The collective space vision of all the world’s countries at the moment seems to be Mars, Mars, Mars. The U.S. has two operational rovers on the planet; a NASA probe called MAVEN and an Indian Mars orbiter will both arrive in Mars orbit later this month; and European, Chinese and additional NASA missions are in the works. Meanwhile Mars One is in the process of selecting candidates for the first-ever Martian colony, and NASA’s heavy launch vehicle is being developed specifically to launch human missions into deep space, with Mars as one of the prime potential destinations.

But is the Red Planet really the best target for a human colony, or should we look somewhere else? Should we pick a world closer to Earth, namely the moon? Or a world with a surface gravity close to Earth’s, namely Venus?

To explore this issue, let’s be clear about why we’d want an off-world colony in the first place. It’s not because it would be cool to have people on multiple worlds (although it would). It’s not because Earth is becoming overpopulated with humans (although it is). It’s because off-world colonies would improve the chances of human civilization surviving in the event of a planetary disaster on Earth. Examining things from this perspective, let’s consider what an off-world colony would need, and see how those requirements mesh with different locations.

Creating a Mars Colony

First, let’s take a look at what the mooted Mars settlement schemes are offering. The Red Planet has an atmosphere containing carbon dioxide, which can be converted into fuel while also supporting plants that can make food and oxygen. These features could allow Martian colonists to be self-sufficient. They could live in pressurized habitats underground most of the time, to protect against space radiation, and grow food within pressurized domes at the planet’s surface.

Over decades, continued expansion in that vein could achieve something called paraterraforming. This means creation of an Earthlike environment on the Mars surface that could include not only farms but also parks, forests, and lakes, all enclosed to maintain adequate air pressure. (The natural Martian atmosphere exerts a pressure of only 7 millibars at the planet’s surface – equivalent to being at an altitude of 21 miles on Earth!)

Mars colony illustration. (Credit: NASA)

Furthermore, in addition to adequate pressure, we’d need a specific mixture of gases: enough oxygen to support human life, plus nitrogen to dilute the oxygen to avoid fires and to allow microbes to support plant life. While the small spacecraft in which astronauts fly today carry food and oxygen as consumables and use a simply chemical method to remove carbon dioxide from the air, this type of life-support system will not swing on a colony. As on Earth, air, water, and food will have to come through carbon, nitrogen, and water cycles.

While it would cost a ton of money to build, paraterraforming sections of Mars with a sample of Earth’s biosphere inside pressure domes, caves, and underground caverns is something that we could achieve within years of arrival of the first equipment. Moving beyond paraterraforming is a more ambitious goal that could require centuries, and that’s full-scale terraforming. This means engineering the planet enough to support humans and other Earth life without domes and other enclosed structures.

Terraforming Mars would require that the atmosphere be thickened and enriched with nitrogen and oxygen while the average temperature of the planet must be increased substantially. To get started, terraformers might seed the world with certain microorganisms to increase the amount of methane in the Martian air, because methane is a much stronger greenhouse gas than carbon dioxide. They also would seed dark plants and algae across the surface, thereby darkening the planet so that it absorbs more sunlight.

With the right combination of plants and well-selected microorganisms, planetary engineers could generate the needed oxygen and nitrogen. During all of the centuries needed for terraforming, colonists would inhabit and expand the system of paraterraformed structures.

That’s a vision that’s relatively cohesive. Still, there are some aspects of the plan that are less than ideal – and indeed, might point our skyward gazes toward a different destination altogether.

The Problem of Distance

A colony totally isolated from Earth would need significant genetic diversity to avoid the disease risks that plague smaller populations. According to a study published earlier this year, a multi-generation starship carrying people whose descendants would colonize a planet orbiting a nearby star would need a population of at least 10,000 and possibly closer to 40,000.

