Travel to the stars

Ever since the first space flights in the 1960s I’ve been fascinated by the idea of space travel. Starting with Dan Dare in the Eagle comic, the idea of interstellar journeys seemed commonplace. Isaac Asimov and Star Trek made it seem that humans could populate the galaxy. However, the cold reality may be somewhat different. Clearly it can never happen in my lifetime, but the thought persists, will humanity ever be able to visit and populate our interstellar neighbours. I’m not an astronomer or space-scientist. The speculation that follows in this article has been culled from interesting articles on the internet. If anyone with greater expertise than I have should read it, please excuse any errors I may have made. In summary, I believe the technological issues may be solvable, but I’m not sure about the human issues of sending a small group of highly motivated individuals into space so that their surviving children (if any) of many generations later would arrive at a place they never asked to go to. The alternative approach of recreating humans at the other end would horrify so many that I don’t believe it would ever be contemplated.

My own writing has been italicized. The rest is culled from various websites and is shown in normal text.

There are other risks to manned space flight. The atmosphere and magnetosphere of the earth protects us from harmful radiation which can cause for example, eye cataracts or cancers. Solar storms dramatically increase this danger. During journeys to the moon it may be possible to avoid the worst periods of solar radiation, but the longer trip to Mars make this more hazardous. Protection against radiation adds weight, requiring even more fuel to be carried.

The greatest threat to astronauts en route to Mars is galactic cosmic rays--or "GCRs" for short. These are particles accelerated to almost light speed by distant supernova explosions. The most dangerous GCRs are heavy ionized nuclei such as Fe+26. "They're much more energetic (millions of MeV) than typical protons accelerated by solar flares (tens to hundreds of MeV)," notes Cucinotta. GCRs barrel through the skin of spaceships and people like tiny cannon balls, breaking the strands of DNA molecules, damaging genes and killing cells.

Another problem is the lack of gravity which leads to muscle atrophy. Six months in the International Space Station can leave astronauts barely able to walk on return to Earth. A possible solution would be to spin the spaceship to create an artificial gravity, but this requires a considerably larger vessel than we have been able to launch to date, plus inevitably, extra fuel. So, we need much faster spaceships with heavier protection against radiation and artificial gravity.

A study conducted by Gregory Matioff, an adjunct professor of physics at the New York City College of Technology, suggests the best way to travel to Mars might be to hitch a ride on a passing asteroid.

Future astronauts could hitchhike their way to Mars—without the need for a “Vogon Constructor Fleet”. According to a new paper, space explorers could reach the red planet by riding along inside asteroids

Landing a ship on a space rock would solve a key issue facing Mars travellers: how to shield astronauts from galactic cosmic rays, high-energy particles travelling at near light speed that come from outside the solar system. Cosmic rays can damage DNA, increasing the risks of cancer and cataracts for space travellers. Current research suggests that the amount of radiation that would bombard an astronaut during a thousand-day, round-trip Mars mission increases his or her risk of cancer by 1 to 19 percent.

Radiation shields used on Earth are often too heavy for use on spacecraft, and they don't necessarily block cosmic rays. Many spacecraft use light, thin aluminum shields, but in the long term, aluminum hit with cosmic rays can produce secondary radiation that can be worse than the original blast. During NASA's Apollo missions, moon explorers absorbed high doses of cosmic rays, but only for a short time. Today's space station astronauts are protected from the worst of the radiation by Earth itself—our planet's body and magnetic field block two-thirds of incoming cosmic rays. By contrast, Mars voyagers would be in deep space with no large body to shield them for up to 18 months.

Asteroids
Instead of focusing on building a better shield, engineers should design spaceships that can hop in and out of passing asteroids, argues study author Gregory Matloff, an adjunct professor of physics at the New York City College of Technology. The asteroid itself could then block cosmic rays during the voyage—astronauts could pull a Millennium Falcon and park their ship in a crater, or they could use on-board mining tools to tunnel into the rock.

According to Matloff's calculations, to be published in the March-April 2011 issue of the journal Acta Astronautica, the asteroid "taxi" would need to be about 33 feet (10 meters) wide to provide enough shielding. It would also need to pass close enough to both planets—within a couple million miles—to make the trip feasible.

Already there are five known asteroids that fit the criteria and will pass from Earth to Mars before the year 2100, based on a database of 5,500 near-Earth objects (NEOs), or comets and asteroids whose orbits take them near our planet.
The asteroids 1999YR14 and 2007EE26, for example, will both pass Earth in 2086, and they'll make the journey to Mars in less than a year. The trouble would be getting home: Because of their wide orbits, it'd be five years before either asteroid would swing around Mars as it heads back toward Earth.

Matloff did find a third space rock that will travel from Mars to Earth—but it makes the journey too early, in 2037. For now it seems a space taxi to Mars would be a one-way ride.
However, the number of NEOs has increased since the database was compiled, Matloff said. There are now more than 7,000 known NEOs, so more potential rock taxis could exist.

