Next stop: Alpha Centauri? | Photo: Skatebiker at English Wikipedia, CC BY-SA 3.0
Last week, Stephen Hawking and Yuri Millner announced the “Breakthrough Starshot” project, an idea to send a postage-stamp-sized spacecraft to Alpha Centauri on a flyby mission. I’m not sure I think it’s possible, or even see the point.
While I laud their imagination and drive — and frankly love the idea of sending spacecraft to relativistic speeds just to see if it can be done — I’m not actually sure if (a) this idea will work and (b) what it could tell us even if it does.
First, there are the obvious issues: we haven’t tried to power a solar sail with an external laser before, we haven’t made a spacecraft the size of a postage stamp before, we haven’t put a massive honking laser in orbit before. Those are the “easy” problems.
Even if we solve those, and well we might, there are some harder ones.
See, we know a lot about the Alpha Centauri stars. We can see them well enough to do all sorts of neat things with the data. We know their masses, relative orbits, compositions, temperatures. We’re pretty good on the stars. And I don’t really know if it’s going to help up with the planets thing any more than, say, another Kepler mission would.
We can rule out seeing anything the size of Pluto, for certain. Because the spacecraft is tiny, it can only carry a tiny camera. This means its ability to image things is going to be pretty bad. Those photos of Pluto were taken with very a special camera, tuned to a very specific low level of light, and with a 75mm lens. A cellphone camera is not going to get much in the way of pictures of planets that aren’t seriously lit by their respective stars. Have you ever tried to take a photo of your cat with your cellphone in a dark room? That’s what we’re working with here. Sure, the iPhone 6 camera is pretty sweet and all, but unless we’re up close and personal with the target it’s like taking a picture of a black cat in a black room at night, meanwhile you don’t know where the cat is, or if there even is a cat.
And of course you’re flying through the room at relativistic speeds.
See, compounding the problem would be the duration of our visit. We’ll be going very, very fast, because we won’t be slowing down when we get there. It’s a flyby, like the New Horizons visit to Pluto. The distance between the two major stars of the Alpha Centauri system (Alpha Centauri A and B — we’re not going to count Proxima, because it’s 15,000 AU away and we’re not even sure it’s gravitationally bound to the system) is about 11 AU (just for reference an Astronomical Unit or AU is the distance from the Earth to the Sun). Let’s just say for the purposes of having something to actually look at with an iPhone camera — being generous — our own solar system is 60AU wide. That’s roughly the orbit of Neptune, where, thanks to the inverse square law and a whole lot of distance, the brightest day is about 0.1% of the brightness of noon on terra firma. Given that Alpha Centauri A has about one and a half times the lumosity of the Sun, maybe we can widen that a bit, but not by a hell of a lot. So let’s just be generous and say 80AU is the “we might get close enough to something to see it” diameter of the Alpha Centauri system. I think that’s being exceedingly generous for a camera the size of the one on your smartphone.
At 1/5 the speed of light, you cover a single AU every (roughly) 2500 seconds, or every 41 minutes and 40 seconds. Let’s round that up too, to 42 minutes. You never know, we might be going a teeny tiny bit slower than a fifth the speed of light. At 42 minutes per AU (and a generous 80AU visible diameter) from one side of the visible system to the other takes 3360 minutes. That just 56 hours. For our solar system, that’s 56 hours to go from the sun being a pinprick in the sky to bigger than humanly imaginable, and back to a tiny pinprick again. New Horizons took years to do less than half that distance, and at that speed it still only got about 7 days of really excellent data as it passed Pluto, 3.2 before closest approach and 3.2 after. We get less than half that time to look at an entire solar system with a camera the size of a postage stamp? You’d better hope we’re sending a few dozen of them, if only to raise the chances of us actually seeing anything at all.
So what could we see, planet-wise? Well, there might be a planet in Alpha Centauri B’s habitable zone, between 0.5 and 0.8 AU from the dimmer star. And if we were there we could see it with the naked eye. But again, the speed gets us. The furthest Venus ever gets away from us is 1.7 AU, and it’s a tiny shining dot to the naked eye, so let’s use that as our scale. From dot to dot again — from one Venus-Earth distance from Alpha Centauri Bc to the opposite distance on the other — we get roughly 2 hours and 22 minutes to look at it. And that’s only if our aim is perfect.
Because at 4.3 light years away, any aiming the tiny spacecraft does — I mean both telemetry and just aiming the teeny tiny camera — is going to have to be done either by itself on the fly or 4.3 years ahead of time. Did I mention we don’t know if there’s a planet there, and where in its orbit it might be if there is? With a camera that size and traveling at that speed you’re pretty much on a wing and a prayer. Now maybe we could see something Jupiter-sized, but as yet we don’t have much data to suggest something like that is there to be seen.
And then of course there’s the problem of relaying any images back. If you’re the size of a postage stamp and travelling at 1/5 the speed of light, sending back any images you do happen to take is — well let’s say it’s beyond our current technology. Hawking and Millner admit this, and think we can surmount it, so… fine. Sure. And you know what, if we can do that, maybe we can make iPhone cameras that can see black cats in pitch black rooms and all my skepticism is for nothing.
I will say one thing, however: I’d like to see them doing it if for no other reason than to explore our own solar system. I’d like to send a half dozen of these things out in all directions to measure the heliopause. I’d love to get some kind of observational data on the Oort cloud. And maybe these teeny tiny, one-use spacecraft would be perfect for doing so in a reasonable timescale. But right now I have a hard time imagining their usefulness at Alpha Centauri, even if we can get the thing there in the first place.
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Richard Ford Burley is a human, writer, and doctoral candidate at Boston College, as well as an editor at Ledger, the first academic journal devoted to Bitcoin and other cryptocurrencies. In his spare time he writes about science, skepticism, feminism, and futurism here at This Week In Tomorrow.