Yuri Milner breakthrough mission to Alpha Centauri Options

[Guest_mcmcmc_*]

https://breakthroughinitiatives.org/about

Laser-powered nanocrafts headed to Alpha Centauri:

Breakthrough Starshot is a $100 million research and engineering program aiming to demonstrate proof of concept for a new technology, enabling ultra-light unmanned space flight at 20% of the speed of light; and to lay the foundations for a flyby mission to Alpha Centauri within a generation.

Which are the engineering challenges?
http://breakthroughinitiatives.org/challenges/3

Yuri Milner twitter feed (official?): https://twitter.com/yurimilner

[Gerald]

Frankly speaking, this approach doesn't look like anything feasible to my eyes.
I'd think, that the only technically feasible way to reach 20% of the speed of light would be a linear motor similar to a linear particle accelerator. The probe would need to be really tiny and robust, and take the role of the accelerated particle. Tiny because of the huge amount of energy required, a back-of-an-envelope calculation returned several tera watts for a one gram probe. And robust due to the incredibly high acceleration, at least.
Provided, such a tiny probe can be accelerated to the presumed velocity, and it won't be destroyed by interplanetary or interstellar matter, how will it be able to send back data over a distance of several light years? The incredibly robust tiny bullet would need to unfold into a huge antenna of presumably an average layer thickness of less than an atom, with a well-defined parabolic shape, and pointed accurately to Earth.
I think, it's less than 1% science and more than 99% fiction in a world with finite ressources.

[scalbers]

Hard to say for me at this point, though it seems the antenna would be the same thing as the sail, doing double duty.

[marsbug]

Frankly speaking, this approach doesn't look like anything feasible to my eyes.
I'd think, that the only technically feasible way to reach 20% of the speed of light would be a linear motor similar to a linear particle accelerator. The probe would need to be really tiny and robust, and take the role of the accelerated particle. Tiny because of the huge amount of energy required, a back-of-an-envelope calculation returned several tera watts for a one gram probe. And robust due to the incredibly high acceleration, at least.
Provided, such a tiny probe can be accelerated to the presumed velocity, and it won't be destroyed by interplanetary or interstellar matter, how will it be able to send back data over a distance of several light years? The incredibly robust tiny bullet would need to unfold into a huge antenna of presumably an average layer thickness of less than an atom, with a well-defined parabolic shape, and pointed accurately to Earth.
I think, it's less than 1% science and more than 99% fiction in a world with finite ressources.

My understanding is that the physics is sound and well understood, except perhaps on the matter of the antenna, which I've not done much reading on. The engineering requirements, however, are.... no cause for optimism.I've heard plausible rebuttles to most objections to the physics, again except on the transmission of data to Earth. But, again, the engineering objections are significant. Still, that's the point of the initiative: To research the issues and determine how far away we are - at least to my understanding. I've not heard anyone seriously suggesting we'll get a star-probe from this. It's laying the foundations of the foundations.

[tanjent]

I think, it's less than 1% science and more than 99% fiction in a world with finite ressources.

Yes, but... if only a couple of entries on that long list of desirable breakthroughs could be checked off within a generation, it would greatly facilitate further exploration within our own solar system. And so I am glad to see someone throw some finite resources at an audacious and unrealistic plan like this. I don't expect we'll see any photos from Alpha Centauri in any living person's lifetime, but there's a lot more to be learned closer to home if only a few of those research lines ever pan out.

[Gerald]

Here is a pretty detailed study of a similar approach, called sail beam.
One of my major questions/concerns is the energy conversion efficiency, i.e. which fraction of the energy of the laser beam is converted into kinetic energy of the space probe? The referenced paper says 6.6 N/GW for an idealized mirror. How does this translate into the laser energy required to accelerate a 1 gram sail to 0.2 c? And how does this energy compare to the energy produced by a typical 1 TW power plant within a year? The straightforward idea, that the powerplant is producing the required TJ in one second doesn't hold, since just the momentum of the photons is used for propulsion, not their energy. Another comparison would be the mass equivalent of the laser energy required to accelerate a light sail of a given mass to 0.2 c.
A (sub-) monolayer graphene mesh would probably be considerably lighter, but also pretty translucent, such that only a small fraction of the light would be reflected, and realease its momentum.
The straightforward idea to stack such layers of graphene meshes, however, would result in multiple reflections between these layers and annihilate parts of the momentum of the photons.

[JRehling]

It seems like Eris and Sedna would be a nice intermediary case to test on before leaping all the way to Alpha Centauri.

The biological inspiration of insect (etc.) swarms is interesting for many domains, but the problem of long-distance information transmission is probably much, harder to scale than the physical exploration itself. I suppose that the one approach would be to use optical transmission and count on big light-bucket telescopes to be created while the nanocraft are in flight, but then you'd have to resolve them vs. the Alpha Centauri stars; maybe a hyperbolic trajectory would fling them off to the side eventually, but that would postpone and worsen the distance problem.

