A guest post by Dr. Bryan J. Mendez
Astronomer & Public Education Specialist at Space Sciences Laboratory at University of California at Berkeley

Imagine that a great calamity is afflicting Earth, making life here impossible. In order to survive, we have to leave our home planet behind. How can we evacuate Earth, and where can we go?

Who is “we”?
Can we save everyone? How feasible is a complete exodus from Earth? There are nearly 7.5 billion human beings currently living on Earth. The types of rockets we use now can only lift crews of a handful of people into orbit. With this capacity, we would need something like 750 million rockets to carry away every person (to say nothing of equipment, supplies, or our other biological brethren). Manufacturing and launching that many rockets would take millions of years to accomplish. A full-scale evacuation of everyone from Earth doesn’t seem feasible at all.


We’ll have to make some hard choices about whom to send into the final frontier. Will every nation decide to create their own plans and make their own decisions about how best to preserve their culture and whom to evacuate? Or, will multinational organizations, trying to preserve a more unbiased sample of humankind and the biosphere of Earth, create a cosmic Noah’s Ark? Will others try experiments in social engineering, and attempt to select a sample of those they think represent the best of humanity ?

One intriguing possibility is to preserve the essence of ourselves in an artificial intelligence. By some estimates, computing systems that can match the human brain in intelligence are only a few decades away. If so, then perhaps human memories—or even consciousness—can be stored in computers that are sent off into space. Perhaps we can preserve Earth’s biosphere as information rather than in its physical form. Will future colonization look like little robots with the memories, thoughts, and genetic information of all life on Earth, zipping about the Galaxy, re-creating that life on other Earth-like worlds? Even more surreal is the possibility that this information would be beamed into space in high powered radio transmissions with the hope that someone, somewhere out there will detect it, decode it, and use it to resurrect the lifeforms of planet Earth.

Where to go?
Outer space itself isn’t hospitable to life. We require liquid water to live, which can’t exist without sufficient atmospheric pressure and temperature. Outer space is essentially a vacuum, with the matter between planets and stars spread very thin. Liquid water isn’t possible in that environment. But, many places besides Earth exist where liquid water is actually possible. Places such as several of the moons of Jupiter and Saturn, as well as Mars and Venus, in the distant past. We need to either find a place where water in a liquid state exists naturally, or we need to construct one. We also need the basic elements that make up our bodies and a source of energy. Luckily, both those things are reasonably common out there in the Universe.

Beyond the absolute basics, there are other considerations. Space is filled with dangerous radiation sources from both our Sun and from beyond the Solar System. Without proper shielding, this radiation can damage our cells, sterilizing life. Shielding can be made of thick barriers, hydrogen-rich materials, or magnetic fields (since most cosmic particle radiation is electrically charged). Earth naturally provides both with a thick atmosphere and a global magnetic field.

Our bodies evolved in Earth’s gravitational field. In outer space, far between massive bodies, gravity is relatively weak, which will harm us in a number of different ways: our bones will lose mass and major muscle groups will atrophy. Acid reflux will be a constant problem. We’ll want to have either natural gravity or simulate it with acceleration (linear or centripetal).


There are three classes of places that we can consider for our new home: space stations, enclosed habitats on other worlds, and other Earth-like planets.

Space stations
The biosphere of Earth is a closed ecological system on a planetary scale. To survive in space without the constant delivery of supplies from an external source, we’ll need to mimic Earth’s closed system by constructing space stations that have self-sustaining closed ecological systems. This means that humans will need to live in symbiosis with other organisms aboard such a station, just as we do now, on Earth. We can construct these stations anywhere, but it might be advantageous to build them close to Earth or in Earth’s orbit. Staying relatively close to the Sun is useful, as it can provide most, if not all, of the energy we need. Staying within Earth’s magnetosphere would provide some free protection from cosmic particle radiation.

Gravity can be simulated by rotating the station to create centripetal acceleration along the interior surfaces facing opposite the axis of rotation (like a centrifuge). One challenge is constructing the space station to withstand the stress of this acceleration.

Another challenge of building these stations is that everything, every resource will have to be brought to the station initially. It will take a long time to construct the many stations we need.

This option might be a sustainable one for the long haul. Material to build additional stations can be mined from asteroids, the Moon, Mars, or perhaps even the remains of Earth. The Sun can provide energy for billions of years to come. The human population can continue to grow aboard an ever-growing fleet of space stations orbiting the Sun.

Enclosed habitats
If we can build a self-sustaining closed ecological system in space, then we can also do so on the surface of a moon or a planet. The Biosphere experiments on Earth were examples of this notion. One advantage of this approach is the easier access to raw materials like water and minerals that might be either on the surface or accessible below the surface of a world.

The Moon is the closest option on which to build such a habitat. There’s frozen water hidden in permanent shadows at the bottoms of craters located at the poles of the Moon. Water isn’t only critical in maintaining biological systems, but it’s made of hydrogen and oxygen, which are used in making rocket fuel.

