What is interstellar travel?

Interstellar space travel is manned or unmanned space travel between different star systems, in comparison with space travel to other bodies in the same star system. It is invariably more difficult, as the time and distance scale for such an endeavour is almost unimaginable

Why we want to visit other stars?

The group identified five key factors as high-level motivations for the exploration of distant space:
• Human survival: ideas related to creating a legacy for the human species, backing up the Earth’s biosphere, and enabling long-term survival in the face of catastrophic disasters on Earth.
• Contact with other life: finding answers to whether there is other life in the universe, whether “intelligent” life exists elsewhere in the galaxy, and at a basic level, whether we are alone in the universe.
• Evolution of the human species: exploration as a human imperative, expansion of human understanding and consciousness through space exploration.
• Scientific discovery: breakthroughs in scientific understanding of the natural universe, a pursuit for knowledge.
• Belief and faith: a search for God or the Divine, a need to explore beyond Earth’s atmosphere as a part of natural theology or as found through religious revelation.

These drivers for exploration were identified through group discussion as the most significant, high-level reasons why human societies and individuals would undertake the exploration of space over long timescales and distances.

Survival was identified as a driver for space exploration through the potential to ensure human survival in the face of planetary-wide catastrophes (including asteroid impacts and mass extinction events) and to potentially provide non-terrestrial repositories for biosphere backups. Contact with other life was identified as a fundamental driver for human exploration of space through the potential to provide answers to whether life is unique to Earth, and whether humanity is “unique” as the only intelligent life in the galaxy. Evolution was discussed extensively within the context of whether the need to explore is a fundamental characteristic of the human condition; similarly, expansion of a human presence into and beyond the solar system could be a means to enabling new facets of human consciousness and understanding of the nature of intelligent life. Discovery was discussed as exploration driven by the motivation for scientific discovery and an increased understanding of the natural universe. Finally, Belief was discussed as a motivation for the exploration of space on the basis of religious thought and belief; one participant observed a distinction in the definition of natural theology, which could be identified as the knowledge of God obtained through any process apart from Divine revelation. In this sense, exploration of outer space could be motivated by either a study of natural theology or as a need found through revelation.
Group discussion and interest focused extensively on Evolution and Contact as strong drivers for interstellar exploration by humans. Perspectives on this driver included a set of ideas ranging from exploration as a human imperative and a part of the human experience to a possible evolution of human consciousness that would be concomitant with embarking on interstellar exploration. By majority, the group felt early funding sources could be particularly engaged by the opportunity for discovery and spin-off technologies, survival of the species, and exploration as a human imperative – collected broadly into Discovery, Survival, and to a somewhat lesser extent, Evolution. Similarly, there was consensus that Contact with discovery of other life in the universe could be a catalyst for broad social engagement with and support for the exploration of deep space.
There was broad agreement that all high-level drivers for long-term human exploration would include direct human involvement in the discovery, exploration, and expansion process. This appears to be fundamental to motivations captured in each of the drivers.

What we have done until now? – Moon landing, discovery of multiple star systems (http://www.bbc.co.uk/news/science-environment-12354390 )
Kepler was launched in 2009 to identify planets by looking for the periodic, tell-tale dips in light as these objects pass in front of their host stars.
The telescope views only a small patch of sky but its findings can be extrapolated across the Milky Way Galaxy. Initial projections would indicate that within about 1,000 light-years of Earth, there may be 30,000 or more planets with potentially habitable conditions.

How far have we gone – the furthest probe and shit!

Voyager 1 probe - the most distant man-made object from Earth.
I've written a couple of articles recently about this veteran explorer. Launched in 1977 on a grand tour of the outer planets, it's now making a push to leave the Solar System. It's getting very close to crossing into interstellar space. Scientists know this from the way particles thrown off our star are behaving in the vicinity of the probe.
Whereas this "solar wind" has always streamed past Voyager, the particles have now slowed and are moving sideways from it. In other words, Voyager has reached the point where the Sun's domain of influence is pressed right up against that of other stars.
And yet, as extraordinary as Voyager is, its efforts to reach out across space still seem quite puny. In 33 years, it has travelled 17.4 billion kilometres. That sounds a lot - and it is. But it's a tiny fraction (1/2,300) of the distance to the nearest star - Proxima Centauri.

