His heart’s in the right place, I will admit that much.

The “Breakthrough Starshot” project was bound to be proposed by someone eventually. Postage stamp-sized space probes riding a beam of light to Alpha Centuri, then beaming messages back to us of what they find there, all with Stephen Hawking’s stamp of approval. Explore another star! Nifty idea, truly, but can we really do it?

Hawking’s intentions are good. From a long-term perspective, there’s a 100% chance that the Earth will endure another extinction-level event. It has happened repeatedly in the history of our world. All the species of life living here and now are dead. Eventually. If we’re here when it happens, we’re dead too. There’s no chance it won’t happen. It may happen tomorrow, next week, next year, maybe next century or even a million years from now. If anything is living here a billion years from now, our sun will have changed its radiant output enough to render this place uninhabitable anyway. Our little oasis in the void won’t last forever. It will happen, even if the probability of it happening in the life span of anybody living now is very very small.

Admittedly, we’re biasing the statistics a little at the moment. There are enough people spread across the face of this planet that everything bad that ever happens will have a human audience caught up in it. And, worse, news of every bad event can now circle the whole planet in minutes by Twitter. It’s also an active discussion that there are enough humans on this planet that our activities are fouling the environment to the point where we may not be able to live here any longer. The notion is scary especially since we really don’t know enough one way or another to tell what can or should be done, if anything. It’s like being diagnosed with cancer, but not having been told yet how serious the disease is or what form the treatment will take.

By truth or distortion, it’s no mistake that we increase the odds of our long term survival by not leaving all the eggs in one basket. This is Stephen Hawking’s thought: we increase our chances of surviving a thousand years from now if we have people living elsewhere… and not merely on Mars, but as far elsewhere as we can get. Hawking is in the unenviable position of being aware that extinction events may originate not here on Earth, or even anywhere in our solar system, but elsewhere nearby in the stars. From his perspective, the further we spread ourselves, the greater the chances we survive that big event that no one can see coming.

Someone eventually has to propose a trip elsewhere beyond the edge of our solar system and it has to happen in such a way that information can return to Earth quickly enough for us to do something about it. Hawking and a couple billionaires proposing it are not surprising to me.

Where I got tripped up is that the proposal was essentially a big piece of Alistair Reynolds Sci-fi. God love him, it’s an interesting read, but not a very plausible one.

The Yahoo article calls it an “Audacious” plan. Postage stamp-sized, laser-beam riding autonomous mini-robots that will get there in 20 years. Seems like they hit all the sci-fi buzz words: nano-beamriding-laser-AIs. Moore’s Law gone crazy. I’m sure it will sell well to the public and they can wave off the ‘audacity’ by pushing the technology requirements into the future. We will develop what’s needed.

Or can we?

If you stop to think about what they’re proposing, they’re talking about essentially being able to detect emissions from a cellphone at a range of 4 light years. That’s essentially all the more power this device can carry with it… that or less depending on how small you build it. I do like lasers, but they aren’t all-powerful. There are no sensors available to detect such a thing. We are not capable of detecting the albedo light reflected off the surface of a planet at a distance of 4 light years and they’re talking about detecting radiant output from an object not able to carry as much battery power as the typical cellphone…

I had no details on the solar sail they’re planning to propel this probe with, but I did some back-of-the-envelop calculations on the power requirements for propelling a ‘stamp-sized’ object with a laser. The concept itself is not really a fictional one: light contains momentum and can be said to have a pressure –I would recommend studying Poynting’s vector for anyone curious. Knowledge that light carries momentum was actually a prerequisite for Einstein’s E=mc^2 postulate (what, you’ve never heard of E=pc?) You shine light on a surface and that surface rebounds from the light it absorbs or reflects. The whole concept underlies the idea of solar sailing. Using a laser as your light source is not a big leap: lasers are more columnated than sunlight and can deliver a great deal of intensity to a tiny spot. Some calculations about the system are easy to make.

