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When you have the oxygen you want, and the dry ice all flat on the lowlands, cover the dry ice with foamed rock, so that the CO 2 is a completely sequestered feature of the lithosphere.

Now take the chunks of Dione you have been saving and crack them against each other in the oxygen-and-buffer atmosphere at the correct height to create steam and rain. This will add back to the planet some heat, which at this point has been taken below the human-friendly range. Possibly some light can be let through the sunshield if needed to help heating. It will only take two years for the greater part of the impact water to rain and snow onto the surface, so be ready to work fast.

The water on the surface after this Dione infusion will be equal to about 10 percent of Earth’s water. It will be freshwater; salt to taste. The water will cover 80 percent of Venus, which is much flatter than Earth, to an average depth of 120 meters. If deeper seas are preferred, but also a maximum amount of land, consider digging an oceanic trench with some of the Dione impactors. Remember this will complicate the CO 2 sequestration if you choose to do it, so make adjustments accordingly. If it is done carefully, however, Venus could ultimately end up with about twice the land surface that Earth has.

At this point (140 years freezing and preparation, 50 years scraping and poaching, so be patient!) you might think that the planet is ready for biological occupation. But remember, combining the Venusian year of 224 days with its daily rotation period of 243 days, you get a screwball curve (retrograde motion, sun rising in the west) in which the solar day for any particular point on the planet is 116.75 days. Tests have long since determined that that’s too long for most Terran life-forms to survive, tweaked or not. So at this point, two main options have been identified. First is to program the sunshield so that it lets through sunlight to the surface and then blocks it off again, flexing like a circular venetian blind to make a more Terran rhythm of night and day. This would make it easy on the new biosphere, but would require that the sunshield work without fail.

The second option would call for another round of impactor bombardments, this time striking the surface of the planet such that their angular momentum spins the planet up to something like a fifty-hour day, which is considered within the tolerance limit for most Terran life-forms. The problem with this option is the way it would delay occupation of the planet’s surface, by its release of a considerable amount of the sequestered dry ice under the foamed rock layer. Biosphere establishment would have to be put off for another two hundred years, effectively doubling the time of terraformation. But there would be no further reliance on a sunshield. And a properly constituted and maintained Venusian atmosphere could handle full sunlight without greenhousing or other spoilage.

Which option you choose is your preference. Think about what you want in the end, or, if you don’t believe in endings, which process you prefer.

KIRAN AND SHUKRA

A few days later they were approaching Venus. Kiran was pleased to see Swan joining him on the ferry ride down. She wanted to talk to a friend; she would introduce Kiran to him, then be on her way.

There were no space elevators on Venus, because the planet rotated too slowly for such a system to work. So their ferry sprouted wings, and as they tore down through the atmosphere the windows torched yellow-white. They landed on a huge runway next to a domed city, then got into a subway car, and came up from that short ride into the city. There they found what seemed like the entire population in the streets. Kiran followed Swan through the crowds, and in a side street they went up some stairs into a little Mercury House, set over a fish shop. They dropped off their bags, then went back out to join the crowd.

The faces of the city were mostly Asian. People shouted, and in the din no one could hear well, so they shouted even louder. Swan looked at Kiran and grinned at his expression. “It isn’t always like this!” she shouted.

“Too bad!” Kiran shouted back.

Two big ice asteroids were apparently headed for a collision at the upper edge of the new Venusian atmosphere, roughly above the equator. This city, Colette, was three hundred kilometers north of the collision, and would therefore be quickly enveloped in a downpour. The rain would not stop for a couple of years, Swan said, after which they would let a little light through their sunscreen and have more ordinary weather.