Beyond: Our Future in Space

by Chris Impey

  Milky white light again. We’re in the shuttle pod buffeting through the atmosphere to scout out a landing site. From Earth, our new home is a pale dot. Previous remote sensing had showed it’s a living world, its air charged by photosynthesis, but we arrive knowing little about a place where we will live and die. Our mission is a huge, expensive gamble, a step across the void, hoping to find safety on the other side.

  The six of us cast nervous, sidelong glances at each other. The pilot stares intently at the screen. Below us is tortured, vertiginous, and unfamiliar terrain—there’s no reassuring plain or prairie, nothing like a savanna, no endless vista.

  Finally, a glimpse of land through swirling clouds. Deceleration. A jolt. We don our suits and enter the air lock, as excited as children about to explore a secret garden.

  It’s difficult to describe the indescribable. We’ve landed in a verdant valley flanked by steep cliffs. Vines cling to every surface. Water drips from the cliff tops, which are partially obscured by thick clouds. There’s a dense mat of vegetation underfoot. We see many plants but no animals. Everything is strange and off-kilter: gravity is weaker than Earth so I have a spring in my step, but the air is thicker so I fight back a smothering sensation. We all wear scrubber masks to keep the air breathable and filter out microbes that may be hazardous. Instinctively, everyone stays close to the lander.

  Is this a swamp or Shangri-la? Either way, there’s no turning back.

  Working efficiently, we unpack the habitat. At the touch of a button, the memory film made of carbon nanotubes unfolds and inflates into a dome that soars twenty feet above our heads. After installing two air locks, we spend the rest of the day setting up a living space. Over the next week, the rest of the crew will join us on the surface, leaving our ark an empty, orbiting hulk, incapable of any more voyages.

  Overwhelmed, exhilarated, anxious. Emotions war inside me. It seems strange to want to be alone since, as a group, we are so utterly alone. But at a break in our construction, I wander away from the dome. The only way to walk through the convoluted landscape is to follow a small stream. Looking around, I notice something strange. There are no trees. I bend down to scoop up some pebbles and rocks. Their mineral forms are familiar and reassuring; at least geology is universal.

  Out of the corner of my eye, movement. I look more closely. What I thought was a mat of moss is actually a delicate web of tendrils that are moving and growing. They undulate and turn, like a carpet that’s weaving itself. It seems chaotic, but suddenly the tendrils form spirals and complex geometric shapes. Then, just as suddenly, the patterns disappear. I stare, transfixed.

  12

  Journey to the Stars

  _______________________

  Home Away from Home

  “Prediction is very difficult, especially about the future,” according to Danish physicist Niels Bohr.1 Prediction is a core part of the scientific method. At a grainy level, scientists predict the outcome of an experiment or a measurement. At a big-picture level, scientists learn about our world by extrapolating laws of nature or predicting how they will operate in unfamiliar situations.

  It’s easy to cherry-pick predictions that make the prognosticator look foolish in hindsight. A classic example is that of Thomas Watson, chairman of IBM, who said in 1943: “I think there is a world market for maybe five computers.” Here’s Ken Olsen, cofounder of Digital Equipment Corporation, in 1977: “There’s no reason for any individual to have a computer in his home.” There are many other such miscalculations in the world of information technology, such as the inventor of Ethernet saying the Internet would collapse and die in 1996, and the founder of YouTube saying in 2002 that his company would go nowhere because there just weren’t many videos to watch.2 For the record, in 2014 there were two billion PCs, two billion websites, and 40 billion hours of YouTube videos watched.

  Predictions on computers and information technology tend to underestimate the rate of progress, while those on space travel tend to overestimate it. In 1952, writer Henry Nicholas collected predictions for the year 2000, based on the “sober conclusions of our greatest scientists, including many of our most famous Nobel laureates.”3 They said interplanetary travel would be common, there would be multiple Moon bases, and city-size space stations would orbit the Earth. Less than ten years ago, Burt Rutan predicted that 100,000 space tourists would have flown by 2018, and we’re still stuck at seven. The reason for this dichotomy is that information technology has gained by exponential progress in miniaturizing the components that go into computers and routers and cell phones. Space travel, on the other hand, has to deal with large objects like people and stubborn laws of physics.