Ideally, astronauts would divert an asteroid so that it cycles permanently between Earth and Mars on a well-timed orbit. Humans could nudge an asteroid into the desired path using a solar sail or gentle propulsion.
Once the asteroid is in a stable orbit, Matloff said, "you'd just jump on it. You could store provisions and spare parts on it and use it for shielding. ... "
Nasser Barghouty, a project scientist at NASA's Space Radiation Shielding Project, said Matloff's idea works in theory. But he thinks having so many extra launches and landings would prove too risky. Like an airline passenger with multiple layovers, "I'd need to hop on so many legs [during the journey]," he said. "That adds to the complexity of the mission, which adds more risk."
A simpler answer is to build lightweight shielding out of something other than aluminum. The International Space Station, for example, uses plastic panels to help protect its inhabitants from the effects of long-term radiation: "Plastic does the trick," Barghouty said.

Solar Sails
Instead of using chemical propulsion, experiments have been conducted using solar sails, effectively using the solar wind to propel the spaceship. The Japan Aerospace Exploration Agency (JAXA) has launched the first spacecraft that will speed across the solar system using a hybrid solar sail—one propelled partly by solar pressure, partly by traditional solar power. Dubbed Ikaros—for Interplanetary Kite-Craft Accelerated by Radiation of the Sun—the experimental craft launched on May 20 2010. Ikaros is hitching a ride into space aboard an H-IIA rocket, piggybacking with JAXA's Akatsuki Venus Climate Orbiter mission.
Once in space, the cylindrical, 677-pound (307-kilogram) craft separated from the rocket and spin itself to unfurl its roughly 46-foot-wide (14-meter-wide) solar sail.

First proposed in the 1920s, solar sails are large reflective membranes that allow a spacecraft to be pushed by radiation pressure from sunlight, negating the need for heavy onboard fuel. "It's the space equivalent of a yacht sailing on the sea," said Yuichi Tsuda, deputy project manager for Ikaros. Like wind filling a boat's sails, particles of light—or photons—streaming from the sun bounce onto a mirrorlike aluminized solar sail. As each photon strikes, its momentum is transmitted to the spacecraft, which begins to gather speed in the almost frictionless environment of space. A solar sail can eventually reach speeds five to ten times greater than a rocket powered by conventional fuels.

Ikaros is considered a hybrid, because the sail's membrane—itself just 0.0075 millimeters thick—sports thin-film solar cells for generating electricity, which will be used to power high-efficiency ion-propulsion engines, Tsuda said. "As soon as the sail has deployed, the craft will be able to start solar sailing," Tsuda said. "Over the six-month scheduled duration of the mission, we believe it will reach a velocity of a hundred meters [328 feet] per second."

After that, lessons learned from Ikaros will be applied to its planned successor, a craft equipped with a 164-foot-wide (50-meter-wide) solar-power sail that will be launched toward Jupiter around 2020.
For the Ikaros mission, JAXA has been working closely with the California-based Planetary Society, which aims to get its own solar sail—LightSail-1—into space before the end of 2011. That mission will be "very different" from Iakros, as it will carry a smaller spacecraft, said Louis Friedman, executive director of the Planetary Society. "But it will be capable of higher accelerations, and it's a step toward developing the very lightweight, pure solar sailing missions of the future."
Friedman said the technology is crucial for the next generation of space travel. "It is the only known technology which may someday enable interstellar flight," he said, if light from onboard lasers could one day replace sunlight as the main propellant.
"On the 2020 mission, we hope to be able to go to the Jupiter system and the concentrated belt of asteroids that exist nearby that are known as the Trojan asteroid region," Tsuda said. "That part of our solar system has never been visited by a man-made craft, and we want to be the first to reach it."

The heliosphere is a region of space dominated by Earth's Sun, a sort of bubble of charged particles in the space surrounding the Solar System, "blown" into the interstellar medium (the hydrogen and helium gas that permeates the galaxy) by the solar wind. Although electrically neutral atoms from interstellar volume can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself. The Sun's Corona is so hot that particles reach escape velocity, streaming outwards at 300 to 800 km/s (1 to 2 million mph).
For the first ten billion kilometers of its radius, the solar wind travels at over 1,000,000 km/h. (This is less than 0.1% of the speed of light). As it begins to interact with the interstellar medium, it slows down before finally ceasing altogether. The point where the solar wind begins to slow is called the termination shock; then the solar wind continues to slow as it passes through the heliosheath leading to a boundary where the interstellar medium and solar wind pressures balance called the heliopause. The termination shock was successfully detected by both Voyager 1 in 2004, and Voyager 2 in 2007.

