Before he died, Stephen Hawking said that humanity must colonise other planets in the next hundred years to avoid extinction.
His fears took the form of super-viruses, nuclear war, asteroid impacts, and global warming. In order to avoid another Dark Age - or altogether extinction of the human species - we need to become a multiplanetary civilisation.
Are We Ditching Earth Now?
No way. Earth is great; there's no chance we're leaving it behind as long as it's a viable home for humanity.
Perhaps the biggest criticism of becoming a multiplanetary civilisation is that it appears we've ruined Earth, and now we're dumping it and looking for more planets to ruin.
The thing to consider here is timescale.
The agricultural and industrial revolutions of humanity have taken a huge toll on the environment and biodiversity. That sucks. We've witnessed wide-scale suffering and extinction at our own hands and we should, as a species, take responsibility for that.
On the broader scale, however, we haven't ruined Earth. The third rock from the sun will still be here doing its thing for billions of years after we've gone. In a few thousands years, the next glacial period will ravish the land - unrelated to humanity - creating catastrophic cooling and range contractions. It will last for 100,000 years. We currently have no influence on that.
Humans are a blip on the geological timeline. And in order to make that blip last longer, we must hedge our bets by establishing independent colonies on other moons and planets.
The plan is to distribute the eggs of humanity across many baskets, so a catastrophic basket loss won't commit our species to extinction.
If the dinosaurs had possessed this capacity, they'd have gone multiplanetary too. It's a matter of survival.
The Benefits of Earth
Life on Earth evolved over billions of years in specific environmental conditions.
Mammals, in particular, have had a presence that dates back 250 million years, in which we survived and adapted to the climate, atmosphere, resources, and competition on Earth.
Any alien world we plan to inhabit must offer similar conditions for our survival, or at the very least we must be able to recreate them with supplies drawn locally or shipped all the way from Earth.
Space, time, distance, and our own human frailties pose significant scientific challenges if we want to explore the final frontier.
Even back in the 1960s, Star Trek referred to terraforming - literally "Earth-shaping" - other worlds by modifying the atmosphere, ecology and surface topography to make them habitable for humans.
But this notion is merely hypothetical until we put it into practice and likely discover a whole host of unknown challenges on the job.
The Search for Earth-like Planets
In the last decade, NASA's Kepler space observatory has identified hundreds of new exoplanets, including some that might be a lot like Earth.
They're all within decent limits of their respective suns and if they possess a stable atmosphere - where all water doesn't freeze or boil away - they may just be habitable.
Humans are real suckers for sunshine and liquid water.
The area studied by Kepler represents just 0.25% of the night sky. And Kepler can only identify planets that happen to pass directly in front of their sun. They're too distant and too dark to be observed directly.
Instead, their transit creates a tiny dip in solar luminescence and this is the tell-tale sign of a planet in orbit. Planets that don't make any transits are simply invisible to us for now.
The prime candidate so far is Kepler-186f, some 490 light years away.
How far is that? Consider that the fastest space probe, Voyager 1, travels at 61,000 km/h (38,000 mph). At this rate our ideal planetary candidate has a journey time of just over 9 million years.
Even if that journey were possible, the species that steps onto Kepler-186f will be very different from the ancestral species that departed from Earth.
Cosmologists reckon there are a theoretical 11 billion rocky planets orbiting Sun-like stars within our galaxy. But that tends to fall on deaf ears, seeing as their proximity to Earth makes them impossible candidates for our ends.
That is, unless we invent intra-galactic teleportation any time soon. Although I think we've all seen The Fly and know how that story ends thank you very much.
Candidates in Our Own Solar System
Our own solar system contains eight planets and 181 moons, some of which might offer us an alternative home-from-home.
Ideally, we'd start by terraforming some of these local worlds and not have to worry about speciation on the journey over.
So which of our solar system planets are viable habitats for humanity?
Closest to the sun are Mercury, Venus and Mars. They're similar to Earth because they're rock-based, and so earn the title of Terrestrial Planets.
The rest of our solar system is made up of the Jovian Planets: the gas giants Jupiter and Saturn, and the ice giants Uranus and Neptune.
Mercury is the closest planet to the sun, orbiting at an average distance of 58 million kilometres. Too bad this means the surface heat and radiation is extremely intense; temperatures reach 426°C by day and -173°C by night. Even a lead-based sunblock would melt off your face, so Mercury probably isn't a goer.
Venus orbits further out at 108 million kilometres but is no more inviting. Day or night, north or south, surface temperatures remain a fierce 460°C on this hell ball. This is due to its thick carbon dioxide atmosphere which traps the Sun's heat as per the greenhouse effect.