I estimate that a postage stamp sized probe needs to be hit by something like a 600 megawatt/m^2 intensity laser for the duration of a year in order to achieve the speeds they’re talking about. That’s the average amount of light power a 1 gram mass space probe must absorb to be accelerated to 20% of light speed in the course of one year. Laser lights are extraordinarily intense, but this rating becomes even steeper when you start to realize that even laser light must fall off as 1/distance^2 for the spherical radius of the laser wave fronts, however well columnated that light is. For a high finesse laser cavity, where the mirrors are curved, that radius actually ends up being shorter –corrective lenses can help, but this is still imperfect. For all existent lasers, the spot size disperses to meters across at a distance of thousands of kilometers, at maximum. How far does a little laser pointer reach? For radiation delivered for a year, coping with the inverse square fall-off of light intensity as the object moves away from us ever more rapidly, we’re talking about a laser that is probably as intense as terawatts/m^2 (in order to continue to deliver the average 600 megawatts/m^2 to our postage stamp sized craft when it is a distance of 1/10th of a Light year away from us). The power capacity required to drive that is terawatt*hrs, which is the energy necessary to drive a small (or big) country. The US generates something like 25 terawatt*hrs of energy. It would certainly take a billionaire to buy up 1/20th of the power generation capacity of the US for a  year.

This is not even considering that precarious problem of how to know where to aim the laser. If it gets some other force input (space isn’t completely empty) it might veer out of the laser path –it doesn’t have to move far, just meters, or even centimeters where you aren’t expecting it to. And, it won’t even travel straight with respect to gravitational interactions it encounters from the planets in our solar system as it leaves. We can’t detect objects that are a centimeter square on planetary scales, let alone extra-solar scales. You could possibly track it by back reflection from the laser propulsion beam, but if it falls out of the beam, then what? They talk about Stealth aircraft having radar cross sections the size of a pigeon… this has a cross section the size of a postage stamp. Maybe the best way to go would be lidar using the propulsion laser, just scanning the beam around until you find back scatter again, but that would depend on the probe maintaining its orientation. The whole problem gets harder and harder as a function of a square of the distance with the rate at which light intensity falls off… the solar system is light hours across where the propulsion system would need to be workable out into light months of distance, or about a fifth of a light year given how far and fast they’re talking about. Feedback of how to move the steering laser beam if the probe disappears when it’s light minutes away means that you can’t find it again for at least minutes and then your knowledge is always minutes old. What do you do when it’s light days or even light weeks away?

Another pesky problem is what to do about dust. Like I said before, space isn’t completely empty. The fragility of the probe is somewhat inversely proportional to its size. The smaller the probe is, the less likely it is to hit dust, but the more likely a dust strike can do critical damage. A small, thin, unarmored thing with highly reduced electronics and electromechanics getting hit by a relatively larger piece of dust is serious business. How many of these probes do they plan to fly in order to hedge their bets and how many lasers are they using? The Pluto New Horizon’s mission carried a dust counting experiment and it is known that dust will gradually wear a space probe down given enough time. For a probe so small traveling at tremendous speeds, that time is less.

All of this is still just dealing with departure and says nothing about the complication of how to return data once the probe reaches its destination –which will be a rapid flyby since there won’t be any way to ever slow back down again. Like I said, do they really think they can detect a broadcast, even a laser broadcast, from a cellphone at a distance of 4 light years? The stars themselves are just points of light and many not even visible!

I shook my head, “Surely you must be joking, Professor Hawking!”

I think that the problem with the proposal is that they’re trying to have their cake and eat it too. They want to explore another star and do it in a time horizon that is significant to our civilization. 25 years is a long time, but not really that long. Exploring another star is not a technically insurmountable task, I don’t think, but it is demanding of both technical expertise and patience. Realistically, 25 years may be a bit impatient on the scale of our universe. It’s true that we have to perform this task someday and this proposal itself may well simply be trying to force that ‘someday’ to come sooner, but is this a form in which that task can work? Given our current capabilities, no: it’s betting on what we don’t know yet. Of course the proposal sounds like science fiction… it is science fiction still.

The great problem may be that we don’t know how close that existential threat is. Perhaps the achievable exploration mission demands that our civilization remain stable for a longer duration than perhaps it can. Are we better than the Roman Empire? With 25 years, there’s a chance that all the difficulties we’re currently facing haven’t exacted their existential toll yet… with two or three hundred years, a slightly more reasonable time goal for the exploration mission at hand, maybe our civilization will already have disappeared and won’t remain to learn what information the probe brings back. We live in days where pre-Renaissance barbarians are actually knocking on the door… will we withstand that?

I’m skeptical, but… go for it, guys, I wish you the best of luck.

Published by foolish physicist

Low level academic enthralled with learning how things work.

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