"Size" of satellite

[rick m]

Except for the mass, is there any size limitation for the satellite?

[pauldear]

Hi Rick,

No size limit at all - were you thinking radar reflector?

P

[rick m]

Fine trailing wire for static power generation.

[pauldear]

Wow - that would be cool!

[DaveHein]

It seems like you would need a long wire to generate any useful power. I don't know much about this, but I understand the voltage potential is generated by a vertical wire traveling through the earth's magnetic field. The voltage is on the order of 100V/km, which is 0.1V/m. You would need a 10 meter wire to get 1 volt. That may be hard to fit in the 20-gram mass budget.

Also, the electrical power is generated by reducing the satellite's kinetic energy. This would cause the satellite to de-orbit a bit earlier.

[pauldear]

The voltage must depend on the velocity, no?

As for mass, 0.1mm copper - total mass for a 10m length would be 0.6g. About a third of that for aluminium.

I even found 0.02mm copper wire readily available - that would put your mass at 0.024g for a 10m length. The current would have to be low, but it would still be useable.

[DaveHein]

The velocity of the wire would be the oribital velocity for LEO. If I did the calculation correctly, a 10m length of 0.1mm copper would have a resistance of about 20 ohms. At one volt you could get about 25 mW of power from this. This could be stored in a capacitor and used in short bursts to power a radio transmitter. It might work.

[rick m]

[DaveHein]

With a 25 mW power source you could do many short bursts per orbit. If your transmitter has 25% efficiency you could transmit a 100 mW signal with a duty cycle of 6%, which would be the equivalent of a one-half second beep every 8 seconds.

It might be easier to use a solar cell. A one square centimeter solar cell would generate around 20 mW if pointed directly at the Sun. Of course, it wouldn't generate any power during the half of the orbit that's in the dark, so you would need a battery for that part of the orbit.

[pauldear]

True. But don't forget, you only need to send enough data to confirm orbit. Even if it beeps once per orbit, I *think* (someone help me out here) that that would be enough to confirm the orbit. (Unless we hypothesize that, as soon as the satellite goes round the dark-side, it lands, gets picked up by a very very fast truck, is shipped around the other side and relaunched....)