Physics of a satellite orbiting the sun

[Polama]

One of the proposals for harnessing solar energy is to put solar panels in space. Either orbiting the Earth, or stationed on the moon, light waves could be converted into microwaves that pass through the atmosphere without losing any energy, then on to electricity on the ground.

What I'm wondering is, why the satellites would orbit the Earth and not the sun itself. If I'm not mistaken, the solar conversion is more efficient in more intense light, as would be present closer to the sun. Which makes sense, as a panel closer to the sun would catch a larger fraction of released energy. Plus a satellite of the sun would never get blocked by other objects.

A quick search reveals no discussion of solar satellites, so I'm wondering if I'm missing something about the physics. Would the solar wind blow a small satellite away? Is the heat at a useful distance from the sun more intense than I imagine? Would a microwave retransmission to Earth lose more energy than catching the sunlight closer? Basically, does anyone have any physical reasons a solar satellite would be impractical?

 

[Dale]

Aiming an intense beam of energy is challenging from Earth orbit, let alone from 1 AU away.

 

[SteamKing]

Don't forget, at daytime on Mercury, the temperature is about 400C max and this is at an orbital distance of about 0.4 AU from the sun. It's very difficult to design satellites which must function reliably for long periods at such high temps.

 

[Filip Larsen]

There is a theoretical construct where you place a large "focusing lens" or light-to-microwave converter on the line from Earth to the Sun in such a way that a fraction of the photon pressure is used to keep the orbit synchronized with Earth so that its orbital time is one year.

I am not aware of of the pros and cons any detailed analysis of such a station may reveal relative to the pros and cons of more "traditional" near-Earth places for a solar farm, so I'm not able to conclude anything. However, on the face of it, the further away it is placed and the more technically complex it is, the more likely it is that such a solar farm will be infeasible compared to alternative ways to harvest solar energy. We are still only on the brink of getting solar farms to work well on Earth, so its a good guess it will be quite a while before any space-based farm will be feasible.

 

[Vanadium 50]

Also, it is very, very difficult (which means costly energetically) to put a body in solar orbit inside the Earth's orbit. Think about it this way: if it cost you 100x as much to double your efficiency, wouldn't you just build 2 of whatever it was?

 

[Filip Larsen]

Also, it is very, very difficult (which means costly energetically) to put a body in solar orbit inside the Earth's orbit. Think about it this way: if it cost you 100x as much to double your efficiency, wouldn't you just build 2 of whatever it was?

I think the idea with the construct I mentioned, was to put it just on Earth-escape trajectory (around 3-4 km/s extra from low Earth orbit) and from this initial solar insertion orbit let the array act as a solar sail to spiral into the wanted orbit. I can't remember if the idea also proposed to use the array to spiral out of Earth orbit or to use impulsive maneuver for that, but if we assume that it is possible to use the array for both maneuvers the otherwise high change in orbital energy to go into low solar orbit is of a less concern. Still, even if orbital energy is not a problem in itself, the concept probably still requires a lot complex technology that would make other solutions more feasible in the foreseeable future.

 

[russ_watters]

How can a solar sail move toward the sun?

 

[Filip Larsen]

How can a solar sail move toward the sun?

In short, by decelerating instead of accelerating. Using a simple geometric model, you can imagine the sail pointing at an angle 45 from the radial thus reflecting the light 90 degree. If the reflected light points in the direction of the in-orbit velocity the sail will continuously lower its orbit (the trajectory will be a slow spiral inwards).

In a more detailed model, it may very well be that the optimal angle is not 45 degree, and, depending on the mission, it may also be that other types of hybrid or non-spiral orbits are more optimal.

 

[DickL]

I hope I'm not pulling this off in another direction (hi-jacking), but ...

I have always been under the impression that solar sails only worked to pressure/move in the same direction as the photon flow. That is, they could not tack in the sense of a sail boat. My thinking has been along 2 lines. For a sail boat to tack, it needs resistance in the direction the wind is blowing to allow it to move at an angle to the wind, hence the deep keels. And photons, being pretty small, would strike the atoms in the sail and transfer their force/momentum along the same line as their travel (e.g. a radial from the sun).

Am I missing something.

 

[Filip Larsen]

I have always been under the impression that solar sails only worked to pressure/move in the same direction as the photon flow.

That is not so, primarily because it moves in a gravity field governed mainly by laws of orbital motion.

But let's look at the underlying "propulsion mechanism" of a solar sail, photon reflection. As you may know, a photon has momentum, so imagine a single photon moving to the right towards a mirror hanging at rest in empty space. Now, let the mirror reflect the photon directly upwards so that photon has been deflected 90 degree, but still with same momentum magnitude. If you apply conservation of momentum for the photon and mirror before and after reflection, you can see that the mirror afterwards must be moving down and to the right by an equal speed (i.e. 45 degree down-angle).

That is, they could not tack in the sense of a sail boat.

I'm not a sailor, but I guess you could say that a solar sail cannot tack in the sense that the mirror in the above example cannot reflect the photon such that it can move to the left afterwards.

However, for something in elliptical orbit it is not the radial acceleration that matters when you want to change orbit, but the tangential acceleration, i.e. changing speed in the direction of the velocity vector or opposite it. If you placed a solar sail around the Sun and let it reflect light directly back towards the Sun, the sail would not spiral away from the Sun, but instead orbit the Sun as if the Sun's gravity was reduced a bit (as reflected momentum per time decreases inversely with the square of the distance, just like gravity). To get closer to the Sun the sail has to reduce its tangential speed (just like an impulsive maneuver would, for the sail the maneuver is just "smeared" along the orbit) with the result that the closest point in the orbit (perihelion) is continuously lowered.

 

[BrockLee]

as the state of solar panels is now you cannot heat them above silicons(probably recycled from cheap consumer products) and waters(yes it is just like a mini steam power plant) bonding temperature.