Centuries ago, Polynesian explorers settled on dozens of Pacific islands spanning from New Guinea to Hawaii to Easter Island. Living on several islands provided the Polynesian culture a better chance for survival. If disaster struck one island, the culture could still thrive on the other islands. This is often, as recently expressed by Stephen Hawking, used as a rationalization for space colonization. Is this a realistic model for human survival? The best way to answer that is to understand how Earth protects life, what could endanger life on Earth, and how difficult it would be to migrate into space.
The Sun resides in a relatively quiet area of the galaxy referred to as the Local Bubble. This bubble was created by a series of nearby supernovae events some 10-20 million years ago. Even so, the Solar System is bathed with galactic cosmic rays and ionized solar winds which are harmful to life. The Earth offers protective layers that insulates life from the harsh realities of space. The magnetic field guides ionized particles towards the polar regions. The harmful kinetic energies of these particles are absorbed by oxygen and nitrogen atoms in the upper atmosphere and converted to harmless light radiation in the form of the aurora. The ozone layer blocks harmful ultraviolet (UV) rays from reaching the surface. In fact, the upper atmosphere heats up at the ozone layer where the high energy UV radiation is absorbed.
The Earth’s atmosphere also absorbs high energy x-rays and gamma rays. This is a key point as our first attempt to colonize space will most likely be on Mars. And Mars does not offer the protective layers that Earth does from the harmful radiation of space. Any attempt to colonize the red planet will need to invent technologies to provide protection from space radiation. Also, Mars has only 1/3 the gravity of Earth which can deteriorate body muscle throughout a long duration stay. We cannot change Mars’ gravity and for this reason, some propose to make future Mars missions a one way colonization effort* as returning to Earth’s gravity may be problematic.
How feasible is it to colonize Mars? To put it in perspective, it is much easier to colonize Antarctica. Currently, there are a few dozen scientists who occupy the South Pole station in any given year. Going to Mars is possible, but during the next few decades only a handful, at best, will occupy our nearest neighbor. When evaluating possible disaster scenarios on Earth, what type of timeline are we looking at?
For now, I want to focus on natural, rather than human induced, disasters. First on deck are supervolcanoes. A supervolcano is an eruption that releases at least 500 cubic km of magma. By comparison, this is 500 times larger than the Mt. St. Helen’s eruption in 1980. These events are pretty rare, about once every 100,000 years. To put that in perspective, the Pyramids of Ancient Egypt were built 5,000 years ago. The last supervolcano eruption was Lake Toba 74,000 years ago in Indonesia. These events can lower global temperatures 10 degrees Celsius over a period of ten years. Once believed to have reduced the human population to 11,000, new evidence suggests that Lake Toba was not as catastrophic to humanity as originally thought.
While that is good news, a supervolcano would certainly be disruptive to human civilization. And while the chances are such an eruption in the near future are remote, at some point in time there will be one. One such mantle hotspot resides in Yellowstone. Recently, the Yellowstone magma chamber was mapped.
The newly found lower chamber contains about 11,000 cubic km of magma. The last Yellowstone eruption occurred 640,000 years. The chances of an eruption in the near future is very slim. As destructive as these events can be, it appears that the next such event could be 10,000 or more years in the future. At this point, there seems little that could be done to thwart the threat of a supervolcano. That is not the case with the danger of an asteroid/comet strike.
Impact events are not uncommon. Small meteors collide with Earth everyday, usually burning up in the atmosphere. When they are large enough to survive the frictional forces of the atmosphere, they strike the ground and are then called meteorites. These objects are collector items but also valuable for scientific research. Unlike the Moon, erosion typically wipes away evidence of past large impact events. One exception is in Arizona where the dry climate has kept intact a 1,300 meter wide crater for 50,000 years from a 30-meter meteor impact event.
If a impact is large enough, sizable amounts of material can be ejected into the atmosphere causing global cooling and potential danger to life. Most famously, an impact near the Yucatan Peninsula 65.5 million years ago killed off the dinosaurs. This was one of the largest impacts in the inner Solar System since the heavy bombardment formation stage some four billion years ago. The cause of this was an object 10 km wide. How often does such an event take place?
Fortunately, extinction type impacts are very rare. In fact, the impact causing the dinosaur extinction is the last known event of this magnitude. More common are smaller, but still damaging impacts such as the 1908 Tunguska event in Siberia. In this case, a 120 foot object vaporized some 5 miles above the ground and the concussion was felt dozens of miles away. While potentially devastating on a local scale, these impacts would not present a threat to humanity on a global basis. Impacts of this scale occur around once every 300 years.
