Breakthrough Starshot - Mission: Alpha Centauri in 20 years?

[morphy_richards]

Reminds me of this thread from a while back.
Nicely summarised in this Adafruit blog.

Basically,

• miniaturised space probe, mass < 1g.
• "Solar" (Light) sail
• "BFG" type laser array on Earth, firing at sail on probe to propel it along.
• Tiny space probe accelerates to about 20% light speed.

Definitely worth shouting about. I'm quite surprised though because I thought laser divergence meant this method would not be useful once you got past Jupiter.

[morphy_richards]

Answer to my own point

" Focusing the light beam on the lightsail would accelerate individual nanocrafts to the target speed within minutes."

(So, it doesn't need to go that far)

[rzusman]

What is the point?
How is a 1g device going to transmit a signal back to Earth?

[stderr]

What is the point?
How is a 1g device going to transmit a signal back to Earth?

Active transmissions that distance are pretty hard to do. Swarms of bots or larger physical systems could use passive occultation of the local star on approach, and during the journey, opportunistic occultation of other stars as they are available. On approach, this could be combined with power generation.

But if you are going to get yourself going largely by local laser light here from our system, how do you slow down at the destination? Unless you accept that it is a fly by, this is the hardest part. If the laser is on the probes and uses power generated by its mechanism for occultation, perhaps that could work.

[morphy_richards]

Occultation? All sounds a bit, you know...mystic. If you ask me
I think the plan is to send swarms of thousands. Individual devices are cheap and something to do with the Earth based sending laser being able to receive transmissions somehow.
Need to study the thing properly because I can't imagine for the life of me how that would work as a receiver.

[W. H. Heydt]

Shades of the Niven/Pournelle "The Mote in God's Eye" as a propulsion technique.

[jamesh]

Mesh network, just keep sending these things out, giving a stream of them to pass data back on.

Basic IoT....!

[morphy_richards]

Mesh network, just keep sending these things out, giving a stream of them to pass data back on.

Basic IoT....!

That could really work. One of the brilliant things about this is that you can attempt to solve some of the engineering problems yourself using hobbyist stuff. We could make a model of a starshot long range communication network on the school field using photodiodes, microbits, Pi Zeros (Where are these flippin' Zeros? ) and, um, stuff like that.

There's a list of unsolved engineering challenges here. As it's an open project anyone can chip in.

[rurwin]

Stated mission parameters:
u = 0 (near enough)
v = c/5
s = 2 x 10^9 m

v^2 - u^2 = 2 a s

c^2/25 = 2 a s
c^2 / (50 s) = a
(3 x 10^8)^2 / (50 x 2 x 10 ^9) = 900,000 m/s/s = 91,743 gravities

The 1g spacecraft would weigh nearly 92kgf during the boost. That's just about reasonable but the light-sail and rigging does have sigificant design issues to support it, resist deformation and remain light-weight.

[morphy_richards]

But if you are going to get yourself going largely by local laser light here from our system, how do you slow down at the destination? Unless you accept that it is a fly by, this is the hardest part. If the laser is on the probes and uses power generated by its mechanism for occultation, perhaps that could work.

Apr 13, 2016 15:47 John McLean
At 0.2c, the craft would have no time to analyse the atmosphere before being vapourised. There is no feasible mechanism so far proposed to allow any meaningful amount of braking - bear in mind we are talking about using a 100 GW laser array to accelerate, which is several orders of magnitude greater than solar flux on earth. A flyby within 1AU would allow a spectroscopic analysis of the atmosphere and the opportunity for some photography. You'd hope to be able to identify moons orbiting an exoplanet, surface temperature, presence and composition of atmosphere, maybe evidence of geological activity. A lot of information can be gleaned without smashing into things.

[BMS Doug]

But if you are going to get yourself going largely by local laser light here from our system, how do you slow down at the destination? Unless you accept that it is a fly by, this is the hardest part. If the laser is on the probes and uses power generated by its mechanism for occultation, perhaps that could work.

