Beyond: Our Future in Space (29 page)

12

Journey to the Stars

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Home Away from Home

“Prediction is very difficult, especially about the future,” according to Danish physicist Niels Bohr.
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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.
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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.”
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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.
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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).
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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.

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.
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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.
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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.
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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.

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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.
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The answer is discouraging: All the mass in the universe in the form of rocket fuel couldn’t do it!