Title: Interstellar travel Content: thumb|Artistic depiction of a Bussard Ramjet, a proposed spacecraft thruster that could accelerate an interstellar spacecraft to a very, very high speed. Interstellar space travel is manned or unmanned travel between stars. Interstellar travel is much more difficult than travel within the Solar System, though travel in starships is a staple of science fiction. Actually, there is no suitable technology at present. However, the idea of a probe with an ion engine has been studied. The energy would come through a laser base station. Given sufficient travel time and engineering work, both unmanned and sleeper ship interstellar travel seem possible. Both present considerable technological and economic challenges which are unlikely to be met in the near future, particularly for manned probes. NASA, ESA and other space agencies have done research into these topics for several years, and have worked out some theoretical approaches. Energy requirements appear to make interstellar travel impractical for "generation ships", but less so for heavily shielded sleeper ships. == The difficulties of interstellar travel == The main challenge facing interstellar travel is the vast distances that have to be covered. This means that a very great speed and/or a very long travel time is needed. The travel time with most realistic propulsion methods would be from decades to millennia. Hence an interstellar ship would be much more exposed to the hazards found in interplanetary travel, including vacuum, radiation, weightlessness, and micrometeoroids. At high speeds the vehicle would be penetrated by many microscopic particles of matter unless heavily shielded. Carrying the shield would greatly increase the propulsion problems. === Cosmic rays === Cosmic rays are of great interest because there is no protection outside the atmosphere and magnetic field. The energies of the most energetic ultra-high-energy cosmic rays (UHECRs) have been observed to approach 3 × 1020 eV, about 40 million times the energy of particles accelerated by the Large Hadron Collider. At 50 J, the highest-energy ultra-high-energy cosmic rays have energies comparable to the kinetic energy of a baseball. As a result of these discoveries, there has been interest in investigating cosmic rays of even greater energies. Most cosmic rays, however, do not have such extreme energies. The energy distribution of cosmic rays peaks at . === Required energy === A significant factor is the energy needed for a reasonable travel time. A lower bound for the required energy is the kinetic energy K = ½ mv2 where m is the final mass. If deceleration on arrival is desired and cannot be achieved by any means other than the engines of the ship, then the required energy at least doubles, because the energy needed to halt the ship equals the energy needed to accelerate it to travel speed. The velocity for a manned round trip of a few decades to even the nearest star is thousands of times greater than those of present space vehicles. This means that due to the v2 term in the kinetic energy formula, millions of times as much energy is required. Accelerating one ton to one-tenth of the speed of light requires at least 450 PJ or 4.5 J or 125 billion kWh, not accounting for losses. The source of energy has to be carried, since solar panels do not work far from the Sun and other stars. The magnitude of this energy may make interstellar travel impossible. One engineer stated “At least 100 times the total energy output of the entire world [in a given year] would be required for the voyage (to Alpha Centauri)”. === Interstellar medium === interstellar dust and gas may cause considerable damage to the craft, due to the high relative speeds and large kinetic energies involved. Larger objects (such as bigger dust grains) are far less common, but would be much more destructive. . ===Travel time=== The long travel times make it difficult to design manned missions. The fundamental limits of space-time present another challenge. Also, interstellar trips would be hard to justify for economic reasons. It can be argued that an interstellar mission which cannot be completed within 50 years should not be started at all. Instead, the resources should be invested in designing a better propulsion system. This is because a slow spacecraft would probably be passed by another mission sent later with more advanced propulsion.Kondo, Yoji: Interstellar travel and multi-generation spaceships, p. 31 On the other hand, a case can therefore be made for starting a mission without delay, because the non-propulsion problems may turn out to be more difficult than the propulsion engineering. Intergalactic travel involves distances about a million-fold greater than interstellar distances, making it radically more difficult than even interstellar travel. ==== Kennedy's calculation ==== Andrew Kennedy has shown that voyages undertaken before the minimum wait time will be overtaken by those who leave at the minimum, while those who leave after the minimum will never overtake those who left at the minimum. Kennedy's calculation depends on r, the mean annual increase in world power production. From any point in time to a given destination, there is a minimum to the total time to destination. Voyagers would probably arrive without being overtaken by later voyagers by waiting a time t before leaving. The relation between the time it takes to get to a destination (now, Tnow, or after waiting, Tt, and growth in velocity of travel is ::\frac Existing and near-term astronomical technology is capable of finding planetary systems around these objects, increasing their potential for exploration.