Terrapin Station: An Alternative Approach To Space Exploration

Charles Eric LaForest
Waterloo Space Society
University of Waterloo, Independent Studies
November 2005

A terrapin is a soft-water aquatic turtle. Like a turtle, the model of space exploration proposed here is slow and long-lived. Terrapin Station [1] is a song by The Grateful Dead which contains a couple of verses with astronomical mentions, and could be taken as a reference to a space station at L5:

Counting stars by candlelight
all are dim but one is bright:
the spiral light of Venus
rising first and shining best,
From the northwest corner
of a brand-new crescent moon
crickets and cicadas sing
a rare and different tune

Terrapin Station
in the shadow of the moon
Terrapin Station
and I know we'll be there soon

Words by Robert Hunter. Music by Jerry Garcia.
Copyright Ice Nine Publishing

Current plans for manned Mars missions, or indeed any other manned missions, involve a large rocket, fast travel, and a supply of consumables. A finite supply of power, air, food, and water limits possible travel time, which must then follow a high-energy trajectory, and thus requires a large, high-thrust, low efficiency (low specific impulse) means of propulsion. These requirements increase proportionally with the distance traveled.

I propose an alternative approach to exploring space. It involves smaller rockets, a large space station, slow travel, and a renewable life-support system. A solar-powered, self-sustaining source of air, food, and water makes longer, lower energy trips possible, and thus makes practical the use of small, low-thrust, high-efficiency means of propulsion. These requirements remain constant relative to the distance traveled, with the exception of propellant.

It begins on Earth. The first step is to create a sustainable closed life-support system. There has already been significant past work done in Russia [2] and currently in progress in Europe [3], Canada [4], and the US [5] on how to recycle waste, grow food, and regenerate air in a closed system with only energy as input. Picture a smallish, sealed building with a few windows, running solely off solar panels on the roof, thus approximating the energetics of a space station.

It's important to note that such a system, even with perfect recycling efficiency, will not last indefinitely but will be limited by the wear and tear on the mechanical components of the system. This is an engineering problem of mean time to failure which can be calculated ahead.

Once sufficiently refined, this setup would be reproduced in Low-Earth Orbit (LEO), where the additional challenges of confined space, microgravity (or artificial gravity), micro-meteorites, radiation, repairs, and spares have to be dealt with. Eventually, such a station could function for years without resupply.

Having thus no need to hurry, the station can move itself using some of the many forms of high-efficiency, low-thrust electrical propulsion, one example of which is the ion thruster [6]. In a pinch, resistojet thrusters (which work by electrically heating a fluid) can use a wide variety of propellants, but at a much lower specific impulse. At that point, the only limitations to the location of such a station are available propellant, solar power, and calculated time until a resupply is required.

A Mars mission is a mere afterthought given this. The main factor would be a need for about 2.25 times more solar panel area to take into account the relatively reduced level of solar radiation at Mars orbit. Illumination decreases as a function of the square of the distance, so doubling the distance from the source (in this case, the Sun) divides the available energy by four. Mars is approximately 1.5 times further away from the Sun than is the Earth. This means that solar illumination on Mars is reduced to about 44% of Earth levels, hence requiring a little over twice as much receiving area in order to obtain the same amount of energy.

A bonus of such an approach is that most of the problems involved in establishing permanent bases on moons, planets, or asteroids will have already been solved. It would then actually be easier to establish a fixed base given solid ground and gravity.

However, there is a caveat about such a system: it can never launch or land. The volume required for the life support system will make it too large and too heavy. The large quantity of solar panels will make it too fragile. Finally, a large fraction of the total energetic cost of a mission is the entry into and exit out of a planet's gravity, which exceeds the energy available in this case. For the same energetic reasons, such a station would have to be built in LEO and then "launched".

Thus, if more than fly-by missions are desired, such a traveling station will have to either be reached via a planet-side rocket, or itself be equipped with one or more small spaceships which would do a slow entry into atmosphere and then launch back to the station using a system not unlike SpaceshipOne. The main difference would be that such a ship would have to be able to reach the orbital velocity of the station [7]. This ship has to be a conventional high-thrust rocket system which cannot be regenerated by the station, but whose propellant must be resupplied externally.

As a consequence of this focus on low-energy space exploration, the Space Shuttle (or a similar replacement vehicle) is no longer a sensible choice. The Shuttle can lift up to 28,800kg of cargo to LEO, sustain a crew of 5 to 10 (typically 7) for about two weeks, and function as a platform for experiments and Extra-Vehicular Activity (EVA) such as satellite repairs. It amounts to launching and landing a small space station for each mission, with the resultant extremely high maintenance requirements and propellant costs.

Experiments should be done on a permanently space-borne platform, such as the ISS, where the most efficient choice is to launch supplies and crew via cheap, reliable, single-use rockets like the Soyuz and Proton launchers. These each have a track record of hundreds of successful launches. Part of those supplies would have to include an Apollo-like re-entry capsule for returning crew and objects until the aforementioned small, light reusable system becomes possible.

Without the Shuttle, performing repairs to existing satellites would be best done by launching the required parts to the station or to an orbit close to that of the satellite, which would then be reached using the station's small ship as a mobile base to perform the repairs from.

More so than the development of expedition-style missions, the refinement of self-sustaining space habitation has direct applications on Earth. The same engineering would make it feasible to inhabit previously difficult locations such as the deep desert, the tundra, or other places where waste management and power infrastructure cannot be brought out for engineering or economic reasons. Even within cities, the same technologies could reduce the extent and cost of those utilities.

While by no means a fast-track to space exploration this slow-and-steady approach, like that of the proverbial turtle, would make our next step into space a very solid, sustained, and economical one.

[1] The Annotated Terrapin Station

[2] "Biosphere-3" aka BIOS-3

[3] Micro-Ecological Life Support System Alternative aka MELiSSA

[4] Controlled Environment Systems Research Facility

[5] Advanced Life Support

[6] Ion Thruster

[7] Difference between sub-orbital and orbital spaceflights