  It might be a fool’s errand, but here’s an educated guess about the arc of our near future beyond the Earth.

  In 2035, a vibrant commercial space industry is operating. As efficient, reusable orbital flight becomes routine, prices migrate from high-end tourism to adventures accessible to the middle class. There’s an unsavory underbelly to go with this new capability: reality TV shows in space, garish orbital advertising, and zero-gravity sex motels.

  In 2045, there are small but viable colonies on the Moon and Mars. They depend on resupply and crew rotations from the Earth, but they successfully pioneer techniques for extracting water and oxygen from the soil and living off-Earth with a small environmental footprint. Rich countries with geopolitical ambitions foot the bill.

  In 2065, mining technology advances enough to harvest resources from asteroids and mineral-rich locations on the Moon. A new business model evolves for off-Earth commerce. The United Nations and other international agencies scramble to stop the new frontier from turning into a “Wild West,” but claims are often settled by corporate militias.

  In 2115, a cohort comes of age who were born off-Earth and who have never been home. Colonists gain a high degree of self-governance and autonomy. Off-Earth GNP rivals the GNP of the rich nations on Earth. No economic or political imperative compels us to travel beyond the Solar System, but the visionaries are compelled to try.

  Where would we go? The difficulty of traveling in interstellar space will limit us to the closest habitable location. As we’ve seen, there’s evidence for an Earth-size planet orbiting the closest Sun-like star to the Earth, Alpha Centauri B. That exoplanet is much closer to its star than Mercury is to the Sun and has a surface temperature of 1200°C—so hot that its surface would be magma. Doppler data are not currently good enough to detect Earth-like planets farther out. However, Alpha Centauri B has a binary companion, and the orbit is wide enough that it wouldn’t disrupt the orbits of planets in the habitable zones, which would be 0.7 astronomical units (AU) from Alpha Centauri B and 1.3 AU from the more luminous Alpha Centauri A. The system offers a double shot at finding a habitable planet.4

  Simulations set expectations as we wait for better data. A 2008 study looked at how planets might form from the disk of rocky material around Alpha Centauri B. The orbits of several hundred protoplanetary rocks as large as the Moon were tracked for 200 million years (which takes only a few hours on a powerful computer). Although the number and type of exoplanets formed depended on the initial conditions in the disk, on average the simulations generated twenty rocky planets, ten of which were in the star’s habitable zone. Statistics should be similar for Alpha Centauri A (Figure 48).

  In 2013, Antonin Gonzalez advanced this research when he estimated an “Earth similarity index” for exoplanets in the simulations. This index gauges how Earth-like a planet is—based on surface temperature, escape velocity, size, and density. Zero represents a dissimilar planet, and one would be a planet identical to the Earth. For comparison, Venus has an Earth similarity index of 0.78 (similar in size but much hotter), while Mars has an Earth similarity index of 0.64 (smaller and much colder).5 The calculations assume hospitality for life as we know it. If exoplanets host biology with a radically different chemical or metabolic basis, we may not be able to recognize it, or even know how to define it.


  Figure 48. Results of astrophysical simulations of exoplanet formation in the Alpha Centauri system. Terrestrial planets form readily around either star, with masses and distances similar to the architecture of the inner Solar System (shown at the top for reference).

  Five of the simulated exoplanets were deemed capable of supporting photosynthetic biology. Their Earth similarity indices were 0.86, 0.87, 0.91, 0.92, and 0.93; two of them even had better conditions for life than the Earth.

  That sounds promising, but we can’t travel trillions of miles without being sure. Unless a planet has an oxygen-rich atmosphere, we’d be better off creating an artificial environment in space or even terraforming Mars. We’ve seen that transforming Mars to have a breathable atmosphere has been subject to feasibility studies by NASA. The technical challenges are manageable, but it would take an industrial-scale application of existing technology. The best guesses on time and cost are a millennium and a trillion dollars. The closer an exoplanet is to having Earth-like atmospheric composition, the easier the task of terraforming becomes.

