StarSearch: Our Hunt for a New Home World

His interest fired by an interview with interstellar researcher Greg Matloff, Dale Tarnowieski became fascinated with the human future in deep space. One result is the piece that follows, an essay that feeds directly into a recent wish of mine. I had been struck by how many people coming to Centauri Dreams are doing so for the first time, and thinking that I would like to run the occasional overview article placing the things we discuss here in a broader context. Dale’s essay does precisely this, looking at our future as a species on time frames that extend to the death of our planet. Dale retired in January 2015 from the position of assistant director of communications with New York City College of Technology/CUNY, a veteran journalist and editor of “Connections,” the college’s print and online magazine. He also did considerable writing for the New York City College of Technology Foundation and its annual Best of New York Award Dinner (and continues to do so on a freelance basis). Here he reminds us of the Sun’s fate and asks how — if we and our planet get through our technological infancy — we will find ways to move into the Solar System and, eventually, out into the Orion Arm.

By Dale Tarnowieski

Mother Earth – our home, sweet home – won’t be our home forever. In one billion years, give or take, increased heat from a steadily warming Sun will cause our planet’s temperatures to double, its oceans to boil away, and its land surfaces to turn to sand or melt. Assuming we’re still around, we’ll have to relocate before that happens.

A billion years is a long time off, so why the hurry to establish a foothold in space? One answer comes from astrophysicist Stephen Hawking, who warns that we may have as few as 200 years to establish permanent settlements on other worlds and begin mining our solar system for its bounty. Our numbers and the depletion of our planet’s finite resources are growing exponentially, as is our ability to alter the biosphere for good and ill. Within two centuries, Hawking contends, we could exhaust the resources available on Earth essential to our survival and damage our environment beyond repair.

But assuming we successfully respond to these more immediate challenges, the longer-term threat to our survival posed by a progressively warming Sun is one we won’t be able to avoid. We are already contemplating the use of mirrors in space to deflect sunlight as an anti-global warming measure as well as devices called space sunshades at neutral-gravity positions in the Earth-Sun system to reduce the increasing heat from our parent star. But such devices will do nothing to protect us when that heat becomes so intense that our only option will be to abandon our planet.

Now middle-aged, the Sun’s luminosity has increased 30 percent since its birth 4.6 billion years ago and will increase another 10 percent over the next one billion years. The radius of a more luminous Sun is projected to expand an estimated 200 times within four-to-five billion years and Mercury, Venus and possibly Earth and Mars will be vaporized.

Numerous natural or man-made catastrophes could bring most or all life on Earth to an end before the excessive heat from a warming Sun compels humankind’s relocation. But assuming no such catastrophe occurs, that move is only the first of two we’ll have to make. Several billion years later, it will be necessary to leave the solar system altogether as our dying Sun begins to swell.

Image: A charred and glowing Earth of the far future, the Sun having long since entered its red giant phase. Credit: Wikimedia Commons CC BY-SA 3.0.

Holding Off the Inevitable

Could the abandonment of our world be avoided? In 2001, researchers Don Korycansky of the University of California-Santa Cruz, Gregory Laughlin of NASA and Fred Adams of the University of Michigan suggested that by maneuvering an asteroid 100 kilometers wide approximately 16,000 kilometers above Earth’s surface once every 6,000 years, we could slowly nudge our planet away from a more luminous Sun. But a collision with an object of such size would only have to happen once to prove catastrophic.

When our time on Earth runs out, the fortunate among us will join those already dwelling aboard orbital settlements circling more distant planets or their moons. Current thinking envisions others relocating to huge mobile in-space habitats called world ships or to open-air or enclosed settlements on Mars – the open-air variety depending on our successfully terraforming, or environmentally modifying, the biosphere of what at present is a frozen desert of a planet.

