Interstellar travel

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A Bussard Ramjet, one of many possible methods that could serve as propulsion for a starship.

Interstellar space travel is manned or unmanned travel between stars. Interstellar travel is much more difficult than interplanetary travel: the distances between the planets in the Solar System are typically measured in standard astronomical units (AU)—whereas the distances between stars are typically hundreds of thousands of AU, and usually expressed in light-years. Because of the vastness of those distances, interstellar travel would require either great speed (some percentage of the speed of light) or huge travel time (lasting from years to millennia).

The required speeds for interstellar travel in a human lifespan are far beyond what current methods of spacecraft propulsion can provide. The energy required to propel a spacecraft to these speeds, regardless of the propulsion system used, is enormous by today's standards of energy production. At these speeds, collisions by the spacecraft with interstellar dust and gas can produce very dangerous effects both to any passengers and the spacecraft itself.

A number of widely differing strategies have been proposed to deal with these problems, ranging from giant arks that would carry entire societies and ecosystems very slowly, to microscopic space probes. Many different propulsion systems have been proposed to give spacecraft the required speeds: these range from different forms of nuclear propulsion, to beamed energy methods that would require megascale engineering projects, to methods based on speculative physics.

For both unmanned and manned interstellar travel, considerable technological and economic challenges would need to be met. Even the most optimistic views about interstellar travel are that it might happen decades in the future; the more common view is that it is a century or more away.

Challenges

Interstellar distances

The basic challenge facing interstellar travel is the immense distances between the stars.
Astronomical distances are measured using different units of length, depending on the scale of the distances involved. Between the planets in the Solar System they are often measured in astronomical units (AU), defined as the average distance between the Sun and Earth, some 150 million kilometers (93 million miles). Venus, the closest other planet to Earth is (at closest approach) 0.28 AU away. Neptune, the furthest planet from the Sun, is 29.8 AU away. Voyager 1, the furthest man-made object from Earth, is 129.2 AU away.

The closest known star Proxima Centauri, however, is some 268,332 AU away, or 9000 times further away than even the furthest planet in the Solar System.

Object	A.U.	light time
The Moon	0.0026	1.3 seconds
Venus (nearest planet)	0.28	2.41 minutes
Neptune (furthest planet)	29.8	4.1 hours
Voyager 1	129.2	17.9 hours
Proxima Centauri (nearest star)	268,332	4.24 years

Because of this, distances between stars are usually expressed in light-years, defined as the distance that a ray of light travels in a year. Light in a vacuum travels around 300,000 kilometers (186,000 miles) per second, so this is some 9.46 trillion kilometers (5.87 trillion miles) or 63,241 AU. Proxima Centauri is 4.243 light-years away.

Another way of understanding the vastness of interstellar distances is by scaling: one of the closest stars to the sun, Alpha Centauri A (a Sun-like star), can be pictured by scaling down the Earth–Sun distance to one meter (~3.3 ft). On this scale, the distance to Alpha Centauri A would be 271 kilometers (169 miles).
The fastest outward-bound spacecraft yet sent, Voyager 1, has covered 1/600th of a light-year in 30 years and is currently moving at 1/18,000th the speed of light. At this rate, a journey to Proxima Centauri would take 80,000 years.^[1]

Some combination of great speed and long travel time are required. The time required by propulsion methods based on currently known physical principles would require years to millennia.

Required energy

A significant factor contributing to the difficulty is the energy that must be supplied to obtain 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 is significantly increased.

The velocity for a manned round trip of a few decades to even the nearest star is several thousand 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 ×10¹⁷ J or 125 billion kWh, without factoring in efficiency of the propulsion mechanism. This energy has to be generated on-board from stored fuel, harvested from the interstellar medium, or projected over immense distances.

Manned missions

The mass of any craft capable of carrying humans would inevitably be substantially larger than that necessary for an unmanned interstellar probe. For instance, the first space probe, Sputnik 1, had a payload of 83.6 kg, whereas the first spacecraft carrying a living passenger (the dog Laika), Sputnik 2, had a payload six times that at 508.3 kg. This underestimates the difference in the case of interstellar missions, given the vastly greater travel times involved and the resulting necessity of a closed-cycle life support system. As technology continues to advance, combined with the aggregate risks and support requirements of manned interstellar travel, the first interstellar missions are unlikely to carry life forms.

A manned craft will require more time to reach its top speed as humans have limited tolerance to acceleration.

Interstellar medium

A major issue with traveling at extremely high speeds is that interstellar dust and gas may cause considerable damage to the craft, due to the high relative speeds and large kinetic energies involved.
Various shielding methods to mitigate this problem have been proposed.^[2] Larger objects (such as macroscopic dust grains) are far less common, but would be much more destructive. The risks of impacting such objects, and methods of mitigating these risks, have been discussed in the literature, but many unknowns remain.^[3]

Travel time

An interstellar ship would face manifold hazards found in interplanetary travel, including vacuum, radiation, weightlessness, and micrometeoroids. Even the minimum multi-year travel times to the nearest stars are beyond current manned space mission design experience.

The habitual illumination energy requirement for each person is estimated to be 12 kilowatts.^[4][5] Other long-term energy requirements are still being investigated.^[6]

More speculative approaches to interstellar travel offer the possibility of circumventing these difficulties. Special relativity offers the possibility of shortening the travel time through relativistic time dilation: if a starship with could reach velocities approaching the speed of light, the journey time as experienced by the traveler would be greatly reduced (see time dilation section). General relativity offers the theoretical possibility that faster-than-light travel could greatly shorten travel times, both for the traveler and those on Earth (see Faster-than-light travel section).

Wait calculation

It has been argued that an interstellar mission that cannot be completed within 50 years should not be started at all. Instead, assuming that a civilization is still on an increasing curve of propulsion system velocity, not yet having reached the limit, 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 (the incessant obsolescence postulate).^[7] On the other hand, Andrew Kennedy has shown that if one calculates the journey time to a given destination as the rate of travel speed derived from growth (even exponential growth) increases, there is a clear minimum in the total time to that destination from now (see wait calculation).^[8] Voyages undertaken before the minimum will be overtaken by those who leave at the minimum, whereas those who leave after the minimum will never overtake those who left at the minimum.

One argument against the stance of delaying a start until reaching fast propulsion system velocity is that the various other non-technical problems that are specific to long-distance travel at considerably higher speed (such as interstellar particle impact, possible dramatic shortening of average human life span during extended space residence, etc.) may remain obstacles that take much longer time to resolve than the propulsion issue alone, assuming that they can even be solved eventually at all. A case can therefore be made for starting a mission without delay, based on the concept of an achievable and dedicated but relatively slow interstellar mission using the current technological state-of-the-art and at relatively low cost, rather than banking on being able to solve all problems associated with a faster mission without having a reliable time frame for achievability of such.

