I'm continuing questions rooted in the same idea: a manned mission to a planet around 61 Virginis. The first set of related questions was asked here.

We're sending a crew of twelve. They'll be in mild stasis for most of the mission, though will have to wake up and move around in a gravity environment periodically for health reasons. As such, the ship will need a livable environment with some sort of rotational gravity in at least one part, though it won't need to be large (one small room where two to four people can hang out).

So, the question is, how much does the ship need to mass in order to get there safely? I'm looking for large scope numbers sufficient to use with the following graph:

enter image description here

Here are a few things that come to mind:

  • Primary propulsion will be via solar sail, but destination maneuvering, orbital insertion, etc. will be required. Presume deceleration on the other end is provided by the same mechanism as acceleration on this end.

  • Power generation by small nuclear plant is possible (international nuclear treaties are ignored in order to get the development done).

  • People only need to be awake for one month per year, so food stuffs for a crew of twelve can be the same as for one crew member who's awake the entire time.

  • Radiation shielding? Is it reasonable to say that the power plant is sufficient to generate a magnetic shield that will protect sufficiently?

I realize this is a broad question, with a lot of variables. But I'm only looking for "orders of magnitude" accuracy, so "500,000 kg - ish" would be a sufficient answer.

  • 3
    $\begingroup$ I thought the question read 61 Virgins missing... $\endgroup$
    – bowlturner
    Mar 30, 2016 at 21:07
  • $\begingroup$ That's another question entirely. $\endgroup$
    – J.D. Ray
    Mar 30, 2016 at 21:25
  • 1
    $\begingroup$ When you say propulsion will be by solar sail, are you assuming anything about the size of the laser array? The article linked in the other question referred to an array 1-10 km on a side to propel a probe with only a few grams of mass on a relativistic voyage, suggesting that if you wanted to propel something with a mass large enough to serve as habitat and do some maneuvering you'd need a really huge laser array--the space shuttle had a mass of about 100 million grams, would you allow an array 100 million km on a side or larger? $\endgroup$
    – Hypnosifl
    Mar 30, 2016 at 23:11
  • $\begingroup$ I don't know if it works that way (though I keep saying that I'm not a physicist, a phrase almost guaranteed to get a physicist to chime in). I'm writing fiction, so adjustments to the wavelength of the laser can alter the size of the array with a reasonable amount of mumbo jumbo. $\endgroup$
    – J.D. Ray
    Mar 30, 2016 at 23:17
  • $\begingroup$ Hmm, this article mentions that Lubin has talked about a series of larger and larger laser arrays including "DE-STAR 6" which could propel a 10^4 kg (10 ton) probe to near light speed, and this article indicates successive DESTAR numbers indicate size increases by factors of 10, so DESTAR 6 would be an array 1000 km on a side. $\endgroup$
    – Hypnosifl
    Mar 30, 2016 at 23:30

2 Answers 2


Robert L Forward wrote a paper which laid out most of the parameters for a laser driven interstellar spacecraft, so you can refer to this for your first cut of the design: http://www.lunarsail.com/LightSail/rit-1.pdf

To give you an idea of the scale and scope of a "realistic" design, Forward considered a manned spacecraft capable of a round trip to e Eridani and travelling at .5*c*. The ship at launch weighs 78,500 metric tons, and the sail is powered by 43,000TW of laser energy, increasing to over 70,000TW by the end of the acceleration phase. At launch, the sail is 1000 km in diameter (Forward's lightsails are separable during flight to reflect laser energy back to the smaller, remaining portion of the sail for deceleration and to accelerate back to Earth at the end of the mission). An immense lens 15 AU from the sun helps focus the beam onto the lightsail as it heads towards its destination.

Forward lightsail

Now Forward does not specifically outline how the ship itself is laid out, the size of the crew etc., but at .5*c*, radiation and interstellar dust is going to be a big concern. The dust might be handled by using the launch laser to "clear" a path prior to launching (the vast energies could ionize molecules and heat dust enough to drive them out of the ship's path), while the crew module itself would be buried under layers of shielding and ship's supplies to stop incoming radiation. Since the amount of energy being beamed at the ship is simply staggering, there is almost no need for a nuclear reactor (except as a backup), solar cells on the outer surface of the ship can absorb a fraction of the 70,000TW of energy illuminating the sail.

So these parameters should give you an idea of where to get started. Your ship could be considerably smaller given the size of the crew, but the size and mass of the sail isn't going to change that much unless you are either going for a smaller sail and slower cruise speed, or a much larger sail to handle even more power and get to your destination much faster.

  • $\begingroup$ Also see the discussion here of more recent analyses of Forward's idea, suggesting some possible improvements. $\endgroup$
    – Hypnosifl
    Mar 31, 2016 at 4:54
  • $\begingroup$ You still need a reactor of some sort for the vast majority of the mission. After a relatively small time the ship gets far enough away, and the beam spreads out enough, that solar cells on the ship become useless. $\endgroup$ Apr 2, 2016 at 3:54
  • $\begingroup$ You have a laser of 73,000 Terrawatts illuminating the sail for the majority of the mission, and the beam is being focused by a 1000km diameter lens to stay tight on the sail, so there should be plenty of energy for the solar cells.... $\endgroup$
    – Thucydides
    Apr 2, 2016 at 14:14

Forget the graphs. First, you need to specify exactly what the mission is: orbital exploration, surface exploration, colonization? Regardless of mission, the ship needs in-system maneuvering capability, and that means reaction drives. It also needs a power source such a nuclear reactor. You can specify a fusion reactor if you prefer, but you still need to come up with some sort of number for the reactor and the power generation gear. If the goal is planetary surface exploration you also need to carry a lander, and if you want manned surface exploration the lander also needs launch capability. The requirement for centrifugal pseudogravity implies a large structure, on the order of 100 feet across or more. Since the crew of 12 will be active at the target, you'll need living quarters, work areas and food production for all of them. Carrying food is not an option - it weighs too much. A reasonable mass for food is about 1 kg per day. A 10-year flight with one person active will require about 4 tons of food. With the crew active at the target you're talking another 5 tons per year.

Let's take a really liberal number like 10 tons per person, and round it to a total of 100 tons. As a comparison, the ISS, with a crew of six, masses 420 tons, so two ISS's would be 840 tons. Close enough, but please note that this is finessing the question of sail construction. Sail weight is included in payload weight, and square kilometers of almost anything will not be negligible in terms of mass. This mass of 100 tons is about $10^5$ kg, and is an order of magnitude larger than the largest mass on your charts. Also note that, per your charts, the final velocity is going to be very low, and the trip time very long. As I pointed out the previous question, $10^6$ meter/sec implies a trip time of more than 8000 years.

And finally, I have to ask - is this a suicide mission? How, exactly, are the crew going to get home? Set up industry on the target planet and manufacture a laser launch facility? From scratch? With 12 people?

  • $\begingroup$ If they have self-replicating machines like the ones postulated here, building a gigantic laser in the target system might not be so difficult. $\endgroup$
    – Hypnosifl
    Apr 2, 2016 at 18:28
  • $\begingroup$ The target planet is known to be inhabited and known to be able to support human life (we found aliens from there living here and they want to go home); we're going with them, but need to supply the ship. All we need to do is get there, though a lander will be convenient. $\endgroup$
    – J.D. Ray
    Apr 4, 2016 at 21:01

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