Interstellar Travel: Laser Sails and Tractor Beams
If Einstein is right, then it’s impossible to get to Alpha Centauri in less than 4.3 years.
If Newton is right, then getting there in a fraction of a human lifetime, though possible, is extremely difficult. Tsiolkovsky’s Rocket Equation tells us you would need an immense amount of propellant and an immense amount of energy to throw that propellant at high speed out the back of your ship. As in, $100 quadrillion dollars worth of antimatter per passenger.
It would be much simpler if you could leave the energy and propellant source behind. But how?
Enter the laser sail. Nutshell: light exerts pressure when it reflects off an object. Assemble a bunch of lasers and aim them at your ship. Turn the lasers on and they’ll accelerate your ship. Keep the lasers turned on until you reach your cruising velocity, then turn the lasers off. Your ship will coast at your cruising velocity until you reach Alpha Centauri.
Problem. How do you slow down?
In Avatar, ISV Venture Star carries antimatter engines to slow down on the approach to Pandora. Other powerful rocket technologies would work. By only needing to fire the engines when arriving, you cut your fuel requirement by 75%. We’ve cut the cost to a mere $25 quadrillion dollars of antimatter per passenger. Maybe there’s a cheaper way.
You could use aerobraking. The ship dips into the atmosphere of a planet or other large body and uses atmospheric drag to slow down. The Motie ship in The Mote in God’s Eye dives into the atmosphere of its destinatation star. Can it work? In theory, but you’re coming in at 40,000 miles per second or faster, and missing your insertion trajectory by just a few degrees either way can send you into a fiery death (and catastrophe for the planet or star you’re impacting at 0.2c) or hurtling off into space with no chance to get back to your destination.
Back in the ‘60s, Dr. Philip Norem pointed out that if you charge your ship, the galaxy’s magnetic field will turn it. You can approach Alpha Centauri from behind. The lasers back at Earth will know when you’ll be in position, and can fire again. This time, the Earth lasers will slow you down. The only problem is if you haven’t mapped the galaxy’s magnetic field well enough, your ship wouldn’t turn onto the correct trajectory, the Earth lasers will miss, and your ship will coast into space with no chance to get back to your destination. Even if you can tune your ship’s charge to deal with unexpected galactic magnetic field strengths along your flight path, you still have to go a long way out of your way. (guesstimating from the picture to the right, 15-20 light years).
Physicist and science fiction writers Dr. Robert Forward came up with another way to use Earth-based lasers to slow your ship. It requires two concentric disc mirrors stacked together, with the smaller disc on the Earth side and the larger disc on the destination side. The smaller disc needs mirrors on both sides.
To decelerate at the destination, first detach the larger disc from the smaller. When the laser beam strikes the larger disc, light reflects and hits the smaller disc. This slows the smaller disc to a stop at the destination. The larger disc sails off at a fraction of light-speed into interstellar space.
And if instead of one small disc, you use a medium and a small disc, you can make a round trip. Use your laser to return the small disc to Earth while the medium disc remains at your destination. See the picture to the left.
The biggest drawback is that the large (and medium) discs can only be used once. Thousands of square miles of mirrors wouldn’t be cheap. Still, it’s the best of these options.
Is there any other way to slow down at Alpha Centauri?
Yes. Tractor beam. (Ctrl-F for “tractor beam” after you follow the link. Huge thanks to Winchell Chung, Atomic Rockets’ proprietor, for all his work, including the three images I grabbed for this post).
Wait, aren’t tractor beams as impossible as faster-than-light travel? And even if they weren’t, how much energy would it take for a tractor beam on your ship to grab Alpha Centauri or one of its planets to slow down?
The answers, respectively, are no and wrong question.
Tractor beams may be possible. If a light beam with carefully tuned properties hits an object, the beam can pull the object toward the light source. Or, as first envisioned by physicist Victor Veselago, surfaces with negative refractive indices (NRI) are another. If you hit an NRI surface with light, the light would exert negative pressure on the surface, pulling it closer rather than pushing it away.
Granted, both concepts are far from implementation. But if either one works…
Scenario: lasers near Earth accelerate the ship toward Alpha Centauri. The ship coasts for a while. Then one of the following scenarios occurs:
- The laser controllers at Earth calculate the date the ship needs to start decelerating, and about 4.3 years before that date, the laser controllers fire a beam of tuned laser light that will pull the ship toward Earth, i.e., decelerate it.
- Just before the deceleration start date, the ship deploys an NRI sheet. Earth’s lasers fired 4.3 years ago. The laser light strikes the NRI sheet and decelerates the ship.
- Both tuned light and an NRI surface are used.