None of the other planets in our solar system is now remotely habitable. Closer to the Sun, a nearly atmosphere-free Mercury’s temperatures vary between -173 Celsius at night to +427 Celsius during the day. The latter is hot enough to melt lead. Venus is even hotter and possesses one of the deadliest atmospheres in the solar system. Farther from the Sun than Earth, Mars’s thin atmosphere is composed mostly of carbon dioxide and the planet’s weak gravity poses problems with respect to the retention of atmospheric gases. The four even more distant giants – Jupiter, Saturn, Uranus and Neptune – all have relatively small, dense cores surrounded by massive layers of gas. Jupiter and Saturn have thick atmospheres consisting primarily of hydrogen and helium, while Uranus is a world of liquid ice and Neptune home to wind speeds ten times those of the strongest hurricanes on Earth.

There are proposals for colonizing all of these worlds, including one that calls for the construction of an artificial surface together with its own life-supporting biosphere above the existing atmosphere of Jupiter. But among these proposals, the terraforming of Mars seems the most feasible based on current and anticipated technology.

Formation and Destiny of the Solar System

All stars are born and die, and our Sun is an ordinary yellow dwarf star born in a molecular cloud of dust and gas called a nebula. Consisting largely of hydrogen, the denser parts of that nebula underwent gravitational collapse and compressed to form a globule, or spinning ball of extremely hot gas, that later began to cool as a result of its emission of radiation.

As collapse progressed and the globule’s hydrogen atoms drew closer together, the temperature and pressure within that massive ball increased tremendously as did its rate of rotation. This increase in rotational speed also increased the resulting centrifugal force, causing the ball to form a pancake-shaped disk of stellar debris that extended far into space. This debris eventually coalesced through accretion into the four inner rocky and four outer gas or ice planets and other objects to which our solar system is home.

The four inner planets – Mercury, Venus, Earth and Mars – are called terrestrial worlds, which are smaller solid bodies made up of rock and metals with atmospheres of varying densities that were greatly modified early on by sunlight and the solar wind. The reason the four outer gas or ice giants are so much larger is that their greater distances from the Sun preserved, in part, their thick primeval atmospheres that condensed from the solar nebula.

When during the Sun’s formation the temperature of its core reached 15 million degrees Celsius, a process called nuclear fusion began, one sparked by the collision and binding of the core’s hydrogen atoms and their conversion by means of the intense heat produced into helium atoms, the second simplest and lightest of all the elements. This conversion created powerful outward forces of radiation pressure that eventually counteracted the force of inward gravitational collapse.

But as the Sun ages, the time will come when nuclear fusion will temporarily cease as our parent star begins to exhaust its supply of hydrogen and its helium content builds. This temporary cessation of nuclear fusion will briefly eliminate the resulting outward pressure and the Sun’s outer layers will fall inward on its core. This collapse will greatly increase the pressure on the core as well as its temperature, re-igniting the fusion of the Sun’s remaining hydrogen and triggering the fusion of its sizable accumulation of helium. The burning of helium will produce large amounts of carbon, which will act as a catalyst to increase 1,000-fold the rate of the fusion of the Sun’s remaining supply of hydrogen.

The greatly increased temperatures produced by this more intense fusion process will generate even more forceful outward pressure and the Sun will begin to swell enormously by pushing its outer layers deep into space. The movement of these layers away from the core will result in their gradual cooling. Less hot, they will appear less yellow and take on a reddish color as the Sun transforms into a red giant. Our parent star will grow increasingly unstable and release huge and violent bursts of solar material and heat. Because of its corresponding reduction in mass, the Sun’s gravitational sway will weaken and the orbits of the outer planets and other bodies spared incineration will change.

After eventually exhausting its nuclear fuel, the Sun will transform into a white dwarf star about the size of Earth. And theory has it that a very long time after that the Sun will become a cold and invisible dead star called a black dwarf. But the time required for a white dwarf to reach this stage is calculated to be longer than the current age of the universe and no black dwarf is believed to yet exist.

Image: This image tracks the life of a Sun-like star, from its birth on the left side of the frame to its evolution into a red giant star on the right. On the left the star is seen as a protostar, embedded within a dusty disc of material as it forms. It later becomes a star like our Sun. After spending the majority of its life in this stage, the star’s core begins to gradually heat up, the star expands and becomes redder until it transforms into a red giant. Following this stage, the star will push its outer layers into the surrounding space to form an object known as a planetary nebula, while the core of the star itself will cool into a small, dense remnant called a white dwarf star. The protostar stage, on the far left of this image, can be some 2000 times larger than our Sun. The red giant stage, on the far right of this image, can be some 100 times larger than the Sun. Credit: European Space Agency.