Beyond the heliopause, where the interstellar medium collides with the heliosphere, it was once thought there was a bow shock. However, data from the Interstellar Boundary Explorer suggests that the velocity of the Sun through the interstellar medium is too low for a bow shock to form. Also, Cassini and IBEX data challenged the "heliotail" theory in 2009. Voyager data led to a new theory that the heliosheath has "magnetic bubbles" and a stagnation zone.
The 'stagnation region' within the heliosheath, starting around 113 AU, was detected by Voyager 1 in 2010. There the solar wind velocity drops to zero, the magnetic field intensity doubles, and high-energy electrons from the galaxy increase 100-fold. Starting in May 2012 at 120 AU, Voyager 1 detected a sudden increase in cosmic rays, an apparent signature of approach to the heliopause. In December 2012 NASA announced that in late August 2012 Voyager 1, at about 122 AU from the Sun, entered a new region they called the "magnetic highway", an area still under the influence of the Sun, but with some dramatic differences.
So if the heliopause marks the end of the effective power of the solar wind, solar sails may not be of use outside the solar system. 
So, what about the stars? Jean-Luc Picard might be able to order warp factor 5 and reach the next system in a day or so, but the reality is rather different. The S-F idea of warping space may be permissible according to theoretical physicists, but it seems the total energy output of a star might be necessary to achieve it.

The Warp Drive
The warp drive, one of Star Trek's hallmark inventions, could someday become science instead of science fiction.

Some physicists say the faster-than-light travel  may one day enable humans to jet between stars for weekend getaways. Clearly it won't be an easy task. The science is complex, but not strictly impossible, according to some researchers studying how to make it happen.

The trick seems to be to find some other means of propulsion besides rockets, which would never be able to accelerate a ship to velocities faster than that of light, the fundamental speed limit set by Einstein's General Relativity. Luckily for us, this speed limit only applies within space-time (the continuum of three dimensions of space plus one of time that we live in). While any given object can't travel faster than light speed within space-time, theory holds, perhaps space-time itself could travel.
"The idea is that you take a chunk of space-time and move it," said Marc Millis, former head of NASA's Breakthrough Propulsion Physics Project. "The vehicle inside that bubble thinks that it's not moving at all. It's the space-time that's moving."

One reason this idea seems credible is that scientists think it may already have happened. Some models suggest that space-time expanded at a rate faster than light speed during a period of rapid inflation shortly after the Big Bang. "If it could do it for the Big Bang, why not for our space drives?" Millis said.
To make the technique feasible, scientists will have to think of some creative new means of propulsion to move space-time rather than a spaceship.
According to General Relativity, any concentration of mass or energy warps space-time around it (by this reasoning, gravity is simply the curvature of space-time that causes smaller masses to fall inward toward larger masses). So perhaps some unique geometry of mass or exotic form of energy can manipulate a bubble of space-time so that it moves faster than light-speed, and carries any objects within it along for the ride.

"If we find some way to alter the properties of space-time in an imbalanced fashion, so behind the spacecraft it's doing one thing and in front of it it's doing something else, will then space-time push on the craft and move it?" Millis said. This idea was first proposed in 1994 by physicist Miguel Alcubierre.
Already some studies have claimed to find possible signatures of moving space-time. For example, scientists rotated super-cold rings in a lab. They found that still gyroscopes placed above the rings seem to think they themselves are rotating simply because of the presence of the spinning rings beneath. The researchers postulated that the ultra-cold rings were somehow dragging space-time, and the gyroscope was detecting the effect.

Other studies found that the region between two parallel uncharged metal plates seems to have less energy than the surrounding space. Scientists have termed this a kind of "negative energy," which might be just the thing needed to move space-time.

The catch is that massive amounts of this negative energy would probably be required to warp space-time enough to transport a bubble faster than light speed. Huge breakthroughs will be needed not just in propulsion but in energy. Some experts think harnessing the mysterious force called dark energy — thought to power the acceleration of the universe's expansion — could provide the key.
Even though it's a far cry between these preliminary lab results and actual warp drives, some physicists are optimistic. "We still don't even know if those things are possible or impossible, but at least we've progressed far enough to where there are things that we can actually research to chip away at the unknowns," Millis told SPACE.com. "Even if they turn out to be impossible, by asking these questions, we're likely to discover things that otherwise we might overlook."

Well, maybe sometime, but don’t hold your breath!

Returning to reality, light travels at 300,000 km per second. Even approaching the speed of light has consequences, including the almost infinite amounts of energy needed for a body of significant mass travelling at light speed. For the purposes of the rest of this exercise, I am going to assume that my spaceship will travel at just 1% of the speed of light. Still that is 3000 Km/s or 6,750,000 mph! If Proxima Centauri is 4.3 light years away how long will it take? That’s easy – 430 years at 1% of light speed. However the spaceship has to accelerate and decelerate at both ends so it would be somewhat more. In any case, it appears that Centauri is a binary or triple star system and it seems unlikely that they would have a habitable planet in their system. Never mind there are more suitable candidates within a 10 light year radius. But 10 light years means 1000 years of travelling. My descendants might eventually get there! A return journey would take as long as the time elapsed since the death of Augustus Caesar, first Emperor of Rome in 14AD!

kepler Space Telescope
The kepler space telescope has been searching for planetary systems within our galactic neighbourhood. On January 7 2013, NASA announce that kepler had discovered 467 solar systems with 2740 planets. Far from our solar system being unique, a recent estimate has suggested there could be more planets in the galaxy than stars. Our Milky Way galaxy is estimated to have around 200,000,000,000 stars. The galactic core with its super-massive black hole may not be a pleasant neighbourhood but it seems there are many planets to choose from and some may be suitable for humans.

