- Elon Musk, founder of SpaceX,
interview in Ad Astra, 2006
The quality of a civilization is
measured not by what it has to do, but by what it wants to do.
- Bruce Murray, research scientist,
Exploring Space, 1991
Terraforming is the process of modifying
a planet, moon or any other suitable object in order to make it habitable for
humans. The word terraforming literally means “Earth-shaping”. Mars is often
regarded as the first candidate for terraforming because it is the most
Earth-like planet in the Solar System. Early in its history, Mars is believed
to be a lot more like Earth with a significantly thicker atmosphere and
abundant liquid water on its surface. Today, transforming Mars into an
Earth-like world through terraforming will require thickening its atmosphere,
warming up the planet and keeping the atmospheric constituents from escaping
into space. Such an endeavour will require numerous technological breakthroughs
and demand huge economic resources. Furthermore, terraforming Mars is a gradual
process which is expected to occur over a timescale that is likely to exceed a
human lifespan and the lack of gratification in return for investment will
deter initial investors. For these reasons, paraterraforming serves as an attractive
intermediate step before full terraforming is achieved.
Figure 1: Artist’s impression of a
terraformed Mars.
Figure 2: Artist’s impression of a
terraformed Venus.
Paraterraforming involves the
construction of a pressurised habitat in the form of an air-tight roof over a
particular area on a planet where an Earth-like environment is completely enclosed
within the habitat. A modular, “pay-as-you-go” approach of expansion is possible
for paraterraforming and this allows it to appropriately meet increasing
demand. As the population increases, more pressurised habitats can be
constructed over the surface of a planet until most of the planet’s surface is
covered or until full terraforming of the planet is eventually achieved.
A typical habitat can consists of an
ultra-strong membrane that is primarily supported by the air pressure contained
within it. The membrane acts as a roof over the surface of the planet, allowing
sunlight in and preventing the atmosphere from escaping. At regular intervals,
tension cables or support towers can anchor the membrane to the surface of the
planet. To support a more Earth-like hydrological cycle, the membrane needs to
be at least a few kilometres above the planet’s surface so as to provide sufficient
altitude for clouds to form. A higher membrane height is also advantages for
aerial transport within the habitat. To deal with the occasional meteor strike,
the membrane should be designed to be capable of self-repairing.
An object that is somewhat less massive
than Mars will be unsuitable for terraforming since its gravity will be too
weak to hold on to an Earth-like atmosphere for long. Therefore, one key
advantage of paraterraforming over terraforming is that paraterraforming can be
done on objects much less massive than Mars since the atmosphere is contained
by the overlying membrane rather than by gravity. As a result, even small
objects such as asteroids can be paraterraformed since an Earth-like atmosphere
can be contained around an asteroid by a membrane which completely envelops the
asteroid. This creates a habitable environment which fully encompasses the
asteroid. A paraterraformed asteroid will be rather interesting as a habitat due
to its very low gravity environment.
Paraterraforming also allows the
creation of Earth-like habitable environments on worlds that are too cold to
support life. Such places include asteroids beyond the orbit of Mars, the
satellites of the gas giant planets, comets and Kuiper Belt Objects (KBOs). The
paraterraforming membranes that are used to envelope such objects can be
designed to provide the necessary super-greenhouse effect to create Earth-like
habitable environments at large distances from the Sun. Many of these objects
are small enough that a single membrane is sufficient to completely envelope
such an object. A paraterraformed comet completely enveloped within a membrane
that provides a strong greenhouse effect will melt and eventually settle as a
sphere of water with denser rocky material gravitationally settling in its
core.