A light sail or magnetic sail powered by a massive laser or particle accelerator in the home star system could potentially reach even greater speeds than rocket- or pulse
propulsion methods, because it would not need to carry its own reaction mass and therefore would only need to accelerate the craft’s payload.
Electrically powered spacecraft propulsion powered by a portable power-source, say a nuclear reactor, producing only small accelerations, would take centuries to reach for
example 15% of the velocity of light, thus unsuitable for interstellar flight during a single human lifetime.
Robert L. Forward proposed a means for decelerating an interstellar craft with a light sail of 100 kilometers in the destination star system without requiring a laser array
to be present in that system.
 Most interstellar travel concepts require a developed space logistics system capable of moving millions of tonnes to a construction / operating location, and most would
require gigawatt-scale power for construction or power (such as Star Wisp– or Light Sail–type concepts).
 Yet the idea is attractive because the fuel would be collected en route (commensurate with the concept of energy harvesting), so the craft could theoretically
accelerate to near the speed of light.
 Whether antimatter propulsion could lead to the higher speeds (>90% that of light) at which relativistic time dilation would become more noticeable, thus making time
pass at a slower rate for the travelers as perceived by an outside observer, is doubtful owing to the large quantity of antimatter that would be required.
In theory, a large number of stages could push a vehicle arbitrarily close to the speed of light.
• Area of the Lightsail, about • Velocity up to (12.5% c) Pre-accelerated fuel Achieving start-stop interstellar trip times of less than a human lifetime require mass-ratios
of between 1,000 and 1,000,000, even for the nearer stars.
Proposed methods Slow, uncrewed probes Main article: Interstellar probe “Slow” interstellar missions (still fast by other standards) based on current and near-future
propulsion technologies are associated with trip times starting from about several decades to thousands of years.
Such vehicles probably have the potential to power solar system exploration with reasonable trip times within the current century.
 A magnetic sail could also decelerate at its destination without depending on carried fuel or a driving beam in the destination system, by interacting with the plasma
found in the solar wind of the destination star and the interstellar medium.
This propulsion system contains the prospect of very high specific impulse (space travel’s equivalent of fuel economy) and high specific power.
If this were performed with an acceleration similar to that experienced at the Earth’s surface, it would have the added advantage of producing artificial “gravity” for the
Very high specific power, the ratio of thrust to total vehicle mass, is required to reach interstellar targets within sub-century time-frames.
 If energy resources and efficient production methods are found to make antimatter in the quantities required and store it safely, it would be theoretically possible
to reach speeds of several tens of percent that of light.
Because of the vastness of these distances, non-generational interstellar travel based on known physics would need to occur at a high percentage of the speed of light; even
so, travel times would be long, at least decades and perhaps millennia or longer.
 As of 2022, five uncrewed spacecraft, all launched and operated by the United States, have achieved the escape velocity required to leave the Solar System as part of missions
to explore parts of the outer system.
Clocks aboard an interstellar ship would run slower than Earth clocks, so if a ship’s engines were capable of continuously generating around 1 g of acceleration (which is
comfortable for humans), the ship could reach almost anywhere in the galaxy and return to Earth within 40 years ship-time (see diagram).
The concept of using a magnetic sail to decelerate the spacecraft as it approaches its destination has been discussed as an alternative to using propellant, this would allow
the ship to travel near the maximum theoretical velocity.
Thus, for interstellar rocket concepts of all technologies, a key engineering problem (seldom explicitly discussed) is limiting the heat transfer from the exhaust stream back
into the vehicle.
Because of their low-thrust propulsion, they would be limited to off-planet, deep-space operation.
 Rockets with an external energy source Rockets deriving their power from external sources, such as a laser, could replace their internal energy source with an energy
collector, potentially reducing the mass of the ship greatly and allowing much higher travel speeds.
 By taking along no crew, the cost and complexity of the mission is significantly reduced, as is the mass that needs to be accelerated, although technology lifetime is
still a significant issue next to obtaining a reasonable speed of travel.
Another issue to be considered, would be the g-forces imparted to a rapidly accelerated spacecraft, cargo, and passengers inside (see Inertia negation).
 Non-rocket concepts A problem with all traditional rocket propulsion methods is that the spacecraft would need to carry its fuel with it, thus making it very massive,
in accordance with the rocket equation.
Such a system could grow organically if space-based solar power became a significant component of Earth’s energy mix.
 Later studies indicate that the top cruise velocity that can theoretically be achieved by a Teller-Ulam thermonuclear unit powered Orion starship, assuming no fuel is
saved for slowing back down, is about 8% to 10% of the speed of light (0.08-0.1c).
Since particles traveling at such speeds acquire more mass, it is believed that this mass change could create acceleration.
Instead, assuming that a civilization is still on an increasing curve of propulsion system velocity and not yet having reached the limit, the resources should be invested
in designing a better propulsion system.
According to Burns, the spacecraft could theoretically reach 99% the speed of light.
Kaku also notes that a large number of nanoprobes would need to be sent due to the vulnerability of very small probes to be easily deflected by magnetic fields, micrometeorites
and other dangers to ensure the chances that at least one nanoprobe will survive the journey and reach the destination.
The principle of external nuclear pulse propulsion to maximize survivable power has remained common among serious concepts for interstellar flight without external power beaming
and for very high-performance interplanetary flight.