Communications

The round-trip delay time is the minimum time between an observation by the probe and the moment the probe can receive instructions from Earth reacting to the observation. Given that information can travel no faster than the speed of light, this is for the Voyager 1 about 36 hours, and near Proxima Centauri it would be 8 years. Faster reaction would have to be programmed to be carried out automatically. Of course, in the case of a manned flight the crew can respond immediately to their observations. However, the round-trip delay time makes them not only extremely distant from, but, in terms of communication, also extremely isolated from Earth (analogous to how past long distance explorers were similarly isolated before the invention of the electrical telegraph).

Interstellar communication is still problematic – even if a probe could reach the nearest star, its ability to communicate back to Earth would be difficult given the extreme distance. See Interstellar communication.

Prime targets for interstellar travel

There are 59 known stellar systems within 20 light years of the Sun, containing 81 visible stars. The following could be considered prime targets for interstellar missions:^[9]

Stellar system	Distance (ly)	Remarks
Alpha Centauri	4.3	Closest system. Three stars (G2, K1, M5). Component A is similar to the Sun (a G2 star). Alpha Centauri B has one confirmed planet.[10]
Barnard's Star	6	Small, low-luminosity M5 red dwarf. Second closest to Solar System.
Sirius	8.7	Large, very bright A1 star with a white dwarf companion.
Epsilon Eridani	10.8	Single K2 star slightly smaller and colder than the Sun. Has two asteroid belts, might have a giant and one much smaller planet,[11] and may possess a Solar-System-type planetary system.
Tau Ceti	11.8	Single G8 star similar to the Sun. High probability of possessing a Solar-System-type planetary system: current evidence shows 5 planets with potentially two in the habitable zone.
Gliese 581	20.3	Multiple planet system. The unconfirmed exoplanet Gliese 581 g and the confirmed exoplanet Gliese 581 d are in the star's habitable zone.
Gliese 667C	22	A system with at least six planets. A record-breaking three of these planets are super-Earths lying in the zone around the star where liquid water could exist, making them possible candidates for the presence of life.[12]
Vega	25	At least one planet, and of a suitable age to have evolved primitive life [13]

Existing and near-term astronomical technology is capable of finding planetary systems around these objects, increasing their potential for exploration.

Proposed methods

Slow, uncrewed probes

Slow interstellar missions based on current and near-future propulsion technologies are associated with trip times starting from about one hundred years to thousands of years. These missions consist of sending a robotic probe to a nearby star for exploration, similar to interplanetary probes such as used in the voyager program. By taking along no crew, the cost and complexity of the mission is significantly reduced although technology lifetime is still a significant issue next to obtaining a reasonable speed of travel. Proposed concepts include Project Daedalus, Project Icarus and Project Longshot.

Fast, uncrewed probes

Nanoprobes

Near-lightspeed nanospacecraft might be possible within the near future built on existing microchip technology with a newly developed nanoscale thruster. Researchers at the University of Michigan are developing thrusters that use nanoparticles as propellant. Their technology is called “nanoparticle field extraction thruster”, or nanoFET. These devices act like small particle accelerators shooting conductive nanoparticles out into space.^[14]

Michio Kaku, a theoretical physicist, has suggested that clouds of "smart dust" be sent to the stars, which may become possible with advances in nanotechnology. Kaku also notes that a large amount 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.^[15]

Given the light weight of these probes, it would take much less energy to accelerate them. With on board solar cells they could continually accelerate using solar power. One can envision a day when a fleet of millions or even billions of these particles swarm to distant stars at nearly the speed of light and relay signals back to Earth through a vast interstellar communication network.

Slow, manned missions

In crewed missions, the duration of a slow interstellar journey presents a major obstacle and existing concepts deal with this problem in different ways.^[16] They can be distinguished by the "state" in which humans are transported on-board of the spacecraft.

Generation ships

A generation ship (or world ship) is a type of interstellar ark in which the crew that arrives at the destination is descended from those who started the journey. Generation ships are not currently feasible because of the difficulty of constructing a ship of the enormous required scale and the great biological and sociological problems that life aboard such a ship raises.^{[17][18][19][20]}

Suspended animation

Scientists and writers have postulated various techniques for suspended animation. These include human hibernation and cryonic preservation. Although neither is currently practical, they offer the possibility of sleeper ships in which the passengers lie inert for the long duration of the voyage.^[21]

Extended human lifespan

A variant on this possibility is based on the development of substantial human life extension, such as the "Strategies for Engineered Negligible Senescence" proposed by Dr. Aubrey de Grey. If a ship crew had lifespans of some thousands of years, or had artificial bodies, they could traverse interstellar distances without the need to replace the crew in generations. The psychological effects of such an extended period of travel would potentially still pose a problem.

Frozen embryos

A robotic space mission carrying some number of frozen early stage human embryos is another theoretical possibility. This method of space colonization requires, among other things, the development of an artificial uterus, the prior detection of a habitable terrestrial planet, and advances in the field of fully autonomous mobile robots and educational robots that would replace human parents.^[22]

Mind uploading

A more speculative method of transporting humans to the stars is by using mind uploading or also called brain emulation.^[23][24] Frank J. Tipler speculates about the colonization of the universe by starships transporting uploaded humans.^[25] Hein presents a range of concepts how such missions could be conducted, using more or less speculative technologies, for example self-replicating machines, wormholes, and teleportation.^[23][26] One of the major challenges besides mind uploading itself are the means for downloading the uploads into physical entities, which can be biological or artficial or both.

Island hopping through interstellar space

Interstellar space is not completely empty; it contains trillions of icy bodies ranging from small asteroids (Oort cloud) to possible rogue planets. There may be ways to take advantage of these resources for a good part of an interstellar trip, slowly hopping from body to body or setting up waystations along the way.^[27]

Fast missions

If a spaceship could average 10 percent of light speed (and decelerate at the destination, for manned missions), this would be enough to reach Proxima Centauri in forty years. Several propulsion concepts are proposed that might be eventually developed to accomplish this (see section below on propulsion methods), but none of them are ready for near-term (few decades) development at acceptable cost.^{[citation needed]}

Time dilation

Assuming one cannot travel faster than light, one might conclude that a human can never make a round-trip further from Earth than 40 light years if the traveler is active between the ages of 20 and 60. In this example a traveler would never be able to reach more than the very few star systems that exist within the limit of 10–20 light years from Earth. This, however, fails to take into account time dilation. Clocks aboard an interstellar ship would run slower than Earth clocks, so if a ship's engines were powerful enough the ship could reach mostly anywhere in the galaxy and return to Earth within 40 years ship-time. Upon return, there would be a difference between the time elapsed on the astronaut's ship and the time elapsed on Earth. If a spaceship travels to a star 32 light-years away and initially accelerates at a constant 1.03g (i.e. 10.1 m/s²) for 1.32 years (ship time) then stops its engines and coasts for the next 17.3 years (ship time) at a constant speed then decelerates again for 1.32 ship-years and comes to a stop at the destination. After a short visit the astronaut returns to Earth the same way.
After the full round-trip, the clocks on board the ship show that 40 years have passed, but according to those on Earth, the ship comes back 76 years after launch.

