Friday, May 3, 2013

Ice Domes as Space Habitats

Enceladus, planet Saturn’s brightest moon,
sprints seven miles per second ‘round his captor -
the massive puffed, yellow-buff gas balloon
but wobbles as if hooked by a one-clawed raptor.

He whips past his sister moons, scarred Dione
and pocked Tethys in a frenzied orbital race.
Saturn's affectionate attendant cronies
pull with a fly-by, gut-churning embrace.

Ice-shelled Enceladus roils at the core;
fissures and resurfaces his shining skin.
From chasms, cryovolcanic geysers soar
and paint the nebulous ice-crystal E ring.

He's wrenched, twisted and warped - the treatment’s rough,
but renders heat through internal commotion.
Enceladus has all the fundamental stuff
to foster life in an under-ice ocean.

- Diane Hine, Enceladus

Figure 1: This image was captured by NASA’s Cassini spacecraft in orbit around Saturn. Shown here is Titan together with the icy moon Dione. The bulk of Saturn fills the background. Here, Saturn’s rings are viewed nearly edge-on and the dark bands at the bottom are shadows cast by Saturn’s rings onto the planet’s cloud-tops. Saturn is also home to Enceladus - an icy moon with active geysers at its south pole. Credit: NASA/JPL-Caltech, Space Science Institute.

Creating large volumes of habitable space is a key requirement for manned exploration of the numerous and diverse worlds found throughout the solar system. On many of these worlds, water in the form of ice exists as a relatively abundant resource. A paper recently published on arXiv investigates the idea of using water ice to construct domes housing large volumes of habitable space on places with abundant water ice and where temperatures are low enough such that water ice always stays solid. These places include the cold permanently shadowed crater floors at the poles of the Moon and Mercury; the Galilean satellites - Europa, Ganymede and Callisto; the icy moons of Saturn and Uranus; and the Kuiper Belt Objects. The construction of ice domes from locally available water ice will allow manned exploration and colonization of these worlds without having to transport large amounts of construction equipment from Earth or elsewhere.

An ice dome basically consists of a shell of water ice of some thickness which forms the main structure. Enclosed within the dome is a large habitable volume with an Earth-like atmospheric pressure. The temperature within this habitable volume will depend on the properties of the thermal insulation covering the internal surface of the ice dome. This temperature should not be too high so as to prevent melting of the ice dome from inside out. An ice dome can be constructed to a very large size, containing thousands to millions of cubic metres of habitable volume. Its construction begins with the deposition of water vapour directly onto the internal surface of a thin, weakly inflated and dome-shaped film of material. Ethylene tetrafluoroethylene (ETFE) can serve as a possible film material. It can be prefabricated on Earth and then transported to site where it is weakly inflated to achieve its desired dome-shape. Since the outer surface of the film is exposed to the cold vacuum of space, any water vapour that gets deposited onto the film’s inner surface quickly freezes into a layer of solid ice. Over a period of time, the layer of ice thickens and eventually forms a dome of ice with sufficient strength to support an Earth-like atmospheric pressure within it.

As a result, the only equipment required for the construction of an ice dome is a thin film to serve as a deposition surface and a device that can spray water vapour. This alleviates the need to transport large amount of construction material and equipment from Earth. In fact, a number of other benefits can be derived from constructing ice domes as pressurized habitats. The huge amount of habitable space enclosed within an ice dome will improve living conditions and enable new possibilities such as large scale agriculture and recreational sports. Water ice also possesses good radiation shielding properties. A one metre thick layer of water ice can effectively attenuate incoming gamma radiation to less than one percent its original intensity. The ice layer of a typical ice dome is likely to be on the order of a few metres thick. Although very large quantities of water are required to construct an ice dome, all the water can be derived locally and is 100 percent recyclable. If the ice dome needs to be scrapped for any reason, it can be slowly sublimated and the water vapour pumped to construct another ice dome nearby.

Figure 2: Halving thickness (cm) of various shielding materials versus gamma radiation energy. Source: Gamma Ray Attenuation Properties of Common Shielding Materials by Daniel R. McAlister, Ph.D., January 2012

An ice dome offers excellent protection against incoming meteoroids. Due to the thickness of the ice layer and the high strength of ice on the dome’s external surface, meteoroids are expected to cause nothing more than minor surface damage. Such damage can be easily repaired by spraying liquid water into the crack where the water will quickly freeze and restore the ice to its original strength. In the unlikely event that a meteoroid is large and/or energetic enough to punch a hole through the ice dome, venting of atmosphere through the hole can cause moisture to freeze around the hole and eventually plug it. Liquid water can also be sprayed into such a hole to quickly seal it.

Stefan Harsan Farr (2013), “Ice Dome Construction for Large Scale Habitats on Atmosphereless Bodies”, arXiv:1303.5356 [physics.pop-ph]