/* Getting to Know Our Neighbours at the Speed of Gaia */
Size
37435 bytes
Delta
—
''This is Chapter 7 of the [[SolSeed book]].''
===The Misinformation of Science Fiction ===
Science fiction has taught us that planets are where everything happens, and that space is small and fast. Captain Kirk beamed down to a planet almost every week. The formats of television shows, movies and even novels force directors and authors to make space seem small and fast or come up with 'magic' reasons why space might seem smaller than it is. Kirk and Spock beam down to Sigma Iotia II and all the action takes place in a single city that looks like Chicago in the 1930s. Luke Skywalker crash lands at a seemingly random point on Degoba and it turns out that he is just a few hundred meters from Yoda. Of course, they were drawn together by the 'force'. Without it Luke might have spent his whole life looking for Yoda without success; Degoba was an entire planet covered in swamp. The imaginations of most science fiction writers seems to be limited. Instead of describing whole new planets, they describe planets as expanded versions of a tiny part of Earth; swamp planets, tundra planets, city planets, desert planets. But every planet we go to will be a whole world of its own, with a whole selection of Earth-like climates, and ecosystems we have never imagined.
Planets, especially Earth-like planets orbiting Sol-like suns are a vanishingly small fraction of what is out there. Scientists estimate that on average each star in the galaxy will have 1.6 planets but only maybe 2 in one thousand are likely to be Earth-like. On the other hand there are likely a trillion worlds orbiting each star in the galaxy. So for every Earth-like world in the galaxy there are likely a thousand trillion non-Earth-like worlds. I use the term world because the vast majority of these will not be planets but what astronomers call minor planets, asteroids and comet-nuclei. In addition there are vast clouds of dust and gas, nebula, so huge that light takes hundreds of years to cross them. A lot may be happening on these worlds and in these clouds. Just because we, and the other members of our biosphere, are not well adapted to living in these places does not mean that there is no action or never will be action in these places.
So space is an much more alien and varied and rich place than science fiction tells us. But it is also a very slow place. Space is so vast that light takes centuries, millennia and even eons to cross it. Science fiction has invented many 'technologies' in order to get their characters from one planet to another quickly so that a single human drama can be painted across the many stars. These technologies are effectively just magic to us here in the 21st century. We have no technology and no idea how to build technology that folds space or digs worm holes through it. Hyperspace might exist but so may pixies. There is no evidence for its existence. We cannot make plans around its existence. If we are going to travel to other stars using technologies we at least have some conception of how to create then the trip will take a long time. The nearest star is 4.4 light years away. At one tenth the speed of light it would take 44 years to travel there. To travel for so long, you would want a large ship to travel in; you would need to be able to raise children on board because those who started the voyage would be senior citizens by the time they arrived. You would need to carry enough people to establish a colony; it would hardly be worth making an 88 year round trip unless you were establishing a permanent presence. You would need food, supplies, equipment and supplies to maintain the space craft, equipment and materials to establish your colony. Lets imagine that somehow you managed to make do with 500 000 tons. The energy needed to accelerate that mass to one tenth the speed of light would be about equivalent to the total energy consumption of every human being on Earth for a year. And then you would need that much energy again to stop. If you carried that energy with you in the form of fuel then you would need even more energy in order to carry the fuel needed to slow down at the end. The project will be a massive one far beyond our reach for many decades. And even then it will be a slow one by human standards.
Sometimes science fiction uses Einstein's relativity to get around the problem of the time it takes to travel between stars. Star ships accelerate to close to the speed of light; time dilation means that the crew experience barely any time while the ship crosses the vast distances between stars. For instance, if we go 99% of the speed of light then we will experience only one seventh as much time as the spacecraft takes to travel. To go to the nearest star we will take only about 4.4 years but the crew would only experience about 7 months. The problem is that it will also take 70 times as much energy to accomplish the trip. Science fiction also usually uses some kind of magic 'technology' called inertial dampers to allow the spacecraft to reach these velocities in a reasonable amount of time without the crew being crushed by g-forces. We don't have that kind of technology. To reach such speeds in reasonable comfort would take a year with another year needed to reduce speed at the other end. In effect despite spending 70 times the energy we might only reduce the apparent duration of the trip for the crew from 44 years to about three. Still a very long time to live in a small ship; for 3 years one might find even a 100 000 ton ship rather small shared with 500 other colonists. For three years you would be dependant on complex systems, the failure of which could leave you stranded in space light years from the resources needed for your colonists to live. Another lie that is often told in science fiction is that if your engines fail in space, your spacecraft will grind to a halt and stop. But objects in space, due to a lack of air resistance, will continue coasting forever. Traveling at 99% the speed of light, the crew would be trapped, passing stars every few years but unable to slow down or get off.
