I can imagine a satellite with a solar sail it rolls when on the night side of Earth, then unrolls near "afternoon", propelled on each past of its "sunset side" of travel. Would such system be a viable method of keeping the satellite in orbit indefinitely (or until actuators fail) or would some other factors (e.g. atmospheric drag) make it impossible?
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3$\begingroup$ Yes (provided an improbably light sail is used). Shameless self-promotion: physics.stackexchange.com/questions/71582/… $\endgroup$– Deer HunterAug 26, 2013 at 14:41
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$\begingroup$ @DeerHunter: Your idea with solar sail is to counteract gravity of Earth entirely by using enormous solar sail as propulsion in equilibrium with gravitational pull. My idea is to merely counteract losses due to air and magnetic drag, that cause orbital decay; not nearly as much energy required. It's the caveats - won't these same losses escalate rapidly with use of the sail? $\endgroup$– SF.Aug 26, 2013 at 15:03
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$\begingroup$ SF: If you have considerable aerodynamic drag, solar pressure simply isn't going to work. No way for solar sailing in LEO. $\endgroup$– Deer HunterAug 26, 2013 at 15:11
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1$\begingroup$ While not a duplicate, the question is of a similar nature to space.stackexchange.com/questions/370/… whereas here we're asking about solar sails for Orbital station-keeping (I might make a tag for this). This initially seems plausible but there are technical complications due to the elliptic-ness of the orbit. You will probably have to toggle back and fourth to prevent this. Also, drag is proportional to area just like solar radiation pressure is - thus viability is exclusively a function of altitude. $\endgroup$– AlanSEAug 26, 2013 at 15:15
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1$\begingroup$ @DeerHunter: You'd need to at least reorient the sail to "sideways" before entering the day side - otherwise on the morning side at best the sail would slow you down, at worst - entangle around you beyond hope, blown in your face. So, turn sideways at night, fly sideways through morning, turn back to gathering the wind near noon, accelerate during evening. $\endgroup$– SF.Aug 26, 2013 at 19:14
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3 Answers
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First, there are several forces acting on a sailcraft.
- Aerodynamic drag
- Solar radiation pressure
- Gravity field non-sphericity
- Electrodynamic drag
If you go anywhere lower than 770 km, atmospheric drag is by far the largest force that pulls your apoapsis down.
Quoting Vulpetti (2008):
When unfurled in LEO, the drag on the sail produced by its flight through this residual atmosphere can be very high; larger in magnitude than the thrust the sail experiences by reflecting sunlight. Simply put, a sail flown in LEO will very quickly lose energy by interacting with the ionosphere (despite the fact that it is getting accelerated by reflected sunlight), and find itself on a reentry trajectory. It is easy to compute that a sail shall operate beyond 700 km (nominally); if one takes the upper-atmosphere changes into account, the previous lower limit increases to 750 to 770 km.
Second, deploying a sail is a risky and not fully understood maneuver. You don't want to stow and deploy it repeatedly.
Third, when there's no solar pressure, you don't need to stow the sail. Provided you operate beyond the 770 km limit, the only thing you have to do is rotate the sailcraft. Now, ensuring necessary rotation rates (attitude control authority) is a problem if you take into account that the whole sailcraft must be very light to rely on solar radiation pressure.
answered Aug 26, 2013 at 15:42
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$\begingroup$ There is just one piece of information missing that would be the full answer to the title question: Time of decay in an orbit where the solar sail can work at all is of order of 25000 years. So where it can be used, it's just no longer needed. $\endgroup$– SF.Feb 19, 2016 at 8:44
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$\begingroup$ @SF. - the chart is not directly applicable to sailcraft. $\endgroup$ Feb 19, 2016 at 8:50
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$\begingroup$ But it is for a satellite we want to keep in a stable orbit for a long time. The whole purpose for making it a sailcraft was this. $\endgroup$– SF.Feb 19, 2016 at 8:56
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$\begingroup$ @SF I'm pretty sure that's just a decimal point, not actually thousands of years for spacecraft lifetime. $\endgroup$– costromOct 30, 2017 at 23:01
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Between atmospheric drag out through almost 1000km, there's the issue that solar light and solar wind are both cut by the passing into the shadow of a planet.
Since this puts more of the energy on the solar side, even for high orbits, the sail is, if passively deployed, making the orbit more ellipsoid and thrusting it planetward on the sunward lobe.
Let's assume a flat sail perpendicular to wind: as you come sunward, it slows you, lowering your periapsis by both slowing you down and by direct thrust, and increasing your apoapsis slightly, but less, due to the shadow.
Now, we tilt it, for outward tack on the inbound: you still slow, widening and shallowing the inbound arc. On the outbound, it's pulling you towards the planet, narrowing it and accelerating you back slightly. At the chunk closest to the sun, it's accelerating you counter-orbitally; that's slowing your orbit, and lowering your periapsis. At away-from sun, it's getting increased speed except in shadow - so it doesn't equal the amount from them sunward arc. This skews the orbit's long axis, but still lowers periapsis.
Tilting the other way drives the approach downward and anti-spinward, lowering periapsis more radically, and the sunward side and half the retreat is acceleration; the away-from sun is slowing but interrupted, slightly increasing apoapsis. It also skews the long axis of the ellipse of the orbit.
Now, even if it's tacked properly (inbound arm outward, ourbound arm outward, solar-side arm spinward) it's still consistently pushing towards the planet. It might be able, if the orbit is high enough, to push the apoapsis up and maintain enough acceleration to overcome the outward force of solar energies.
As a passive system, it's going to eventually push you into atmosphere, and at that point, it is a liability.
As an active system, it can theoretically be used to increase orbital velocity for the majority of the orbit, and slowly accelerate one outward. This could even — in theory — be used to accelerate to breakaway, provided one starts in an orbit well above 1000km, and can tack the sail fast enough to not be a problem on the sail-turn at the sunward extreme.
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There is also the issue of eccentricity- accelerating only on one side of a planet would, as far as I know, gradually increase your eccentricity, until it eventually would escape the orbit it was in entirely. However, I'm no expert, so I could easily be wrong.