Unlike supervolcanoes, it is feasible to mount a defense against a possible asteroid or comet impact. NASA now has Planetary Defense Coordination Office whose mission is to locate, track, and devise efforts to defend against collisions with Near Earth Objects (NEO). NASA has discovered over 13,000 NEO’s and detects an additional 1,500 NEO’s per year. The program’s budget is $50 million annually. That is 25% the cost to make the most recent Star Wars movie. NASA’s goal is to have a test mission to redirect an asteroid during the 2020’s. While we can plan to defend against impact events, the stellar evolution of the Sun is much more problematic.
The Sun is an average sized main sequence star halfway through its expected lifespan of 10 billion years. Main sequence stars like the Sun fuse hydrogen into helium in their cores. Over the next billion years, the Sun will become hotter and more luminous. As a star ages, the rate of fusion in its core rises. Some 3.5 billion years from now, the Sun will emit enough energy to vaporize the oceans and propel Earth’s remaining water vapor into space. And it doesn’t stop there. About 5 billion years from now, the Sun’s core will run out of hydrogen and begin fusing helium into carbon. The core will become hotter causing the Sun to expand into a red giant. At this point, the Sun will consume the inner planets including Earth. Here, humankind will need to develop interstellar travel or cease to exist.
Would interstellar travel guarantee our survival as a species? Not quite. The universe itself is evolving and has a life span. Currently, the universe is expanding at an accelerating rate. If this trend were to continue, 22 billion years from now some models predict the Big Rip will occur. In this state, all matter down to sub-atomic particles will have been shredded apart, making life in our universe impossible. Unlike stellar evolution, the eventual outcome of the universe is not completely known. While we can observe other sun-like stars to see how they live and die, we do not have the ability to observe other universes to do the same. And in fact, we do not even know what 95% of our own universe is made of. Nonetheless, physicists, such as Michio Kaku, have floated proposals for life to escape to a parallel universe when ours becomes uninhabitable.
So, how should we plan for the future of humanity and where do we place our priorities? Lets take a look at a potential timeline of possible threats.
Climate change: 0-100 years
Nuclear proliferation: 0-100 years
Supervolcanoes: 0-30,000 years
Impact: 0-tens of millions of years
Sun: 3.5 billion years
Universe: Tens of billions of years
The most imminent threats are human made, rather than natural. It is technically feasible to defend against meteor/comet strikes while not the case with supervolcanoes. More than likely, that leaves us with a few thousand years to figure out how to establish a permanent human presence on Mars. Certainly, going to Mars is doable if the incentive is there to devote resources and funding. It will not be possible to defend Earth against the Sun’s stellar evolution. If interstellar travel is possible, and that’s a big if, we have on the order of a billion years to find a way to do that. Like a lot of space enthusiasts, I’d like to see that happen in the 23rd century just as in Star Trek. However, unlike exploring the Solar System, the distances involved with interstellar travel will require a far-reaching advancement of physics and engineering that is not guaranteed to happen. So what do we do now?
Hollywood blockbusters notwithstanding, the major priority should be getting a handle on human induced dangers such as climate change and nuclear proliferation. Concurrently, we can continue our efforts to begin human exploration of Mars. All this can take place in the next century but it must be stressed a human settlement on Mars is not a substitute for cleaning up our act on our home planet. During this time, we will begin to discover Earth-like planets, and possibly, life beyond our Solar System.
Efforts to begin interstellar exploration are in a very, very prototype stage. Relativity places a limit on velocity at the speed of light. Concepts to bypass this limit are speculative at best and are why, as mentioned earlier, will require a deeper understanding of physics to accomplish. This advancement will have to be on the order of what Issac Newton achieved in the late 1600’s and modern (both Einstein and the quantum physicists) physics in the 20th century. When and if this happens, we can plan for humanity’s migration to the stars to escape the Sun’s vaporization of Earth. If we are fortunate enough to accomplish that, we can owe it to the same spirit that carried the Polynesians in their wooden canoes across the vast expanse of the Pacific.
*To be clear on this point, the Mars One initiative is not realistic in its timeline or funding. However, other proposals, such as offered by Buzz Aldrin, may be more realistic.
**Image on top of post is the famous Earthrise photograph from Apollo 8, the first space mission to carry humans to another celestial object. Taken on Christmas Eve, 1968. Credit: NASA.