===The Speed of Gaia ===
The more realistic and safer option is to travel slowly, living off the land as we go. To travel slowly in interstellar terms might mean measuring your speed in tens of kilometers per second but your travel times will be measured in tens of thousands of years. To 'live off the land' for such a long time requires that we bring a world with us. Earth is the most reliable space craft we have ever travelled on. We would be well advised to try to replicate its robust nature for our trips between stars. Again, contrary to all of our instincts and direct experience, objects in space will continue coasting forever once given a solid push, providing the push imparts enough speed to escape the local gravity. It doesn't matter if the object is a mote of dust or a galaxy or anything in between. To move a small world between stars might seem like an impossible task but in actual fact it should be quite doable with only minor improvements in reliability of technologies we already possess and dogged determination over generations. Two technologies are necessary. One is gravitational tractors and the other is gravitational sling shots.
====Gravitational Tractors====
Gravitational tractors are space craft that use their engines over long time spans to maintain their positions. This is harder than it sounds but still doable. Again the fact that once given a push objects in outer space continue to move and the fact that the gravity of Sol and all the other objects in the Solar system are continuously pulling at spacecraft make it harder to stay still in space than to move. However, in the vicinity of a small world in the outer solar system gravitational pulls are quite small. Ion drives would be quite capable of station keeping a space craft there. The key would be to use reflectors to concentrate distant Sol's light onto solar collectors in order to provide the power necessary. Then the minute but ever present gravitational pull of the spacecraft on the world would slowly change the orbit of the world in which ever direction we desired. Could a gravitational tractor pull a world right out of the solar system? Probably, if we were patient enough but that much patience isn't necessary. Instead let us consider how the Oort cloud was discovered.
====The 'Discovery' of the Oort Cloud====
No object in the Oort cloud has ever been seen. We have seen more than 1200 objects in the much closer Kuiper belt and one object in the intermediate scattered disk. But the objects in the Oort cloud are much too faint and too slow moving to detect with modern technology. At the rate telescopes are improving it might be possible to detect them in a few decades. So how do we know they are there? The answer is the statistics, long period comets and solar system modelling. Periodic comets don't last long. At best they may last millions of years. Repeated close passes to Sol melt them away. So, the question was asked, why are there still comets after billions of years of Sol's life. The answer is that there must be a source of comets. There must be a massive pool of icy objects in the outer solar system. As time passes, their orbits are perturbed, perhaps by passing stars, perhaps by close passes with each other and some of them fall toward the centre of the solar system. For centuries, astronomers have carefully measured the number of comets that appear each year and their distribution and their speeds and sizes and directions. From this data and based on the same computer models that are used to accurately predict the orbits of space probes and asteroids, we have worked out the sizes and shapes and make-up of the multiple pools that are out there. We knew the Kuiper belt was there before we detected the 1200 objects we have seen to date. We knew the scattered disk must be there before we detected Sedna. We know the Oort cloud is there even though we have yet to detect an object. But the same models that tell us the current shape and size of each of these clouds also tell us how they got there and how they have been thinning these billions of years. Early in Sol's life all those icy worlds were much closer to her, still far enough to be icy but closer never-the-less. They made regular passes close to the young gas giants, Jupiter, Saturn, Uranus and Neptune. The gravitational pulls of these planets threw all the worlds in these clouds out into the orbits in which they revolve today. Many more crashed into the Gas Giants as we saw Comet Shoemaker–Levy 9 do in 1994. Others crashed into the smaller rocky planets and the moons of all eight planets leaving the craters that in many cases can still be seen today. But, most intriguing of all, Jupiter threw many worlds right out of the solar system and continues to do so today. The effect that Jupiter used to do this is called the gravitational sling shot.
