OOT: Proposed Interstellar Mission To Alpha Centauri! (Kinda!)

Submitted by superstringer on
Since prior posts on all things super-sciency and space-related seem to be something of a hit, and given the low level of non-satellite camp activity today, here's something trendy in space news.
 
An uber-rich Russian, with some props from Stephen Hawking and That Facebook Guy and others -- wants to spend some of his wealth on a massive project -- AN INTERSTELLER MISSION TO THE NEAREST STAR SYSTEM.  He proposes launching it in 20 years, for a 20 year trip.
 
 
 
I think it's cool if we break this down.  It's got a lot of, uh, let's call them interesting aspects to it.
 
THE DESTINATION -- Alpha Centauri, which is about 4 lightyears away.  It's actually a binary system, with a third star (Proxima Centauri IIRC) somewhat close.  We have detected more than one planet around one of the two stars, but not remotely habitable.  So, while it's close, I'm not sure what the attraction is to this particular destination.
 
THE "VEHICLES" -- These aren't quite what you see in the movies.  The vehicles he wants to send would be really tiny, like postage-stamp sized, or a few centimeters across at most.  And they'd be FLAT -- little squares or disks -- but unfurl little reflective "sails."  And he'd send THOUSANDS of them, because not all will survive the trip or make it straight to the destination.  All the electronics (and power-production) would need to be nano-sized, way below current technology but not out of the realm of question.
 
THE JOURNEY -- To get to Alpha Centauri in 20 years, and given that it's 4 lightyears away ... hmm ... let me ask Siri ... ok, you need to go 20% the speed of light.  Yikes.  The speed of light is 670 million miles per hour.  So 20% of that is 134 million miles per hour.  Needless to say, humans have never, ever sped up any vehicle anywhere close to that speed.  We've sped up packets of individual atoms in particle accelerators, yes, but... wow.
 
THE PROPULSION -- How do you get a small vehicle (which can carry no engine or fuel) up to that speed?  Here's the idea:  you continually blast it with a super-powered laser (orbiting the Sun, probably far away, like past Jupiter or something).  Light has pressure; if you shine a laser on something, you "push" it.  The effect is very, very little; it's unnoticeable on the surface of the Earth.  (Shining a flashlight on a carefully-balanced object, for instance, won't knock it over.)  But a spaceship with a "sail" (flat surface to reflect the light) would be pushed by, say, sunlight hitting that sail.  If you intensify that "push" with not sunlight but a really really really powerful laser, you can add momentum to the spaceship.  
 
THE LASER -- Ok, so this whole trick really is about building a massive, space-based laser that is so powerful it can accelerate these postage-stamp spacecraft to 20% the speed of light.  It would be a laser far beyond anything humans have ever built.  It would probably orbit far away from the Sun, and remain pointed at the spacecraft, and just keep blasting them to push them away, building up their speed over time.
 
THE PROJECT -- So it turns out, $100M doesn't get you an intersteller space mission.  It gets you some initial design work.  Actually building this project would cost ... well, a lot more.
 
I HAVE QUESTIONS!!! -- Don't we all.  Some of the challenges I see:
 
-- This laser, you know, if you turned that sucker away from space and towards a city of an unfriendly country... yes, this would be the most massive weapon ever, too.  So, you know, I'm not sure about anyone allowing this to actually happen.
 
-- If you accelerate something up to 20% the speed of light... uh, how does it stop at its destination?  You'd need to decelerated it by 20% the speed of light, too, which will require exactly the same amount of push (in the opposite direction) that the craft initially received to speed UP to 20%.  Shining a laser on it to push it up to 20% is only half the challenge; with no laser at Alpha Centauri pointing in the other direction, it'll never slow down.  It's going to zoom past its destination at 134 million mph.  That's not a lot of time to take a photo; assuming you need to be within 25,000 miles for a "close up" picture, you'll be within that distance (assuming you zoom super-close to the surface without hitting it) for only 0.6 seconds!  "Hey ok we're approaching our tarrrrr... rats, we just passed it, see that tiny tiny dot fading away behind us?"
 
-- How do you change the path of the spacecraft?  What if, when it leaves the solar system, its path is 0.000001% off-target?  It would miss its target by millions of miles.  Or, what if interstellar winds blow it off course.  (Not making that up -- this is a real problem -- intersteller space has lots of tiny dust and debris and charged particles whizzing around.)  Clearly these little vehicles might need to "turn" a bit in space, to re-orient themselves.  How's that happen?  The article linked above says they'd have "photon thrusters," but the amount of thrust from those (basically shining a small light in one direction, to push away into the other direction) is going to be tinnnnnnnnnnny.  At the speeds they'll be moving, it won't allow for much ability to change course.
 
-- How do you ensure the laser is pointed at the spacecraft?  This is the ultimate problem with this plan.  If the direction of the laser is off-target, it won't shine directly the spacecraft (and the laser, being in orbit, is continuously moving; so is the Sun, too, around the center of the galaxy); and if the laser stops blasting the craft, the craft doesn't accelerate (and probably loses its power too).  As the spacecraft get farther and farther away, the accuracy has to get better and better because the target (spacecraft) gets smaller and smaller.  Imagine how accurate the laser has to be, when a postage-stamp spacecraft is trillions of miles away.  But here's the rub:  you won't know exactly where that spacecraft is at any moment.  Since light takes time to travel, when you "see" the spacecraft, you only know where it WAS when the light left it.  For instance, if the spacecraft is 1 light-month from Earth, it takes 1 month for the light to get to us.  So when you "see" it, you see where it WAS a month ago.  How do you keep the laser aimed at it?  Not where you see it now; it's not there anymore.  It's moved somewhere else, and it's going to be somewhere else by the time the laser's re-oriented light gets to it.  For a craft 1 lightmonth away, the laser light will take a month to get back to it; you have to aim at a spot where it'll be a month FROM NOW.  (Like leading the receiver.)  This problem gets worse and worse the farther the craft moves away, because you NEVER have real-time information as to exactly where the craft is, or whether your laser is currently pointed at it.  If you change the laser's direction, it'll be 2 months (for a craft 1 month away) to find out if the laser is back on target.  If you guessed wrong, you try again, but now the craft is 2 months away, etc. etc.   Even with a wide-area laser beam, so your accuracy doesn't need to be pinpoint, this seems like a show-stopper to me.
 
