On 20 May, 2010, Japan launched IKAROS probe with primary propulsion by a solar sail. If the radiation pressure, that helps the sail to move, comes from the sun, then shouldn't it be pushed away from the sun? But the IKAROS probe actually passed by venus, which is nearer to sun as compared to earth by any means. Can someone please expalain that?

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    $\begingroup$ Keep in mind that light sails are generally reflective. So they experience a momentum transfer that depend on both the incident and out-going light directions. Now, you can't tack "upwind" but you can get a significant non-radial impulse. Then you combine that with the usual "East takes you Out, Out takes you West, West takes you In, In takes you East. Port and Starboard bring you back. business of orbits and you can do some surprising things. $\endgroup$ – dmckee Dec 10 '18 at 19:50
  • $\begingroup$ A bit of clarification to @dmckee's point: when the sail is tilted relative to the outward light radiation from the sun, the net push can have a component that goes in the same direction or opposite to the orbital velocity. If it's in the same direction, the sail will rise to a higher orbit; if it's against the orbital velocity, the sail will drop to a lower orbit. $\endgroup$ – S. McGrew Dec 10 '18 at 20:01
  • $\begingroup$ So, the sail is not used to push the probe closer to the Sun - it is used to slow it down in its orbit which causes it to fall toward the sun! Cool! $\endgroup$ – DaveC426913 Dec 11 '18 at 1:32
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    $\begingroup$ @dmckee why dont you write an answer , the question migh get many hits in searches. $\endgroup$ – anna v Dec 11 '18 at 16:21
  • $\begingroup$ Frankly I was hoping someone a little more expert would chime in, but I've written something. $\endgroup$ – dmckee Dec 12 '18 at 3:17

Light sails are generally reflective, so the impulse provided by the light depends on the difference of the momentum of the incoming light (directed radially outward), and that of the outgoing light (which can point in any direction, depending on how you tilt the sail, though your effective area drops significantly if you tilt too much).

The result is that the sail can be used, not only to push you outward but also to increase or decrease your orbital angular momentum, which will change the shape of your obit. (For that matter you can change the plane of your orbit as well.)

Reflect the light "forward" (i.e. in the direction of your orbital motion) and your orbital motion will slow and your path will bend more toward the primary. Reflect it "backward" and your orbit speed will increase and your path will bend away from the primary.

Getting into a lower orbit (as in going to the neighborhood of Venus from the neighborhood or Earth) means first slowing yourself so that you can fall in toward the primary, then speeding up again to match velocity with the target. Always keeping in mind that light sails provide little thrust so these evolutions are not at all fast.

That said, you can't get the sail to push you radially inward: you must rely on gravity to move you in.


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