According to Nasa, barely 20 years ago, scientists wondered if other stars even had planets at all — and many had resigned themselves to thinking that if they did, we'd never find them. Ten years later, dozens of exoplanets had been discovered, but they were all massive, gas giant planets like Jupiter, many arranged in punishingly hot orbits that practically grazed the surfaces of their stars. We'd found planets, but they were so alien and unlike Earth that scientists wondered if planets were common, but friendly ones like ours were not.

Flash forward to 2012 — 17 years after the first exoplanet discovery. A cosmos that many astronomers once thought was barren has revealed itself to be practically chock-full of planets. Not only does the galaxy appear to be crammed with planets and solar systems, but the kepler mission's broad planet-finding net has found that small, rocky planets are likely to be much more common than big ones like Jupiter. Which means that the pictures we're seeing from Curiosity on Mars could be representative of millions, if not billions of similar viewpoints on planets strewn across the Milky Way and if even a small percentage of those rocky worlds happens to be the right distance from their stars and have the right mix of volcanic activity, stellar radiation, and liquid water to support life as we know it?

But if the greatest astronomical disappointment of the 20th century was finally reaching other worlds, only to discover them barren and lifeless, then perhaps the greatest accomplishment of the 21st is finding that our own solar system is just the beginning — one outpost in a galaxy teeming with possibility.

Some more assumptions.
Professor Seager explains the goal in studying exoplanet atmospheres is to understand the atmospheric composition and temperature. We want to be able to recognize planet atmospheres like Earth’s: with water vapour, oxygen, ozone, and carbon dioxide. These strong absorbers would make the major contributions to the spectrum we could observe from afar. While the detection of true Earth twins is some time off, we are busy trying to understand hot Jupiter and hot Neptune atmospheres observed by primary and secondary eclipses for transiting exoplanets. Professor Seager’s group’s research focuses on computer models of exoplanet atmospheres and interpretation of data from space telescopes.

Let’s assume that we find an earth-like planet orbiting a star at 10 light-years from earth and studies indicate there is oxygen and water vapour in its atmosphere. It may be difficult to determine the atmospheric density and percentage of oxygen, (15-20% would be nice), but this could be a candidate for spending trillions to send our descendants to find out.
One potential problem, is that geologists tell us that the oxygen in earth’s atmosphere was created by life. Where did earth’s oxygen come from? It is thought that, initially, oxygen produced by cyanobacteria was used up reacting with iron in soils, rocks, and the ocean, forming iron oxide compounds and minerals. Geologists can estimate the amount of oxygen in the atmosphere in ancient times by looking at the kinds of iron compounds in rocks. In the absence of oxygen, iron tends to combine with sulphur, forming sulphides such as pyrites. When it is present, however, these compounds break down and the iron combines with oxygen, forming oxides. As a result, pyrites in ancient rocks indicates low oxygen levels, whereas oxides indicate the presence of significant amounts of the gas.
Once most of the available iron had combined with oxygen, the gas was able to accumulate in the atmosphere. It is thought that by about 2.3 billion years ago, levels had risen from a tiny trace to about 1% of the atmosphere. Things then seemed to balance out for a long period as other organisms evolved to use oxygen to provide energy by the oxidation of carbon, producing carbon dioxide (CO2). They achieved this by eating carbon-rich organic plant material, either living or dead. This created a balance, with oxygen production through photosynthesis matched by its consumption by oxygen-breathing organisms.

It seems that, because of this balance, photosynthesis alone cannot account for the initial rise in oxygen. One explanation is that some dead organic matter became buried in mud or other sediment and was not available to aerobic organisms. This matter could not combine with atmospheric oxygen, so not all the element produced was used up in this way, allowing levels to rise.
At some point later in the Earth’s history, oxygen levels rose dramatically to around their present level. Some scientists believe this may have happened around 600 million years ago. Around this time, a great many relatively large, complex, multicellular organisms appeared that would have required much higher oxygen levels. It is not clear what caused this change, however. Interestingly, it occurred as the Earth seemed to be emerging from a massive ice age, during which most of the planet was covered by ice.

One theory is that the action of glaciers, when advancing and retreating, ground up rock rich in phosphorus and released huge amounts of it into the oceans. Phosphorus is an essential nutrient for phytoplankton, so this may have caused an explosion of this form of life. This would, in turn, lead to increased production of oxygen, with probably very little land-based life to use it up. Not all scientists agree with this theory, however, and as of 2012, the issue remains unresolved.

So if the oxygen in our atmosphere has been created by life, an earth-like planet with an atmosphere containing oxygen would almost certainly have been created by life. This raises ethical issues but perhaps even more importantly what if the planet had a population of creatures further advanced that we are?

Anyway, let’s ignore that for the present. We’ve found a planet 10 light-years away, circling a sun-like star in the goldilocks zone with oxygen and water-vapour in its atmosphere. What are we waiting for?