How difficult it is – vastness of space, time scale, top 10 problems

(Faster than light) Many stories include zany explanations of how faster-than-light travel is possible. The reality is that physics prevents this. There are no cheats. Even close-to-light travel runs into all sorts of interesting relativistic problems involving mass and energy. Our only possibility is to use wormhole portals. Such a wormhole would have to be carefully controlled, which is beyond our present capabilities, and we would have to somehow manage to create a twin wormhole far off at our desired destination, which might require someone else at the other end. Needing someone else to be there beforehand is not feasible for the first interstellar flight. Worse, the physical effects of traveling through a permanent or semi-permanent wormhole would warp and destroy any matter. You would arrive at your destination as a plasma.
(Generation Ships) If faster-than-light travel is impossible or impractical, we might look towards generation ships. Even though our nearest star takes light only four years to reach, heavy objects would take much longer. Most stars would take hundreds of years to reach at least. Generation ships are designed for a population to live in for generations until the destination is reached by the descendants many years later. There are several problems with a generation ship. The descendants might forget the original purpose of the mission as it fades into legend over the years. A cleverly-designed computer system might be able to educate people born on the ship to avoid this, but it still becomes increasingly difficult to predict what might occur as the generations pass. If there is a problem with the ship, a population which has descended into savagery over the centuries will be helpless.
(Stasis) When longevity and using another generation are not possible, many films and stories use humans kept in suspended animation to explain long trips. People would not be able to age in such a state, or would age very slowly, and it would be much like hibernation. Unfortunately, telomeres again present a problem. Our bodies always contain a small number of radioactive elements. These emit tiny amounts of radiation, which are harmless because our cells continually replace damaged ones. If a person does not age in stasis, then their telomeres cannot be shortening and so their cells cannot be dividing. It follows that any radioactive elements would cause permanent damage to the body, and if given enough time, could result in death. Even slow aging would not keep up with radioactive damage over long periods of time. We need our cells to divide at a normal rate.
(Damage) Our closest stars are Alpha Centauri, four light years away. Traveling at standard car speed, 60km/h, this would take 72 million years to reach. Even overcoming all of the above arguments, such a time frame is impossible due to natural wear and decay, let alone the almost zero probability of arriving at all after such a long time. Speed is needed, even if it is limited by the speed of light. Due to the tiny atoms scattered throughout space, any ship traveling at speed will be impacted by them with such force that they would tear through even the strongest steel. Tiny pinholes going right through a ship are hardly a good thing. Two options remain: humans or machines constantly patch the damage, which would require impractically large amounts of repair material to be brought, or the ship is made of elastic material which self-heals. The good news is NASA has done research into such materials. The bad news is that they do not think them feasible.

Where should we go first – probably the nearest star- Alpha Centauri

What technology can we use? – 3 types of propulsion (nuclear fusion, proton something, solar sails..); generation ships and shit (BBC articles) + Icarus Project

The performance of Daedalus was set by a decision, early in the study, that flight to Barnard's Star (less than 6 light years distant) must be accomplished within the lifetimes of the people involved with the project, about 50 years. This meant that Daedalus must reach a velocity of some 24,000 miles/sec (38,600 km/sec).
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|7 |[pi|Frozen nuclear pellet |
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|8 |[pi|Nuclear explosion |
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|9 |[pi|Reaction chamber |
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After a full examination of possible propulsion techniques, it was concluded that the best candidate was the nuclear pulse rocket. The engine concept is based on studies of controlled fusion in which pellets of deuterium and helium 3 are injected into the centre of a cusp shaped magnetic field.
As they reach the target point they are hit simultaneously by high-power electron beams. The explosion repition rate is an incredible 250 per second. Principles of the engine which might become practicable in the 21st century are shown in the diagram.
Hence, interest grew in technologies such as solar sails, blown along by streams of light particles. In the 1980s, physicist Robert L Forward even proposed a solar sail driven by a giant laser, which would push a spaceship at close to the speed of light (although some consider even that technology too slow for interstellar exploration). riting about those ideas is one thing, but building them is another, says Mark Lewis, a professor of aerospace engineering at the University of Maryland and a former US Air Force chief scientist. The laser-powered light sail, for example, would require planet-sized optics as big as Jupiter. “One can write the equations to demonstrate how it would be done, but the resources needed to actually build it would be enormous,” he says. “We know how to build them, but that doesn’t mean we could.” proposed a low-power warp drive (a faster-than light propulsion system) that involves the warping of spacetime around the starship. His idea involves using new meta-materials – which change how light is refracted – to slow the speed of light, while also creating a repulsive anti-gravity effect, creating a warp bubble surrounding the starship. This approach, Sarfatti anticipates, would break the space-time barrier, allowing the spaceship to travel beyond the speed of light in its own bubble. (A related Sarfatti proposal involves using a “quantum entanglement communicator”, which would allow the ship’s crew to speak with people back on Earth).
Eric W Davis, a physicist at the Texas-based Institute for Advanced Studies at Austin, an independent think tank, proposed a traversable wormhole, which would provide a hyperspace shortcut for a spaceship travelling to the stars. The idea would be to harness the energy found in free space - known as negative vacuum energy - to shape the wormhole, allowing the spaceship to travel to Alpha Centauri in just a few days. The difficulty, however, is producing enough negative vacuum energy to create the wormhole.