Apr 13, 2016 15:47 John McLean
At 0.2c, the craft would have no time to analyse the atmosphere before being vapourised. There is no feasible mechanism so far proposed to allow any meaningful amount of braking - bear in mind we are talking about using a 100 GW laser array to accelerate, which is several orders of magnitude greater than solar flux on earth. A flyby within 1AU would allow a spectroscopic analysis of the atmosphere and the opportunity for some photography. You'd hope to be able to identify moons orbiting an exoplanet, surface temperature, presence and composition of atmosphere, maybe evidence of geological activity. A lot of information can be gleaned without smashing into things.

I hope there is no space based infrastructure in the system where we are proposing to shotgun thousands of C fractional bullets.

that could be considered "unfriendly".

[Burngate]

I hope there is no space based infrastructure in the system where we are proposing to shotgun thousands of C fractional bullets.
That could be considered "unfriendly".

It'll be okay.

Twenty years or so from now, while you're watching out for any replies from the 'bots, keep an eye out for a high power laser flash.
If you see one, you've got another fifteen or so years to sort out some counter-measures.

Meanwhile, you've got positive evidence that life exists elsewhere - and that's a worthwhile result, isn't it?

[morphy_richards]

But if you are going to get yourself going largely by local laser light here from our system, how do you slow down at the destination? Unless you accept that it is a fly by, this is the hardest part. If the laser is on the probes and uses power generated by its mechanism for occultation, perhaps that could work.

Apr 13, 2016 15:47 John McLean
At 0.2c, the craft would have no time to analyse the atmosphere before being vapourised. There is no feasible mechanism so far proposed to allow any meaningful amount of braking - bear in mind we are talking about using a 100 GW laser array to accelerate, which is several orders of magnitude greater than solar flux on earth. A flyby within 1AU would allow a spectroscopic analysis of the atmosphere and the opportunity for some photography. You'd hope to be able to identify moons orbiting an exoplanet, surface temperature, presence and composition of atmosphere, maybe evidence of geological activity. A lot of information can be gleaned without smashing into things.

I hope there is no space based infrastructure in the system where we are proposing to shotgun thousands of C fractional bullets.

that could be considered "unfriendly".

Space is vastly hugely mindbogglingly big. I mean you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space. *
So they would probably just miss.
*Douglas Adams HHGTTG

[stderr]

I hope there is no space based infrastructure in the system where we are proposing to shotgun thousands of C fractional bullets.
that could be considered "unfriendly".

Space is vastly hugely mindbogglingly big. So they would probably just miss.

But this presupposes all parts of space are equally likely to receive the barrage, yet we are directing it exactly at a specific real target. Furthermore, it presupposes that gravity will not have any effect though it will, even given that the objects are perhaps flying at greater than the escape velocity of the Milky Way and especially if they are not, to wit, as time goes to infinity, it becomes increasingly likely, even outside of that spaceship that Zaphod stole, that it will be in fact right round one to one. And the expansion of the universe to make even more space in space will not save you for that expansion isn't in all things, not in us or anything else that is gravitationally bound, such as the galaxies and their clusters, rather only in those things that are not. So if the objects, the shards, the shrapnel, if they stay within the galaxy, they will fly around until they hit something, perhaps that something will be mad about it. Now do we have objects in the galaxy that are flying around at some fraction of C? The answer is yes we do, we call them "cosmic rays". Would you be mad at someone shooting at you with cosmic rays millions of times bigger than the normal ones? I know I would be. For the love of all things good in the world, halt this insane project right now!

[morphy_richards]

Well, there is something called a blazar, which is mass "ejected" * from a super massive black hole. That stuff travels at 99.9℅ the speed of light.
OK, that's just gas (plasma) but anyone who has ever stood in front of a blank round thats been fired at them (is dead) can tell you very fast gas is bad. And that's a natural occurrence. There must be other bits of stuff amongst all that pelting along at a fair old pace too.
I think these space craft have on board ion engines so I suppose they will all be able to correct their course from shotgun cone into a perfect narrow cylinder. If that can't happen then I think the mission could not hope to succeed.
You're right, given enough time the likely hood of a collision with something else is 1. I reckon if you worked out the maths it would be a star or black hole or a really really big thing like Betelgeuse (OK that's another star) with overwhelming odds.
Even if it was a planet with a little bit of atmosphere they would be vapourised well before hitting anything firm.
I dunno what the odds are exactly but holding back from humanities next step (and only chance of our long term survival as a species) because of the risk of upsetting the locals has never stopped us exploring before.