It’s been reported that Elon Musk wants to build a Mars colony with a population of 80,000. This certainly would fulfill the population requirement, but a further distance is a challenge both in fuel and in time. First, fuel. The Musk plan involves sending multiple crafts each with a total payload of 15 tons per trip. To convert that to people onboard, consider that that’s just under half the tonnage of NASA’s new Orion spacecraft which carries a maximum of six astronauts. This gives us a ratio of approximately 5 tons per person.

Some of the tonnage is due to the fuel needed to accelerate the ship from low Earth orbit to escape velocity, and this may not differ between Mars and closer sites, such as the moon. But the tonnage per person also depends greatly on the travel time, because of life support and other issues related to consumables, so it’s fair to say that for a given number of voyages we’ll be able to relocate more people to sites in the Earth-moon system than to Mars.

Second, the time it takes to transport settlers. A colonization program will be efficient only if each transport ship is designed to make multiple trips back and forth. A 15-ton payload of the Musk plan currently translates into three colonists per ship, but to be optimistic let’s imagine that we could increase that number to 20 people. In that case, transporting 10,000 people to Mars (the minimum number needed for healthy genetic diversity) requires 500 voyages from Earth, while 4,000 voyages would be needed to reach the 80,000 colonist milestone. Assuming that we’d build only a fraction of that number and have the ships go back and forth, we can expect to be waiting around for ships taking a year or two to return to Earth to pick up a new load of settlers. Certainly, the advent of advanced propulsion technologies, shrinking the travel time between Earth and Mars from a year or so down to weeks would change these considerations, but right now the various Mars colonization proposals (at least the developed ones) are based on the old-fashioned chemical engines that have sent the current MAVEN probe toward Mars at turtle speed.

Artist's impression of the MAVEN spacecraft orbiting Mars. (Credit: NASA/Goddard Space Flight Center)

A two-year round trip time and a fleet of 25 ships transport ships gives us 50 years to relocate 10,000 people, and 400 years for 80,000 people. Certainly the time frame would shrink due to early waves of colonists having babies, and certainly technology could accelerate the program, but given that we’re talking about many decades to reach the genetic diversity milestone, it seems worthwhile to make a similar calculation for the moon, for which the round trip time is only a week. Doing this, with the same type of program (25 ships each carrying 20 people), we get the first 10,000 to the moon in less than six months, and the first 80,000 in less than four years.

And, finally, being closer would help with ongoing rapid access to and from Earth. That may sound contradictory, given that the goal is to build a colony that’s self-sufficient. But getting to that point could take some time, and at the beginning some colonists might need to be evacuated. There should be a growing medical capability on the colony, but initially cases of very serious illness and certain injuries might be better handled on Earth. This would not be an option if the travel time were measured in months, or even weeks. And what if there were a planetary disaster on Earth in the early decades of the colony? From a location close to Earth, the colony might actually be able to provide some help.

Close to Home

A colony on the moon, on the other hand, would be within easy reach. Like Mars, the moon has caverns and caves that can be sealed for paraterraforming, along with craters that can be enclosed with pressure domes.

One fascinating lunar colony proposal would utilize the Shackleton crater at the moon’s south pole, enclosing a domed city with a 5,000-foot ceiling and a diameter of 25 miles. A colony in that location would have access to large deposits of water ice and would be situated on the boundary between lunar sunlight and darkness. Its proponents estimate a Shackleton dome colony could support 10,000 settlers after just 15 years of assembly by autonomous robots.

Artist's depiction of a pole colony on the moon. (Credit: Asa Schultz)

In the event of an Earth-wide disaster, evacuating people to the moon would be far easier than to Mars. Another, even nearer option would be free space colonies. These would be built using materials mined from the moon or from near-Earth asteroids. The colonies could be located in the Earth-moon system at sites that are gravitationally advantageous, known as Lagrangian points. In these regions, a colony’s distance and orientation to both the Earth and the moon, or to the Earth and the sun, would remain constant. Utilizing Earth-moon Lagrangian points, it would be relatively easy to transport lunar materials to the site of the planned colony and build it, and the travel time from Earth would be similar to the travel time to the moon, meaning a few days with current technology.