From the viewpoint of the astronaut, on-board clocks seem to be running normally. The star ahead seems to be approaching at a speed of 0.87 lightyears per ship-year. 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.

At higher speeds, the time onboard will run even slower, so the astronaut could travel to the center of the Milky Way (30 kly from Earth) and back in 40 years ship-time. But the speed according to Earth clocks will always be less than 1 lightyear per Earth year, so, when back home, the astronaut will find that 60 thousand years will have passed on Earth.^{[citation needed]}

Constant acceleration

This plot shows a ship capable of 1-gee (10 m/s² or about 1.0 ly/y²) "felt" or proper-acceleration^[28] can go far, except for the problem of accelerating on-board propellant.

Regardless of how it is achieved, if a propulsion system can produce acceleration continuously from departure to destination, then this will be the fastest method of travel. If the propulsion system drives the ship faster and faster for the first half of the journey, then turns around and brakes the craft so that it arrives at the destination at a standstill, this is a constant acceleration journey. If this was performed at nearly 1g, this would have the added advantage of producing artificial "gravity". This is, however, largely unfeasible with current technology because of the difficulty in maintaining acceleration the closer one gets to the speed of light. This is illustrated by the definition of force: F=dp/dt. This is also a part of Newton's second law of motion.^[29]

From the planetary observer perspective the ship will appear to steadily accelerate but more slowly as it approaches the speed of light. 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.

From the ship perspective there will be no top limit on speed – the ship keeps going faster and faster the whole first half. This happens because the ship's time sense slows down – relative to the planetary observer – the more it approaches the speed of light.

The result is an impressively fast journey if you are in the ship.

By transmission

If physical entities could be transmitted as information and reconstructed at a destination, travel at nearly the speed of light would be possible, which for the "travelers" would be instantaneous. However, sending an atom-by-atom description of (say) a human body would be a daunting task. Extracting and sending only a computer brain simulation is a significant part of that problem. "Journey" time would be the light-travel time plus the time needed to encode, send and reconstruct the whole transmission.^[30]

Propulsion

Rocket concepts

All rocket concepts are limited by the rocket equation, which sets the characteristic velocity available as a function of exhaust velocity and mass ratio, the ratio of initial (M₀, including fuel) to final (M₁, fuel depleted) mass.

Very high specific power, the ratio of thrust to total vehicle mass, is required to reach interstellar targets within sub-century time-frames.^[31] Some heat transfer is inevitable and a tremendous heating load must be adequately handled.

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.^[32]

Nuclear fission powered

Fission-electric

Nuclear-electric or plasma engines, operating for long periods at low thrust and powered by fission reactors, have the potential to reach speeds much greater than chemically powered vehicles or nuclear-thermal rockets. Such vehicles probably have the potential to power Solar System exploration with reasonable trip times within the current century. Because of their low-thrust propulsion, they would be limited to off-planet, deep-space operation. 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.^[33]

Fission-fragment

Fission-fragment rockets use nuclear fission to create high-speed jets of fission fragments, which are ejected at speeds of up to 12,000 km/s. With fission, the energy output is approximately 0.1% of the total mass-energy of the reactor fuel and limits the effective exhaust velocity to about 5% of the velocity of light. For maximum velocity, the reaction mass should optimally consist of fission products, the "ash" of the primary energy source, in order that no extra reaction mass need be book-kept in the mass ratio. This is known as a fission-fragment rocket. thermal-propulsion engines such as NERVA produce sufficient thrust, but can only achieve relatively low-velocity exhaust jets, so to accelerate to the desired speed would require an enormous amount of fuel.

Nuclear pulse

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. driven by a series of nuclear explosions. This propulsion system contains the prospect of very high specific impulse (space travel's equivalent of fuel economy) and high specific power.^[34]
Project Orion team member, Freeman Dyson, proposed in 1968 an interstellar spacecraft using nuclear pulse propulsion that used pure deuterium fusion detonations with a very high fuel-burnup fraction. He computed an exhaust velocity of 15,000 km/s and a 100,000-tonne space vehicle able to achieve a 20,000 km/s delta-v allowing a flight-time to Alpha Centauri of 130 years.^[35] 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).^[36] An atomic (fission) Orion can achieve perhaps 3%-5% of the speed of light. A nuclear pulse drive starship powered by Fusion-antimatter catalyzed nuclear pulse propulsion units would be similarly in the 10% range and pure Matter-antimatter annihilation rockets would be theoretically capable of obtaining a velocity between 50% to 80% of the speed of light. In each case saving fuel for slowing down halves the max. speed. 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.^[37] Alternative designs utilizing similar principles include Project Longshot, Project Daedalus, and Mini-Mag Orion. 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.

In the 1970s the Nuclear Pulse Propulsion concept further was refined by Project Daedalus by use of externally triggered inertial confinement fusion, in this case producing fusion explosions via compressing fusion fuel pellets with high-powered electron beams. Since then, lasers, ion beams, neutral particle beams and hyper-kinetic projectiles have been suggested to produce nuclear pulses for propulsion purposes.^[38]

A current impediment to the development of any nuclear-explosion-powered spacecraft is the 1963 Partial Test Ban Treaty, which includes a prohibition on the detonation of any nuclear devices (even non-weapon based) in outer space. 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.

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. In theory, a large number of stages could push a vehicle arbitrarily close to the speed of light.^[39] These would "burn" such light element fuels as deuterium, tritium, ³He, ¹¹B, and ⁷Li. Because fusion yields about 0.3–0.9% of the mass of the nuclear fuel as released energy, it is energetically more favorable than fission, which releases <0 .1="" 4="" a="" achievable="" although="" and="" are="" as="" available="" be="" best="" c.="" centuries.="" concepts="" correspondingly="" decades="" difficulties="" easily="" energetically="" energy="" engineering="" exhaust="" fission="" for="" fraction="" fuel="" fusion="" high-energy="" higher="" however="" human="" intractable="" involve="" large="" lifetime="" long="" loss.="" mass-energy.="" massive="" maximum="" may="" most="" nearest-term="" nearest="" neutrons="" of="" offer="" or="" out="" p="" potentially="" prospects="" reactions="" release="" s="" seem="" significant="" source="" stars="" still="" technological="" than="" the="" their="" these="" they="" thus="" to="" travel="" turn="" typically="" velocities="" which="" within="">
Early studies include Project Daedalus, performed by the British Interplanetary Society in 1973–1978, and Project Longshot, a student project sponsored by NASA and the US Naval Academy, completed in 1988. Another fairly detailed vehicle system, "Discovery II",^[40] designed and optimized for crewed Solar System exploration, based on the D³He reaction but using hydrogen as reaction mass, has been described by a team from NASA's Glenn Research Center. It achieves characteristic velocities of >300 km/s with an acceleration of ~1.7•10⁻³ g, with a ship initial mass of ~1700 metric tons, and payload fraction above 10%. Although these are still far short of the requirements for interstellar travel on human timescales, the study seems to represent a reasonable benchmark towards what may be approachable within several decades, which is not impossibly beyond the current state-of-the-art. Based on the concept's 2.2% burnup fraction it could achieve a pure fusion product exhaust velocity of ~3,000 km/s.