At higher speeds, the time on board will run even slower, so the astronaut could travel to the center of the Milky Way (30,000 light years from Earth) and back in 40 years
As part of this, distances between objects in the direction of the ship’s motion will gradually contract until the ship begins to decelerate, at which time an onboard observer’s
experience of the gravitational field will be reversed.
This treaty would, therefore, need to be renegotiated, although a project on the scale of an interstellar mission using currently foreseeable technology would probably require
international cooperation on at least the scale of the International Space Station.
The universe would appear contracted along the direction of travel to half the size it had when the ship was at rest; the distance between that star and the Sun would seem
to be 16 light years as measured by the astronaut.
 A major issue with traveling at extremely high speeds is that due to the requisite high relative speeds and large kinetic energies, collisions with interstellar dust
could cause considerable damage to the craft.
Another fairly detailed vehicle system, “Discovery II”, designed and optimized for crewed Solar System exploration, based on the reaction but using hydrogen as reaction mass,
has been described by a team from NASA’s Glenn Research Center.
Several propulsion concepts have been proposed that might be eventually developed to accomplish this (see Propulsion below), but none of them are ready for near-term (few
decades) developments at acceptable cost.
 With this proposal, this interstellar ship would, theoretically, be able to reach 10 percent the speed of light.
By contrast, ion engines have low force, but the top speed in principle is limited only by the electrical power available on the spacecraft and on the gas ions being accelerated.
Constant acceleration Regardless of how it is achieved, a propulsion system that could produce acceleration continuously from departure to arrival would be the fastest
method of travel.
For example, a spaceship could travel to a star 32 light-years away, initially accelerating at a constant 1.03g for 1.32 years (ship time), then stopping its engines and coasting
for the next 17.3 years (ship time) at a constant speed, then decelerating again for 1.32 ship-years, and coming to a stop at the destination.
However, they will not approach another star for hundreds of thousands of years, long after they have ceased to operate (though in theory the Voyager Golden Record would be
playable in the highly unlikely event that the spacecraft is retrieved by an extraterrestrial civilization).
Based on work in the late 1950s to the early 1960s, it has been technically possible to build spaceships with nuclear pulse propulsion engines, i.e.
Whereas the distance between any two planets in the Solar System is less than 30 astronomical units (AU), stars are typically separated by hundreds of thousands of AU, causing
these distances to typically be expressed instead in light-years.
Even assuming shielding was provided to protect the payload (and passengers on a crewed vehicle), some of the energy would inevitably heat the vehicle, and may thereby prove
a limiting factor if useful accelerations are to be achieved.
 From the perspective of a planetary observer, the ship will appear to accelerate steadily at first, but then more gradually as it approaches the speed of light (which
it cannot exceed).
 The velocity for a crewed round trip of a few decades to even the nearest star is several thousand times greater than those of present space vehicles.
 An additional consideration is that due the non-homogeneous distribution of interstellar matter around the Sun, these risks would vary between different trajectories.
The speeds required for interstellar travel in a human lifetime far exceed what current methods of space travel can provide.
First, in the annihilation of antimatter, much of the energy is lost as high-energy gamma radiation, and especially also as neutrinos, so that only about 40% of would actually
be available if the antimatter were simply allowed to annihilate into radiations thermally.
Prime targets for interstellar travel There are 59 known stellar systems within 40 light years of the Sun, containing 81 visible stars.
 Fast, uncrewed probes Main article: Interstellar probe Nanoprobes Near-lightspeed nano spacecraft might be possible within the near future built on existing
microchip technology with a newly developed nanoscale thruster.
Second, heat transfer from the exhaust to the vehicle seems likely to transfer enormous wasted energy into the ship (e.g.
If deceleration on arrival is desired and cannot be achieved by any means other than the engines of the ship, then the lower bound for the required energy is doubled to .
This means that due to the term in the kinetic energy formula, millions of times as much energy is required.
When the ship reaches its destination, if it were to exchange a message with its origin planet, it would find that less time had elapsed on board than had elapsed for the
planetary observer, due to time dilation and length contraction.
But the speed according to Earth clocks will always be less than 1 light year per Earth year, so, when back home, the astronaut will find that more than 60 thousand years
will have passed on Earth.
Accelerating one ton to one-tenth of the speed of light requires at least 450 petajoules or125 terawatt-hours (world energy consumption 2008 was 143,851 terawatt-hours),
without factoring in efficiency of the propulsion mechanism.
 As a near-term solution, small, laser-propelled interstellar probes, based on current CubeSat technology were proposed in the context of Project Dragonfly.
Nuclear fusion rockets Fusion rocket starships, powered by nuclear fusion reactions, should conceivably be able to reach speeds of the order of 10% of that of light,
based on energy considerations alone.
Interstellar travel is expected to prove much more difficult than interplanetary spaceflight due to the vast difference in the scale of the involved distances.
 Fast, crewed missions If a spaceship could average 10 percent of light speed (and decelerate at the destination, for human crewed missions), this would be enough
to reach Proxima Centauri in forty years.
 Even so, the energy available for propulsion would be substantially higher than the ~1% of yield of nuclear fusion, the next-best rival candidate.
 The ship will be close to the speed of light after about a year of accelerating and remain at that speed until it brakes for the end of the journey.
Thus, although these concepts seem to offer the best (nearest-term) prospects for travel to the nearest stars within a (long) human lifetime, they still involve massive technological
and engineering difficulties, which may turn out to be intractable for decades or centuries.
Even with a hypothetically perfectly efficient propulsion system, the kinetic energy corresponding to those speeds is enormous by today’s standards of energy development.
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