5 ms·

The physics doesn't stack up for me. The spacecraft must be lightweight so it can be accelerated to high speeds but it must also have a very large area such tha

The physics doesn't stack up for me. The spacecraft must be lightweight so it can be accelerated to high speeds but it must also have a very large area such that a large proportion of the laser photons hit it. This means a very thin disc shaped spacecraft (a few grams but with a few m2 of area). Whilst this may be able to be constructed it would be incredibly delicate for launch and I can't see how it would have the required strength to avoid it tearing apart under the force applied by the laser photons. A paper produced looking at the feasibility of such a system https://arxiv.org/pdf/1604.01356.pdf https://arxiv.org/pdf/1604.01356.pdf says >A critical issue will be the stability of the sail. There are a number of perturbative effects. These include laser instabilities and laser mode issues, differential forces on the sail and mechanical modes in the sail, heating of the sail and laser pointing instability. >This is a complex sets of issues that requires a significant amount of research and development. This will not be trivial. I believe these issues are show stopping rather than "not trivial" You can never fabricate a probe of small enough mass which has enough area for laser propulsion to work effectively whilst not tearing itself to pieces or overheating (at least not to anywhere near relativistic speeds).

fnordpiglet 4y ago
I have over 500 hours on kerbal space program and have been playing with the KSPIE mod that enables beamed power and sails. They definitely work.
- barbegal 4y ago
  Oh yeah they work fine when you neglect certain critical parameters. How hot do your solar sails get in Kerbal? And what is the maximum torque between the sail and the spacecraft in Kerbal?
  - simonh 4y ago
    I suspect a close proximity of tongue and cheek in that comment.
  - fnordpiglet 4y ago
    They do actually. It’s a really good game and mod. Highly recommend it. Especially if you need to use a solar sail that works
- LegitShady 4y ago
  (in kerbal space program)
  - fnordpiglet 4y ago
    I mount my games.
marcosdumay 4y ago
Well, you are referencing a paper that says they already got material good enough for 1% of c. The fact that the target of 25% of c is still not achieved isn't as large a showstopper as you are making it to be.
usrusr 4y ago
> “and the data they produce will be sent back to Earth“ That's where my optimism limiters were triggered: sending something in the general direction of earth is all nice and well, but receiving that signal, sent from the confines of a "DVD sized" emitter, against the backdrop of a nearby star? They should try to find a way to harness optimism for propulsion...
- cee_el123 4y ago
  perhaps that can be overcome by a train/succession of such probes relaying the signals over shorter distances ?
  - usrusr 4y ago
    But how much shorter? Light years are a long distance to cover with relay stations if their peer to peer range is not unlimited. And when "DVD sized" should include both sender and receiver, p2p range most certainly won't be "astronomical" (which it would quite literally have to be). The relay approach implies symmetry and usually what we do with probes is the opposite, compensate a smaller device on one end with a bigger device on the other end.
    - sliken 4y ago
      Voyager 1 uses 22 watts and 1970s technology to communicate from 21 light hours away. Radio encoding has come a long way since then. In particular ft8/js8call uses an encoding that can communicate at over 20 db below the noise floor, allowing communications across the earth with a few watts which involves many 1000s of miles of atmosphere, bouncing (poorly) off the upper atmosphere and the ground many times. If we launched one an hour at 5% of C (better than today, but much worse than the goals of the project) they would only have to communicate across hops of 3 light minutes, er about half that since 5% of C is the final velocity not the average. Considering voyagers success at 21 light hours and the improvements in encoding that sounds quite feasible.
usefulcat 4y ago
Let's say they make a sail that is large enough and light enough to allow the craft to accelerate to very high speeds (say, 0.1 - 0.2c?). It's quite a long way to Alpha Centauri, and even though interstellar space is very empty, it's not completely empty. As the sail hits the odd spec of dust here and there at those speeds it seems like it will be shredded. Maybe it's ok as long as it lasts long enough for it to be accelerated to a 'fast enough' speed?
- Ancalagon 4y ago
  The original idea I recall for this was to send hundreds of these probes with the hope that a few might make it (because this launch method and the probes would be relatively cheap once you built out the laser system). But also, interstellar vacuum is very, very empty. Encountering a speck of dust even in that distance with that size of a probe is a pretty negligible risk if I recall correctly.

fnordpiglet 4y ago

I have over 500 hours on kerbal space program and have been playing with the KSPIE mod that enables beamed power and sails. They definitely work.

barbegal 4y ago
Oh yeah they work fine when you neglect certain critical parameters. How hot do your solar sails get in Kerbal? And what is the maximum torque between the sail and the spacecraft in Kerbal?
- simonh 4y ago
  I suspect a close proximity of tongue and cheek in that comment.
- fnordpiglet 4y ago
  They do actually. It’s a really good game and mod. Highly recommend it. Especially if you need to use a solar sail that works
LegitShady 4y ago
(in kerbal space program)
- fnordpiglet 4y ago
  I mount my games.

marcosdumay 4y ago

Well, you are referencing a paper that says they already got material good enough for 1% of c. The fact that the target of 25% of c is still not achieved isn't as large a showstopper as you are making it to be.

usrusr 4y ago

> “and the data they produce will be sent back to Earth“ That's where my optimism limiters were triggered: sending something in the general direction of earth is all nice and well, but receiving that signal, sent from the confines of a "DVD sized" emitter, against the backdrop of a nearby star? They should try to find a way to harness optimism for propulsion...

cee_el123 4y ago
perhaps that can be overcome by a train/succession of such probes relaying the signals over shorter distances ?
- usrusr 4y ago
  But how much shorter? Light years are a long distance to cover with relay stations if their peer to peer range is not unlimited. And when "DVD sized" should include both sender and receiver, p2p range most certainly won't be "astronomical" (which it would quite literally have to be). The relay approach implies symmetry and usually what we do with probes is the opposite, compensate a smaller device on one end with a bigger device on the other end.
  - sliken 4y ago
    Voyager 1 uses 22 watts and 1970s technology to communicate from 21 light hours away. Radio encoding has come a long way since then. In particular ft8/js8call uses an encoding that can communicate at over 20 db below the noise floor, allowing communications across the earth with a few watts which involves many 1000s of miles of atmosphere, bouncing (poorly) off the upper atmosphere and the ground many times. If we launched one an hour at 5% of C (better than today, but much worse than the goals of the project) they would only have to communicate across hops of 3 light minutes, er about half that since 5% of C is the final velocity not the average. Considering voyagers success at 21 light hours and the improvements in encoding that sounds quite feasible.

usefulcat 4y ago

Let's say they make a sail that is large enough and light enough to allow the craft to accelerate to very high speeds (say, 0.1 - 0.2c?). It's quite a long way to Alpha Centauri, and even though interstellar space is very empty, it's not completely empty. As the sail hits the odd spec of dust here and there at those speeds it seems like it will be shredded. Maybe it's ok as long as it lasts long enough for it to be accelerated to a 'fast enough' speed?

Ancalagon 4y ago
The original idea I recall for this was to send hundreds of these probes with the hope that a few might make it (because this launch method and the probes would be relatively cheap once you built out the laser system). But also, interstellar vacuum is very, very empty. Encountering a speck of dust even in that distance with that size of a probe is a pretty negligible risk if I recall correctly.