https://en.wikipedia.org/wiki/Exposing_Microorganisms_in_the...

https://en.wikipedia.org/wiki/Gerard_K._O%27Neill#Space_colo...

you could, for example, send a million settlers to Kepler-69420, and with the TFR of 1.5 - an unrealistically high number - the colony would be extinct in just a few centuries. 1m becomes 100k in 200 years and 10k in 400 years.

* Sun shade/sail near L1 tipped up to 35 degrees to remain still: 5 micron polymer film (1.5-3.5 billion tons or 10-25 million SpaceX Starship launches at 150 tons each) or 50 layer graphene (15 thousand tons or 100 launches). Liquid CO2 ocean forms at 31 C or 88 F, or dry ice glaciers at -78 C or -108 F result in nitrogen atmosphere dropped from 92 times pressure to close to Earth's pressure. Shade rotation can simulate a 24 hour day.

* Comets to increase water and spin rate: 50-100 100 km diameter comets from Kuiper Belt at 30 AU, nuclear rocket using 1% of water to gravitationally slingshot comets by planets over 20-100 years to impact at equator, resulting in 50 day retrograde or 64 day prograde rotation (down from 243 days). Decreases temperature and sulphuric acid enough for microbes to start fixing CO2 and acid.

The "hard" parts are getting bots into orbit to blow graphene bubbles to form a honeycomb, and inventing open-ended fusion rockets to avoid containment issues.

5 cm by 50 cm graphene sheet grown in 20 minutes:

https://www.nature.com/articles/srep21152.pdf (warning PDF)

Direct fusion drive:

https://www.sciencedirect.com/science/article/pii/S009457652... (PDF available)

Magnetic mirror concept for open-ended fusion rocket:

https://en.wikipedia.org/wiki/Magnetic_mirror

Magnetic reconnection thruster:

https://www.youtube.com/watch?v=caM94mem5K4

I think the sun shade is probably how we'll slow global climate change until we can plant the 1-10 trillion trees it will take to reverse it (mechanical carbon capture can't be scaled enough practically), but I digress.

Note that the blocker is actually getting to low Earth orbit (LEO) since delta V is straightforward with ion engines. That will arguably be a solved problem once big "dumb" rockets like Starship scale. I'm a big fan of JP Aerospace's airship to orbit concept and other magnetohydrodynamic (MHD) craft, but it's unclear if they will be able to achieve heavy lift. Aerospike engines and exotic rockets are being evolved by AI currently.

Humans cannot survive long-term space travel, yet, the technology does not exist

The radiation alone will kill you and then there is the problem is you will go blind from changes in your body and brain fluids

I still think humanity's far future is in orbitals in space, not on planets and certainly not on planets as hostile as Venus is. I'm not sure how well living at 50km above the surface would work. You still need a lot of buoyance to float large structures.

The atmosphere is also a solvable problem. One idea I've heard is using so-called "fusion candles". That is a fusion-powered device in the atmosphere that sends waste gas into space and waste matter to the ground in an equilibrium that keeps them airborne, all powered by fusion. You could extract carbon and/or oxygen this way from the plentiful atmospheric CO2.

Still, if you ever got the atmosphere down to a non-hellish level at surface, the surface would still be covered with all sorts of exotics and metals, many of them toxic. You'd probably be looking at geologic timescales to rehabilitate it.

But whenever these terraforming questions come up (often with respect to Mars), people really don't appreciate the scale and the energy budget required. The energy budget is many orders of magntidue what our civilization currently uses. If you have access to that much energy, there are far better options.

[1]: https://www.youtube.com/watch?v=BI-old7YI4I