====Gravitational Sling Shots====
We have already used the gravitational sling shot effect to launch space craft on interstellar voyages. The NASA space probes, Voyager I and II are on their way to the stars courtesy of momentum stolen from these planets. We could simply wait for the next world to make its final pass by Jupiter and jump on board in order to get a free lift to the stars. But if we use gravitational tractors to shift the orbit of a likely world we won't have to wait nearly as long. The gentle push of a gravitational tractor can push a near miss into the perfect trajectory and we will be on our way. The long gentle push of a gravitational tractor can steer that world toward the star of our choice so that our trip need not be millions of years but perhaps only tens of thousands. The speed gained by a pass with Jupiter can be as much as 26 kilometers per second, almost a ten thousandth of the speed of light. As we pass Jupiter we can add energy to the pass with the use of a gravitational tractor or a mass driver. In theory we could make two passes by Jupiter, one on the way in and one on the way out, and add to that passes by other gas giants and we might make a thousandth of the speed of light. We could travel to the nearest star in just thousands of years and to anyone of thousands of stars in just hundreds of thousands of years. Although this is a much longer time than we would need if we tried the tenth the speed of light scenario or 99% the speed of light scenario, the energy required is much less and the crew would have an entire world with them, with all the resources they need to survive for millions of years.
====Gaia Time====
A well-adapted biosphere living on a world can survive for hundreds of millions of years. Gaia took billions of years to conceive of multicellular life. Once she did, she conserved it for hundreds of millions of years despite asteroids crashing into the Earth, solar variation bringing about intensely warm periods and freezing cold periods, super-volcanos releasing massive clouds of poison gas. She creates species at a rate of one per lineage per million years. The pulse of a biosphere is slow and if we learn to live on Gaia's time scales and forget about the myths of science fiction, we might find that travel between stars is easy. To succeed we need to practice growing biospheres and living on frozen worlds for hundreds of thousands maybe millions of years. But as we learn to understand the pulse of Gaia and our place in the biosphere this will become natural to us.
===The Icy Outer Worlds of Solar Systems ===
The vast majority of the solid surface area in the universe seems to be airless ice. This is not true in the part of the solar system where we live; we live in the realm of rock and air. The inner solar system which is the home of Earth and the three other rocky planets is a central but tiny part of the Sol's domain. We have been taught from early ages that the solar system has one sun, Sol and eight planets. However, to understand how small the inner solar system is compared to the rest of the solar system lets describe the solar system in a different way:
The solar system is really made up of 5 main gas objects; Sol, Jupiter, Saturn, Uranus and Neptune. Sol, a star, is the largest by far with a mass roughly 750 times that of the other four put together. But the other four, the gas giant planets, have a combined mass 100 times all of the rest of the solar system put together. Around each of these 5 objects there is a number of tightly orbiting solid objects. We call these tightly orbiting objects different names depending on whether they are orbiting Sol or one of the gas-giant planets but perhaps that is just an accident of history. In the case of Sol, we call the objects asteroids and the four inner planets; in the case of the gas giants we call the objects ring particles and moons. But what if we demoted Mercury, Venus, Earth and Mars to Moons of Sol and what if we called the asteroid belt, Sol's ring system. Now you can look at the solar system as 5 large gas objects each with ring systems and moons. Just as Europa and Io are moons of Jupiter, Earth and Mars would be moons of Sol. Of course, Sol being a thousand times larger than Jupiter has larger moons with bigger orbits but the orbit of Mars is only about 8 times bigger than the orbit of Jupiter's outer moon. The smallest of Sol's Moons, Mercury, is smaller than the largest of the gas giants’ moons, Titan, and Ganymede. It is not that I am suggesting that anyone adopt this new description of the solar system. But to consider this description lets us see the Solar System in a new light; one that is closer to reality. Earth has more in common with Europa than it does with Jupiter. Jupiter in many ways has more in common with Sol than it does with Earth. Sol's moons are different from Jupiter's moons because Sol is so much bigger and because Sol is a star; Sol's moons are hot; their ice mostly melted, and even vaporized, long ago. Even Earth with all its surface oceans has more iron than it does water.