The Red Planet
Mars has received a lot of our space-faring attention. To date, 40 attempts to send exploration vessels have resulted in 18 successful missions (if you define successful as not disappearing without a trace, crash landing, or breaking down soon after arrival). But that's par for the course: this is interplanetary exploration we're talking about. There are a lot of unknowns.
Elon Musk has set his sights on a manned mission to Mars in 2024 through his company SpaceX, initially to check on water resources and local hazards, as well as to establish mining, power and life support infrastructure. Colonisation will occur rapidly thereafter.
But what will it be like for the first astronauts to travel to Mars?
The Journey to Mars
Once we leave the protective shroud of the Earth's atmosphere and magnetic field, conditions in space become pretty inhospitable.
We need an intricately well-designed and well-equipped spacecraft to overcome challenges as wide-ranging as baseball-sized space debris ripping through the hull, to safely disposing of astronaut poo.
Still, humans have a decent amount of experience in near-Earth space, so one of the new challenges in going to Mars is embarking on such a long-haul voyage and still having your sanity on arrival. Researchers have created a fake Martian habitat on a mountain in Hawaii purely to test this premise.
How far away is Mars? The distance between Earth and Mars varies due to their independent orbits, so the journey time really depends on when you set off. Mars will make a close approach to Earth in July 2018, bringing them 58 million kilometres apart, and this is due to reoccur in October 2020. So there are predictable windows for which we can aim.
On average, though, the separation is more like 225 million kilometres. And these are just straight line measurements. A realistic trajectory must take into account the continual movement of Mars and avoid passing too close to the sun. Luckily rocket scientists love working this stuff out and have calculated the optimum journey time to take around nine months.
Mars-bound travellers need to deal with all and any issues that arise, without relying on real-time conversation with support crews on Earth. Communications are delayed by 3-22 minutes which is the time if takes for sound signals to bounce to and fro. Interactions with loved ones on Earth, as well as requests for technical help, will be limited to one-way messages and long tense silences.
And then there's the physical and psychological isolation that helps drive the plot of every single space movie ever.
Astronauts are locked in to cramped conditions together with no escape for the better part of a year, along with all the joys and pitfalls of weightlessness. Alien taught us that in space, no-one can hear you scream. However your fellow crew mates may beg to differ and request you keep your psychosis to yourself.
Welcome, Future Martians
So, we've made it to Mars. What's it like down on the ground?
Mars is about half the size of Earth (but with the same amount of dry land) and takes almost twice as long to orbit the sun.
Both Mars and Earth have similar axial tilts which give rise to seasons on both planets.
Mars has two small moons, Phobos and Deimos, however unlike our Moon they have no stabilising effect, so the tilt of Mars does wobble. Phobos is expected to be torn apart by gravitational pressures in the next 20 to 40 million years; a date in the diary for future Martians.
Mars is made chiefly of iron-rich rock and the resulting dust is what gives it the nickname of the Red Planet.
Vast dust storms - the largest observed in our solar system - can rage for months, sometimes covering the whole planet in a thick red shroud.
As I write, NASA's old and trusted Opportunity rover is currently devoid of solar power because of a dust storm that's encircling the entire planet. (The other Martian rover, Curiosity, is nuclear-powered and having a grand old time monitoring the storm.)
Adding to the dust problem is Olympus Mons, a massive volcano about three times the height of Mount Everest, and thought to be active. Being a shield volcano with gently sloping sides, an observer on the ground wouldn't be able to see its entire profile, even from a distance. The curvature of the planet and the vastness of the volcano would obscure such a view. In other words, Olympus Mons is so ridiculously big it curves visibly around the planet.
When the dust on Mars settles, the sun returns - but at only half the size as it appears on Earth.
What's more, the Martian atmosphere is 100 times thinner than ours and is comprised of 95% carbon dioxide (vs our 0.04%). The rest of Martian air is made up of argon, nitrogen and 0.02% oxygen (vs our 21%).
Without specialised breathing equipment and oxygen supplies, humans on Mars would quickly suffocate.
Of course, science and technology are rising to meet the challenges posed by the Martian environment. Soon we will have devices that can extract oxygen from the plentiful supply of carbon dioxide in the Martian atmosphere, and this can be filtered into the habitats and resupply breathing apparatus.
The thin atmosphere means Mars can't retain heat or moisture, and levels of UV radiation are high. Any water is likely to be salty thus enabling it to avoid freezing or vaporising.
Temperatures are cold at an average of -60°C. Only in summer at the equator can it reach comfortable temperatures of 20°C, but this drops rapidly to -73°C by nightfall.