Antimatter rockets

An antimatter rocket would have a far higher energy density and specific impulse than any other proposed class of rocket. 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 approaching that of light. Then relativistic time dilation would become more noticeable, thus making time pass at a slower rate for the travelers as perceived by an outside observer, reducing the trip time experienced by human travelers.

Supposing the production and storage of antimatter should become practical, two further problems would present and need to be solved. First, in the annihilation of antimatter, much of the energy is lost in very penetrating high-energy gamma radiation, and especially also in neutrinos, so that substantially less than mc² would actually be available if the antimatter were simply allowed to annihilate into radiations thermally. Even so, the energy available for propulsion would probably be substantially higher than the ~1% of mc² yield of nuclear fusion, the next-best rival candidate.

Second, once again heat transfer from exhaust to vehicle seems likely to deposit enormous wasted energy into the ship, considering the large fraction of the energy that goes into penetrating gamma rays. Even assuming biological shielding were provided to protect the passengers, some of the energy would inevitably heat the vehicle, and may thereby prove limiting. This requires consideration for serious proposals if useful accelerations are to be achieved, because the energies involved (e.g. for 0.1g ship acceleration, approaching 0.3 trillion watts per ton of ship mass) are very large.

Rockets with an external energy source

Rockets deriving their power from external sources, such as a laser, could bypass the ordinary rocket equation, potentially reducing the mass of the ship greatly and allowing much higher travel speeds. Geoffrey A. Landis has proposed for an interstellar probe, with energy supplied by an external laser from a base station powering an Ion thruster.^[41]

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. Some concepts attempt to escape from this problem (^[42]):

Interstellar ramjets

In 1960, Robert W. Bussard proposed the Bussard ramjet, a fusion rocket in which a huge scoop would collect the diffuse hydrogen in interstellar space, "burn" it on the fly using a proton–proton fusion reaction, and expel it out of the back. Later calculations with more accurate estimates suggest that the thrust generated would be less than the drag caused by any conceivable scoop design. 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.

Beamed propulsion

This diagram illustrates Robert L. Forward's scheme for slowing down an interstellar light-sail at the destination ^[43] system.

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. Robert L. Forward proposed a means for decelerating an interstellar light sail in the destination star system without requiring a laser array to be present in that system. In this scheme, a smaller secondary sail is deployed to the rear of the spacecraft, whereas the large primary sail is detached from the craft to keep moving forward on its own. Light is reflected from the large primary sail to the secondary sail, which is used to decelerate the secondary sail and the spacecraft payload.^[44]

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.^[45][46]

The following table lists some example concepts using beamed laser propulsion as proposed by the physicist Robert L. Forward:^[47]

Mission	Laser Power	Vehicle Mass	Acceleration	Sail Diameter	Maximum Velocity (% of the speed of light)
1. Flyby - Alpha Centauri, 40 years
outbound stage	65 GW	1 t	0.036 g	3.6 km	11% @ 0.17 ly
2. Rendezvous - Alpha Centauri, 41 years
outbound stage	7,200 GW	785 t	0.005 g	100 km	21% @ 4.29 ly
deceleration stage	26,000 GW	71 t	0.2 g	30 km	21% @ 4.29 ly
3. Manned - Epsilon Eridani, 51 years (including 5 years exploring star system)
outbound stage	75,000,000 GW	78,500 t	0.3 g	1000 km	50% @ 0.4 ly
deceleration stage	21,500,000 GW	7,850 t	0.3 g	320 km	50% @ 10.4 ly
return stage	710,000 GW	785 t	0.3 g	100 km	50% @ 10.4 ly
deceleration stage	60,000 GW	785 t	0.3 g	100 km	50% @ 0.4 ly

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. This could be achieved by multi-staged vehicles on a vast scale.^[39] Alternatively large linear accelerators could propel fuel to fission propelled space-vehicles, avoiding the limitations of the Rocket equation.^[48]

Speculative methods

Quark matter

Scientist T. Marshall Eubanks thinks that nuggets of condensed quark matter may exist at the centers of some asteroids, created during the Big Bang and each nugget with a mass of 10¹⁰ to 10¹¹ kg.^[49] If so these could be an enormous source of energy, as the nuggets could be used to generate huge quantities of antimatter—about a million tonnes of antimatter per nugget. This would be enough to propel a spacecraft close to the speed of light.^[50]

Hawking radiation rockets

In a black hole starship, a parabolic reflector would reflect Hawking radiation from an artificial black hole. In 2009, Louis Crane and Shawn Westmoreland of Kansas State University published a paper investigating the feasibility of this idea. Their conclusion was that it was on the edge of possibility, but that quantum gravity effects that are presently unknown may make it easier or make it impossible.^[51][52]

Magnetic monopole rockets

If some of the Grand unification models are correct, e.g. 't Hooft–Polyakov, it would be possible to construct a photonic engine that uses no antimatter thanks to the magnetic monopole that hypothetically can catalyze the decay of a proton to a positron and π⁰-meson:^[53][54]

π⁰ decays rapidly to two photons, and the positron annihilates with an electron to give two more photons. As a result, a hydrogen atom turns into four photons and only the problem of a mirror remains unresolved.

A magnetic monopole engine could also work on a once-through scheme such as the Bussard ramjet (see below).

At the same time, most of the modern Grand unification theories such as M-theory predict no magnetic monopoles, which casts doubt on this attractive idea.

Faster-than-light travel

Artist's depiction of a hypothetical Wormhole Induction Propelled Spacecraft, based loosely on the 1994 "warp drive" paper of Miguel Alcubierre. Credit: NASA CD-98-76634 by Les Bossinas.

Scientists and authors have postulated a number of ways by which it might be possible to surpass the speed of light. Even the most serious-minded of these are speculative.

It is also debated whether this is possible, in part, because of causality concerns, because in essence travel faster than light is equivalent to going back in time. Proposed mechanisms for faster-than-light travel within the theory of general relativity require the existence of exotic matter.

Alcubierre drive

According to Einstein's equation of general relativity, spacetime is curved:

General relativity may permit the travel of an object faster than light in curved spacetime.^[55] One could imagine exploiting the curvature to take a "shortcut" from one point to another. This is one form of the warp drive concept.

In physics, the Alcubierre drive is based on an argument that the curvature could take the form of a wave in which a spaceship might be carried in a "bubble". Space would be collapsing at one end of the bubble and expanding at the other end. The motion of the wave would carry a spaceship from one space point to another in less time than light would take through unwarped space. Nevertheless, the spaceship would not be moving faster than light within the bubble. This concept would require the spaceship to incorporate a region of exotic matter, or "negative mass".

