: http://adsabs.harvard.edu/abs/2001AIPC..552..402K.

When I first read it, I thought: These microsails have to have sensors and propulsion to keep focused, why not also equip them with communications, to relay data back along the beam? In fact, why can't we do away with the massive spaceship the sails are supposed to sacrifice themselves to propel, and use the beam itself instead to do the observing and relaying of data?

This leads to something quite similar to what you describe, except the microsails are at relativistic speed and incredibly lightweight and tenuous.

I like the idea of chains of slow probes you propose, because the hardware is more realistic. Also, being slow, the chains can be redirected from one star system to the next by gravitational bending.

I suppose them main problem is keeping the probes operational for very long times. Another problem is the very long lead time from when the project is started to the first returns. That Tony Stark will long be dead when the first probe arrives at Alpha Centauri...

Anonymous said...

'....don't expect probe-1 to reach Alpha Centauri for a few tens of thousands of years.'

What were humans doing a few tens of thousands of years ago? Do we find any of the utensils our predecessors created at the time relevant or useful today. Is there a continuity in our development of technological tools? If we will be bound in our biological life span, the time span translates into a little over 200 generations. I just cannot imagine the continuously warring humans to be able to consummate such a grandeur time scale project. It might first require a political evolution of human society.

Eniac said...

The closest to what Anonymous #2 is looking for is probably Stonehenge. While we do not exactly find it useful, we do admire it and go there a lot to take pictures...

I agree completely, though, that a project with returns after thousands of years would require a radical change in humanity.

An alternative is to justify the project by much more short-term purposes, perhaps as a deep space communications backbone, or a very long baseline telescope array. The latter actually makes a lot of sense. If you send, say, four streams out in tetrahedral directions, that could make a very formidable array in a fairly short time span, and its ever expanding reach would keep it exciting all the way to Alpha Centauri (and three other stars).

Also, within a relatively short time, you would get to the sun's gravitational focus with all the exciting science that that offers. And then there is the near-interstellar environment and the Oort cloud, worthy subjects of study by themselves.

Contrary to the original assertion, I think fission is probably the only currently feasible way to power such probes. U235 lasts 100 million years, and can be burned with a fairly high yield of energy per mass. Certainly many orders of magnitude higher than any chemical fuel. Of course, the design of a space probe that lasts 10,000 years is a field of research that is wide open, never mind the energy source...

Caleb Scharf said...

All great comments. The Daedalus type idea is certainly still interesting, and conceivably less of a challenge than this kind of multi-multi-generational project (although it might be easier to figure out how to increase human longevity first). But there are I think equally huge engineering challenges to overcome. I guess my first instinct is to do Voyager on steroids.

I still think fission power is a problem, unless you carry along a *huge* supply of unenriched U235/238 or the ability to process spent fuel then the duty cycle of a loaded reactor is not so very long. Additionally, the neutron flux tends to degrade materials in a reactor, so eventually things start to crumble.

Paul Gilster said...

Caleb, fascinating stuff. Do be aware, though, of James Lesh's work at JPL. Back in 1996, Lesh analyzed a 20-watt laser system with 3-meter telescope as its transmitting aperture and found it would be workable if placed in Centauri space. Getting it there is as much an issue as getting a probe there in the first place, but Lesh is confident that if we can solve the propulsion issue, communications won't necessarily be a problem.

On the other hand, that deep space backbone you write about has plenty of advantages of its own! We should also keep Claudio Maccone's ideas about gravitational lensing in mind here. His numbers on communicating between two stations -- one at the Sun's 550 AU grav lens distance, the other at the equivalent lensing distance around Centauri A and B, are amazing -- essentially you need something with the power of a cell phone!

Eniac said...

Yes, fission power has its problems, but there really is no less problematic alternative, I am afraid.

You would probably carry a supply of pure U235 (as opposed to unenriched U235/238), because it should be easier to burn with high fuel utilization. However large the needed supply, it would be many orders of magnitude smaller than that of any chemical fuel.

I like the idea of a radiation cooled solid state reactor, with thermionic energy conversion. It has no liquids and no moving parts, and requires no hardware for cooling. It's essentially a critical piece of highly refractive fuel (e.g. Uranium carbide) with some sort of (here the miracle occurs...) temperature dependent moderation that keeps the reaction limited at a constant, very high temperature (2000K-3000K). At such temperatures, thermionic conversion is quite efficient, as is radiative cooling. Most of the reactor's mass could be fuel (with a little bit of tungsten for the electrodes), so your fuel supply could be a supply of reactors, eliminating permanent components that could be radiation degraded. Dangle it out on a long string, and very little shielding is required to protect the rest of the craft. Change it every 50 years or so, and with 200 you could go for 10,000 years.

Eniac said...

Ooops, correction: That should be "refractory" instead of "refractive", above.