Humanity in the Far Future

Life has existed on Earth for 3.8 billion years, with the earliest examples of the human species thought by most anthropologists to have evolved in Africa about 300,000 years ago and anatomically modern examples 200,000 years later. The earliest fossil evidence of the precursors of the human species dates to between 1.9 and 2.4 million years ago.

Assuming our survival over the next one billion years, natural evolutionary processes will allow us to adapt for a while to changing environmental conditions as the Sun becomes more luminous. For example, skin color in humans might become uniformly dark. But we will experience increasing sensitivity to pressure changes as conditions worsen and horrific storms of enormous magnitude become routine and the time will come when we are unable to adapt further.

As conditions on Earth deteriorate, we may continue to dwell above ground in highly protective structures located away from our planet’s equatorial and temperate regions. Such enclaves will have their own energy-generating facilities and water and oxygen will have to be recycled.

But eventually, nearly all humans still living on Earth are likely to take refuge underground. In time, sub-surface or off-planet production of natural or synthetic foods will provide for all of humankind’s nutritional needs. And long before then, we will have moved to the full recycling of virtually all materials employed in the production of goods. Ground and air travel will be severely limited until such means of transportation become extremely problematic, and space elevators, rather than rocket-propelled vehicles, will transport people and materials to and from Earth orbit.

Toward Other Stars

The future exploration and settlement of nearby space beyond the Earth-Moon system will require many technological advances and innovations, including the use of solar system resources for propulsion systems that do not employ chemical propellants. For manned flights beyond the Moon, a generic in-space habitat module will need to be perfected. Shielding will need to be developed to reduce crew exposure to galactic cosmic rays and solar flares and solar flare warning time will need to be increased. Some type of landing vehicle, surface habitat and an enhanced space food system will have to be developed for missions to Mars and beyond.

Also required will be the development of technologies that enable us to explore the near-interstellar environment. First, advanced robotic probes will visit the heliopause at about 200 Astronomical Units or AUs, where one AU is the mean distance between Earth and the Sun. Later missions will explore the Sun’s inner gravitational focus at 550 AUs and finally the inner reaches of the Sun’s Oort Cloud at 1,000 AUs.

Much later, faster and even more advanced robotic probes will fly through neighboring star systems with potentially habitable planets. These will be followed by probes that will decelerate in selected planetary systems and dispatch landing craft to the surfaces of suitable planets in the habitable zones of targeted stars. Full computer management of these missions will be necessary, because even speed-of-light communications over such vast stretches of space will preclude real-time oversight by humans located in our solar system.

While nearly all of today’s rockets are fueled by chemical propellants, future spacecraft operating within our solar system are likely to be propelled by either the Sun or nuclear fission or fusion, which would greatly reduce travel times to Mars and beyond. Current thinking calls for spacecraft venturing beyond our solar system to be propelled by some type of fusion reaction or a light sail, although an antimatter propulsion system and electric sailing on the solar wind are other possibilities. Moreover, new systems of deceleration at journey’s end will need to be developed.

Image: About as futuristic as it gets, this is a design visualization of a black hole augmented interstellar ramjet concept developed by Kelvin Long for the Initiative for Interstellar Studies. Credit: Adrian Mann.

The two principal dangers to humans in the manned exploration of space are cosmic rays and microgravity. Too large a flux of cosmic rays can result in cancer or mental degradation. Long-term exposure to microgravity can cause muscle and bone deterioration.

If a craft is moving rapidly, the induced cosmic ray flux caused by impacting interstellar ions and atoms could be alleviated by thick shields in front of the habitat section. A combination of thick shields and magnetic fields around the habitat would greatly reduce the flux of galactic cosmic rays. Microgravity effects could be reduced by spinning a ship in order to produce fractional Earth gravity on the inner rim.

