# Once lightyears out of the solar system, could you expect to get feasible propulsion with a solar sail?

It's been said before that a solar sail could be used for solar and extrasolar missions - if you aren't in a hurry.

However, as you can't spell solar sail without the solar part, I'm wondering if you could expect to get reasonable propulsion through the universe once you are lightyears away from the sun.

It seems that eventually the propulsion coming from the sun would die down, and all that would be left is noise coming from other sources, all trying to push you one way or another.

Wouldn't a solar sail's value diminish greatly once you are in the middle of nowhere?

• Beyond the orbit of Jupiter, solar propulsion (depending on lightness number) is pointless. Most designs of interstellar missions involving solar sails shed the sail around that distance from the Sun. – Deer Hunter Aug 3 '13 at 4:22
• If you want to slow down, you might want to store it rather then shedding it. – James Jenkins Aug 3 '13 at 11:41
• @JamesJenkins - the ability to stow the sail comes at a cost of further mass increase. Only the largest probes will be able to afford that. – Deer Hunter Aug 3 '13 at 16:51
• The noise component may be reduced by stowing the sail after pre-defined velocity is achieved. Of-course the sail would have to be 'furled' carefully, else the craft vector may change – Everyone Nov 2 '13 at 6:09

You could expect to get a reasonable propulsion only if you plan ahead. Specifically, if you were to shine a very high powered laser on said solar sail from somewhere, you might be able to get propulsion even further out.

Without a laser, you really can't do much once you get far away. As this table shows, here's a few solar constants at the orbits of particular planets (W/m2):

Planet       Mean      Perihelion       Aphelion
―――――――――――――――――――――――――――――――――――――――――――――――――
Mercury      9116.4       14447.5         6271.1
Venus        2611.0        2646.4         2575.7
Earth        1366.1        1412.5         1321.7
Mars          588.6         715.9          491.7
Jupiter        50.5          55.7           45.9
Saturn        15.04         16.76          13.53
Uranus         3.72          4.11           3.37
Neptune       1.510         1.515          1.507
Pluto         0.878         1.571          0.560


At Neptune, you are at less than a tenth of a percent of what you'd get at Earth. At that point, it just doesn't make any sense to keep trying, unless you can increase it artificially somehow. One could even argue that it's not worth it at a point far before reaching Neptune as well.

• Surely you will only get propulsion up to and around Neptune. Surely at that point the acceleration will be negligible but surely it would have maintained its velocity from all the points getting to there. Making it more of an aim and fire kind of mechanism? – RhysW Aug 5 '13 at 9:03
• @RhysW What you say is exactly true, but it wasn't what was asked. – PearsonArtPhoto Jun 19 '17 at 1:38

Relying on solar radiation alone is ineffectual for interstellar missions. As PearsonArtPhoto commented, the right way is relying on lasers.

If you see the Wikipedia page for laser propulsion you'll read the main references to the literature. Robert Forward even came up with a scheme using multi-staged sails and a huge Fresnel lens that enabled the mission to brake up at destination

But the problem with such missions is not only the huge sails required because of diffraction-limited beam divergence, but the huge optical accuracies required to point a laser correctly so that it is kept aimed where the sail is.

I've recently published an alternative proposal for using multiple self-stabilised optical beam relays for beam refocusing. It doesn't solve all the problems, but at least enables one to consider additional possibilities in the design space of beamed propulsion concepts, basically trading-off interstellar-scale beam divergences and optical accuracies with only interplanetary-scale beam divergences and optical accuracies. The downside is that the relay mirrors need to have outstanding optical performance (400 ppm scattering losses or less) and they need to be maintained and repaired in-situ in interstellar space. We are not there yet

But if that approach could be workable, then you could have beamed power all the way between the two stars. Since the structure will not be stable unless there is also a laser at destination, you will be able to brake as quickly as you were able to accelerate