Although very efficient, an IPS does require a significant power source.  Solar-powered IPSs are fine for small spacecraft such as satellites and probes, but if humans want to use this technology to explore the Solar System, power systems with greater output capacities will be required.
28 June 2011 -a NASA engineer has come up with a new way to fling satellites through space on mere grams of fuel, tens of times as efficiently as today’s best space probe thrusters. The answer, he says, is fusion. You might be thinking, "Fusion? Really?" But it’s not as far-fetched as it sounds at first blush.

Instead of using deuterium and tritium as the fuel stocks, the new motor extracts energy from boron fuel. Using boron, an "aneutronic" fuel, yields several advantages over conventional nuclear fusion. Aneutronic fusion, in which neutrons represent less than 1 percent of the energy-carrying particles that are the result of a reaction, is easier to manage. "Neutrons are problematic, because for one thing they’re difficult to harness," says John J. Chapman, the concept’s inventor and a physicist and electronics engineer at NASA’s Langley Research Center, in Virginia. To make use of neutrons, "you need an absorbing wall that converts the kinetic energy of the particles to thermal energy," he says. "In effect, all you’ve got is a fancy heat engine, with all its resultant losses and limitations."

In Chapman’s aneutronic fusion reactor scheme, a commercially available benchtop laser starts the reaction. A beam with energy on the order of 2 x 1018 watts per square centimeter, pulse frequencies up to 75 megahertz, and wavelengths between 1 and 10 micrometers is aimed at a two-layer, 20-centimeter-diameter target.

The first layer is a 5- to 10-µm-thick sheet of conductive metal foil. It responds to the teravolt-per-meter electric field created by the laser pulse by "acting as a de facto proton accelerator," says Chapman. The electric field releases a shower of highly energetic electrons from the foil, leaving behind a tremendous net positive charge. The result is a massive self-repulsive force between the protons that causes the metal material to explode. The explosion accelerates protons in the direction of the target’s second layer, a film of boron-11.

There, a complicated nuclear dance begins. The protons (which carry energy on the order of roughly 163 kiloelectron volts) strike boron nuclei to form excited carbon nuclei. The carbons immediately decay, each into a helium-4 nucleus (an alpha particle) and a beryllium nucleus. Almost instantaneously, the beryllium nuclei decay, with each one breaking into two more alpha particles. So for each proton-boron pair that reacts, you get three alpha particles, each with a kinetic energy of 2.9 megaelectron volts.

Electromagnetic forces push the target and the alpha particles in the opposite directions, and the particles exit the spacecraft through a nozzle, providing the vehicle’s thrust. Each pulse of the laser should generate roughly 100 000 particles, making the method tremendously efficient, says Chapman. And according to his calculations, improvements in short-pulse laser systems could make this form of thruster more than 40 times as efficient as even the best of today’s ionic propulsion systems that push spacecraft around.  Even at 50 percent efficiency, burning off 40 milligrams of the boron fuel would deliver a gigajoule of energy. The amount of power depends on the laser pulse rate. The motor could generate 1 megawatt per second if the pulses are frequent enough to start reactions that consume that amount of boron in 1000 seconds. (According to Chapman, using this aneutronic fusion technique with helium-3 isotopes would yield roughly 60 percent more energy per unit mass. But boron is a more attractive fuel source because it is abundant on Earth and helium-3 is scarce.)

Another big advantage of fusion space propulsion, Chapman claims, is that some of the energy can be converted into electricity to power a spacecraft’s onboard control systems. "A travelling wave tube—basically an inverse klystron—captures most of the particles’ flux kinetic energy and efficiently converts it into electrical energy," says Chapman. The process, he says, is 60 to 70 percent efficient.

The NASA engineer acknowledges that this collection of ideas is still a long way from being a practical device. For example, losses from the alpha particles striking the walls of the exhaust nozzle or each other lower the net power output. Figuring out how to control the particles’ path is an important consideration.

Asked how long it will be before his fusion reactor is pushing spacecraft toward Mars, Chapman acknowledges that a decade of work might be required before that happens. "It takes teamwork to get something to the point where you put it in space," he says. His aim so far is "to get the idea out so other minds can begin thinking about it."
So, if we can use a fusion reactor with Boron as its fuel, what about the spaceship. Previously the possible use of earth-crossing asteroids was mentioned. For a very-long duration inter-stellar space flight, we may have to be more ambitious.

Asteroids as space ships
Asteroids are small, airless rocky worlds revolving around the sun that are too small to be called planets. Most asteroids lie in a vast ring between the orbits of Mars and Jupiter. This main belt holds more than 200 asteroids larger than 100 kilometers in diameter. Scientists estimate the asteroid belt also contains more than 750,000 asteroids larger than 1 kilometer in diameter and many millions of smaller ones. Not everything in the main belt is an asteroid — for instance, comets have recently been discovered there, and Ceres, once thought of only as an asteroid, is now also considered a dwarf planet.