On the other hand the moons of the gas giants are mostly ice. Among the 17 largest moons of the Gas Giants, fifteen have airless icy surfaces. Only Titan has an atmosphere and Io surface is covered in sulphur compounds not ice. But Ganymede, Calisto, Europa, Rhea, Iapetus, Dione, Tethys, Enceladus, Mimas, Titania, Oberon, Umbriel, Ariel, Miranda, Triton are each spheres of ice and few among them have even thin atmospheres. Yet each is a whole world, waiting for explorers and perhaps colonists. Each will have its unique sights and opportunities. They will not be bland undifferentiated worlds. Within each world there will be colder and hotter places, places with unique topographies and varied resources. If you try to meet a friend on one of these worlds, but, instead, crash land in a random place, it will be unlikely that you are anywhere near your friend. Each is large enough that you could walk all your life and never cover the whole world. Yet the requirements for survival on each of these worlds may be similar. You will need to be able to survive cold, vacuum and a certain amount of hard radiation. Once you learn to live on the surface of one you should be able to live on the surface of any other.
Beyond Neptune is Pluto. Recently Pluto was kicked out of the Planet Club. But Pluto didn't cease to exist. It wasn't even demoted from Planet status to something lesser. Instead it became the type object for a whole class of world, the Trans-Neptunian Object or TNO. Pluto was the first TNO to be discovered (in 1930). Since 1992, over 1200 other TNO's have been discovered. We are discovering them so quickly, that for the most part we aren't naming them; we are numbering them. But the largest ones and those that were discovered earliest have been named. They include, Eris, Makemake, Haumea, Sedna, Quaoar and Orcus. Each of these is a unique world, as yet unexplored except by artists’ imaginations. Each has been no more to us, yet, than a faint dot on a photograph taken by telescope. But just the eight largest discovered to date have a combined surface area almost as large as the land surface of the Earth. Scientists believe that we will eventually discover a trillion worlds out there with a combined surface area of 10000 Earths. There is room in the solar system beyond imagining. And all of these trillion worlds will be similar enough that once we know how to survive on one of them, we will know how to survive on them all.
Compare this richness to the inner worlds. There are 5 of them other than Earth; Mercury, Venus, the Moon, Mars and Ceres. Mercury is blazing hot but airless, Venus is blazing hot with a corrosive crushing atmosphere, the Moon is airless but with a temperature that hovers around Earth's, Mars is frozen with a thin but useful atmosphere and Ceres is frozen and airless. Learning to live on one of these will teach us little about living on another. Organisms adapted to living on Ceres may be able to learn to live on the two million or so asteroids in the main asteroid belt but most of these have experienced enough heating to have lost their ices. Living on airless rock is a very different problem from living on airless ice. In fact, Ceres fits into the category of worlds that includes the TNO's and most of the Gas giant moons, airless ice worlds. On the other hand, organisms that live on Mars will be unlikely to survive on the Moon or vice versa, and it is hard to imagine any organism that could survive on Venus or Mercury. Terraforming Mars will be a beautiful project but after huge amounts of effort it won't even double the living area for the Biosphere; Mars is smaller than the Earth. Mars is a tremendous opportunity, not to be missed. We can create a second Earth. Or if we find there are organisms hidden on Mars and in need of help we may choose to Areoform Mars as suggested in Ben Bova's Mars Life. But the bigger project with much grander returns will be the colonizing of the airless ice.
===Dyson Trees ===
So how can we make a living on a ball of ice in the dark, cold, outer regions of the solar system? Our first and most important task is to establish an ecology and the basis of all ecologies are the autotrophs, the organisms who convert ambient energy into biological energy. On Earth, this role is mostly filled by the plants.
Plants arn't the first stars of the Epic of Evolution. Mention the word evolution and most people will picture the following string of organisms: fish, coelecanth, amphibian, lizard, mammalian reptile, monkey, ape, Neanderthal, accountant. This image of evolution or variations of it show up on T-shirts all the time. But a truer image would include blue green algae, algal mat, kelp, moss, horsetail, fern, pine and oak with sidebars about coral and lichen. This view, that the ultimate product of evolution todate is the oak rather than humanity, is rare only because of human vanity. Without plants there would be no fish, no lizards and no accountants. This fact is often lost on accountants but rarely is it lost on farmers. It must not be lost on the pioneers of Trans-Neptunia.
