by Sean McMullen
Sean McMullen extols the potential virtues of a planet-sized space telescope in his novelette “Mirrorstar”, available to read in our [November/December issue, on sale now!]
My most recent Analog story, Mirrorstar, is set in the control and maintenance modules of a space telescope whose primary mirror is ten thousand miles in diameter. Is there a sound case for building such a large telescope? Would it be better to explore nearby planetary systems with starships? The distances between stars are pretty confronting, yet the urge to explore seems to be a built-in feature of we humans. Can we satisfy that urge without sending astronauts? After all, thanks to the James Webb Space Telescope we can now see images of the earliest galaxies without anyone having to leave Earth.
The exploration of Mars is quite a good example of what can be done without any human going anywhere. Several dozen probes have been launched to explore the planet, and excluding those that blew up on the launch pad, failed to reach Earth orbit, or just stopped working on the way, about three dozen have returned results. Mars has been mapped in detail, and thanks to the landers we have a pretty good idea of the surface conditions, geology, meteorology, colours and sounds on Mars. We even know that a Marian sunset looks like. You don’t need to send a shipload of humans to explore a planet because robots are very cheap by comparison. They don’t eat, drink or breathe, they can last a long time, they don’t have to be brought home, and they do some pretty good science.
I have already explored the idea of robots being sent to extra-solar planetary systems in earlier stories like Two Hours at Frontier (Analog November/December 2017) and The Firewall and the Door (Analog, March 2013), so I wondered what other types of exploration might be viable. So far our real life probes have found no evidence of alien visitors within our own planetary system. Those potential visitors have had 4.5 billion years to check the place, so if no visitors have ever done any visiting, what should we conclude? Assuming that alien scientists have ever existed, they might have explored the sun’s planets pretty thoroughly, but never made the trip. That idea led to me writing Mirrorstar.
Let us start with current technology. Telescopic examination of planets at light-year distances is no longer a ridiculous idea. Large space telescopes and very large terrestrial telescopes are now able to detect extra-solar planets by Doppler shift, transits, eclipses and gravitational perturbations, and these techniques tell us that such planets exist, how big they are, and what their orbits look like. Spectroscopic analysis can tell us something about atmospheric composition, whether or not there is water present, and if they have ring systems. We also have the technology to identify atmospheric pollutants from industrial civilizations, past or present. Do a google search and you can even view the first direct images of extra-solar worlds.
Could a stupendously large telescope do any better than all the foregoing? Think about how much we have learned about Mars from orbiting space probes, then imagine a truly enormous telescope pointed at a planet from light years away. A space telescope with a mirror ten thousand miles in diameter, as in my Mirrorstar story, could be large enough to show us surface details including mountains, seas, clouds, weather systems, forests, and even evidence of civilizations.
At first sight this seems very exciting, so is there any limit to the size of a space telescope? Quite possibly not. Ice becomes harder than steel once it gets cold enough, and out past Neptune conditions get very cold indeed. Ice is also plentiful out there, so why not use ice to make a telescope whose mirror’s diameter is greater than that of Earth? Better still, why not build two of them, orbiting on opposite sides of the solar system as an immense interferometer? By now we are starting to look at enormous costs and engineering challenges, yet would it be harder than building a starship?
You can explore multiple systems with one telescope, but under the existing laws of physics, even the sorts of starships that we can only dream about would take lifetimes to reach even one extra-solar system. Telescopes are bound to be cheaper than starships, can examine whatever system they are pointed at, will be operational earlier, and in principle can last forever.
For a start, you can build such telescopes with known technology, while starship technology is still theoretical at best and science fiction at worst. This brings one immediate benefit: we are liable to see results years, decades or even centuries earlier than from sending a crew, or even a robot. You can explore multiple systems with one telescope, but under the existing laws of physics, even the sorts of starships that we can only dream about would take lifetimes to reach even one extra-solar system. Telescopes are bound to be cheaper than starships, can examine whatever system they are pointed at, will be operational earlier, and in principle can last forever. Even better, any encounter between a speck of dust and a starship travelling at even a tenth of light-speed is going to be a catastrophe, but a meteor strike on a large telescope’s mirror array can be repaired with just a maintenance crew and a few spare hexagonal ice panels.
Telescopes do have drawbacks and problems, however. When confined to a telescope one cannot land on a planet and take samples, and the images are never going to be as good as you can get from having a camera on the surface. Steering is probably they most serious problem of all for a telescope with the diameter of a planet. It would require latticework supporting billions of ice hexagons, and this sort of structure will have an immense amount of inertia. Think about using a speedboat to turn an aircraft carrier around and you get the idea.
Still, compared to developing starship drives, shielding and maintenance systems, the problems involved with pointing a planet-sized telescope at some particular star are a lot less daunting. The actual mechanisms to orient the immense mirror and supporting optics might be a combination of gyroscopic rings and steering jets, but even these come with their own unique problems. Try to turn anything of this size, and materials science will need to be developed way beyond what we can do now. Apply pressure from a (very large) steering jet to a structure thousands of miles across, and parts will bend a little. That will store energy, and that energy will bounce around within the structure, causing vibrations which will spoil the images of whatever is being studied. When looking at something hundreds of light years away, you really do need zero tolerance when it comes to vibrations. Turning the telescope is always going to be a major undertaking, but it will be easier and quicker than developing a warp drive for a starship.
A way to lessen this problem is to use the really big telescope as a backup to a collection of smaller telescopes, only turning it when there is something particularly interesting to examine. If it were only meant for exploring Earth-like planets in the Goldilocks zone of nearby stars, that would cut down on the number of times it needs to be turned – and increase the time available at each world. Even a program to point the thing at all the promising planets within, say, 200 light years could be completed within the working life of an astronomer. Of course the ruins of a civilization would require a lot more scrutiny, and discovering a functioning civilization would be an absolute game changer. We could try to establish communications, and communicating with cooperative aliens would give us close-up views of alien worlds.
All of this leads us back to the question of why there is no evidence of aliens visiting our own planetary system over its 4.5 billion year existence. It could be that contamination from alien organisms has turned out to be a major problem in the history of our galaxy, and that the control of intrusive species is best done by quarantine. In other words, send pictures but don’t visit. Interstellar travel may have turned out to be too hard, too expensive, and far too dangerous. If enormous space telescopes were discovered in other planetary systems, this would be visible evidence for interstellar travel not being a winner.
Given the difficulties and dangers of interstellar travel, Mirrorstar-sized telescopes could be the best alternative to actually travelling to extra-solar worlds. They are cost effective, return results faster, the technical problems are not ludicrously difficult, and they are a lot less vulnerable to catastrophic accidents. Besides, present indications are that we might not be welcome out there.
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