Great News for Breakthrough Starshot

[Shpaget]

1 hour ago, wumpus said:

I'm not kidding about "recondensed plasma".

I'm not sure the term "plasma" would still apply XD.

If all that energy was delivered instantly, it would raise the temperature of the probe (very generously assuming specific heat capacity of water) to about 430 000 000 000 (yeah, that's 430 trillion).

Perhaps we're looking at this the wrong way. Maybe it's not a solar/laser sail, maybe it's a fusion drive.

[monophonic]

Wait, were the lasers supposed to be built on Earth? What does dumping that much power through the atmosphere years on end do to the local climate? At least I'm hoping the power isn't enough to have a significant effect on global climate but will accept any corrections.

[Shpaget]

Well, Earth gets about 1 kW per m²  (1 GW/km²) from the Sun, so if you use solar panels there is no net input, but you'd need to cover much more than 1 km²  with solar panels to get to 1 GW of laser energy delivered. Solar panels are about 20% efficient, so you need 5 km². High power lasers are around 30% efficient, so our solar panel array climbs to about 15 km^2., and then there's the beam width losses. You'd probably need to have your laser beam much much wider than the target to ensure the alignment. A factor of 10 seems highly optimistic (factor of 1 000 000 seems closer, since Voyagers parabolic antenna can aim "only" at particular hemispheres on Earth, not individual antennas), but let's go with 10 anyway - the array is now 150 km²  for 1 GW of energy delivered.

So you need to take all the sunlight that usually falls on 150 km²  and dump it over just 1 km². Yeah, it could cause some serious weather issues over a couple of decades, not to mention instantly kill every flying thing that crosses the beam.

 

Ha! When I started to write this post, I intended to write "Nah! The Earth gets 1 GW per km²  from the Sun anyway", but just ballparking the math we get some very different answers.

Edited July 30, 2017 by Shpaget
typo

[monstah]

I'm reading a lot of comments here on why solar sails and solar panels can't work in interstellar space... but aren't these laser sails, presumably with laser panels? That's how I understood it.

Anyway, I'm interested in this. Not hopeful, or hyped... just interested.

[Bill Phil]

11 minutes ago, monstah said:

I'm reading a lot of comments here on why solar sails and solar panels can't work in interstellar space... but aren't these laser sails, presumably with laser panels? That's how I understood it.

Anyway, I'm interested in this. Not hopeful, or hyped... just interested.

Solar sails are photon sails. Lasers emit photons. Solar sails are the same as laser sails. Usually...

Photovoltaics can be powered by lasers, too.

[Green Baron]

I would think that in interstellar space there is only the omnidirectional light of the surrounding stars. A single star's radiation pressure a few light hours away even on a humongous "sail"area is not enough to accelerate something. Forces go down with the square of the distance.

I must say that i have my difficulties with the word "sail" in this context, since i do sail and know how our different kinds of sail work to propel a boat on the ocean.

This starshot thing (which is just a crazy idea well marketed without a fundament in reality in my eyes) shall be "blown" by well aimed radiation pressure from a single point (laser park) on earth in a concentrated, short term act (earth must face in the direction of the chips), more like grains of pepper on the hand that you blow away. The rest of the chips' journeys are drifting.

Edited July 31, 2017 by Green Baron

[Shpaget]

28 minutes ago, monstah said:

I'm reading a lot of comments here on why solar sails and solar panels can't work in interstellar space... but aren't these laser sails, presumably with laser panels? That's how I understood it.

Anyway, I'm interested in this. Not hopeful, or hyped... just interested.

If you think that we are bashing this without reason, don't. We are calling the project out on what we see as fundamental flaws that are not just one discovery away from being solved, but deeply rooted in the laws of physics.

The problem with the proposed laser launch is that this system needs mind boggling acceleration for couple of reasons.

1. They need to have a steady stream of probes to maintain the relay network (or is it a relay line?), with individual probes not too far from each other due to the problems with coms over vast distances and their fundamental lack of proper parabolic dishes and strong transmitters (they want their probes to be absolutely tiny).

2. It's not possible focus lasers over very large distances, so they need to deliver all the energy before the probe gets beyond the usable focus range of the laser (we've talked about this multiple times in the many space warfare topics that popped up on these forums over the past years).

Those two facts and their wish to achieve relativistic speeds directly lead to the requirement that they deliver insane amounts of energy to the probes in very short time, which leads to several problems. The acceleration of the probes needs to be very aggressive. It is questionable if they could survive it, but even if they could, the sheer amount of energy over the short period of time will result in heating beyond any hope of ever being manageable. Earlier in this thread I said 430 trillion degrees. I did not pull that number out of my derrière, I actually did some rough math, but even if I'm three orders of magnitudes wrong, it's still hopelessly beyond being in the domain of solvable.

But those two issues are far from the only ones. Every time I sit on a toilet, I find a new hole in the project. So far, these are the issues mentioned in this thread that would have to be addressed in order to achieve the goal:

1. Giant freaking lasers to push the probes (giant as in orders of magnitude larger than anything else existing).

2. Power supply for those lasers (while Earth would probably have enough grid capacity to supply it, I don't see any particular country/region on Earth that could do it by itself, and sharing the load just doesn't work).

3. Probe design that can survive the launch (so far we have no material that can even remotely approach surviving the conditions during the launch).

4. Communications between the probes (such tiny probes have no chance of transmitting and receiving over distances we are talking about).

5. Power supply for probes on their way while in interstellar space (solar panels are not an option, there is no sunlight in deep space, and laser power is out of the picture since we can neither focus the laser at the distances required nor provide laser power to all the probes in the relay line at once since the ones that are closer would cast a shadow on the ones further out).