So our original question becomes, "How do we create autotrophs capable of living in Trans-Neptunia?" The human-quick answer is to build heated and lighted green houses and grow Earth-plants. Fusion reactors could supply the energy; Trans-Neptunia is rich in hydrogen, the fuel of fusion. The minerals that make up the rocks of the inner planets are hydrogen-poor, instead made up mostly of silicon, iron, oxygen and aluminium. But the compounds that make up the ices in Trans-Neptunia are composed of the elements of life, Carbon, Hydrogen, Oxygen and Nitrogen (CHON). Hydrogen predominates in terms of number of atoms if not mass. So, yes, the human-quick way will work, with fusion reactors and green houses and artificial light and heat. But in such a future, all life will live in human-quick cages, cages maintained by humans, cages which will fail when human's fail to maintain them, and cages in which ill-adapted life will die in cold vacuum when failure occurs.
Nothing humans have built has survived the tests of deep time. Our spacecraft survive for months, years, or decades and then their signals are lost, their power cells fail, they are abandoned in deep space, they burn up in the atmosphere, they sit rusting in the deserts of Mars or they are placed on display in the Smithsonian. Our civilisations last centuries or millenia and then they are pillaged by barbarrians or fall prey to short-sighted leaders who lack the vision to lead them through times of change. As individuals we spend most of our time worrying about hours and days.
But what Gaia has built on Earth has lasted not just millenia but millions of millenia. At the outer edge of Trans-Neptunia, there are worlds whose years are ten million Earth-years. To Gaia, ten million years is just enough time to try out a new idea. To Gaia, we are a new idea and to us ten million years is unimaginably long. Ten million years ago is seven million years before anything even close to human first existed. If we can give Gaia the responsibility for maintaining life in Trans-Neptunia, then the chances that our creations will last to fulfill the Destiny will be increased greatly. If life is to take root amongst the stars it needs to learn to grow roots in the ices of Trans-Neptunia. The Gaia-slow answer to the question is Dyson Trees. Freeman Dyson, a life-long member of the Institude for Advanced Studies at Princeton University, imagined trees which could grow their own greenhouses living on Trans-Neptunian worlds. Others have named the concept, Dyson Trees. Various calculations have been made which suggest that such trees, given time, could grow to immense sizes due to the low gravity, using star light or radioactive isotopes to power their growth. In the cold, they could use a mixture of increased salinity, pressure and waste heat to keep their cytoplasm liquid. As Dr. Dyson has pointed out, fur is an even more effective insulator in a vacuum than it is in air. Anyone who has seen a pussy willow knows that trees are perfectly capable of growing hair. With enough insulation, even a tiny amount of waste heat could keep an organism warm even in Trans-Neptunia. Given the tiny amount of energy available to the trees, their growth will be slow, but nature is capable of finding a balance. Herbivores that travel through the Oort cloud will also have slow metabolisms and will keep their reproduction in check so that their offspring do not strip worlds bare and then starve. Here on Earth we call organisms that keep their reproductive rates tuned to the availability of food, K-type organisms. One trend in evolution is that K-type organisms have become more and more successful relative to their oposites, the r-type organisms who reproduce at full speed until they have eaten all available food and then starve in large numbers.
Whenever two options exist, there is always a compromise, and between the human-quick and the Gaia-slow answers to living in Trans-Neptunia there are probably several compromises possible. For example, for tens of thousands of years, human-quick greenhouses and Gaia-slow Dyson Trees may coexist peacefully. The immense sizes of Dyson Trees made possible by the low gravity may make it possible for Dyson Trees to be genetically engineered to produce their own fusion reactors. How convenient it will be to arrive on a new world and find a Dyson Tree has already grown an organic particle accelerator clear around the equator of the world and is busily fusing hydrogen into carbon to produce energy and then using the carbon to grow more seeds. Perhaps we will learn to package our own DNA in hidden organelles inside the cells of Dyson Trees and their seeds. A new seedling tree growing on an ice world will grow until it is capable of producing seeds of its own and then it will unpack our DNA from its own cells and grow its own humans to help calculate the best seed trajectories to send its seeds to new worlds.