Most asteroids fall into three classes based on composition. The C-type or carbonaceous are greyish in color and are the most common, including more than 75 percent of known asteroids. They probably consist of clay and stony silicate rocks, and inhabit the main belt's outer regions. The S-type or silicaceous asteroids are greenish to reddish in color, account for about 17 percent of known asteroids, and dominate the inner asteroid belt. They appear to be made of silicate materials and nickel-iron. The M-type or metallic asteroids are reddish in color, make up most of the rest of the asteroids, and dwell in the middle region of the main belt. They seem to be made up of nickel-iron.

An asteroid spaceship
What we need is a nickel-iron asteroid capable of being fashioned into a space-ship. Assuming it’s spherical (it won’t be, but it helps the calculation), a one km diameter nickel-iron asteroid will have a volume of 524,000,000 cubic metres. As the density of iron is 7870 kg / cubic metre, this give a mass of 4121 million tonnes. Let’s hollow it out and make a spaceship. Say we leave a 100m wall, that means hollowing out a 800m wide void, or 268 million cubic metres, 2,100 million tonnes, which could be used to build a second spaceship.
OK, we are in the asteroid belt, 300 million km from earth, no problem! I suspect we will need smart machines to achieve this and lots of them. How about super-intelligent, self-repairing and replicating robots able to operate in space without the life-support systems that humans need and with power tools to match? We’ll probably need these machines to operate and repair the space ship on its 1000 year voyage as well!

There will be an immense cost in developing these robots and energy systems. But once set to work on a suitable asteroid, the work becomes self-perpetuating. The asteroid belt contains an immense store of metals, water-ice and other elements necessary for our space-ship. Smart robots would construct factories in space to produce everything necessary with very little need for human involvement. (The alternative scenario, popularised by various S-F writers, is of a wild-west lawless frontier on the asteroid belt!)

As well as building our spaceship in the asteroid belt, mining the belt could answer the predicted resource shortage on earth. Minerals of all kinds should be plentiful, and there are no neighbours to complain about the noise and environmental pollution. Autonomous factories in the belt could extract and refine the resources needed for the future of earth as well as building and provisioning interstellar ships.

Without attempting to go into details, let’s assume that we can develop these machines within the coming century along with an effective fusion reactor to give them virtually limitless power. The robots will hollow out the interior of the asteroid, extracting valuable elements along with the nickel-iron. Some of the extracted nickel-iron would be fused to the exterior of the asteroid, smoothing out the craters to create a uniform shell for our spaceship, perhaps up to 100 metres thick for maximum protection against cosmic radiation. Internally a habitable volume would be created. If humans are to live their lives inside, the interior could be made to spin on a longitudinal axle in order to create a 1g gravitational field. It may even be possible to create a magnetic field around the asteroid, similar to the earth’s which would further protect the inhabitants. At the rear, would be the engines.

 I’ve already postulated a fusion reactor for the construction process, along with the boron fusion drive mentioned above, this would be housed at the rear, behind shielding many metres thick. So far on earth, we have made the JET reactor produce fusion power from deuterium for seconds at a time. However, on earth, it has been suggested would, like a nuclear reactor, produce steam to drive a turbine. In space this would not be a practical approach. Perhaps by the end of the 21st century, we might have a reliable fusion reactor producing power more directly.

Cold Fusion power

But, cold fusion could be the answer Experimenters inserted a small pyroelectric crystal (lithium tantalite) inside a chamber filled with hydrogen. Warming the crystal by about 100 degrees (from -30 F to 45F) produced a huge electrical field of about 100,000 volts across the small crystal.

The tip of a metal wire was inserted near the crystal, which concentrated the charge to a single, powerful point. The huge electric field sent the nuclei careening away, smacking into other hydrogen nuclei on their way out. Instead of using intense heat or pressure to get nuclei close enough together to fuse, this new experiment used a very powerful electric field to slam atoms together.
Unlike some previous claims of room-temperature fusion, this one makes intuitive sense: its just another way to get atoms close enough together for the strong force to take over and do the rest. Once the reaction got going, the scientists observed not only the production of helium nuclei, but other tell-tale signs of fusion such as free neutrons and high energy radiation.

This experiment has been repeated successfully and other scientists have reviewed the results: it looks like the real thing this time.

So now we have our spaceship (or two), with a drive system fuelled for interstellar travel, a sufficiently thick wall and a magnetic shield to protect generations of inhabitants, comfortable gravity and robots to operate and repair all the systems on board for a very long time.

The Journey begins
So let’s get going. We have a plasma ion drive, much larger but the same in principle as that already tested by NASA to reach the asteroid belt. The experimenters used Argon for the plasma, I suspect there may be a problem obtaining enough but I don’t know enough about the physics to say why it would not be possible to use some of the waste asteroid material as reaction mass. A chemical rocket propels its cargo by shooting out material from the rocket motor. According to Newton’s third law, “For every action, there is an equal and opposite reaction”. The ion drive also obeys the third law, but the reaction material is much hotter and travels a great deal faster, hence much less of it is required. I’ll skip the maths and assume that the spaceship can carry sufficient reaction mass for the ion drive to reach the speeds we need. Once the spaceship is ready we use a devlopment of the boron fusion drive. Presumably there will be enough boron in the asteroid belt to fuel the spaceship.