New propulsion technologies are key, of course. The feeble chemical rocketry that sent Voyager on its way in 1977 will not do. Most favoured are the emerging electric propulsion systems.
These rely on the motion of highly excited gases, or plasmas, moulded by magnetic fields to provide thrust. Although they don't give the initial big kick you get from chemical combustion, their supreme efficiency means they can go on thrusting for extended periods, achieving far more acceleration per kilogram of fuel consumed.
A glimpse of what may be possible in the future can be seen in what appears to be the current pacesetter - a type of electric engine called Vasimr (Variable Specific Impulse Magnetoplasma Rocket).
Vasmir prototypes have already produced remarkable performance in laboratory tests. The developers, Ad Astra or Texas, believe their megawatt-class units could get a ship to Mars in as little as three months.
Project Icarus envisages bigger systems that could push deep into interstellar space in just a few decades. Nuclear fusion reactors that drive gigawatt-class vessels may eventually get robots and even humans to other star systems.

How long it would take us to accomplish this (current estimates) – in at least 200 years

Programs that have been initiated – Daedalus, Icarus, 100 year starship

The world's first engineering study of an unmanned spaceship to explore one of the nearer stars was made by a technical group of the British Interplanetary Society between 1973-77. The target selected for the exercise was Barnard's Star, nearly 6 light years distant from Earth. The contributors recognised that the work, based on the technology extrapolated to the beginning of the 21st Century, could represent only a first approximation to the solution of starflight.

The result was a two stage, nuclear fusion powered vehicle, unmanned and under autonomous operation due to the distances involved, accelerated to 16% of the speed of light, and armed with a variety of probes, sensors, robotic wardens and intelligent decision making computers. Although the journey could take as long as 40 years, a flyby at such speeds would be over in 70 hours.
Although the study was conducted during the 70's, it's still referred to today, even in NASA, as a baseline study. Any future mission to the stars probably won't look anything like Daedalus, but it gives a good idea of the complexity and scale of task, and the length of time it would take to get to even the closest stars.
No estimate of the cost of such an enterprise could be made, but it would be way beyond the capacity of an individual nation, and would probably need a period of world stability unlike any we have seen to date.
The results showed it would be a formidable task requiring a massive craft that would dwarf the Saturn V moon rocket, the largest space vehicle yet flown by man. Daedalus, as currently conceived, would weigh some 54,000 tonnes, nearly 20 times the weight of the Saturn V, carrying nearly 500 tonnes of fully automated payload. Because of the enromous time lag involved in radion communications between the Earth and the ship, a semi-intelligent computer would have to control the entire ship and work out all actions necessary for the exploration phase of the mission. http://www.bisbos.com/rocketscience/spacecraft/daedalus/daedalus.html
Project Icarus, as they call their venture, have tried to envisage the ships we could be building in the decades and centuries ahead that might just get us a decent distance across space in a time which means something on a human scale.

The 100 Year Starship™ (100YSS™) study is an effort seeded by DARPA to develop a viable and sustainable model for persistent, long-term, private-sector investment into the myriad of disciplines needed to make long-distance space travel practicable and feasible.