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In Entering Space, Robert Zubrin mentioned disassembling Venus and forming a ringworld out of it a 1 AU radius.
Niven's Ringworld is thus clearly a much more practical concept than a Dyson sphere, but its obviously a rather big project. To see how big, let's calculate just the energy requirements to disassemble Venus and then move its mass from its present orbit (at 0.72 AU) to the 1-AU distance required for the Ringworld, to melt it into a solid ring, and then to accelerate the ring so that it spins about the Sun with sufficient velocity to create Earth-normal gravity at its inner surface.
Well, Venus has a mass (M) of 4.87 x 10^24 kg and a radius (R) of 6,051,000 m, so its total gravitational energy (given by GM^2/R) of 2.6 x 10^32 joules (where G is the universal gravitational constant). To move the mass from Venus to Earth's orbit requires a delta-V of 5,355 m/s, which implies an energy (given by (MV^2)/2) of another 0.7 x 10^32 joules. If we assume a megajoule to melt every kilogram, that's another 4.87 x 10^30 joules. But then to spin it up to create earth-normal gravity, we need to accelerate the ring to a speed of 1,212,435 m/s. This will need an energy of 3.6 x 10^36 joules, a requirement that makes the rest insignificant. To put the mater in perspective, 3.6 x 10^35 joules is 11 million billion times the amount of power humanity currently uses in a year; it is equal to the total amount of power output from the Sun over a period of three centuries! So short of the development of picotechnology, not even a Ringworld (Let alone a Dyson sphere) will ever be constructed.
Lets play around with this idea for a bit.
What if instead of moving Venus' mass to Earth orbit, we left it in Venusian orbit and constructed a ringworld there? With a shorter radius, the rotational velocity will be less, we would also spread Venus' mass less thinly
What if we used Mercury instead and build a ringworld at Mercury's average radius.
granted those locations get too much Sun, but ringworlds typically have shadow squares anyway to produce night, we can control the amount of light the ringworld receives if we assume he ability to build one.
What if we built a ringworld with a radius of 3,678,100 km using the mass of Mercury? This is the 1-G distance. The radius of the Sun is by comparison 695,842 km, this is 5.285826 solar radii. To spin at 2-G would require a tangential velocity of 268.588 km/sec with an angular velocity of 0.004183947262292175 degrees per second. or 86043 seconds per rotation, or 23.9 hours rotation! The outward centripedal force is 1-g after subtracting the Sun's gravity. We can split the ring into two parts, one that is spinning at this speed and an equal mass ring on the outside that is not spinning. The outer ring experiences the 1-g pull of the Sun, the inner ring experiences the 1-g of outward net centrifugal force. and of course we'll need a shield to block most of the Sun's light. I think Tungsten might do the trick, we need something solid to block the sunlight from the inner surface. Maybe reflect it at an angle into space.
Last edited by Tom Kalbfus (2016-12-17 18:34:21)
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