Although the surface of each world in Trans-Neptunia is likely to be unique, they will at least be similar. A tree adapted to living on one world in Trans-Neptunia will probably be able to adapt itself to living on another. The worlds of Trans-Neptunia are mostly low gravity worlds without atmospheres. Except on the largest of them, the equipement needed to launch a seed into space from one world so that it lands on another would be no more powerful than a mideval catepult. With no atmosphere to slow down the seeds or divert them from their courses, from most worlds in Trans-Neptunia just bending back a seed laden tree branch and then letting it go would be enough to sow a thousands worlds. As these trees adapt to each world they colonize, they will diversify. And if our DNA is packed inside them we will spread from world to world with them. We will create animals adapted to living amongst their branches and pack the DNA of those animals into the tree cells also. We may teach these trees to create living spaces inside themselves for us. In return we will help them launch their seeds accurately toward new worlds and help them to grow. We will become symbionts with them. As they diversify, so will the animals and even the humans that live with them. Imagine the diversity of life in the solar system with a trillion worlds with the surface area of 10000 Earths.
===Living Seed Catapults ===
But how will these trees spread to other stars? We have already seen that Dyson trees will spread themselves from world to world in Trans-Neptunia using seed catapults. Those which become trapped on the surface of large Trans-Neptunian Worlds such a Pluto or Eris may evolve, through the deep future, more sophisticated catapults capable of launching seeds into space off of even these worlds. The escape velocity from Pluto is only about 1200 m/s compared to Earth's 11 km/s. Consider that the tip of a whip can reach speeds over 300 m/s. It is not hard to imagine that a tree might learn to accelerate its seeds to 4 times that speed by a similar but scaled-up mechanism. Pluto's atmosphere is almost one hundred thousand times thinner than Earth's, so once a seed is accelerated the atmosphere would not slow it down much. Pluto is on the warmest inner edge of the Trans-Neptunia; it only gets colder further out. Further from Sol fewer and fewer atmospheres won't be frozen to the surface of their worlds as simply another layer of ice. As our trees jump to worlds more and more distant from the sun, it is unlikely that they will meet many atmospheres thicker than Pluto's. The trillion-or-so worlds in Trans-Neptunia will provide easy stepping stones right out to the edge of the Oort cloud.
The outer edge of the Oort cloud is estimated to be so far away that Sol's light may take nine and a half months to reach it. Once Dyson Trees have spread to the edge of Oort they will already be almost 20% of the distance to the currently closest star, Proxima Centauri. At this distance, once an object has escaped the surface of a Sol orbiting world, escape velocity from Sol is just another 200 km/hr. (about 50 m/s). Those trees with the ability to escape Pluto's gravity well, will have the ability to launch their seeds on interstellar voyages from here. Interstellar seeds will need greater abilities than those used to jump between Trans-Neptunian Worlds. First they will need to survive a long time. Even starting out from a tiny nearly gravity-free world at 1200 m/s, it would take seventy thousand years to travel to the nearest star.
Secondly, they will need more perceptive senses and more powerful maneuverability because there is no way that a tree will be able to aim its seed at a particular world in a cloud around another star; with all of our technology we have been unable to detect a single Oort world in our own Sol's cloud. In fact, technology is almost irrelevant to the issue. Because of the cold and dark, the light reflected and emitted by even the largest Oort world is so dim and from such a tiny region of the sky that the laws of physics dictate that the telescope needed to see it would have to be 1000 km in diameter. The law of physics in question, the Raleigh Criterion, is based on the wavelength of the light used and the distance to the object. The best light available is the reflected light of the world's sun. The telescope would need to be even larger to see a similar object orbiting in the cold, dark outer regions of another star's system. It will not be feasible for Dyson Trees living on worlds only a few km in diameter to build telescopes tens of thousands of km in diameter. If the world-clouds around other stars are like Oort, then they are so thin that the trees could launch a million seeds at them and every one would pass right through without coming near a world. Instead their seeds will need to place themselves in wide elliptical orbits around the other star and begin searching for worlds to land on. The search might last millions of years.
On the other hand the trees might just wait. Consider the star Gliese 710, a K-type star, 60% the size of Sol. It is currently 60 light years distant but it is moving rapidly toward us. In 1.4 million years, it will pass within one light year of Sol. We don't know if Gliese 710 has an world-cloud like Oort but if it does then its world-cloud will pass right through Oort. Again, each cloud will in all likelihood just pass right through the other without a single world to world collision but worlds will pass close to each other. In many cases, they will pass close enough to make sighting a Gliese-710-orbitting world from a Sol-orbiting world quite easy. If by that time our Oort cloud is well colonized by Dyson Trees then many will find it possible to fling their seeds onto Gliese worlds. As the two solar systems move apart again, life will go with each of them.