Besides the risk of hypothermia after dark, the extra 38 minutes in each Martian day would cumulatively impact on your circadian rhythm, triggering sleep disruption and insomnia. Cardiovascular problems could arise and kidney stones more likely because of the potential for dehydration and increased excretion of calcium from bones.
That's because of the gravity problem. Mars has a mass of 10% that of Earth, giving it around 38% of our gravity. In other words, if you weigh 80 kg (175 lbs) on Earth you'd only weigh 30 kg (60 lbs) on Mars. Moving around in Martian gravity would be awesome fun - but not without its downsides.
Your body is perfectly adapted to Earth's gravity, so on Mars your bones and muscles would quickly degrade. Not to mention that you just spent nine months in zero gravity; by comparison Mars would actually make you feel pretty heavy.
Astronauts lose minerals from their bones with density falling by 1% per month. So without proper exercise and nutrition, you'd lose bone and muscle strength, and would have a hell of a time undergoing physiological rehabilitation after returning to Earth.
So while Mars is relatively "nearby" and "similar" to Earth in the grand scheme, colonising the planet poses tremendous challenges.
It's really cold, with low gravity, immense dust storms, and limited directly available oxygen. The first Martians will need to transport huge quantities of hardware from Earth and be extremely mentally and physically resourceful to survive.
Nevertheless, the race is on for governments and private companies to take humanity to Mars within the next few years.
Mars is happening - and sooner than you think.
What About The Jovian Planets?
Let's think even bigger for a moment. Mars could be our first home-from-home, but a true space-faring civilisation needs to establish itself on multiple planets, moons, asteroids and space stations.
Exploration missions have gone to the Jovian planets (Jupiter, Saturn, Uranus and Neptune) and some of their moons. Nine spacecraft have already travelled to Jupiter, sometimes using its gravity as a slingshot to even further flung destinations. We've obtained some amazing images and scientific insights, coming to the realisation that these are not at all Earth-like planets.
Their very description as "ice" or "gas" giants provides the first clue as to how difficult their colonisation might be.
Jupiter is the largest planet in our solar system, with a mass 2.5 times greater than all of the other planets combined. The outer atmosphere comprises thick gas which has literally crushed exploratory spacecrafts under the pressure.
Eventually, Jupiter becomes liquid, and then solid at the very core. There, pressures are immense: estimated to be 50-100 million times that of Earth's pressure at sea level.
Jupiter doesn't rotate like the terrestrial planets. Made up of mostly hydrogen and helium gas, Jupiter spins faster at the equator than the poles. In fact, it has the fastest rotational speed in the solar system, producing 10-hour days and winds of 480 km/hour.
Its famous Great Red Spot is an area of high pressure: a continuous giant spinning storm first sighted in the 17th century. The storm itself is more than twice the size of Earth.
Don't Forget The Moons
Colonising such gas giants may well be impossible, but let's not forget about Jupiter's 69 moons, the most famous of which are the ice moons, Europa and Callisto.
In fact, scientists reckon the ocean underneath Europa's icy crust could well harbour life, since all the necessary conditions are present.
NASA has funded the initial development of a number of high-tech concepts including the amphibious squid rover designed to explore the deep oceans under Europa's ice.
So far, eight spacecraft have visited Europa and photographed 15% of its surface at a decent resolution. The surface is smooth, lacking craters thanks to the ocean currents continually recycling the ice.
Europa is also subject to deadly radiation on one face, so human habitats should be built on the protected opposite face.
Of course, there would be the chill-factor to deal with. The average temperature is -160°C at the equator and -220°C at the poles. Ice quakes would pose a hazard, as would the sudden explosion of violent water plumes from the ice.
The gravity on Europa is 13% that of Earth and it has no atmosphere - and therefore no weather.
The sky is always as black as night.
Meanwhile, fellow moon Callisto is also covered with ice and is the most heavily cratered object in the solar system, thanks to its ancient 4-billion-year-old landscape and lack of geological activity.
Like Europa, it too could support an ocean filled with cold-tolerant life forms. It's certainly an enticing driver to get a permanent human base set up on the surface in order to find out.
For the first time in 3.8 billion years, life on Earth is developing the capacity to explore and put down roots on other planets. It's not merely a giant leap for mankind, it's an advancement for all life that we take with us.
Think beyond domestic animals and lab mice. I'm talking about the plants, bacteria and fungi that fuel our ecosystems and will be vital to our nourishment on other worlds.
Once the life support infrastructure is established on Mars, Jupiter's moons, and beyond, going multiplanetary will be a one-way trip for millions of humans - including your descendants who may already be alive today.