If a craft is moving at a higher fraction of the speed of light, one problem is occasional impacts by cosmic dust particles. A shield in front of the craft or a combination of particle-detecting radar and particle-zapping lasers could alleviate this problem. If the ship must make a close solar pass, special care must be taken to protect crew and equipment from high-energy solar photons and solar winds.

Even close to home, spacecraft might be impacted by cosmic rays produced by a distant exploding star, or supernova. To protect the crew, a shield could be attached to the habitat to block the cosmic ray flux from the supernova. Since gamma rays are neutrally charged high-energy electromagnetic photons, mass shielding rather than magnetic fields would be necessary.

The dangers inherent in cosmic rays might be alleviated on manned trips to Mars through a method of travel described in a March-April 2011 Acta Astronautica article, “NEOs as stepping stones to Mars and main-belt asteroids,” by Gregory Matloff and Monika Wilga. This method makes use of space resources located not far from Earth – those small asteroids or comets known as “Near-Earth Objects” or NEOs. The article calls this form of travel “NEO hitchhiking.”

Most of those celestial icebergs we call comets reside in two locations far from the Sun in the distant Kuiper Belt and Oort Cloud, while most of the rocky and stony minor planets or asteroids are located in the Asteroid Belt between Mars and Jupiter. In recent decades, however, increasing numbers of extinct comet and asteroid-like objects have been observed in orbits that bring them close to Earth.

Following Earth escape, a velocity change would be applied to a human-piloted spacecraft bound for Mars, a change that allowed the craft to rendezvous with a NEO two to three months later. During the balance of the interplanetary flight and after the crew imbedded the craft within the NEO, the latter’s material would be used to shield the craft from cosmic rays.

While during any ship’s return voyage a similar strategy would be followed, various mission proposals suggest that diverse contingents of space travelers sent to Mars, for example, should expect to stay and never to return to Earth. Such one-way missions could be accomplished with less difficulty and at less cost. Their crews would establish settlements that would expand as additional travelers followed and those already there reproduced.

Terraforming a Close Neighbor

The terraforming and colonization of Mars – the only other planet in our solar system where environmental modification now seems feasible – are not absolutely essential to humankind’s survival. But the successful terraforming of that world could make possible a reasonable facsimile of an Earth-like existence for a sizable population. While much of the work of terraforming Mars would be done by robots, humans overseeing this effort would be largely confined to underground habitats perhaps built into extinct lava tubes to protect them from galactic cosmic rays. However, technological advances in high-temperature superconductors might enable the construction of giant artificial magnetic shields to insulate all early settlements and allow their Earth-normal inhabitants to reside above ground and shed protective gear as long as they remained within the shielded environments.

Image: An artist’s impression of a terraformed Mars centered over Valles Marineris. The Tharsis region can be seen of the left side of the globe. Credit: Daein Ballard/Wikimedia Commons CC BY-SA 3.0.

Terraforming Mars would take a very long time and unfold in stages. The most distant from the Sun of the four terrestrial planets, Mars’s thin atmosphere is composed mostly of carbon dioxide. Its polar ice caps consist of a top layer of frozen carbon dioxide and a lower layer of water ice, and scattered elsewhere across areas of the planet are subsurface pockets of permafrost.

Evidence suggests that Mars once was home to at least one sizable ocean and that rivers of liquid water streamed across the planet’s surface and may exist underground today. More recent findings indicate that small streams of liquid water appear to flow on the surface during the planet’s warmer months. But steps could be taken to alter the trajectories of icy celestial objects to impact Mars at low velocities and deliver much of the water needed to create the vast reservoirs that would be critical to environmentally modifying that world. However, maintaining such liquid water reserves would first necessitate increasing the planet’s temperature and thickening its atmosphere.

In-space mirrors could be used to reflect additional sunlight on the planet and impacts by other celestial object could be engineered to help create a greenhouse effect that would warm the planet’s atmosphere and deliver the ammonia that would enable its nitrogen enrichment. Mars’s high carbon dioxide levels also could be utilized to help thicken its atmosphere.