As mentioned previously, it will not be practical to travel anywhere near the speed of light, so I have chosen 1% of that – 3,000 Km / second as a target speed. I’ve also assumed that the drive will not be capable of accelerating at anything close to 1g, so quite arbitrarily I’ve chosen just over 1% of that, 0.1 metres per second per second. This may or may not be a practical speed considering the 2100 million tonne mass of our spaceship, but let’s assume it is.

What does that mean then? From our starting point in the asteroid belt around 450 million kilometres from the sun, the engines are switched and after the first second we will be travelling at 0.1 metres/sec. After a minute we will have reached the speed of 6 metres/sec or 13.4 mph! But after a day at this rate the speed will have reached 8640 metres/sec or 16159 mph and it keeps on going, day after day. To reach our target speed of 3,000 km/s after which the engines can be switched off will take 347 days, about three weeks short of an earth year. As there is little resistance to the space ship, according to Newton’s first law this speed will be maintained indefinitely (subject of course to gravitational drag from the sun and outer planets). After 347 days the space ship will have travelled approximately 44 thousand million kilometres or 293 Astronomical Units (AU). A light year is 9.46 million million km. (9.46 trillion km) The distance of 10 light years at 1% of the speed of light was deliberately chosen so I did not have to resort to my calculator to state with assurance that the journey would take 1000 years at this speed, plus of course another 347 days of reverse thrust when approaching journey’s end.

A thousand year journey may not be appealing, so what about going faster. Let’s say that our Boron drive is capable of pushing the ship up to 10% of light speed and it can carry enough fuel to accelerate up to 30,000 km/sec and decelerate at the other end. What does that do to the duration of the journey. Now it will take 9.5 years to reach the target speed and decelerate. In 9.5 years the spaceship will have travelled 4.5 trillion km of our 94.6 trillion km, with another 4.5 trillion in 9.5 years at the other end. 85.6 trillion km @ 30,000 km/sec would take 90.46 years for a total of just under 110 years. Four generations of space travelling is more appealing than 40! Presumably, prior to beginning deceleration, onboard instruments will be able to discover much more about the target planet and if it turns out not to be suitable, divert to an alternative star system.

Still a very long time. Could we push the speed up to 15% of light speed, or 45,000 km/sec at the same rate of acceleration? That takes 14.25 years to reach in which time the spaceship will travel 10 trillion km with the same deceleration at the other end, leaving 74.6 trillion at 45,000 km/sec which would take 52.5 years for a total 81 years. Now it is just feasible that it could be achieved in a lifetime, but it could be a very boring lifetime with little time left to appreciate the destination! At least it would be possible for one or more survivors of the original crew to have a personal relationship with their children and grandchildren when they reached their destination.

Hazards
Beyond Neptune, the Kuiper belt lies at 30-50 AU or 4500 to 7500 million km from the sun. Further out is the Oort Cloud which begins at 2000-5000 AU or between 300 and 750 thousand million km from the sun. Extending out as far as 100,000 AU or 15 million million km. There could be a risk of colliding with a large object in either belt. At 10 million Km per hour, very long distance radar will be needed to provide sufficient time for a course correction to avoid large objects. Indeed at this speed a collision with a piece of ice several metre across could damage our spaceship, despite its 100m thick walls. After leaving the Oort cloud which could extend up to 3 light years from the sun, it is possible we would find a similar belt of objects around our target stellar system, so there could be a risk of collision up to 6 light years out of the total 10 to be travelled.

There appear to be potentially billions of objects in the Kuiper belt and Oort cloud but they are dispersed over a vast area. There will be a risk, but how great is it? I don’t believe there is a possibility of shielding, as in Star Trek, but rather than changing course, perhaps a space torpedo to break up and disperse the object may be feasible. Perhaps we already have a method of doing this. A missile with low-yield nuclear warhead would not be subject to air-resistance or gravity in space. Propellant sufficient for about one hour of powered flight would put it a few thousand km ahead when the fuel ran out, thereafter it could be designed to have sufficient remaining fuel for final course correction to impact the object. Detonation would occur only seconds before the spaceship arrived but that should be sufficient to destroy frozen volatile objects. Anything larger than a few metres across would have to be avoided by course correction using lateral thrusters.

The Kuiper belt sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets, extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but it is far larger—20 times as wide and 20 to 200 times as massive. Like the asteroid belt, it consists mainly of small bodies, or remnants from the Solar System's formation. While most asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles (termed "ices"), such as methane, ammonia and water. The classical belt is home to at least three dwarf planets: Pluto, Haumea, and Makemake. Some of the Solar System's moons, such as Neptune's Triton and Saturn's Phoebe, are also believed to have originated in the region.

