The amount of water available in comets is laughably inadequate to fill Venus's shortfall. But anyway its day length would make it very close to useless to try to terraform.
Shipping in enough hydrogen from (say) Neptune might be possible in principle using billions of automated, self-reproducing nuclear powered spacecraft.
If its atmosphere could be precipitated and the carbon freed of oxygen and somehow permanently protected from runaway combustion, the planet's low (2.64°) axial tilt and solar proximity might make a polar existence possible. But the overwhelming excess of oxygen would need to be removed or bound up in water. (The carbon might then be safely kept under water. Or, be crystallized out as diamond, which is hard to ignite.)
Quadrillions of aluminum foil balloons full of nitrogen bobbing in the stratosphere might suffice to bring temperature down.
Perhaps surprisingly, the present 3.5% of its atmosphere that is nitrogen is more than Earth's total.
As noted elsewhere, Venus's magnetic field is not much like Earth's, although it stretches almost to Earth's orbit, and might have crossed it in the past. (Such events might account for Venus's baleful reputation in to the oldest myths.)
Shipping in enough hydrogen from (say) Neptune might be possible in principle using billions of automated, self-reproducing nuclear powered spacecraft.
If its atmosphere could be precipitated and the carbon freed of oxygen and somehow permanently protected from runaway combustion, the planet's low (2.64°) axial tilt and solar proximity might make a polar existence possible. But the overwhelming excess of oxygen would need to be removed or bound up in water. (The carbon might then be safely kept under water. Or, be crystallized out as diamond, which is hard to ignite.)
Quadrillions of aluminum foil balloons full of nitrogen bobbing in the stratosphere might suffice to bring temperature down.
Perhaps surprisingly, the present 3.5% of its atmosphere that is nitrogen is more than Earth's total.
As noted elsewhere, Venus's magnetic field is not much like Earth's, although it stretches almost to Earth's orbit, and might have crossed it in the past. (Such events might account for Venus's baleful reputation in to the oldest myths.)