Starshot

[PromoTheRobot]

DC Tom raised the same braking dilemma that occurred to me. Then again you are braking something the size of an iPhone, not a space station.

 

Obviously one of the challenges of this mission is creating fully autonomous devices. AI-capable micro-craft that can brake, assess a situation and formulate a plan on their own because they can't wait 9 years for us to tell it what to do.

 

But back to braking, couldn't they use their solar sails to capture light from Alpha/Beta/Proxima Centauri to accomplish that?

[sodbuster]

But back to braking, couldn't they use their solar sails to capture light from Alpha/Beta/Proxima Centauri to accomplish that?

I wonder how much, if at all, the solar winds from that star system would slow it down. Correct me if I'm wrong, but aren't they going to be facing resistance, and basically swimming upstream once they cross the termination shock of the system? Or would that resistance be negligible? Edited April 13, 2016 by sodbuster

[4merper4mer]

 

Shielding and redundancy. I suspect the radiation risk is lower in the interstellar medium than it is in a stellar neighborhood - cosmic rays are much more energetic than stellar (i.e. solar) radiation, but there's also far fewer of them. I'm not interested enough in that problem to actually do the calculation, though.

 

The biggest handicap to the project is the speed of the probe - not getting it fast enough to get there, but keeping it slow enough to collect meaningful data. They're talking about 20 years to Alpha Centauri, which is about 4 light years away. If you don't have a way to slow the probe down at the destination (hint: you don't. The fuel you'd need for that delta-V is mass-prohibitive just to get off the earth, never mind carry to Alpha Centauri), you send it screaming through the system at a minimum of a fifth of the speed of light. That's fast enough to cross our entire solar system in two days.

 

That may seem like a lot...but consider New Horizons: it had a 22-hour flyby dedicated solely to data collection (basically, went into an automatic mode and collected and stored everything it could for later transmission.) For this plan, we're talking about a data collection window a little more than twice as long for an entire solar system. What's more, that's for an entire solar system where we don't know where anything is. You can't even do the sort of mission planning it took to pull of the New Horizons mission, because you can't predict where to point the cameras. (And you can't use any sort of command guidance because, y'know, any command you send takes four years to get there.) It's analogous to me asking you to drive down a road at 300mph and read the house numbers off mailboxes, when we don't even know if there's any mailboxes on the road. And if I want to ask you to do something different while you're driving, I have to mail you a letter.

 

It's a ridiculous engineering challenge. Probably not insoluble...but nearly so. The biggest benefit of a program like this would probably be ancillary, like how the massive investment in SDI gave us the Clementine lunar mission.

 

 

I didn't need to read this post because sitcom math is enough to know that this mission is folly.

 

I am glad I did read it though because I learned that those little oranges that are easy to peel come from the moon.

 

 

I guess it makes sense because the lower gravity brings the peel away from the rest of the orange. Cool.

[DC Tom]

DC Tom raised the same braking dilemma that occurred to me. Then again you are braking something the size of an iPhone, not a space station.

 

Obviously one of the challenges of this mission is creating fully autonomous devices. AI-capable micro-craft that can brake, assess a situation and formulate a plan on their own because they can't wait 9 years for us to tell it what to do.

 

But back to braking, couldn't they use their solar sails to capture light from Alpha/Beta/Proxima Centauri to accomplish that?

 

1) You're braking something that has to generate enough power to send a signal back to earth. That has to be far closer to "space station" size than "iPhone." It's a pointless exercise if you can't actually get the data back.light yea

 

2) Any form of braking you might use is highly dependent on the mission profile. The simplest profile is: accelerate at a constant rate for half the distance there, then turn around a decelerate at that same rate for the other half. That's only about a fifth of a g of acceleration to get you there in 20 years. But that also requires an engine that will run flawlessly for 20 years, with enough thrust to accelerate a craft at 2m/s^2 that is big enough to contain all the fuel to maintain that thrust for for 20 years. Calculating the size of that craft is left as an exercise to the student (i.e. I'm too busy to do the integration, do it yourself.)

 

And no, solar sails won't work. You need a square kilometer of sail are to accelerate a 50lb object at a fifth of a gravity at the distance of the earth from the sun (about 6 newtons per square kilometer). That force falls off with the square of the distance from the light source (i.e. the sun)...so once that craft gets to Jupiter, the thrust has already fallen off to about a quarter of a newton (and the acceleration a hundredth of a gravity). Taking the "edge" of our solar system to be about 50 AU (the Kuiper belt), you'd get a force of 0.2% of a newton and an acceleration of about one ten-thousandth of a gravity.

 

That's the force Mt. Everest exerts on a building 10 miles away. It's actually a measurable force (old buildings in Nepal are measurably slated because the plumb bobs weren't plumb, because of the sideways force Everest exerted on them). But it's practically negligible. It's also a value that lets you calculate the top speed of your probe that allows it to be captured by Alpha Centauri (there's a certain speed v that, when decelerated from 50 AU to 1 AU from the star at a deceleration dependent on the square of the distance, gives a final speed vf that allows capture by the stellar system. Integrate over distance, from 50 to 1 AU. Or, more easily, calculate the problem in reverse: how fast is a 1km solar sail pushing 50 lb going when it accelerates from earth to the Kuiper belt - since you're starting at a solar orbit of one AU, you can practically ignore the question of "what is the capture speed vf?") Again, the problem is left to the student - I'm not going to bother, since I'm certain the answer leads to a mission time somewhere between "pigs fly" and "hell freezes over." And that's just to send a bag of flour, never mind something useful.

I wonder how much, if at all, the solar winds from that star system would slow it down. Correct me if I'm wrong, but aren't they going to be facing resistance, and basically swimming upstream once they cross the termination shock of the system? Or would that resistance be negligible?

 

See above. I used the Kuiper belt as an arbitrary measure, simply because it's known. The boundary between stellar and interstellar space may seem better...but we simply don't know what it is (we don't even understand our own), and if the force at the Kuiper belt is so low, the force at the stellar bow shock is ridiculously negligible (in fact, it's likely a turbulent zone than a sharp boundary, so you'd have to be well through it before you could feel a predictably consistent force.)

[unbillievable]

Just use the solar sails to capture high speed tachyon eddies to accelerate to warp 1.