Bodies in the inner Solar System have been considered for terraforming and space colonization. The main candidates for colonization in the inner Solar System are Mars and Venus. Other possible candidates for colonization include the Moon and even Mercury.
The short distance between the Earth and the Moon makes it a natural expansion after Earth.
A number of government space agencies have periodically floated lunar plans such as Russia (2014), China (2012)[needs update] and[when?] the US have made plans in constructing the first lunar outpost.
In a May 2018 interview, Blue CEO Jeff Bezos indicated Blue would build and fly the Blue Moon lunar lander on its own, with private funding, but that they would build it a lot faster, and accomplish more, if it were done in a partnership with existing government space agencies. Bezos specifically mentioned the December 2017 NASA direction and the ESA Moon Village concepts.
The hypothetical Colonization of Mars has received interest from public space agencies and private corporations, and has received extensive treatment in science fiction writing, film, and art.
The most recent[when?] commitments to researching permanent settlement include those by public space agencies—NASA, ESA, Roscosmos, ISRO and the CNSA—and private organizations—SpaceX, Lockheed Martin, and Boeing.
The colonization of Venus has been a subject of many works of science fiction since before the dawn of spaceflight, and is still discussed from both a fictional and a scientific standpoint. Proposals for Venus are focused on colonies floating in the upper-middle atmosphere and on terraforming.
In addition to the aerostats we can use on Earth, of all known planets and moons in the Solar system, only the Venusian atmosphere has a Lana Coefficient[clarification needed] that allows for the use vacuum airships made of some composites (that will work up to an altitude of 15 km) and graphene (up to an altitude of 40 km).
With discoveries as of 2020 traces of possibly indigenous life in the atmosphere of Venus, attempts of any humanization of Venus have become an increased issue of planetary protection, since uncontrolled effects of human presence might endanger such life.
Once thought to be a volatile depleted body like our Moon, Mercury is now known to be richer in minerals than any other terrestrial body in the inner solar system. The planet also receives almost seven times the solar flux as the Earth/Moon system and also has a magnetosphere, unlike Mars and Venus.
Mercury is an ideal place[according to whom?] to build and launch solar sail spacecraft, which could theoretically launch as folded up, by a mass driver from Mercury's surface.[clarification needed] This could also make Mercury an ideal place to acquire materials useful in building hardware to send to (and terraform) Venus.
As Mercury has essentially no axial tilt, crater floors near its poles lie in eternal darkness, never seeing the Sun. They function as cold traps, trapping volatiles for geological periods. It is estimated that the poles of Mercury contain 1014–1015 kg of water, likely covered by about 5.65×109 m3 of hydrocarbons. This would make agriculture possible. It has been suggested that plant varieties could be developed to take advantage of the high light intensity and the long day of Mercury. The poles do not experience the significant day-night variations the rest of Mercury do, making them the best place on the planet to begin a colony. Underground temperatures in a ring around Mercury's poles can even reach room temperature on Earth, 22±1°C. For these reasons Alexander Bolonkin and James Shifflett consider Mercury the most suitable planet for colonization, rather than Mars.
Because Mercury is very dense, its surface gravity is 0.38g like Mars, even though it is a smaller planet.