Regardless of scenario, the result is the Earth-based laser acts as a “tractor beam” and slows down the ship at Alpha Centauri. No shipboard rockets, no aerobraking, and no galactic magnetic field surfing required. You could also use the Earth-based laser to accelerate the ship back to Earth at the end of the mission.
(Of course, the ship is trusting that Earth will turn on the laser beam at the right time. But for my purposes, that’s great. Stories come from where things can go wrong).
Whether the tractor beam approach is better than Dr. Forward’s multi-stage structure would come down to cost. Big throwaway mirrors may well be cheaper than giant banks of tunable lasers or big NRI sheets. Especially because we would only need Earth-based lasers to slow our ships until the human colony at Alpha Centauri builds its own bank of lasers to decelerate incoming ships and accelerate ships back to Earth: not many missions to pay off the R&D costs for tunable lasers or NRI sheets.
Even in deep space, money talks.
Quoting Nick Land, expat Brit philosopher in Shanghai, “The cosmic reality visible to us is characterized by an intense, efficient aversion to the existence of advanced civilizations.” He calls whatever it is that prevents the existence of advanced civilizations “The Great Filter.” Longtime science fiction readers familiar with Gregory Benford’s Galactic Center series or Fred Saberhagen’s Berserker universe will understand what Land means when he dubs the Great Filter “the Exterminators:” Killer robots sent out to destroy advanced civilizations.
But on the bright side, Exterminators probably don’t exist, because if they did, the human race would already be extinct.
You might have heard of Von Neumann probes. A self-replicating interstellar probe journeys to a nearby star, makes copies of itself, and those copies journey to nearby stars. Repeat until you have a probe in every stellar system in the galaxy. Even if the probes’ net velocity is only 1% of the speed of light, they would reach every star system in the galaxy within 10 million years.
Given that the galaxy is about 13.2 billion years old, filling the galaxy with self-replicating probes would take but a moment of astronomical time. Look at it this way: if Earth is typical, and a planet needs to exist for (rounding) 4.2 billion years for intelligent life to develop a civilization capable of launchign a self-replicating probe, then it would take just one alien civilization arising in our galaxy in the last 9 billion years for there to be a probe somewhere in the solar system right now.
Now suppose that one alien civilization built probes with a straightforward mission: destroy other intelligent species while those intelligent species are stuck on their home planet, to ensure that one civilization can exploit all the resources of the galaxy. If true, their killer probes would have destroyed us a long time ago. Maybe even before there was an us.
Since that clearly didn’t happen, we conclude that zero alien civilizations built Exterminators.
Wait, their killer probe might be here, waiting to destroy us
No, because the Exterminator has nothing to gain by waiting. Over three thousand years ago, human beings built plainly artificial objects visible from low earth orbit. A clear signal that a species had evolved tool use and enough social organization to engage in massive engineering projects. Why wait to destroy that species? Maybe it will take four thousand years for that species to build its own Von Neumann probes, but what if it takes them four hundred? Or forty? Don’t take that chance. Destroy them now.
Since ancient Egypt wasn’t wiped out by a hundred-mile-wide asteroid impact, the sun going nova, or a never-ending army of implacable battle robots, “no killer probe” is the safe bet.
What does this mean?
Looks like the Great Filter lies behind us. Whether life is rare, or planets rarely stay habitable for billions of years, or the metabolic expense of intelligence rarely conveys a selective advantage, or tool use is rare, doesn’t matter. We are probably the only intelligent tool-using species in the galaxy. The handful of human beings who will ever get past low earth orbit will be like the Aborigines crossing the Torres Strait or the First Nations pushing south of the Ice Age glaciers, entering a vast, resource-rich realm without competition.
Except with each other, which for science fiction writers is a good thing. Fodder for a million stories….
Speaking of which, I should get back to work. Till next time.
While researching a story I’m currently writing about terraforming, I found The Paul Birch Web Archive. If you’re looking for big, hard science fiction ideas, follow the link now and get your mind expanded. Want to speed up Venus’ rotation to give it a day 24 hours long, at the low low price of $20 trillion? Want to build a bridge across the Pacific Ocean or up to low Earth orbit? Want to sail the solar wind and interstellar medium, and reach Alpha Centauri in 400 years? Want to take a one-way faster-than-light trip into the future?
The link again is The Paul Birch Web Archive. Enjoy!
I’m pleased to let you know that I have a new science fiction short novel coming out on September 25, 2014. It’s got intrigue, a love story, and an homage to Lawrence Durrell’s tetralogy The Alexandria Quartet, all wrapped up in my distinctive flavor of sf speculation. You can preorder the ebook now or buy the trade paperback at better booksellers on the release date.