Resolving all of these issues is absolutely necessary for the accomplishment of this mission, with only the number 2. and 5. being in the realm of possible, if you could convince some country to dedicate a significant chunk of their electricity production to this project continuously for couple of decades and if we could develop some ultralight solid state radioisotope power generator for the probes themselves (which raises the issue of radiation shielding for the rest of the electronics). The rest of the issues are much harder to solve.

[HebaruSan]

3 minutes ago, Shpaget said:

5. Power supply for probes on their way while in interstellar space (solar panels are not an option, there is no sunlight in deep space, and laser power is out of the picture since we can neither focus the laser at the distances required nor provide laser power to all the probes in the relay line at once since the ones that are closer would cast a shadow on the ones further out).

There is sunlight at the destination, where the science is to be performed. Why not hibernate till Alpha Centauri is bright enough?

[monstah]

1 hour ago, Bill Phil said:

Solar sails are photon sails. Lasers emit photons. Solar sails are the same as laser sails. Usually...

Photovoltaics can be powered by lasers, too.

My point was it's not limited to solar output at 1/r^2 fallout. Anyway. Whatever, this is probably just more space debris.

[Shpaget]

29 minutes ago, HebaruSan said:

There is sunlight at the destination, where the science is to be performed. Why not hibernate till Alpha Centauri is bright enough?

Because you need power for all the probes en route that act as relays.

[HebaruSan]

4 minutes ago, Shpaget said:

Because you need power for all the probes en route that act as relays.

Is that actually part of their mission architecture? I'm not finding the word "relay" on any of the pages at https://breakthroughinitiatives.org.

[Shpaget]

That's the impression I got from their press material and stuff, I could be wrong, of course, but consider this:

He talks about "sending hundreds and maybe thousands" of probes for each target.

He also talks about 1 W lasers mounted on the probes, and I don't see how those can work over 4 light years. At those distances, a perfect diffraction limited red laser with a lens diameter of 3 m would have a minimum beam width of around 10 million kilometers. To achieve a 1 km diameter laser spot, we would need to go to x ray wavelengths (1 nm) and scale up the size of the lens to 50 m. All that is assuming a diffraction limited optics, which a fresnel lens made out of a flimsy solar sail material that is changing shape mid-flight can't realistically be.

However, let's consider that they managed to do it and get a 1 km wide laser dot smack right on top of their receivers on Earth. You are now receiving a 1 W signal over 1 km² that is competing with natural starlight and man made interference. I'll let somebody else to figure out if that sort of signal/noise ratio is usable.

Relays would make more sense.

[HebaruSan]

58 minutes ago, Shpaget said:

He talks about "sending hundreds and maybe thousands" of probes for each target.

For "redundancy." No mention of relays.

58 minutes ago, Shpaget said:

Relays would make more sense.

The members of this project don't seem to agree with you, though. Possibly because they know relays would need power in interstellar space.

[kerbiloid]

Even on a successful targeting, this will look like.

Spoiler

Will they fly around right near the Earth or closer to Pluto?

[wumpus]

17 hours ago, monstah said:

I'm reading a lot of comments here on why solar sails and solar panels can't work in interstellar space... but aren't these laser sails, presumably with laser panels? That's how I understood it.

Anyway, I'm interested in this. Not hopeful, or hyped... just interested.

One thing to remember about Shpaget's post is that all the propulsion issues happen because the stated goal is relativistic speed.  Interplanetary speed isn't a problem, and extremely slow trips (somewhat faster than the Voyagers) to the stars are possible, but relativistic speed is right out (and thus ever living to see the thing get there).

There are other issues with solar panels for interplanetary missions as well.  If you are using solar panels for propulsion, you are obviously going to have low thrust.  Going from LEO to escape velocity will be *slow* (probably months) and a huge amount of that time will be spent in the Van Allen belts.  The Van Allen belts tend to destroy solar panels (and other things, but the solar panels will go first and are the hardest to shield).  I've suggested beaming microwave power to ferries that move cargo from LEO to escape velocity, but really haven't checked the math.

Communications will also be a non-trivial issue.  There are deep mathematical and physical reasons limiting your ability to communicate at extreme ranges and relatively tiny amounts of power (although the Voyagers and Pioneer probes did wonders in pushing the state of the art).  My guess is that any interstellar communication will involve charging a capacitor bank with the local power supply then sending carefully timed bursts of '1s'.  For deep space communication (SETI), ground based radio telescopes could transmit around 1Hz (but for thousands of light years).  Don't expect a tiny craft to be able to transmit at 1bps.  Also remember that the mission like this would like New Horizons only more extreme (the probe desperately downloading everything its sensor see while it rushes past the target, and only then begins to return its treasures as the target recedes into the darkness).  Hope you brought along a nuclear thermal [power] generator, because you won't be in range to get solar power for the whole time you need to transmit back.  (This also hits the problem that current NTGs use isotopes that won't survive the *time* of interstellar distances.  Personally, I don't think that's much a physical issue, but it would take considerable R&D to make effective small reactors.)  None of these are really an option for a 4g probe.

[Shpaget]

8 hours ago, HebaruSan said:

For "redundancy." No mention of relays.

The members of this project don't seem to agree with you, though. Possibly because they know relays would need power in interstellar space.

Could be.

I just don't see direct transmission from light years away as feasible, so I assumed it would use relay system.

[monstah]

3 hours ago, Shpaget said:

I just don't see direct transmission from light years away as feasible, so I assumed it would use relay system.

I've figured transmission would be a problem when I first read about it, until I found the concept of the relay line (on Atomic Rockets? Don't remember), and I assumed that's what they were doing, too.