  To astronomers, oxygen is the best “biomarker,” or tracer of life on another planet. If life on Earth is representative of biology elsewhere, low levels of oxygen indicate microbial photosynthesis and high levels are signatures of plant life. Putting it another way, if all life on Earth died overnight, the one in five oxygen molecules we breathe and depend on would disappear in a few thousand years as they reacted with rock and water. Substantial levels of atmospheric oxygen are difficult to sustain by geological processes alone. A related biomarker is ozone, which has a strong spectral signature although it’s far less abundant than oxygen. Another biomarker is methane, generated by fossil fuels and decaying vegetation. Methane has low concentration today, but between 3.5 and 2.5 billion years ago it was as abundant as oxygen is today, produced by microbes called methanogens. Water vapor is also a biomarker, since we assume that life can’t exist without water.6

  In practice, astronomers will need to detect a suite of biomarkers, and compare the spectra to models of planetary chemistry and geology, before being confident they’ve seen biology.7 The detection will most likely come from dispersing the feeble reflected light of an Earth-like exoplanet into a spectrum. If the light shows absorption by some combination of ozone, oxygen, methane, and water vapor, the result will make immediate headlines as the first detection of life beyond Earth. Since it will be a form of microbial life and there will be no pictures of it, public interest is likely to wane. But for science it will be a momentous discovery. Until we find the second example of life, it’s always possible to argue that life on Earth is a unique accident.

  The challenging observations required to detect biomarkers have only been tested on Jupiter-mass exoplanets, where no life is anticipated. In proof-of-concept measurements of six exoplanets with the Hubble Space Telescope, vapor of sodium, methane, carbon dioxide, carbon monoxide, and water has been seen.8 Making this observation on Earth-mass exoplanets will require a new telescope in space or innovative image-sharpening methods using ground-based telescopes. Most researchers expect the critical observation to be made within the next decade. Based on the frequency of Earths found by Kepler, the nearest clone is likely to be a dozen light years away. If we get lucky, it will be even closer.

  _________

  If we revisit the smaller-scale journey that opened this book, we see that the migration from Africa to Chile was as grandiose as reaching for the stars. The cradle of humanity was in northeastern Africa. A canny hunter-gatherer might roam 10 miles to find food. But humans migrated a distance a thousand times larger, with no certainty of food or shelter—the same ratio as the distance to nearby stars compared to the size of the Solar System.

  After a thousand generations, we had traveled to the dense forests and vertiginous valleys of Southeast Asia. After two thousand more generations, we had roamed into the barren tundra of Siberia and across the land bridge to the Pacific Northwest. After another few hundred generations, we reached the lush rainforest and azure waters of the Central American isthmus. It took a scant hundred generations more to reach the southern tip of Earth’s land mass. To anyone who had a cultural memory of the African savanna, the sight of the wild, windswept shore of Patagonia, with the stars in the night sky wheeling in the opposite direction, would have seemed as alien as an exoplanet.

  Building a Better Engine

  To see that interstellar travel is a stretch goal for space exploration, consider this scale model.

  Shrink the Earth to the size of a Ping-Pong ball and the Moon would be a marble a yard away. If you hold the Earth in front of your nose and the Moon at arm’s length, that’s the full extent of human venturing. In this version of the Solar System, shrunk by a factor of a hundred million, the Sun would be a glowing gas ball eight feet in diameter a hundred yards away and Neptune would be the size of a beach ball four miles away. On this scale, the nearest star to the Sun is 30,000 miles from the little Ping-Pong ball, or more than the Earth’s circumference. To get to the stars in a reasonable time, we need enormous speeds. It would take 50 million years to get to the Alpha Centauri system at the highway speed limit. At the speed Apollo used to get to the Moon, it would take 900,000 years, and even at the speed of the Voyager spacecraft (which left the Solar System traveling at 37,000 mph), it would take 80,000 years.

