Let us terraform the moon.
Mars seems to have stolen the limelight in terms of the first body to terraform (alter to make earthlike surface conditions). The moon seems not to be considered because, I am guessing, it is nearly devoid of volatiles (carbon, oxygen, nitrogen, water) and has no atmosphere at all. I think people assume that, since it is airless, it is incapable of keeping an atmosphere.
However, if the the moon had an atmosphere, it would keep it for millions of years [Calculation. 2 pages. PDF.]. That is short compared to the 4.5 billion year age of the moon, but very, very long compared to a human lifetime. Over time spans of a few millennia, there would be no perceptible change in atmospheric pressure. A lunar atmosphere would be essentially permanent.
The moon has other advantages compared to Mars. It is one light-second away, not 4 to 24 light-minutes. That’s a couple of days by rocket, as opposed to 3 to 12 months. You can have a chat by radio between earth and the moon. It gets the same amount of sunlight as earth, so terrestrial plants could easily adapt and solar power is more efficient.
This article outlines how we can terraform the moon using two basic schemes. First, atomize moon rocks to generate oxygen, and second, bring in water from the outer solar system. The small body 2060 Chiron lends itself as a good example for the latter.
The Atmosphere Furnace
The moon lacks an atmosphere. However, oxygen is very common in the lunar crust. The most common mineral is Anorthite, that is mostly CaAl2Si2O8. If one were to split that mineral one would have a metallic alloy of Ca, Al, and Si, and gaseous oxygen. The energy required to atomize Anorthite is about 4000 kilojoules per mole, which releases 4 moles, or 0.128 kg, of O2.
The mass in O2 atmosphere for a breathable lunar atmosphere is around 5 × 1017 kg [How to calculate this (PDF, 2 pages)], which implies a hefty energy expenditure of 1.5 × 1025 J of energy to get the moon to self-create an atmosphere. Now suppose we cover a 100 km by 100 km patch of the moon with solar panels. That’s 0.3% of the surface; a patch of real estate big enough to be detected by eye from earth. Assuming a conversion efficiency of 50%, we use that energy to start making air. The machines we would invent feed rock into a furnace, then separate the metals from the oxygen, letting the oxygen fly free. The power from the solar panels is 6 × 1014 watts. Power supplied at this rate creates a full atmosphere in 2.5 × 1010 seconds, or 750 years. All of this is trivially scalable. For example, if we covered the whole moon in solar panels, the job would go 300 times faster. In that case, the job would be finished in two and a half years.
We would also produce bricks of silicon, aluminum, sodium, and calcium as a byproduct. Much of this might be landfilled, so to speak, but these metals are excellent structural materials (on the moon) and could be used to build roads and buildings and other infrastructure.
I envision this as a mobile army of truck sized robots, slowly crawling artfully across the regolith, powdering rock and feeding into the furnace-separator unit. The furnace itself is a parabolic mirror that bounces sunlight to a focus point. The tractor treads and rock grinder and element separator require electricity and some photovoltaic panels, but the furnace itself is straight conversion of sunlight to heat, a nearly 100% efficient conversion.
Chiron the Water Tanker
Water is the second volatile that the moon is largely missing, except of course for small polar ice deposits discovered by the Clementine mission. Water is composed of oxygen and hydrogen, and hydrogen is the flightiest, most volatile element of all. However, in the outer solar system there is abundant water. Every small body out there is very icy, and many are more ice than rock.
I propose we bring some of that ice to us. Enter Chiron, one of countless icy remnants left over from solar system formation in the outer solar system. Chiron wobbles around between Saturn and Uranus, and shows cometary outbursts, meaning its water ices turn to steam when the sun heats it, though the sunlight is only 1% as intense as we feel here on earth. As a thought experiment, let us move Chiron to the inner solar system, then crash it on the moon. And so: water. How much water? Chiron is around 110 km in radius, so there is about 5 × 1018 kg of H2O, and a similar amount in carbon- and nitrogen-rich rocky material. Those elements are needed for fertilizer.
The technology to move moonlets is partially in hand. Ion propulsion engines, for example, are clearly the way to move small icy bodies. Their high nozzle velocities mean they don’t consume very much fuel. Their gentle thrusts are perfectly suited for patient orbit changes. The fuel they need can be mined on site. [Calculations (PDF, 2 pages)] Moving Chiron itself, these calculations show, is energetically challenging because it is such a large body. We will have to move smaller bodies, nearer bodies such at the Trojan asteroids in Jupiter’s orbit, or slice chunks off Chiron and float those home, instead, using gravitational de-assists from Jupiter and Saturn to ease the energy requirements. Or, we could pony up the energy, which seems to require a mature fusion technology for energy expenditures much larger than anything in the human experience to date.
Gentle thrusting with ion engines strapped to a moonlet would drive it at a leisurely pace toward the inner solar system. As it nears the earth-moon system, steam will erupt from all exposed surfaces, making a spectacular comet. To bring it all the way home, we will build an array of ordinary chemical rocket motors for short-lived, high-thrust burns. These will fire off for the final orbit insertions, when the moonlet moves from sun-orbit to moon-orbit. They will fire again for the final deceleration phases, to crash-land the icy body into the moon. Gentle landings are best for reasons of safety. It is probably pointless to waste too much energy on deceleration, so the moonlets will impact at speeds around 2 km/s. Impacts will make new craters and be generally messy. But the clouds of steam leftover will soon condense and precipitate. The moon will have its first rain shower.
Project Chiron is feasible with today’s technology plus a few generous oodles of engineering. Technology tests could begin soon, if we, we meaning the global community, had the collective will to try. Given sufficient interest, in 200 years, people could be frolicking in their skivvies on the shores of lunar lakes.