[peterlite]

A Raspberry Pi Zero weighs more than a gram which shoots the plan down in popularity. In fact there is no useful measuring device weighing a gram or less. You are looking at hundreds of grams to measure something useful then convert it to a transmission plus hundreds of grams to form a useful sail.

By the time you add an illuminated sign advertising the sponsor, Nestle, Coca Cola, Ford, whatever, you are up to 20,000 kg.

The grid network would work until their batteries go flat. The grid idea would work better if you shot a decent relay node into orbit around each star on route.

The laser blaster providing propulsion would be more efficient firing from space instead of heating up our atmosphere. Plus it would shoot down less Malaysian Airlines flights.

What if we coordinated all the Lazer Tag players in the world to fire at the same spot in the sky. We throw a Zero in front of the lasers. We could create the critical first stage boost by coordinating some paint ball players to ...

[BMS Doug]

What if we coordinated all the Lazer Tag players in the world to fire at the same spot in the sky. We throw a Zero in front of the lasers. We could create the critical first stage boost by coordinating some paint ball players to ...

You would have better like with power point presentation presenters, at least they actually have lasers, Lazer tag guns don't.

Space is vastly hugely mindbogglingly big. I mean you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space. *
So they would probably just miss.

Sure I know the chances of hitting something are infinitesimal, but consider that a truly space based civilisation could have lots of orbital traffic. What a lovely way to introduce ourselves, shooting a narrow target window (orbital flyby) with a large number of projectiles.

[rurwin]

I hope there is no space based infrastructure in the system where we are proposing to shotgun thousands of C fractional bullets.
that could be considered "unfriendly".

Space is vastly hugely mindbogglingly big. So they would probably just miss.

But this presupposes all parts of space are equally likely to receive the barrage, yet we are directing it exactly at a specific real target. Furthermore, it presupposes that gravity will not have any effect though it will, even given that the objects are perhaps flying at greater than the escape velocity of the Milky Way and especially if they are not, to wit, as time goes to infinity, it becomes increasingly likely, even outside of that spaceship that Zaphod stole, that it will be in fact right round one to one. So if the objects, the shards, the shrapnel, if they stay within the galaxy, they will fly around until they hit something, perhaps that something will be mad about it.

The ship is aimed to fly within 1AU of its target. 1 AU is 92 million miles. We can estimate the chances of a collision by projecting all the obsticles on a circle perpendicular to the flight-path. Given that the light sail is the largest part of the craft and is around 4 square metres, we find that the target circle is 18,000,000,000,000,000,000,000 times bigger than the craft. Of course the planet is probably the largest thing in there, and that's maybe 50,000,000 times as big as the craft. (8000km radius -- much bigger than Earth). Let's assume they have absolutely masses of orbital infrastructure equivilant to a whole other planet. So the target is 100,000,000 times bigger than the craft. But there is 1,800,000,000,000,000 times more room for the craft to be in that does not hit anything. So the chances of hitting anything in the target system is the reciprical of that, or 0.000,000,000,000,05% and the probability of missing everything is 99.999,999,999,999,95%.

The area around the target zone is by far the most chance it will ever have of hitting anything important. Space is on average many orders of magnitude more empty than that.

20% of light-speed is above the escape velocity of the galaxy, which is about 2% c. So the spaceship would just fly in more or less a straight line forever. Even if it continued to fly around in inter-galactic space it would probably be torn to pieces by dust long before it had a remote chance of hitting anything.

You're right, given enough time the likely hood of a collision with something else is 1.