  Chemical energy is just too inefficient to get us to the stars. We have to go beyond rearranging electrons among atoms and unlock the power of the atomic nucleus.

  Let’s revisit the energy available from different fuels. The usual units are millions of Joules per kilogram (MJ/kg). For reference, a million Joules is the energy released by a kilogram of TNT exploding, or the energy expended by running for an hour, or the energy stored in a candy bar. In these units, wood and coal store about 20 MJ/kg, gas and other hydrocarbon fuels store about 40 MJ/kg, and hydrogen has the best energy storage, at 142 MJ/kg. NASA scientists at the Glenn Research Center in Cleveland have worked out how much fuel would be required to get a Space Shuttle payload (think of a fully laden school bus) to Alpha Centauri in 900 years.9 The answer is discouraging: All the mass in the universe in the form of rocket fuel couldn’t do it!

  Figure 49. The energy storage in three different chemical fuels (food, coal, and gasoline) compared with mass-energy release in the fusion process. Matter–antimatter annihilation would be a thousand times more efficient than either fission or fusion.

  If we look beyond chemical energy, the best source is mass itself. The implication of Einstein’s iconic equation E = mc2 is that mass is frozen energy. Because the speed of light is a large number, a tiny amount of mass converts into a huge amount of energy. Mass can be liberated into energy in nuclear reactions with an efficiency of 0.1 percent for fission, 1 percent for fusion, and 100 percent for matter–antimatter annihilation (Figure 49). That represents energy storage of 108 MJ/kg for fission, 109 MJ/kg for fusion, and 1011 MJ/kg for matter–antimatter annihilation. Surely an efficiency millions of times better than chemical fuels can get us to the stars?

  Yes, but not without the necessary technology development. A rocket needs thrust (or force) so it can exert a big push, but it also needs a high specific impulse, which is the force delivered per kilogram of fuel per second, analogous to fuel efficiency. Chemical rockets have high thrust but lousy specific impulse. The ideal interstellar rocket must score well for both quantities. Remember the rocket equation? It says that the final speed of a rocket depends on the fuel exhaust speed and the ratio of the fuel mass to payload mass. Having a nuclear fuel means less mass is needed, but that ratio is inside a logarithm, which suppresses its influence on the final speed, so it’s just as important to increase the exhaust speed.

  None of the rocket engines about to be described have ever been built. They all rely on bleeding edge technology, though they are comfortably within the realm of known physics. Let’s look at the potential performance of rockets that don’t depend on chemical energy.

  F
or nuclear fission, the simplest concept is to put a reactor on top of a rocket nozzle. Conventional fission and fusion concepts—recall that fusion hasn’t yet been used to generate energy on Earth—have ten to twenty times better performance than chemical rockets. This is a key limitation, since the practical gain in a rocket ends up far less than the theoretical gain based on energy density.

  Fusion would still require 1011 kilograms of fuel, equal to a thousand supertankers, to get to Alpha Centauri in less than a thousand years. In the 1960s, Project Orion was developed by Stanislaw Ulam, a brilliant mathematician who had worked on the Manhattan Project. The idea was to use a series of controlled nuclear explosions to propel the spacecraft forward. In the 1970s, the British Interplanetary Society amended the design to use a large number of microfusion explosions (Figure 50).10

  With matter–antimatter engines, we enter the realm of speculation, since antimatter, the quantum shadow partner of matter, has only been produced and contained in tiny quantities. An antimatter engine has a performance gain of a factor of a hundred, and the fuel requirement drops to about 100,000 kilograms or ten railway tankers of propellant to get there in less than a millennium. These numbers double because the spacecraft will need fuel to decelerate when it reaches its destination. Gathering this much antimatter will be impossible for the foreseeable future. At the moment, it would cost $100 billon just to create one milligram of antimatter.11 For those wanting to try this at home—the calculation, not actually building an interstellar rocket—the RAND Corporation used to sell the nifty (and very retro) Rocket Performance Calculator, dating from 1958.12 This circular slide rule incorporates the rocket equation and it can still be found occasionally on eBay, making for a great conversation piece.