Artificial gravity control

Scientist Lance Williams thinks that gravity can be controlled artificially through electromagnetic control.^[56]

Wormholes

Wormholes are conjectural distortions in spacetime that theorists postulate could connect two arbitrary points in the universe, across an Einstein–Rosen Bridge. It is not known whether wormholes are possible in practice. Although there are solutions to the Einstein equation of general relativity that allow for wormholes, all of the currently known solutions involve some assumption, for example the existence of negative mass, which may be unphysical.^[57] However, Cramer et al. argue that such wormholes might have been created in the early universe, stabilized by cosmic string.^[58] The general theory of wormholes is discussed by Visser in the book Lorentzian Wormholes.^[59]

Designs and studies

Enzmann starship

The Enzmann starship, as detailed by G. Harry Stine in the October 1973 issue of Analog, was a design for a future starship, based on the ideas of Dr. Robert Duncan-Enzmann.^[60] The spacecraft itself as proposed used a 12,000,000 ton ball of frozen deuterium to power 12–24 thermonuclear pulse propulsion units.^[60] Twice as long as the Empire State Building and assembled in-orbit, the spacecraft was part of a larger project preceded by interstellar probes and telescopic observation of target star systems.^[60][61]

Project Hyperion

Project Hyperion, one of the projects of Icarus Interstellar.^[62]

NASA research

NASA has been researching interstellar travel since its formation, translating important foreign language papers and conducting early studies on applying fusion propulsion, in the 1960s, and laser propulsion, in the 1970s, to interstellar travel.

The NASA Breakthrough Propulsion Physics Program (terminated in FY 2003 after a 6-year, $1.2-million study, because "No breakthroughs appear imminent.")^[63] identified some breakthroughs that are needed for interstellar travel to be possible.^[64]

Geoffrey A. Landis of NASA's Glenn Research Center states that a laser-powered interstellar sail ship could possibly be launched within 50 years, using new methods of space travel. "I think that ultimately we're going to do it, it's just a question of when and who," Landis said in an interview. Rockets are too slow to send humans on interstellar missions. Instead, he envisions interstellar craft with extensive sails, propelled by laser light to about one-tenth the speed of light. It would take such a ship about 43 years to reach Alpha Centauri, if it passed through the system. Slowing down to stop at Alpha Centauri could increase the trip to 100 years,^[65] whereas a journey without slowing down raises the issue of making sufficiently accurate and useful observations and measurements during a fly-by.

100 Year Starship study

The 100 Year Starship (100YSS) is the name of the overall effort that will, over the next century, work toward achieving interstellar travel. The effort will also go by the moniker 100YSS. The 100 Year Starship study is the name of a one year project to assess the attributes of and lay the groundwork for an organization that can carry forward the 100 Year Starship vision.

Dr. Harold ("Sonny") White^[66] from NASA's Johnson Space Center is a member of Icarus Interstellar,^[67] the nonprofit foundation whose mission is to realize interstellar flight before the year 2100. At the 2012 meeting of 100YSS, he reported using a laser to try to warp spacetime by 1 part in 10 million with the aim of helping to make interstellar travel possible.^[68]

Other designs

• Project Orion, manned interstellar ship (1958–1968).
• Project Daedalus, unmanned interstellar probe (1973–1978).
• Starwisp, unmanned interstellar probe (1985).^[69]
• Project Longshot, unmanned interstellar probe (1987–1988).
• Starseed/launcher, fleet of unmanned interstellar probes (1996)
• Project Valkyrie, manned interstellar ship (2009)
• Project Icarus, unmanned interstellar probe (2009–2014).
• Sun-diver, unmanned interstellar probe^[70]

Non-profit organisations

A few organisations dedicated to interstellar propulsion research and advocacy for the case exist worldwide. These are still in their infancy, but are already backed up by a membership of a wide variety of scientists, students and professionals.

• Icarus Interstellar ^[67]
• Tau Zero Foundation (USA) ^[71]
• Initiative for Interstellar Studies (UK) ^[72]
• Fourth Millennium Foundation (Belgium) ^[73]
• Space Development Cooperative (Canada) ^[74]

Skepticism

The energy requirements make interstellar travel very difficult. It has been reported that at the 2008 Joint Propulsion Conference, multiple experts opined that it was improbable that humans would ever explore beyond the Solar System.^[75] Brice N. Cassenti, an associate professor with the Department of Engineering and Science at Rensselaer Polytechnic Institute, stated at least the total energy output of the entire world [in a given year] would be required to send a probe to the nearest star.^[75]

Space exploration

From Wikipedia, the free encyclopedia

 

Saturn V rocket, used for the American manned lunar landing missions

The Moon as seen in a digitally processed image from data collected during a spacecraft flyby

Space exploration is the ongoing discovery and exploration of celestial structures in outer space by means of continuously evolving and growing space technology. While the study of space is carried out mainly by astronomers with telescopes, the physical exploration of space is conducted both by unmanned robotic probes and human spaceflight.

While the observation of objects in space, known as astronomy, predates reliable recorded history, it was the development of large and relatively efficient rockets during the early 20th century that allowed physical space exploration to become a reality. Common rationales for exploring space include advancing scientific research, uniting different nations, ensuring the future survival of humanity and developing military and strategic advantages against other countries.

Space exploration has often been used as a proxy competition for geopolitical rivalries such as the Cold War. The early era of space exploration was driven by a "Space Race" between the Soviet Union and the United States, the launch of the first man-made object to orbit the Earth, the USSR's Sputnik 1, on 4 October 1957, and the first Moon landing by the American Apollo 11 craft on 20 July 1969 are often taken as landmarks for this initial period. The Soviet space program achieved many of the first milestones, including the first living being in orbit in 1957, the first human spaceflight (Yuri Gagarin aboard Vostok 1) in 1961, the first spacewalk (by Aleksei Leonov) on 18 March 1965, the first automatic landing on another celestial body in 1966, and the launch of the first space station (Salyut 1) in 1971.

After the first 20 years of exploration, focus shifted from one-off flights to renewable hardware, such as the Space Shuttle program, and from competition to cooperation as with the International Space Station (ISS).

With the substantial completion of the ISS^[1] following STS-133 in March 2011, plans for space exploration by the USA remain in flux. Constellation, a Bush Administration program for a return to the Moon by 2020^[2] was judged inadequately funded and unrealistic by an expert review panel reporting in 2009.^[3] The Obama Administration proposed a revision of Constellation in 2010 to focus on the development of the capability for crewed missions beyond low earth orbit (LEO), envisioning extending the operation of the ISS beyond 2020, transferring the development of launch vehicles for human crews from NASA to the private sector, and developing technology to enable missions to beyond LEO, such as Earth/Moon L1, the Moon, Earth/Sun L2, near-earth asteroids, and Phobos or Mars orbit.^[4]

In the 2000s, the People's Republic of China initiated a successful manned spaceflight program, while the European Union, Japan, and India have also planned future manned space missions. China, Russia, Japan, and India have advocated manned missions to the Moon during the 21st century, while the European Union has advocated manned missions to both the Moon and Mars during the 21st century.