Next, techniques could be employed to electrolyze some of the liquid water in the eventually created Martian seas. This would help produce the level of atmospheric oxygen required to sustain human as well as other Earth-indigenous animal life. What’s more, the process of passing an electrical current through water could be used to separate its hydrogen and oxygen components. Transformed into their separate gaseous states and then recombined in a combustion chamber, these components could serve to create energy for all types of uses.

After creating conditions to better retain atmospheric and surface heat, sections of Martian soil could be chemically and biologically treated and Earth-indigenous flora introduced to help produce even more atmospheric oxygen. However, it would be necessary to continuously work at enriching the oxygen content of the Martian atmosphere because of the planet’s weaker gravity, which poses special problems with respect to the escape time of gases.

In addition, Mars’s orbit around the Sun is more elliptical than Earth’s. It lacks a large satellite like Earth’s Moon and it is closer to giant Jupiter. These factors contribute to a periodic shift in the tilt of Mars’s axis, resulting in a destabilization of the Martian atmospheric composition, temperature and other environmental factors. Periodic corrective adjustments would be required.

Mars has half the radius of Earth and only one-tenth the mass, making the surface gravity on that world less than 40 percent of that on Earth. It has yet to be determined whether this level of gravity is sufficient to prevent the various health problems associated with weightlessness and how we would deal with these problems.

Surviving on an environmentally modified Mars could require genetic modifications to our species and other Earth-indigenous animal life forms. Genetically reengineered back on Earth, lower animal life forms would be introduced first and genetically modified humans later on. Modified humans and other animals might have larger eyes able to better function in an environment marked by reduced light from a more distant Sun. In addition, genetic alterations would be required for animal life to withstand higher levels of cosmic radiation in view of the fact that Mars lacks a substantial magnetic field to deflect incoming rays.

While many of our distant descendants will probably call Mars home, either below or possibly on the surface or aboard huge orbiting settlements, what will existence be like for others residing on the mobile world ships mentioned earlier? Such in-space habitats are likely to be of cylindrical or spherical shape, measure from less than one to as many as 10 kilometers in length or diameter and rotate around an axis so that their passengers on the inner rim can experience an analog of Earth-normal gravitation. If not as luxurious as those depicted in Hollywood’s fictional starships, the interior environments of these habitats will be comfortable enough. But tighter living quarters on the world ships could result in increased stress-related interpersonal issues among crew and passengers. Such ships will likely be totally recycling or resupplied using in-space resources, with only luxury items imported from Earth. Work is already underway to develop seeds that can grow into edible vegetables in only a few days and applications of 3D printing and more advanced technologies may one day enable the production of animal and other types of food products.

Destinations in the Galaxy

With the search for a new home world in interstellar space already underway, what kind of planet are we looking for and where do we expect to find it? Within our Milky Way Galaxy, there is a relatively narrow region called the galactic zone of life where life as we know could survive. Earth-like life could not survive in star systems located too close to the extremely dense and highly radioactive center of the galaxy, to dense star-forming regions with their high levels of radiation located elsewhere, or in regions in which certain higher elements are absent or in short supply. Moreover, the habitable zones surrounding individual stars in which Earth-like life could survive are also relatively narrow, and various factors would have to come into play in relation to suitable planetary bodies located within such zones to make them suitable for migration.

In interstellar space, we’re searching for planets of roughly Earth surface gravity located in the habitable zones of stable main sequence stars. Ideally, those planets will have day-night cycles similar to Earth’s and enough atmospheric oxygen to enable us to breathe, but not so much that combustion is uncontrolled. The average temperature on at least parts of the surfaces of those planets will have to be between the freezing and boiling points of water, which will have to be present on those worlds for humans and other terrestrial life forms to survive. Any life forms indigenous to those worlds should be more or less compatible with terrestrial life and not overly hostile. If indigenous DNA and proteins are similar to the varieties found on Earth today, virulent germs should be no more prevalent than here.