The Problem of Gravity

All planets and large moons have enough gravity to hold an atmosphere, so terraforming in theory is widely possible. But in terms of human life not all gravities are created equal.

On Mars you weigh 0.38 your weight on Earth, and we’re not entirely sure what this would do to human health. To keep Mars residents’ bones from demineralizing, for instance, they might need to exercise inside large centrifuges every single day. Thus far, NASA and other organizations have studied effects of partial gravity to a limited extent on humans by producing Mars and lunar gravity for short periods (under a minute) during parabolic flight.

For long-term effects, which in weightlessness involve not only bone demineralization, but also muscle atrophy, immune system effects, and other complications throughout the body, there is no way to replicate partial gravity on Earth. We can simulate it with various contraptions that have allowed researchers to study things like walking on Mars and whatnot. We can put people in bed for long periods with the beds angled so as to simulate the shifting of fluids on Mars or other worlds. But until we actually send animals to those environments, we can’t really be sure what will happen to various systems, including reproduction. The development of embryos depends on gravity and is known to be disrupted in weightlessness, but we don’t know what will happen in environments with a fraction of Earth’s gravity.

And while Martian gravity is low in terms of human physiology and movement (you could jump really high on Mars and that would be fun), it’s high enough that spacecraft would consume a significant amount of energy in taking off from the planet or landing on it. Similarly, while the atmosphere is way too thin to support human life (until we terraform it), it’s still thick enough to cause dust storms that can ruin colonial machinery. So considering the air and gravity along with the distance from Earth, Mars actually may not be the best candidate for an off-world colony.

Lightening the Load

Here Venus has one advantage over other worlds: its gravity, which is just a little less at the surface compared with Earth’s. On the Venusian surface, the pull is approximately 91 percent what it is on the surface of Earth. That’s close enough that it seems unreasonable to predict any long-term detrimental health effects from the gravity difference, which is a nice advantage. On the other hand, Venus would have to be terraformed before anyone could live on the surface at all, since the high pressure and temperature would not allow for paraterraforming. Nevertheless, we might be able to terraform Venus just as easily as Mars.

An extraterrestrial greenhouse illustration. (Credit: Jan Kaliciak)

Going in an opposite direction as Mars terraformation, a Venusian project would begin by having planetary engineers interfere with the runaway greenhouse effect that cooked the planet billions of years ago. The process might start using heat-loving microorganisms and various chemical tricks to remove large amounts of carbon dioxide and other gases that we wouldn’t want there.

Another gravitational fix could be found in free-space colonies. We already said that these could be built using lunar or asteroid materials, but another advantage is that we could build them in any shape. If built in the shape of a doughnut, such a colony could be rotated at the precise speed needed to produce the same gravitational pull as we feel on Earth – meaning that keeping our bones, heart, and other body systems healthy would be as easy as hopping on an Earth-style treadmill, kicking a few handstands, playing tennis, or whatever physical activity you enjoy.

A New Home in the Solar System

I support an aggressive Mars exploration program. We’re sending probe after probe there for good reason: geologically the planet is similar to Earth, and used to be even more similar. Moreover, it’s one of the most interesting and vital sites for astrobiology in the solar system. Very likely, the Red Planet will become the first place where we confirm the existence of extraterrestrial microbial life, providing us with a second datum for biology. Since all life on Earth that we know has basically the same chemistry, comparing it with a newly discovered system could stimulate quantum leap advances in biotechnology and medicine here on Earth.

But while Mars science must advance at full speed, it does not mean that the same world is the best first site to settle families with children. Given all that we’ve discussed, until we have much faster propulsion, I think that colonization should begin closer to Earth, either on the moon, or in free space colonies in the Earth-moon system, depending on what studies on early lunar bases tell us about the long-term effects of lunar gravity – including, importantly, whether healthy pregnancies on the moon are possible.

After all, whichever of these locations we choose, we’ve got a long line of future space descendants to think about.