The ALECS Quartet, by Raymund Eich
He had a month to learn the planet’s secrets – and Juliette’s
His Cover Story
Return to Elard to dismantle his sect’s missionary work to the planet’s natives.
His True Mission
Investigate decades-old mysteries of love and death.
Return to Earth with his discovery – if he can.
Trade paperback edition available for US $10.99 or equivalent from all better booksellers, including Amazon and Barnes & Noble.
Audio edition coming soon.
Find out more at the publisher’s website, cv2books.com.
Quick note: this blog post may look familiar to my mailing list subscribers. It was one of those exclusive, pre-release bonuses my mailing list subscribers received almost three weeks ago, and almost a week before Operation Iago became available.
You can get similar bonus content about my next books by subscribing now. Scroll down on the page to “Signed Paperbacks Giveaway” for a chance to win signed copies of both Take the Shilling and Operation Iago. Mailing list signup gives you your best chance to win!
Even though Operation Iago is my fifth science fiction novel, and the second book in the Confederated Worlds series, I still get a thrill when I swipe through the ebook edition or riffle the pages of the trade paperback and see a story of mine in print.
Part of the thrill for me comes from knowing how the story started life. Writers come up with all sorts of metaphors for the process of writing a novel. Running a marathon. Building a house. Giving birth.
One way I look at a novel is as an oak tree. A thick trunk, reaching deep into the earth, extending branches into the sky. Words like leaves, thousands of them working together, creating a shady spot for readers to pause and refresh.
Yet large as it is, and long as it may take to grow, the oak tree starts as a single acorn. So too does a novel. An acorn of an idea, dropped on a fertile spot of the subconscious, and watered by new notions about characters, locales, and events, can grow into a novel.
The acorn from which Operation Iago grew
So far in the series, we’ve gotten a range for N, the number of detectable civilizations in the galaxy, to [5e-7 to 8e-6] * f_c * L. Today’s post will estimate the value of f_c, the fraction of intelligent species that go on to develop a civilization detectable (through electromagnetic transmissions and/or probes travelling at a sizable fraction of lightspeed) across interstellar distances.
A common assumption in science fiction is that intelligent life forms will inevitably build high-tech civilizations. Similarly to the typical view of inevitable evolutionary progress toward intelligence which I demolished previously, this common assumption smacks of whig history. Of course progress is a law of nature. It gave rise to the pinnacle of existence: us.
As a longtime sf fan, one of the toughest realizations I ever came to is that Space settlements will never happen for economic reasons.
In part, the costs of getting to space are too high. Charles Stross has discussed the costs at great length here. To get one person to the Moon, bringing along the life support he needs for the trip, using advanced versions of the rocket technology we have today, would cost about US$400,000 as an optimistic estimate.
That’s far too expensive for anything except government boondoggles or multimillionaire’s larks, i.e., the current state of space travel.
Things get worse as go further in the solar system, even keeping in mind Heinlein’s comment that “Earth orbit is halfway to anywhere.” The cost of travel to Mars or any other place in the solar system would be even higher than $400,000, for at least two reasons: (1) you have to carry the fuel for the return trip, and (2) you have to carry more life support infrastructure for the years of round-trip travel time forced on you by Hohmann transfer orbits.
Interstellar travel? Alpha Centauri is about 250,000 times further away than Mars. The energy cost to get a solitary explorer there in less than one lifetime (at 0.1 c, 40 years in transit) is comparable to the yield of all nuclear weapons ever built, or the energy consumption of the entire world for a couple of weeks. Generation ships are even worse: the energy savings from their slower speed (call it 0.01c, 400 years in transit) is offset by the mass of hundreds of people and the infrastructure needed to keep them alive and safe for four centuries. And we haven’t even touched on the individual and social psychology issues these avenues would bring up. How well would you do living in your car for four decades?
So nevermind settling the solar system; the idea of normal people going into space is so expensive, it’s a non-starter.
About now, a reader might protest, “But what about nanotechnology? Advanced materials and cheap energy production will lower all those costs dramatically.”
I read Stan Schmidt’s mid-’80s Analog editorials on nanotechnology, and K. Eric Drexler’s Engines of Creation. Although I think Drexler is intoxicated with his ideas, I completely agree that some of the fruits of nanotechnology–the super-strong, super-light materials and cheap energy referred to above–are entirely possible, and are in fact likely to appear somewhere on Earth in the coming decades. Yes, those advances will make space elevators and fusion-powered torchships possible. Yes, nanotechnology would greatly lower the costs of space travel and space settlements.
But. Nanotechnology would also greatly lower the benefits of space settlement, leaving the prospect as uneconomical as it is today. More on that point in my next post.