What planets or satellites are possible for terraforming?

Well, ..., Here's my list ...

1) Mars: and obvious choice it looks to be a viable first step. Maneuvering a large comet into the planet would add to the much needed atmospheric pressure and density of gases and much water vapor.

2) Venus: Believe it or not. Maneuvering a large asteroid into the L1 Position could diminish the Sun affect and help us get the runaway greenhouse effect under control.

3) Europa, Ganymede, and Callisto are great choices. However, it's is we that may adapt to the planet by creating vast floating or submersible cities. With all of the water on each there'd be more than enough energy and atmosphere.

Well if we learned how to move things out of their current orbits, closer to the habitable zone perhaps, then virtually any body large enough to hold an atmosphere could be "terraformed". But assuming this never happens or we don't wait some billions of years until the moons of the gas giants become "balmy" from the red giant sun, then here's my list:

-Venus, pump out most of its atmosphere using mass drivers, and use the "propulsion" to speed up its rotation (or we could push large moon like Io into a 30 day orbit around Venus to speed up its rotation, but then again I said we'd ignore that possibilty for now). A faster rotation period should hopefully equal magnetic field, and then we can add water without it instantly vaporizing. Already nitrogen and CO2 there.

-Earth, its becoming quite changed by humans, so before as we all blast off to space we should clean it up and return it to its former glory)

-Mars, of course, we just need to solve the dynamo problem by building orbiting electromagnets (this seems more feasible on Mars than Venus) and heat it up with tons of greenhouse gases, a thick atmosphere, and the oceans will make themselves.

-Ganymede, probably better to terraform than Europa, receiving only a fraction of the lethal radiation Europa gets from Jupiter, plus Ganymede boasts its own magnetic field. Callisto even less radiation.

-Saturn's moon Titan is also a good candidate, but like the other icy moons of the gas giants, would become an ocean planet if terraformed, with absolutely no land sticking out to colonize.

-Triton (Neptune's moon)is also pretty big, but is also the coldest place in the solar system (-391 F).

Those are good answers. None that we know of can be even sort of terraformed with present technology, and made exactly like Earth will likely remain impractical for ever. The polar craters of Mercury are possibly easiest for tiny colonies due to abundant energy available 24/7/365 at the crater rim, but cool at the bottom of these craters. Mercury has 0.38 g surface gravity, the same as Mars.

Earth's Moon also has some similar polar craters, but huge towers would be needed to collect the Sun's energy even most of the year. Balloon colonies are likely practical for Venus, Saturn, Uranus and Neptune, but the cloud top gravity of Jupiter is too high for humans = more than twice the surface gravity of Earth. Pluto and Sedna, may be almost as habitable as Mars. Mass center of small asteroids may prove practical, as that solves the radiation problem. Neil

I find it odd that folks keep talking about Mars as a prime terraforming candidate. Regardless of all the possible ways to add an atmosphere, everyone seems to ignore the fact that you cannot do any of those things without first having a magnetic field. It is the main reason why Mars dried up in the first place. No force field, no life.

All of them are "possible."

Probable is what you are looking for.

And what do you mean by terraforming?

The whole planet/moon or living under domes?

For the whole planet/moon you need something big enough to hold an atmosphere or far enough from the sun to make it sluggish enough for a small body to hold (like Titan).

We could get rid of about 95% if Venus' atmosphere and start working on her, but unless she has a strong magnetic field the sun's radiation would cook us by Christmas.

See, there are a lot of factors to consider.

It's only a lack of imagination that we have a planetary bias and can only think o colonizing and terraforming planets and Moons. It would make far more sense to colonize space habitats such as O'Neill Cylinders, Bernal Spere's and Stanford Torus's

You have to build habitats anyways to colonize the Moon or Mars, in space you can build much larger ones because weight need not be supported till the structure is complete. Larger habitats are easier to balance than smaller ones. Material is available from asteroids, comets and moons. There is no planetary gravity well to launch against and no dangerous re-entry to endure. Most importantly, full 1-g artificial gravity can be achieved by rotation.

Self sustaining habitats are the basis of generation ships so eventually small fleets of habitats will decide to live between the stars or travel to nearby planets as only asteroids and comets are needed as resources.

You would not be able to spin Venus faster or remove much of it's atmosphere so colonizing Venus can only be in aerostats which unfortunately does not bring you access to the resources of Venus except for the CO2 and sulfuric acid.

Mars, the Moon, Europa, Enceladus, Titan, Ceres, Ganymede,... they all have very low gravity. Adults can reduce the effects of low gravity with exercise and medication but children would likely develop with deformities and health problems. It isn't clear how much gravity is needed for reasonable development but we know from micro gravity experiments that we can't address these issues in microgravity.

Saturn, Uranus and Neptune could be colonized in aerostats but again the only resources that can be claimed after the dangers of re-entry, docking with a fragile blimp and launching against a large gravity well would only be the gases in the atmosphere. On these planets, helium 3 could be harvested but that's only of value if we perfect fusion reactors which after many decades and trillions of dollars, we haven't done.

Planets and Moons are not good colonization sites, it's just not worth the risk of re-entry and the gravitational penalty of take off when you can build larger habitats in space.

It is an idea and a great idea but there is no money for it yet. Yes. It would be wonderful if we can have spaceship in for of terraformed. Hopefully by the year 3000 we might have it but I can assure you not in our life time here on earth. At least not yet!

You mean inside the Solar System? Mars and Venus only.

Europa is not a realistic proposition. It is too small, the Sun is too far away, and the particle radiation is too severe.

Terraforming is PURE Science Fiction. Even if it was possible, VENUS could never be.

We will never cause that to happen anywhere.