A solar-system consists of the sun and noise -- everything else is negligible. So to hit our sun it would have to hit a circle of radius 695,700 metres in a circle of around 43,000,000,000,000,000 metres. That's a 99.999999998% chance of missing everything. It would have that chance on average once every twenty years. So the chance of missing everything for n years is 0.99999999998^(n/20). A brief bit of experimentation shows that in 20 billion years, it still has a 98% chance of missing everything.

And that assumes that it will still be within the Milky Way galaxy, which it won't be; the galaxy is only 100,000 light-years across. Inter-galactic space is even more empty.

So we don't need to worry about someone suing us for at least twenty billion years and the Sun will eat the Earth in only four billion. By the time the miniscule chances of a hit are increased by the passage of time, the Sun will have burned out and we will either be dead or living somewhere else.

[BMS Doug]

So we don't need to worry about someone suing us for at least twenty billion years and the Sun will eat the Earth in only four billion. By the time the miniscule chances of a hit are increased by the passage of time, the Sun will have burned out and we will either be dead or living somewhere else.

when our great great great grandchildren are killed by angry aliens out for revenge I'm going to blame you!

[rurwin]

"great great great grandchildren" is only a century or two. Assuming you mean something more like a billion years...

"Oh no; it wan't us. Must have been the dinosurs."

[morphy_richards]

I was trying to work out what the effect of time dilation would be on the probes.
Using

t'=t √1-(v^2/c^2)

And (pleasingly) it's quite easy to do. As v to c is a simple ratio we can drop exact figures and use percentages, therefore full light speed "c" can be represented as 100 (%) and v at a quarter light speed can be represented as 25.
If we drop the time part as well and put in 1 then we can work out the time difference from t' to t as a simple ratio too.
So
t'=1(√1-(625/10000))
t'=1(√0.9375)
t'=0.96824583655

So, for every unit of time from our perspective, 0.96824583655 units of time will pass for the probe (which is a hell of a lot less than I was expecting)
If the probe had a 1GHz processor then to us it would only be 0.97 GHz.
If the journey takes 25 years of our time then it will seem to take 24.2 years to the probe.

Not a massive difference really.
Hmm..
Well, there you go

edit. oops its 20% c not 25% c
So the time difference is actually 0.97979589711
25 years our time will take 24.49 years for the probe.
I'm staggered by how surprisingly un-staggering those figures are. Time dilation. It's not all that really.

[rurwin]

You need to get a lot faster to get the good figures for dilation. Like over 80 or 90% of c. That's why I used good old Newton for my acceleration calculation. I'd need general relativity if the final speed was too much higher. (Special relativity only handles constant speed.) And that's unfortunate because that's where it gets horribly complex.

You can get a feel for it by taking a snapshot of the dilation at the final velocity. The dilation factor is 0.98, so the effective mass of the probe is 1g/0.98 = 1.02g Force = mass x acceleration. For the present purposes the force from the laser can be assumed to be constant. If the mass increases by 2% then the acceleration must reduce by 2%. So the final acceleration will be 2% lower than the 900,000 m/s/s that I calculated.

In actual fact, the acceleration will start high and reduce toward the end because the laser beam width will increase over the two million kilometres. Relativity will probably be a negligable effect.


Here's an interesting fact. I weigh about 100,000 times more than the space-ship. (Maybe only 50,000 if the sail and the craft both weigh 1g.) So if I painted myself silver and stood in front of the beam, assuming I don't get instantly vapourised, I would be accelerated at 9m/s/s -- 0-60mph in less than 3 seconds. That's a quick acceleration but it is less than 1 gravity. Ignoring air friction, in 38 seconds I would be going faster than the speed of sound. After the five minutes that the speacecraft boost takes, I would be doing over 6,000 mph. Fast enough to go around the world in 8.2 seconds.

[rzusman]

We should aim aim the probes at the Monk's planet - they would know how to slow them down!

http://www.e-reading.club/chapter.php/7 ... Space.html

[peterlite]

if I painted myself silver and stood in front of the beam

What will happen to aeroplanes flying over the beam?

[rurwin]

What will happen to aeroplanes flying over the beam?

It wouldn't be pretty, but 5 minutes is not long to maintain an exclusion zone and there is a research project to look into the requirements.