Regarding delta Vs to get to near-Earth asteroids:
From the Google Books copy of
Space Resources: Breaking the Bonds of Earth by John S. Lewis & Ruth A. Lewis, Chapter 9, page 224-225 (edit: huh, for some reason most of the relevant section didn't appear when I tried to make a link to it, but I can see it on the book preview I got on a Google search - well, you'll just have to trust me, or look up a copy of the book yourself):
For many of the most accessible known asteroids, the sum of the out-bound delta Vs is between 4.5 and 5.5 kilometers per second. The lowest reasonable outbound delta V for any asteroid in a plausible orbit is about 3.4 km/s. The easiest known asteroid to get to, 1982 DB, requires about 4.4 km/s outbound. We may recall from the discussion in chapter 7 that the outbound delta V from LEO to the lunar orbit is 3.1 km/s, the delta V to match speeds with the Moon is 1.0 km/s, and the delta V to land on the moon (through the inner Lagrange point, the gravitational saddle point between the Earth and the Moon), is another 1.9 km/s. Thus the total out-bound delta V from LEO to the lunar surface is 6.0 km/s. Thus, as far as outbound propulsion requirements are concerned, the nearby asteroids are clearly superior to the Moon.
For many near-Earth asteroids, the total inbound delta V for return from the surface of the asteroid to LEO is under .4 km/s, with the very best candidates close to .1 km/s. For the return from the Moon to LEO, 1.9 km/s is needed to depart through the inner Lagrange point, 1 km/s is needed to kill enough of the Moon's orbital speed to let the payload drop its orbital perigee into the Earth's atmosphere for aerobraking, and about 0.05 km/s should be budgeted for circularizing the orbit and rendezvousing with the Space Station after aerobraking. Thus the return delta V from the Moon to LEO is 3 km/s. The comparable figure for the near-Earth asteroid 1982 DB is 0.1 km/s. Thus the best asteroids are spectacularly superior to the Moon for the return leg of the journey: the propulsion energy required per ton of payload is 900 times larger for return from the Moon than from 1982 DB!
It is important to realize that we are not talking about two or three asteroids with fortuitously good orbits, Even if only 20 percent of the Earth-crossing asteroids have round-trip delta Vs smaller than that for the Moon, then some 60,000 asteroids larger than 100 meters in diameter would be easier to get to than the Moon (see table 9.1).
There is another major difference between lunar and asteroidal propulsion requirements: for the trip to the Moon and back only about 4 km/s of the total delta V could possibly be done with an efficient, low-thrust, high-specific-impulse propulsion system. This means that specific impulses for most of the burns would be near 400 seconds (for a hydrogen-oxygen chemical rocket) rather than near 4000 seconds (for a low-thrust, high-specific-impulse mercury ion engine). This combination of much higher delta Vs and much lower specific impulse has a devastating effect on the payload mass that can be returned from the Moon.
Quickly skimming
this paper and looking at figure 3 in this paper, most near-Earth asteroids have LEO-NEA delta Vs of 6-8 km/s - similar to Earth's surface to LEO delta V, but with the advantage that you can use low-thrust fuel-efficient rockets that would literally never be able to get off the ground on Earth's surface. And asteroid material return missions have smaller delta Vs, because asteroids don't have much of a gravity well to get out of, which is good because the mining equipment will be what leaves LEO and goes to the asteroid and the product the mining company wants to sell will be what leaves the asteroid and goes to LEO.
Looking at
the Atomic Rocket mission table, delta Vs from LEO to
main belt asteroids are in the range of 10 km/s. Early asteroid mining will probably focus on Earth-crossing asteroids, which are much closer and easier to reach. Generally LEO is not really close to Earth's surface in terms of delta V; there's a cliché that once you're in orbit you're "halfway to anywhere," and looking at the Atomic Rocket mission table that's not exactly true but it's not that far from the truth: LEO to Mars delta V is less than 6 km/s, and Hohmann transfers to the outer planets are mostly less than 20 km/s. And a big part of the problem with Earth's surface to Earth's orbit missions is Earth's atmosphere and the need for a high-thrust engine to literally get off the ground makes everything
much harder. Fuel efficiency and thrust tend to trade off against each other in rockets, and if you need to get off the ground on a big terrestrial planet like Earth you're stuck using the rocket equivalent of gas-guzzling SUVs (that's what chemical rockets are). In deep space you can use fuel-efficient rockets that use very gentle pushes sustained for a long time to accelerate the spacecraft.
A major probable application of early asteroid mining is bringing construction material and station-keeping fuel for satellites and space habitats to Earth orbit, because asteroid material in Earth orbit is likely to be cheaper than Earth material in Earth orbit. Now, I'm not going to say whether asteroid mining is "realistic" or not, I don't think anyone can really answer that question and the technical obstacles are certainly formidable (if it was easy somebody would probably have started doing it already), but I think in terms of delta Vs the math looks fairly favorable for an asteroid mining industry servicing satellite construction and maintenance and space habitats and refueling/ship-to-shuttle transfer stations for interplanetary missions in LEO, with an important sideline in returning valuable metals to Earth itself.
This is kind of tangential to a habitable Venus. Looking at the Atomic Rocket mission table, Earth surface to Venus surface is 21.7 km/s for a Hohmann transfer, which is the most efficient but slowest transfer orbit (an Earth to Venus Hohmann transfer takes 1 year and 7 months according to the same table). Well, if anyone cares about getting to Venus that may stimulate interest in asteroid mining, because orbiting refueling stations will be
very useful if you want to have regular Earth-Venus traffic. If you're writing this as an alternate history story, I'd say you have a lot of room to pick the scenario you think is most interesting on this issue.
You're ignoring a number of factors such as greenhouse effect, the planet's Albedo and planetary mass.
In my post I talked about how Venus could have less carbon dioxide in its atmosphere and a higher albedo and hence be cooler. It's just that those assumptions have their own implications that might give you a planet that's somewhat less Earth-like in other ways.
However it also has less mass to heat up 4.868 x 1024 vs. Earth's 5.98 x 1024 or 81.4% of the mass. Assuming the same thermal resistance as Earth that means it gets the effect of 1.89 times the amount of Earth's sunlight.
Earth and Venus have very hot interiors mostly heated by radioactive decay; I don't think you should model them as primarily externally warmed masses.
Wait- so in effect, by burning all of the world's fossil fuel reserves, and returning all of that bound-up, accumulated CO2 to the atmosphere to enter the carbon cycle once more, we're actually improving the long-term sustainability of life on Earth (even if we're causing a mass extinction event in the process)? That's- actually pretty cool. Poignant.
I read a paper on the long-term fate of anthropogenic carbon dioxide. Most of it will go into the oceans within a few hundred years, and its eventual fate will be to get incorporated into calcium carbonate (i.e. limestone) and other carbon-based rocks over the next 100,000 years or so. 100,000 years is an eye-blink in geologic time; basically the carbon cycle will be back to normal in a very short time compared to the age of the Earth, and the biggest long-term effect of humanity on Earth's carbon cycle will be to transfer carbon from coal to limestone.