From the 1990s onwards, private interests began promoting space tourism and then private space exploration of the Moon (see Google Lunar X Prize).

History of exploration in the 20th century

Most orbital flight actually takes place in upper layers of the atmosphere, especially in the thermosphere (not to scale)

Timeline of Solar System exploration.

In July 1950 the first Bumper rocket is launched from Cape Canaveral, Florida. The Bumper was a two-stage rocket consisting of a Post-War V-2 topped by a WAC Corporal rocket. It could reach then-record altitudes of almost 400 km. Launched by General Electric Company, this Bumper was used primarily for testing rocket systems and for research on the upper atmosphere. They carried small payloads that allowed them to measure attributes including air temperature and cosmic ray impacts.

The first steps of putting a man-made object into space were taken by German scientists during World War II while testing the V-2 rocket, which became the first man-made object in space on 3 October 1942 with the launching of the A-4. After the war, the U.S. used German scientists and their captured rockets in programs for both military and civilian research. The first scientific exploration from space was the cosmic radiation experiment launched by the U.S. on a V-2 rocket on 10 May 1946.^[5] The first images of Earth taken from space followed the same year^[6][7] while the first animal experiment saw fruit flies lifted into space in 1947, both also on modified V-2s launched by Americans. Starting in 1947, the Soviets, also with the help of German teams, launched sub-orbital V-2 rockets and their own variant, the R-1, including radiation and animal experiments on some flights. These suborbital experiments only allowed a very short time in space which limited their usefulness.

First flights

Sputnik 1, the first artificial satellite orbited earth at 939 to 215 km (583 to 134 mi) in 1957, and was soon followed by Sputnik 2. See First satellite by country (Replica Pictured)

Apollo CSM in lunar orbit

Apollo 17 astronaut Harrison Schmitt standing next to a boulder at Taurus-Littrow.

The first successful orbital launch was of the Soviet unmanned Sputnik 1 ("Satellite 1") mission on 4 October 1957. The satellite weighed about 83 kg (183 lb), and is believed to have orbited Earth at a height of about 250 km (160 mi). It had two radio transmitters (20 and 40 MHz), which emitted "beeps" that could be heard by radios around the globe. Analysis of the radio signals was used to gather information about the electron density of the ionosphere, while temperature and pressure data was encoded in the duration of radio beeps. The results indicated that the satellite was not punctured by a meteoroid. Sputnik 1 was launched by an R-7 rocket. It burned up upon re-entry on 3 January 1958.

This success led to an escalation of the American space program, which unsuccessfully attempted to launch a Vanguard satellite into orbit two months later. On 31 January 1958, the U.S. successfully orbited Explorer 1 on a Juno rocket. In the meantime, the Soviet dog Laika became the first animal in orbit on 3 November 1957.

First human flights

The first successful human spaceflight was Vostok 1 ("East 1"), carrying 27 year old Russian cosmonaut Yuri Gagarin on 12 April 1961. The spacecraft completed one orbit around the globe, lasting about 1 hour and 48 minutes. Gagarin's flight resonated around the world; it was a demonstration of the advanced Soviet space program and it opened an entirely new era in space exploration: human spaceflight.

The U.S. first launched a person into space within a month of Vostok 1 with Alan Shepard's suborbital flight in Mercury-Redstone 3. Orbital flight was achieved by the United States when John Glenn's Mercury-Atlas 6 orbited the Earth on 20 February 1962.

Valentina Tereshkova, the first woman in space, orbited the Earth 48 times aboard Vostok 6 on 16 June 1963.

China first launched a person into space 42 years after the launch of Vostok 1, on 15 October 2003, with the flight of Yang Liwei aboard the Shenzhou 5 (Spaceboat 5) spacecraft.

First planetary explorations

The first artificial object to reach another celestial body was Luna 2 in 1959.^[8] The first automatic landing on another celestial body was performed by Luna 9^[9] in 1966. Luna 10 became the first artificial satellite of the Moon.^[10]

The first manned landing on another celestial body was performed by Apollo 11 in its lunar landing on 20 July 1969.

The first successful interplanetary flyby was the 1962 Mariner 2 flyby of Venus (closest approach 34,773 kilometers). Flybys for the other planets were first achieved in 1965 for Mars by Mariner 4, 1973 for Jupiter by Pioneer 10, 1974 for Mercury by Mariner 10, 1979 for Saturn by Pioneer 11, 1986 for Uranus by Voyager 2, and 1989 for Neptune by Voyager 2.

The first interplanetary surface mission to return at least limited surface data from another planet was the 1970 landing of Venera 7 on Venus which returned data to earth for 23 minutes. In 1971 the Mars 3 mission achieved the first soft landing on Mars returning data for almost 20 seconds. Later much longer duration surface missions were achieved, including over 6 years of Mars surface operation by Viking 1 from 1975 to 1982 and over 2 hours of transmission from the surface of Venus by Venera 13 in 1982, the longest ever Soviet planetary surface mission.

Key people in early space exploration

The dream of stepping into the outer reaches of the Earth's atmosphere was driven by the fiction of Jules Verne^[11][12][13] and H.G.Wells,^[14] and rocket technology was developed to try to realise this vision.

The German V-2 was the first rocket to travel into space, overcoming the problems of thrust and material failure. During the final days of World War II this technology was obtained by both the Americans and Soviets as were its designers. The initial driving force for further development of the technology was a weapons race for intercontinental ballistic missiles (ICBMs) to be used as long-range carriers for fast nuclear weapon delivery, but in 1961 when USSR launched the first man into space, the U.S. declared itself to be in a "Space Race" with the Soviets.

Konstantin Tsiolkovsky, Robert Goddard, Hermann Oberth, and Reinhold Tiling laid the groundwork of rocketry in the early years of the 20th century.

Wernher von Braun was the lead rocket engineer for Nazi Germany's World War II V-2 rocket project. In the last days of the war he led a caravan of workers in the German rocket program to the American lines, where they surrendered and were brought to the USA to work on U.S. rocket development ("Operation Paperclip"). He acquired American citizenship and led the team that developed and launched Explorer 1, the first American satellite. Von Braun later led the team at NASA's Marshall Space Flight Center which developed the Saturn V moon rocket.

