- Surface temperature (mean): 134 K ± 11 K
- Escape velocity: 2.440 km/s
- Equatorial surface gravity: 1.235 m/s2 (0.126 g)
-Surface pressure: 7.5 pbar
The terraforming of Callisto would be very difficult with current technology. Nevertheless here's one theory. The terraforming of Callisto would require vast amounts of nitrogen gas to be deposited or created as its buffer gas; bacteria can help add nitrogen by converting ammonia found on Callisto's surface into nitrates, then nitrogen. After this, the entire Earth's supply and reserves of a potent green house gas, say Sulfur Hexafluoride would have to be sent to Callisto. As Callisto has a relative abundance of water-ice, terraforming might not be as difficult as it seems. After the greenhouse gases begin heating Callisto, the water would melt to form oceans (The Oxygen part of the atmosphere could be created by electrolysis of some of this water). Another advantage with the moon is the low radiation levels it receives, making a magnetic field, or the move of its orbit, unnecessary to the terraforming process. Callisto's day is over 16 times longer than Earth's, so its atmosphere would have to be thick enough to distribute heat from the day to the night side.
The other theoretical plan, however, is more practical. Because Callisto is farther away from the sun than the Moon, it would be less problematic for Callisto to hold an atmosphere. The best way to create a stable atmosphere, is to fill it with heavy inert gases (Xenon, Krypton). However, those gasses only exist in trace amounts throughout the Solar System, so the only possible way to create them in great amounts, is via nuclear fusion. That, however, is beyond the current technology.
If we do create a stable atmosphere above the Armstrong's Limit, our next challenge would be warming up the satellite. The strongest known greenhouse gas is Sulfur Hexafluoride. Sulphur is relatively common in the Solar System. It is abundant on Io, wich is also it the Jovian system. Fluorine, however, is quite rare and it makes up most atomic mass of SF6. For that reason, we cannot rely on SF6 alone.
It is theorized, that below the ice, Callisto holds "bubbles" of Oxygen and Hydrogen mixture. If those were reacted with carbon, than they would form CO2 and CH4 respectively. Both are greenhouse gases. It is also theorized that Callisto contains large amounts of Ammonia ice, which is both a source of nitrogen, and a powerful greenhouse gas at the same time. Oxygen, if needed, could be easily created by electrolysis of water.
If we bring the surface temperature above the melting point of water, then our next problem would be the lack of a solid surface; about half of the mass of Callisto is ice, and it is very reasonable to believe that having melted, it would cover the whole surface of Callisto with a global ocean. Constructing artificial continents is very impractical. But it would also be possible to evaporate some of that water to expose land. Though it is much easier to build small floating bases.