Since the belt was discovered in 1992, the number of known Kuiper belt objects (KBOs) has increased to over a thousand, and more than 100,000 KBOs over 100 km (62 mi) in diameter are believed to exist. The Kuiper belt was initially thought to be the main repository for periodic comets, those with orbits lasting less than 200 years. However, studies since the mid-1990s have shown that the classical belt is dynamically stable, and that comets' true place of origin is the scattered disc, a dynamically active zone created by the outward motion of Neptune 4.5 billion years ago; scattered disc objects such as Eris have extremely eccentric orbits that take them as far as 100 AU from the Sun.

The Kuiper belt should not be confused with the hypothesized Oort cloud, which is a thousand times more distant. The objects within the Kuiper belt, together with the members of the scattered disc and any potential Hills cloud or Oort cloud objects, are collectively referred to as trans-Neptunian objects (TNOs).

The Oort cloud is thought to occupy a vast space from somewhere between 2,000 and 5,000 AU (0.03 and 0.08 ly) to as far as 50,000 AU (0.79 ly) from the Sun. Some estimates place the outer edge at between 100,000 and 200,000 AU (1.58 and 3.16 ly). The region can be subdivided into a spherical outer Oort cloud of 20,000–50,000 AU (0.32–0.79 ly), and a doughnut-shaped inner Oort cloud of 2,000–20,000 AU (0.03–0.32 ly). The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets to inside the orbit of Neptune. The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981. Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo; it is seen as a possible source of new comets to resupply the relatively tenuous outer cloud as the latter's numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years.

The outer Oort cloud is believed to contain several trillion individual objects larger than approximately 1 km (0.62 mi) (with many billions with absolute magnitudes brighter than 11—corresponding to approximately 20 km (12 mi) diameter), with neighboring objects typically tens of millions of kilometres apart. Its total mass is not known with certainty, but, assuming that Halley's comet is a suitable prototype for all comets within the outer Oort cloud, the estimated combined mass is 3×1025 kg (7×1025 lb or roughly five times the mass of the Earth). Earlier it was thought to be more massive (up to 380 Earth masses), but improved knowledge of the size distribution of long-period comets has led to much lower estimates. The mass of the inner Oort Cloud is not currently known.

If analyses of comets are representative of the whole, the vast majority of Oort-cloud objects consist of various ices such as water, methane, ethane, carbon monoxide and hydrogen cyanide. However, the discovery of the object 1996 PW, an asteroid in an orbit more typical of a long-period comet, suggests that the cloud may also contain rocky objects. Analysis of the carbon and nitrogen isotope ratios in both the Oort cloud and Jupiter-family comets shows little difference between the two, despite their vastly separate regions of origin. This suggests that both originated from the original protosolar cloud, a conclusion also supported by studies of granular size in Oort-cloud comets and by the recent impact study of Jupiter-family comet Tempel 1.

The Human Dimension
So far I have considered the issues of building the ship, the distances involved, propulsion systems, and space hazards. What of the human element. If this space ship is to be populated by humans for 1000 years, or some 40 generations there will be huge social problems to be overcome. There will be a closed community which will have to keep its population stable over many generations, recycle everything (including the bodies of the passengers), maintain an ecology to grow all food and materials required for sustaining life. None of the furnishings, equipment and technology on board can be expected to last the duration of the voyage so all the material and tools required for every aspect of life on board will have to be produced, used and recycled. This includes all the electronics. All the information available on earth will need to be stored and maintained on board. This also applies to the robotic devices I mentioned earlier as being necessary to build and maintain the spaceship. They too will wear out and need to be replaced. These thoughts were partly behind my reasoning for such a large space ship, sufficient to carry all the material necessary for maintaining life and technologies over such a long period.

However, it could be the issues of maintaining a stable community with sufficient genetic diversity over 40 generations in a space only a few hundred metres across which render the project unachievable. If a greater speed of 10% of light-speed could be achieved, then travel to a star 10 light years distance could be achieved in around 110 years. At 15% it would take 81 years.

The solution may lie in a different technology which raises huge ethical concerns on earth. That is to leave the space ship inhabited only by robots for almost all of its journey. Instead the DNA from a few thousand individuals could be stored on board. This could be stored in digital form. When the time came, the DNA would be recreated in synthesised eggs using in-vitro techniques. Robotic nursemaids would raise the infants to adulthood and educate them for their role in populating the new world. We already have the technology for sequencing the entire human genome and may soon be capable of recreating the DNA and growing the resulting embryos. Genetic abnormalities could be screened out. The same would apply to food plants and any other organism we may desire to take with us to the new world (not forgetting our symbiotic intestinal flora!). This approach would have the added benefit of ensuring no unwelcome pathogens accompanied the crew.  This sounds very like eugenics! Perhaps we need to leave some of the decisions to the super-intelligent robots which have operated the ship on its journey and would recreate humanity at the other end.

One could argue that humanity should not travel to the stars, instead, humans could imprint their consciousness on robotic devices more suited to alien worlds as a way of propagating our species across the galaxy. If we followed this route, then the massive spaceships I have suggested would not be necessary. Without the need for life-support and organic food, a pure machine complex, hardened against radiation and able to autonomously replicate and repair itself would need a much smaller, cheaper vessel.