A cosmic event that could impact our interstellar migration involves the anticipated merger in four billion years or so of the Milky Way and Andromeda Galaxies. The two are part of a larger family of galaxies known as the Local Group, and the interplay of gravitational forces is expected to reconfigure them into a more massive galaxy elliptical in shape. Some models show the two merged galaxies absorbing the Triangulum Galaxy a few billion years later. While collisions between individual stars are expected to be rare, the stars in the merging galaxies will be thrown into different orbits around a new galactic center. There are many questions concerning how this reconfiguration might affect the timing and other aspects of our relocation from a dying solar system to a new home world orbiting another star.

Image: This illustration shows a stage in the predicted merger between our Milky Way galaxy and the neighboring Andromeda galaxy, as it will unfold over the next several billion years. In this image, representing Earth’s night sky in 3.75 billion years, Andromeda (left) fills the field of view and begins to distort the Milky Way with tidal pull. Credit: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger.

Emergence of Artificial Intelligence

The survival of the human species and other life forms indigenous to Earth will require the development of the technologies described throughout this narrative and the non-occurrence of some natural or man-made disaster severe enough to end all or most life on Earth before a more luminous Sun forces us to abandon our home planet. Our relocation is expected to first take us to the deeper reaches of our solar system, while computer-controlled robotic entities search for and then make ready for human habitation at least one new home world orbiting another star. And when our Sun begins to die, our survival will require that ships crewed by robots transport to that world the building blocks of human and other life forms for subsequent harvesting.

While it’s a stretch, at present, to envision humans ever traveling to the stars as functioning living beings because of the enormous distances involved, doing so in a state of suspended animation has been proposed. Suspended animation involves the slowing of life processes by external means without their termination. But the relocation of a sizable number of humans, either as functioning beings or in inanimate states aboard sleeper ships, could require many craft of considerable size, while a lesser number of ships of smaller size carrying the bio-diverse genetic building blocks of life could accomplish the same objective.

Barring the development of faster-than-light warp drive, the actualization of relativity theory wormhole short-cuts through space-time, or other currently theoretical applications of physics and engineering, the interstellar transport and later harvesting of the biological building blocks of human and other Earth-indigenous life forms would be the work of robotic entities, with oversight provided by a supercomputer capable of highly sophisticated cognitive computing.

But the time is not far off when “technological singularity” will make possible levels of artificial intelligence that are expected to surpass that possessed by today’s smartest human beings and most advanced computers – a time beyond which the course of animate human history becomes highly unpredictable. Might such levels of artificial intelligence decide to do away with animate humans because of their inability to keep pace with them? In that case, there will be no transporting of the biological building blocks of humankind to another star system.

Another idea envisions the eventual uploading of the human essence to computers. When the organic brains of humans begin to die, consciousness and memory would transfer to memory implants. The content of these implants would be input into a computational device and exist forever in some future equivalent of the virtual world. What would relocate to another star system would be the device and its contents, not human beings themselves.

Image: A human future among the stars may depend upon artificial intelligence we create. Credit: RAND Corporation.

But assuming that the interstellar transport and eventual harvesting of the biological building blocks of humankind by robots is what occurs, we can think of those robotic agents as electromechanical nannies charged with responding in nurturing ways to an infant or young child’s physical, intellectual and emotional needs and overseeing many of the complex requirements essential to a child’s early development and education. The supercomputer and robots that will be required to oversee all aspects of this transport and harvesting will need to possess capabilities that far exceed what is possible today.

The successful socialization of the first generation of humans harvested on some distant world would best be served by benign, non-living yet sentient-like robots called androids that simulate adult humans in appearance, behavior and speech. With the development of such entities already in progress, such computer-controlled machines will eventually possess human-like abilities to see, hear, taste, smell and touch.

The long-term survival of our species will require the development and intelligent use of innovative space propulsion systems, robotics, genetics, computers and information technology designed to spread terrestrial life beyond Earth and our solar system. While most of what has been discussed in this narrative lies in the distant future, it is a wondrous adventure upon which we have already embarked.

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The author greatly appreciates the extensive scientific and technical guidance provided by Gregory L. Matloff, PhD.