Gliese 710 is the only star we have detected so far that is on a trajectory that will pass this close to Sol. However, in order to know whether a star is on a collision course toward us we need to know how quickly it is moving toward or away from us (radial velocity) compared to how quickly it is moving across our field of view (tangential velocity) in addition to how far away it is. But we have measured the radial velocities of only a small fraction of stars. Two programs, RAVE and GCS have tried to systematically determine radial velocities of Milky Way stars. But RAVE has focussed on stars 100s or thousands of light years away and while GCS has focussed on nearby stars, it has also focused on F and G class stars, more or less ignoring the much more common K- and, especially, M-type stars. M-type stars or red dwarfs are about 10% to 20% the mass of Sol but are only between .01 % and 7% as bright as Sol. This makes them hard to detect. It may be that, for every Gliese 710 we find, there are dozens of M-class stars on collision courses with Sol's Oort cloud. To determine the radial velocity of a star is relatively time consuming. The distances of to thousands of stars, along with their tangential velocities can be calculated by comparing a few pictures of the a small section of sky taken every 6 months or so. But radial velocities require that the light from a particular star is put through a spectrograph, more or less one star at a time. The technology to do 150 stars at a time (such as is being done in RAVE) is complex and expensive.
So perhaps the trees will not have to wait for Gliese 710 but will have opportunities to colonise other stars every ten or one hundred thousand years. If that is the case, and if they are able to repeat the process at each star they colonize, then they may double the number of stars they have colonized every hundred thousand years. This would mean that every hundred million years they would multiply by 1000 the number of stars they have colonized. Within four hundred million years they would have colonized every star in the galaxy. Given that the galaxy takes 200 million years to revolve, this means that within two revolutions they may have completed the colonization of the galaxy. And all we have to do to start this process irrevocably, is create one well adapted species of tree.
===Getting to Know Our Neighbours at the Speed of Gaia ===
Many of our fears about the harm we could do to other biospheres can be relaxed when we consider the strategy of focusing on small icy worlds. Life taken from Earth and transplanted directly to other Earth-like worlds might conceivably be adapted to other similar worlds. But life adapted to living on Trans-Neptunian objects will not be well adapted to living on naturally evolved worlds. If our life spreads to other stars' cold outer object clouds, it will not then be well placed to land on the inner worlds of those stars. It will be adapted to low gravity, zero atmospheric pressure (vacuum) and frigidly low temperatures while those inner worlds will be have high Earth-like gravity, high Earth-like atmospheric pressure and high Earth-like temperatures. Even enterring such an atmosphere, would mostly burn their seeds up. With luck they might colonize vacant inner worlds after millions of years of trying but anything already living will be safe from them; the well-adapted native organisms would rapidly out-compete the invaders as the invaders colapsed under their own weight in the high gravity, were crushed by the high pressure and baked in the high-temperature.
===Diversity Through Sluggishness ===
===Imagining a human culture with the patience of Gaia ===
===Parallel destinies===
From our current vantage point in the Internet era, with its breathtaking pace of change that seems to bring a new revolution every week, it's hard to imagine the patience all this would take. Maybe the space-adapted plants and animals we engineer won't long for a faster pace of life, but some of our descendents surely will. And in a solar system filled with people, scattered among myriad worlds yet interconnected through the interplanetary equivalent of the Internet (despite its hours-long lightspeed transmission delays), it will probably only take a few centuries for someone to get the people and resources together to launch an interstellar colonization effort like the ones described in the previous chapter.
As we've discussed, there are plenty of risks entailed in the higher-speed colonization approach, both to ourselves and to the alien biospheres we may encounter. But when such a mission meets with some accidentally self-inflicted catastrophe in a distant star system, the survivors might take comfort in knowing that in a few million years, the second wave of life will arrive, moving at its own slower and more careful pace. And the inhabitants of little icy worlds drifting slowly between the stars may occasionally make the joyful discovery that the system they're approaching holds a friendly civilization, established there long ago by distant cousins from the same original home.