We’ll want to partially bury these habitats beneath the surface of the planet, or in caves. This will help provide shielding from space radiation and also aid in maintaining steady temperatures. Energy can be generated from passive solar and photovoltaic systems. However, a solar cycle on the Moon lasts just over 27 days, with two weeks in daylight and two weeks in night. During the long nights we’ll need energy stored in batteries or generated another way. Fossil fuels are out of the question, as they only exist where there was once a large amount of life. Without an atmosphere there’s no wind, so wind power is also not an option. Nuclear power is an option in these cases. Nuclear fusion reactors are still decades away from becoming a viable solution, so the first choice will have to be nuclear fission reactors. The fuel and waste of fission reactors is radioactive, so care needs to be taken to keep from contaminating the habitats. Fusion reactors, on the other hand, will need only hydrogen and will produce helium as waste (if they used the same reaction as the Sun does).

Mars is another option for surface-based habitats, but it’s farther away and would take months to reach, rather than days. This will limit the number of people who can be moved there. Solar days on Mars are very much like Earth—just over 24 hours. Plus, there’s an atmosphere with winds to access for power. Mars has higher surface gravity than the Moon, making it a closer experience to that of Earth. Mars also has large stores of frozen water just beneath the surface in a permafrost and in its polar ice caps. The atmospheric pressure is very low, with little free oxygen, and temperatures are quite cold. Mars has no global magnetic field and the radiation levels at its surface are potentially lethal. So an enclosed habitat with radiation shielding will still be needed.

Humans can live in habitats on Mars comfortably and indefinitely. But some have entertained the notion of attempting to transform the Martian environment into something more like it was billions of years ago, to match the conditions on Earth more closely. This process is called terraforming. We begin by placing a spacecraft between the Sun and Mars, upstream of the solar wind, which would generate a large magnetic field. This magnetic field will shield Mars’ atmosphere from the bombardment it receives from the solar wind, allowing it to slowly build higher density and pressure. Greater amounts of carbon dioxide in the atmosphere will enhance the greenhouse effect and help to warm the surface, releasing more CO2 from the frozen polar ice caps and permafrost. This will eventually also start to melt the planet’s stores of water. Water added to the atmosphere will further enhance the greenhouse effect. It might take many thousands of years to get Mars to reach something approximating Earth, but there would be hope for an Earth-like environment in our own Solar System, if we’re willing to do the hard work.

Other Earths
But if it’s another Earth-like planet that we’re looking for (rocky with liquid water on the surface and an atmosphere), we’ll have to look beyond the Solar System. Over the past couple of decades, astronomers have discovered thousands of planets orbiting other stars near us in the Milky Way Galaxy; we call them exoplanets. We can’t yet determine the detailed properties of these planets so we don’t know if any are Earth-like and can sustain human life. But our observations have allowed us to identify dozens of worlds that may be habitable, and we’re able to estimate that such worlds are probably very common in the Galaxy. The Milky Way could be home to some 40 billion worlds like Earth. That’s a lot of options!

The problem is their distance. The nearest potentially habitable exoplanet actually orbits Proxima Centauri, the nearest star to our Sun. This system is 4.25 light-years away, meaning it takes light 4.25 years to make the journey. But we can’t currently build any spacecraft that can achieve speeds anywhere approaching light (300,000 km/s). The fastest spacecraft we’ve ever built moved at about 253,000 km/h. At that speed, it would take 18,170 years to reach Proxima Centauri.


Using current or even cutting-edge propulsion methods, we’ll need to construct spacecraft that are self-contained enclosed ecosystems, like the space stations. They’ll need nuclear fuel, as solar power will become too weak once we reach even the outer Solar System. These craft will have to be home to thousands of generations of people before they ever reach an exoplanet. While they’re in the vast empty space between the stars, they’ll need to be completely self-sufficient; remember, the matter between the stars is spread too thin to be of much use. Hydrogen can potentially be collected by giant scoops, which will be useful as fuel .

Many science fiction writers have imagined that a way to make such a journey without consuming resources along the way would be to place all life in a cryogenic freeze, and then thaw everyone once the spacecraft reaches its destination. We’ve been able to successfully freeze and thaw simple lifeforms and there have been some indications that the technique might be possible for more complex lifeforms, like humans.

An interstellar journey like this would surely be a one-way trip into the unknown.

Living out there
If we have enough time and resources, there are many options for humanity to survive beyond Earth. The smartest thing is to not put all of our cosmic eggs in one space-basket and try several or all of these survival options. That way if one space colony fails, there would be many others to carry on.

Reading List
We asked Bryan to suggest additional readings for those of you that want to dig a little deeper into the ideas presented in this post.

About the Author: Dr. Bryan J. Mendez is the Astronomer & Public Education Specialist at Space Sciences Laboratory at University of California at Berkeley

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