# Is it possible to have smaller solar sails and increase the amount of energy they receive with a constellation of mirrors?

Since a solar sail needs to have a huge area to capture enough energy from the Sun, but also be thin to make it easier to move, its design and materials are a big engineering challenge.

Is it possible to reduce its size, but instead increase the amount of energy it receives?

Increasing the amount of energy could be done with a constellation of mirrors+lenses in orbit to deflect the light from the Sun to the sail. These network of mirrors could be used for different sails (reusable), and making the sails easier to build, as some of the complexity is translated to the mirrors. Even the mirrors could have long lifetime, given that they should be easier to maintain than a sail that is travelling. Even lasers could be used to propel the sail. Specially as the energy received from the Sun is decreased as the sail travels.

Sorry for the lack of details around this idea, I'm wondering about the constraints to implement something like this, like restrictions in the amount of energy that can be transmitted from the mirrors to the sail, or collected by the mirrors, or economic costs of such a satellite constellation.

• The idea of using big lasers that stay at home to propel a lightsail is key to the plot of the Niven/Pournelle novel The Mote in God's Eye. Jun 11 at 12:18
• @OrganicMarble and more recently, Stross' Accelerando. Jun 11 at 14:39

### Yes!

As long as the question is about possibility rather than practicality, something conceptually a bit like this; the mirrors would receive only a small fraction of the light's forward momentum, and as the spacecraft moved (roughly) along the optical axis they would have to refocus each time the beam got larger than the spacecraft's sail.

Since the Sun is a half-degree wide, your solar sail would always have to be larger than the size of the image of the Sun that the optical system produces. As a random example, if it's 100 km wide and f/10 at 1 AU, your sail spacecraft's sail will still have to be ~ 10 km in diameter and able to withstand 100x concentrated sunlight.

But it will continue to substantially enhance illumination for many AU as long as it can continue to refocus its own optical surfaces.

Assuming they are both in heliocentric orbits (of different shapes and types) the mirrors would also have to tilt laterally to follow the mis-matched arcs.

Put a series of these in increasingly large heliocentric orbits phased properly and your spacecraft would get a boost from each one like a proton in a linear accelerator!

From Are X-ray telescopes with glancing angle surfaces basically "funny-looking" Cassegrain telescopes mathematically? The answer there shows several alternative designs for a high f/no imager.

Source

• nice! now I'm curious about practicality, but I guess is not all fun and games right? Jun 11 at 14:47
• yeah makes sense, how about posting a new question? it could extend this topic to practicality Jun 12 at 13:40
• @germanio I think if you have an interest in this you should ask it yourself. I think it will be well received as long as you explain how it's different from this question. Include the history and solar-sail and reference-request tags.
– uhoh
Jun 12 at 14:01
• thanks! I have it here: space.stackexchange.com/q/53684/16583 Jun 12 at 14:44
• @germanio I hope you don't mind that I've made an edit there, please feel free to edit further or roll it back, and thanks for posting it!
– uhoh
Jun 12 at 14:54