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Simply put, would it make sense to place laser in space to propel a solar sail? Thinking about solar sails, and the inefficiencies of sending a laser through the atmosphere, I wondered if it made sense to position the laser outside the atmosphere.

Power would be an issue, but since ideally we would not be firing this continuously (just while a craft was accelerating), we could collect solar energy and store it for use during the acceleration phase.

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  • $\begingroup$ Yes, this is a thing people have thought about, although in most proposals, the acceleration phase is half the total distance and the other half is deceleration. Unfortunately, the subject is a rather broad one, so you'll need to be more specific about what you're asking. $\endgroup$ – Nathan Tuggy Apr 12 '16 at 21:03
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What I am concerned about is how you are planning to store that energy. Just for reference, 1kWh of energy is only going to push the spacecraft an extra 0.024 Ns. That is assuming perfect conversion to laser, the sail being perfectly reflecting, and all the beamed power hitting the sail head on. Storing the amount of energy required to do significant propulsion is difficult, your best options being chemical batteries and fuel cells.

From a sample energy density for Lithium ion batteries of 1.8 MJ/kg, you are going to need 40 kg of batteries per kg of spacecraft you want to accelerate by 1 m/s. (In practice, at least 100 kg including some energy losses).

Generating the energy on-the-fly may be easier, but there are still issues. If for example solar power is being used, you are still relying on the energy flux from the Sun like the solar sail, but are effectively losing 95% of the energy from converting it first to electricity, then back to light again. The laser station will in any case massively out weight the sail craft.

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  • $\begingroup$ Ideally the laser station would be permanent and useful for multiple missions. $\endgroup$ – mklauber Apr 12 '16 at 21:19
  • $\begingroup$ An advantage of converting sunlight to laser light is that it can be nearly parallel and not diverge for a long distance, while sunlight is dropping at 1/r^2 everywhere. For example, at 4.5AU sunlight is 95% dimmer than at 1AU, but a 2km wide laser beam won't start broadening due to diffraction until roughly 10AU (depending on the shape). Of course someone has to make a many-multi-giga-watt laser and solar panels to feed it if you want to compete with the sun! Sounds like a lot of work to me. $\endgroup$ – uhoh Apr 13 '16 at 1:01
  • $\begingroup$ This may be a stupid question, but would there be some kind of "recoil" that an orbital laser would have to compensate for? $\endgroup$ – Joe L. Apr 13 '16 at 4:12
  • $\begingroup$ @JoeL. Actually, yes. Similar principle as a light sail. $\endgroup$ – SE - stop firing the good guys Apr 13 '16 at 4:33
  • $\begingroup$ @JoeL. not a stupid question at all! For every action there is a reaction (Newton's Third Law). Now let me see, where else does that come in handy in space? $\endgroup$ – uhoh Apr 13 '16 at 17:10

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