Initially the race for space was often led by Sergei Korolyov, whose legacy includes both the R7 and Soyuz—which remain in service to this day. Korolev was the mastermind behind the first satellite, first man (and first woman) in orbit and first spacewalk. Until his death his identity was a closely guarded state secret; not even his mother knew that he was responsible for creating the Soviet space program.
Kerim Kerimov was one of the founders of the Soviet space program and was one of the lead architects behind the first human spaceflight (Vostok 1) alongside Sergey Korolyov. After Korolyov's death in 1966, Kerimov became the lead scientist of the Soviet space program and was responsible for the launch of the first space stations from 1971 to 1991, including the Salyut and Mir series, and their precursors in 1967, the Cosmos 186 and Cosmos 188.^[15][16]

Other key people

• Valentin Glushko held the role of Chief Engine Designer for USSR. Glushko designed many of the engines used on the early Soviet rockets, but was constantly at odds with Korolyov.
• Vasily Mishin was Chief Designer working under Sergey Korolyov and one of first Soviets to inspect the captured German V-2 design. Following the death of Sergei Korolev, Mishin was held responsible for the Soviet failure to be first country to place a man on the moon.
• Robert Gilruth was the NASA head of the Space Task Force and director of 25 manned space flights. Gilruth was the person who suggested to John F. Kennedy that the Americans take the bold step of reaching the Moon in an attempt to reclaim space superiority from the Soviets.
• Christopher C. Kraft, Jr. was NASA's first flight director, who oversaw development of Mission Control and associated technologies and procedures.
• Maxime Faget was the designer of the Mercury capsule; he played a key role in designing the Gemini and Apollo spacecraft, and contributed to the design of the Space Shuttle.

Targets of exploration

Image of the Sun from 7 June 1992 showing some sunspots

The Sun

While the Sun will probably not be physically explored in the close future, one of the reasons for going into space is to know more about the Sun. Once above the atmosphere in particular and the Earth's magnetic field, this gives access to the Solar wind and infrared and ultraviolet radiations that cannot reach the surface of the Earth. The Sun generates most space weather, which can affect power generation and transmission systems on Earth and interfere with, and even damage, satellites and space probes.

MESSENGER image of Mercury

Mercury

Mercury remains the least explored of the inner planets. As of May 2013, the Mariner 10 and MESSENGER missions have been the only missions that have made close observations of Mercury. MESSENGER entered orbit around Mercury in March 2011, to further investigate the observations made by Mariner 10 in 1975 (Munsell, 2006b).

A MESSENGER image from 18,000 km showing a region about 500 km across

A third mission to Mercury, scheduled to arrive in 2020, BepiColombo is to include two probes. BepiColombo is a joint mission between Japan and the European Space Agency. MESSENGER and BepiColombo are intended to gather complementary data to help scientists understand many of the mysteries discovered by Mariner 10's flybys.

Flights to other planets within the Solar System are accomplished at a cost in energy, which is described by the net change in velocity of the spacecraft, or delta-v. Due to the relatively high delta-v to reach Mercury and its proximity to the Sun, it is difficult to explore and orbits around it are rather unstable.

Mariner 10 image of Venus

Venus

Venus was the first target of interplanetary flyby and lander missions and, despite one of the most hostile surface environments in the solar system, has had more landers sent to it (nearly all from the Soviet Union) than any other planet in the solar system. The first successful Venus flyby was the American Mariner 2 spacecraft, which flew past Venus in 1962. Mariner 2 has been followed by several other flybys by multiple space agencies often as part of missions using a Venus flyby to provide a gravitational assist en route to other celestial bodies. In 1967 Venera 4 became the first probe to enter and directly examine the atmosphere of Venus. In 1970 Venera 7 became the first successful lander to reach the surface of Venus and by 1985 it had been followed by eight additional successful Soviet Venus landers which provided images and other direct surface data. Starting in 1975 with the Soviet orbiter Venera 9 some ten successful orbiter missions have been sent to Venus, including later missions which were able to map the surface of Venus using radar to pierce the obscuring atmosphere.

The "marble" Earth picture taken by Apollo 17

First television image of Earth from space

Earth

Space exploration has been used as a tool to understand the Earth as a celestial object in its own right. Orbital missions can provide data for the Earth that can be difficult or impossible to obtain from a purely ground-based point of reference.
For example, the existence of the Van Allen belts was unknown until their discovery by the United States' first artificial satellite, Explorer 1. These belts contain radiation trapped by the Earth's magnetic fields, which currently renders construction of habitable space stations above 1000 km impractical. Following this early unexpected discovery, a large number of Earth observation satellites have been deployed specifically to explore the Earth from a space based perspective. These satellites have significantly contributed to the understanding of a variety of earth based phenomena. For instance, the hole in the ozone layer was found by an artificial satellite that was exploring Earth's atmosphere, and satellites have allowed for the discovery of archeological sites or geological formations that were difficult or impossible to otherwise identify.

The Moon as seen from the Earth

Apollo 16 astronaut John Young

Earth's Moon

Earth's Moon was the first celestial body to be the object of space exploration. It holds the distinctions of being the first remote celestial object to be flown by, orbited, and landed upon by spacecraft, and the only remote celestial object ever to be visited by humans.
In 1959 the Soviets obtained the first images of the far side of the Moon, never previously visible to humans. The U.S. exploration of the Moon began with the Ranger 4 impactor in 1962. Starting in 1966 the Soviets successfully deployed a number of landers to the Moon which were able to obtain data directly from the Moon's surface; just four months later, Surveyor 1 marked the debut of a successful series of U.S. landers. The Soviet unmanned missions culminated in the Lunokhod program in the early '70s which included the first unmanned rovers and also successfully returned lunar soil samples to the Earth for study. This marked the first (and to date the only) automated return of extraterrestrial soil samples to the Earth. Unmanned exploration of the Moon continues with various nations periodically deploying lunar orbiters, and in 2008 the Indian Moon Impact Probe.

Manned exploration of the Moon began in 1968 with the Apollo 8 mission that successfully orbited the Moon, the first time any extraterrestrial object was orbited by humans. In 1969 the Apollo 11 mission marked the first time humans set foot upon another world. Manned exploration of the Moon did not continue for long, however. The Apollo 17 mission in 1972 marked the most recent human visit there, and the next, Exploration Mission 2, is due to orbit the Moon in 2019. Robotic missions are still pursued vigorously.

