The Solar-Electric Sail: Application to Interstellar Migration and Consequences for SETI

Round 1

Reviewer 1 Report

I have read the paper “The Solar-Electric Sail: Application to Interstellar Migration and Consequences for SETI” by G.L. Matloff. The paper discusses the application of sails powered by solar (stellar) wind for the purpose of interstellar travel. The main point of the paper is that this may be an effective propulsion system for civilizations around M dwarf stars and, if stellar encounters are considered, a single civilization could use it effectively to send probes into many other stellar systems over the history of the galaxy. 

Overall, this is a topic that has been explored extensively in the past, for example regarding the so called Von Neumann probes, with ramifications on the Fermi paradox (i.e., if there are long-lived civilizations with interstellar travel capabilities, why have we not been visited?). I think that the main novelties of the paper are (1) the focus on electric sails as opposed to solar sails (although there were already various papers on the subject, some of them listed in the reference list) (2) the comparative strength of the former for M-like stars, and (3) the fact that stellar encounters are taken into account. 

The physics involved in the calculations is rather elementary but, as far as I can tell, is sound. Most of the paper is highly speculative, to say the least, as we know very little about habitability of planets around M dwarfs. Also, the technical feasibility of sails on the scale required by the paper (of order 10^5 km) is all but obvious. However, it can be taken as a conceptual exercise. 

One way to make the article more substantial would have to have some idea (and perhaps an accompanying table or plot) of the scaling involved under various hypothesis. In particular, I think that the assumption of a spaceship mass of 10^7 kg is rather extreme (also, at the beginning the mass is set at 100 kg, which is a bit inconsistent with the rest of the paper). It may be interesting to see some estimates for smaller payloads, for example for microprobes (such as those considered by Breakthrough Starshot) that might be more feasible. What would the sail area required in that case, and how it would impact on other estimates, and so on? This would have an obvious impact for searches of artifacts in the solar system, etc. It would also be relevant in scenarios of directed panspermia as opposed to colonization. 

Finally, I would suggest the author to have a careful reading of the manuscript because it seems a bit unpolished and it contains many small typos that make it confusing to read (e.g., just to name a couple of the most apparent, “5 billion tears” at line 60-61, “500,000 stars” at line 333, and so on). Also, many formulas are badly rendered or contain typographical mistakes.

Author Response

Thank you for the candid review. Please see the file.

Author Response File: Author Response.pdf

Reviewer 2 Report

This paper explores the use of solar electric sails to accelerate large worldships to the stars over timescales of centuries to millennia.That’s quite a big matter, so should be supported well. It looks doubtful to me because the approximations that are assumed could make the entire matter quite risky. Think of it: would you risk your future on a fluctuating solar wind? What if the wind falls? Specific comments:

1) The proposal of Perakis and Hein (N. Perakis, A. M. Hein, Combining Magnetic and Electric Sails for Interstellar Deceleration, Acta Astronaut. 128 (2016) 13–20 (Jul 2016). doi:10.1016/j.actaastro.2016.07.005.) would give some balance to the options, but is not mentioned.

2) The statement “It is therefore inferred that about 2.5 encounters within 0.5 parsecs will occur every 328 million years. On average, 400,000 years will elapse between close stellar encounters.” is unsupported by any scaling: does the encounter rate scale as the distance  between stars to what power, perhaps 2, 3?

3) And what does “the volatile-rich outskirts of our solar system” referred to in the end of the conclusion have to do with it? 

Author Response

Thank you for the candid review. I have replaced the self-citation in Ref. 3 with the Perakis/Hein paper. The typos you found (and one other) have been corrected. Formulas are now easier to read.

Author Response File: Author Response.pdf