So this is a fun mind-game, but, this joins the space elevator as a mind-numbingly unworkable idea that people are actually going to waste money on.  If he really wants to spend $100M on something useful, design a space-based solar shade to cool the Earth and reverse global warming.  Or, Hugh Freeze needs a sunroom on his house for those summer siestas he seems to enjoy, and the leftover cash can sent Paul Feinbaum to some world where he doesn't have to hear about Jim Harbaugh (hey, that superhot planet orbiting Alpha Centauri might be one!)
 
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ST3

April 13th, 2016 at 1:39 PM ^

a series of lasers instead of one giant laser? You put a laser on your postage stamp and point it at the postage stamp in front of it. You end up with a sequence of laser postage stamps pushing other laser postage stamps. This might just work.

Why are we putting arbitrary criteria on this? Why do we have to go 20% the speed of light and get there in 20 years. Would the effort be any less awesome if it took 30 or 40 or even 100 years? Yeah, none of us would be around to see it, but future generations could look back at our foresight and wisdom and label us the greatest generation because we studied the universe using laser propelled postage stamps.

P.S. and in the effort of studying this concept, we might just end up with Tang 2.0

Joined: 09/11/2010

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Bryan

April 13th, 2016 at 1:50 PM ^

We still wouldn't know whether the mission was a success or not for many more years, right? How fast could the data travel back to earth?

 

Joined: 07/10/2009

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mgobleu

April 13th, 2016 at 2:02 PM ^

So...then what? Your "postage stamp" is going to be able to support the type of transmission device to reach back to earth to let you know what's happening to it or where it is? That's almost as far fetched as a laser that can produce light over even a tiny fraction of that distance. Or are these the universe's most expensive kites ever? Suppose they do get there, and suppose they can communicate back to earth, so what? We get to look at some big balls of gas and rock not necessarily much unlike the ones we get to see through our telescopes. I don't know, man. You can do what you want but it seems like a waste of money to me. What about solar energy for us earthlings, or clean wells for Africa, or maybe a nice new yacht fleet?

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UM Fan from Sydney

April 13th, 2016 at 2:13 PM ^

I welcome these posts, as the number of satellite camp-related threads has been out of hand this week.

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DualThreat

April 13th, 2016 at 2:17 PM ^

There exists a theory/postulate regarding when is the best time to launch something to other stars based on two competing factors.

1 - The earlier you launch it, the sooner it will reach its destination.

2 - The later you launch it, the more advanced technology there will be and thus your vehicle will get there faster.

Surely 20 years from now will not be this "knee in the curve" moment when we should go to the stars.  1,000 years from now maybe.  If we launch too soon, we'll just smoke past whatever we launch now with whatever we launch 100 or more years from now.

We should focus now on manned exploration of the solar system.  Interstellar exploration will come when the time is right.  But we don't have reasonable technologies yet.

Joined: 05/04/2012

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LSAClassOf2000

April 13th, 2016 at 2:17 PM ^

This reminds me that we're in crunch mode for the Star Trek timeline to actually take place - we've only got about 47 years until we're supposed to test the first warp engines and introduce the Vulcans to hard rock and another 20 or so after that until we start the first colony on an as-yet unspecified planet in the Alpha Centauri system. We need to get with it, folks. 

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MC5-95

April 13th, 2016 at 3:24 PM ^

As a UM undergrad in the early 90s, groups of us used to take over computer labs for all night Spaceward Ho marathons. I have no idea why that game was so addictive, but I now see that there's an ios version so I guess I'll see if I can remember.

Joined: 02/26/2014

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ca_prophet

April 13th, 2016 at 2:57 PM ^

The weaponization is a real concern but you can put it on, say, one of the moons of Saturn such that it can't see Earth. (Larry Niven has a story involving a spacefaring race of merchants called the Monks who use launching lasers, as well as a story around the Kzinti lesson: a communications device like a laser is a weapon in proportion to its power output.) A better approach here is to use a solar sail. The solar wind provides a minuscule amount of acceleration but it's always on and can be caught with a large sail. Orbit the sun a few times, tacking as needed, until you're up to speed and then let her rip. The challenge to the tech won't be acceleration - that's minuscule - and velocity itself will only have relavistic effects. It's radiation-hardening the tech to handle open space for twenty years. Deceleration is a concern but you can do it with the sail as well; the Centauri solar wind also has pressure. That is a "bang-bang" control problem - full acceleration then full deceleration. It's hard on the payload but simplest on the guidance. All of that is an engineering challenge. We won't hit new physics until we put the things into deep space and encounter conditions we've only simulated imperfectly until then.

Joined: 09/07/2010

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aratman

April 13th, 2016 at 5:43 PM ^

by using many lasers circling the sun, run the postage stamp in orbit around the sun so that it passes each laser during its orbit.  time each laser to bump the speed of the stamp as it passes. When you reaches the speed they want have one of the lasers bump it out of its orbit towards the solar system of choice.  That is how a large particle accelerator works.

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