I'd be very enthusiastic about seeing this developed and tested for Eris, etc. first. That solves/reduces the magnitudes of several key variables a LOT.

[marsbug]

Here is a pretty detailed study of a similar approach, called sail beam.
One of my major questions/concerns is the energy conversion efficiency, i.e. which fraction of the energy of the laser beam is converted into kinetic energy of the space probe? The referenced paper says 6.6 N/GW for an idealized mirror. How does this translate into the laser energy required to accelerate a 1 gram sail to 0.2 c? And how does this energy compare to the energy produced by a typical 1 TW power plant within a year? The straightforward idea, that the powerplant is producing the required TJ in one second doesn't hold, since just the momentum of the photons is used for propulsion, not their energy.....

OK, bracing myself to eat my words, and just so I'm sure we're on the same page here: Photon momentum, individually or as a flux, is related to energy by: momentumn = Energy / c . That seems to suggest that getting the momentum of the beam from it's output energy should be (in principle, i realise there are probably real world complications) simple enough. So are you asking about the efficiency of momentum transfer from the photon flux to the sail - which would depend how close to a perfect mirror the sail is, and hence how close to all photon collisions with the sail being perfectly elastic you come? Or have I totally got the wrong end of the stick here?

[Gerald]

I'm asking about the ratio of energy of the laser beam converted into kinetic energy of the probe (relative to Earth) by presumably mere momentum transfer of the photons of the laser beam being reflected by the light sail.
In a first calculation, I think, that one can rely on the figure of 6.6 N/GW provided in the paper as an upper limit. So, this is about physics, assuming perfect engineering.

In the case of Sedna, it may be possible to simplify the technical overhead by first sending the probe to the sun, then unfolding or pointing the light sail, and exploiting the inverse square rule for a push by radiation pressure of up to more than 10,000-fold the 9.08 µPa at 1 a.u.. It's of course challenging for the electronics to survive temperatures of more than 1000 K close to the sun or within an intense laser beam, as well as temperatures of few 10s K in the outer reaches of our solar system, Transport to the sun can be within a heat-protected capsule, but then the nano probe needs to be exposed to the intense radiation.

[Gerald]

If I didn't make a mistake, I got 0.5 v/c for the energy conversion efficiency for a mirror accelerated from 0 to v by a laser beam, neglecting the Doppler shift, hence about 10% for v = 0.2c, which would be pretty good.

[marsbug]

I did some very crude calculations in bed last night and got numbers from 5% - 1%, but that was just working out how much KE the nano probe would be gaining per second given it's acceleration under kind of forces mentioned in the literature, and comparing that to the joules/second output of the power source. I had actually expected it to be much less, though I'm not really sure why. It is probably far too simplistic approach, and I wasn't going to share it, but it provides a bit of context for your numbers so here it is.

[scalbers]

I'm curious if the energy imparted to the craft is balanced by a red shift of the light reflected off? Otherwise it seems like a perpetual motion machine.

[Gerald]

The larger the red shift between the source of light and the mirror the larger the energy transfer from the photon to the mirror, when observed from the source of light, since the momentum vector of the photon is reversed by the mirror (in the mirror's frame). So, for small v/c, the momentum transfer to the mirror is almost independent of velocity v, actually (presumably, without cross-checking) proportional to 1-v/c, but the transfer of kinetic energy from the photon to the probe increases with the velocity of the mirror (seen from the source of light, and for "small" velocities), and hence the energy loss of the photon. (Momentum of an object is mass times velocity, while kinetic energy is proportional to mass times velocity squared. A fixed momentum transfer results in a fixed Δv, but kinetic energy transfer to non-relativistic probe of mass m is ΔE=0.5m x ((v+Δv)²-v²)=0.5m x (2vΔv+(Δv)²), hence approximately proportional to v, for small Δv << v, and for v << c.)
Or, seen from the mirror's perspective, we get red-shifted light with increasing velocity. Only this red-shifted portion of the photon is reflected in the mirror's frame (and presumably red-shifted a second time, when observed from the source of light). Photon energy is proprtional to its wave length (photon energy equals frequency times Planck's constant).
For larger velocites, it's probably more complicated, since the number of reflected photons may be proportional to 1-v/c, and we'll eventually run into relativistic Dopple shift, with an additional Lorentz factor. For v = 0.2c, the Lorentz factor is about 1-0.02, much closer to 1 than 1-v/c, so neglectible for a draft back-of-an-envelope calculation.

[cIclops]

From the Breakthrough Discuss virtual livestream recorded April 13, 2021

https://youtu.be/mTFx5-AMmTk?list=PLyF3OMOi...HERg&t=2356