Mars as seen by the HST

Surface of mars by the Spirit rover in 2004

Mars

The exploration of Mars has been an important part of the space exploration programs of the Soviet Union (later Russia), the United States, Europe, and Japan. Dozens of robotic spacecraft, including orbiters, landers, and rovers, have been launched toward Mars since the 1960s. These missions were aimed at gathering data about current conditions and answering questions about the history of Mars. The questions raised by the scientific community are expected to not only give a better appreciation of the red planet but also yield further insight into the past, and possible future, of Earth.
The exploration of Mars has come at a considerable financial cost with roughly two-thirds of all spacecraft destined for Mars failing before completing their missions, with some failing before they even began. Such a high failure rate can be attributed to the complexity and large number of variables involved in an interplanetary journey, and has led researchers to jokingly speak of The Great Galactic Ghoul^[17] which subsists on a diet of Mars probes. This phenomenon is also informally known as the Mars Curse.^[18] In contrast to overall high failure rates in the exploration of Mars, India has become the first country to achieve success of its maiden attempt. India's Mars Orbiter Mission (MOM)^[19][20][21] is one of the least expensive interplanetary missions ever undertaken with an approximate total cost of 450 Crore (US$73 million).^[22][23]

Phobos

The Russian space mission Fobos-Grunt, which launched on 9 November 2011 experienced a failure leaving it stranded in low Earth orbit.^[24] It was to begin exploration of the Phobos and Martian circumterrestrial orbit, and study whether the moons of Mars, or at least Phobos, could be a "trans-shipment point" for spaceships travelling to Mars.^[25]

Jupiter

Voyager 1 image of Jupiter

Image of Io taken by the Galileo spacecraft

The exploration of Jupiter has consisted solely of a number of automated NASA spacecraft visiting the planet since 1973. A large majority of the missions have been "flybys", in which detailed observations are taken without the probe landing or entering orbit; the Galileo spacecraft is the only one to have orbited the planet. As Jupiter is believed to have only a relatively small rocky core and no real solid surface, a landing mission is nearly impossible.

Reaching Jupiter from Earth requires a delta-v of 9.2 km/s,^[26] which is comparable to the 9.7 km/s delta-v needed to reach low Earth orbit.^[27] Fortunately, gravity assists through planetary flybys can be used to reduce the energy required at launch to reach Jupiter, albeit at the cost of a significantly longer flight duration.^[26]

Jupiter has over 60 known moons, many of which have relatively little known information about them.

A picture of Saturn taken by Voyager 2.

Huygens image from the surface of Titan

Saturn

Saturn has been explored only through unmanned spacecraft launched by NASA, including one mission (Cassini–Huygens) planned and executed in cooperation with other space agencies. These missions consist of flybys in 1979 by Pioneer 11, in 1980 by Voyager 1, in 1982 by Voyager 2 and an orbital mission by the Cassini spacecraft which entered orbit in 2004 and is expected to continue its mission well into 2012.
Saturn has at least 62 known moons, although the exact number is debatable since Saturn's rings are made up of vast numbers of independently orbiting objects of varying sizes. The largest of the moons is Titan. Titan holds the distinction of being the only moon in the solar system with an atmosphere denser and thicker than that of the Earth. As a result of the deployment from the Cassini spacecraft of the Huygens probe and its successful landing on Titan, Titan also holds the distinction of being the only moon (apart from Earth's own Moon) to be successfully explored with a lander.

Uranus from Voyager 2

Voyager 2 image showing the tortured surface of Miranda

Uranus

The exploration of Uranus has been entirely through the Voyager 2 spacecraft, with no other visits currently planned. Given its axial tilt of 97.77°, with its polar regions exposed to sunlight or darkness for long periods, scientists were not sure what to expect at Uranus. The closest approach to Uranus occurred on 24 January 1986. Voyager 2 studied the planet's unique atmosphere and magnetosphere. Voyager 2 also examined its ring system and the moons of Uranus including all five of the previously known moons, while discovering an additional ten previously unknown moons.
Images of Uranus proved to have a very uniform appearance, with no evidence of the dramatic storms or atmospheric banding evident on Jupiter and Saturn. Great effort was required to even identify a few clouds in the images of the planet. The magnetosphere of Uranus, however, proved to be completely unique and proved to be profoundly affected by the planet's unusual axial tilt. In contrast to the bland appearance of Uranus itself, striking images were obtained of the moons of Uranus, including evidence that Miranda had been unusually geologically active.

Picture of Neptune taken by Voyager 2

Triton as imaged by Voyager 2

Neptune

The exploration of Neptune began with the 25 August 1989 Voyager 2 flyby, the sole visit to the system as of 2014. The possibility of a Neptune Orbiter has been discussed, but no other missions have been given serious thought.
Although the extremely uniform appearance of Uranus during Voyager 2's visit in 1986 had led to expectations that Neptune would also have few visible atmospheric phenomena, Voyager 2 found that Neptune had obvious banding, visible clouds, auroras, and even a conspicuous anticyclone storm system rivaled in size only by Jupiter's small Spot. Neptune also proved to have the fastest winds of any planet in the solar system, measured as high as 2,100 km/h.^[28] Voyager 2 also examined Neptune's ring and moon system. It discovered 900 complete rings and additional partial ring "arcs" around Neptune. In addition to examining Neptune's three previously known moons, Voyager 2 also discovered five previously unknown moons, one of which, Proteus, proved to be the last largest moon in the system. Data from Voyager further reinforced the view that Neptune's largest moon, Triton, is a captured Kuiper belt object.^[29]

Other objects in the Solar system

Pluto

Pluto and Charon (1994)

The dwarf planet Pluto (considered a planet until the IAU redefined "planet" in October 2006^[30]) presents significant challenges for spacecraft because of its great distance from Earth (requiring high velocity for reasonable trip times) and small mass (making capture into orbit very difficult at present). Voyager 1 could have visited Pluto, but controllers opted instead for a close flyby of Saturn's moon Titan, resulting in a trajectory incompatible with a Pluto flyby. Voyager 2 never had a plausible trajectory for reaching Pluto.^[31]

Pluto continues to be of great interest, despite its reclassification as the lead and nearest member of a new and growing class of distant icy bodies of intermediate size, in mass between the remaining eight planets and the small rocky objects historically termed asteroids (and also the first member of the important subclass, defined by orbit and known as "Plutinos"). After an intense political battle, a mission to Pluto dubbed New Horizons was granted funding from the US government in 2003.^[32] New Horizons was launched successfully on 19 January 2006. In early 2007 the craft made use of a gravity assist from Jupiter. Its closest approach to Pluto will be on 14 July 2015; scientific observations of Pluto will begin five months prior to closest approach and will continue for at least a month after the encounter.

Asteroids and comets

Comet 103P/Hartley

Asteroid 4 Vesta, imaged by the Dawn spacecraft

Until the advent of space travel, objects in the asteroid belt were merely pinpricks of light in even the largest telescopes, their shapes and terrain remaining a mystery. Several asteroids have now been visited by probes, the first of which was Galileo, which flew past two: 951 Gaspra in 1991, followed by 243 Ida in 1993. Both of these lay near enough to Galileo's planned trajectory to Jupiter that they could be visited at acceptable cost. The first landing on an asteroid was performed by the NEAR Shoemaker probe in 2000, following an orbital survey of the object. The dwarf planet Ceres and the asteroid 4 Vesta, two of the three largest asteroids, are targets of NASA's Dawn mission, launched in 2007.

While many comets have been closely studied from Earth sometimes with centuries-worth of observations, only a few comets have been closely visited. In 1985, the International Cometary Explorer conducted the first comet fly-by (21P/Giacobini-Zinner) before joining the Halley Armada studying the famous comet. The Deep Impact probe smashed into 9P/Tempel to learn more about its structure and composition while the Stardust mission returned samples of another comet's tail. The Philae lander successfully landed on comet 67P/Churyumov–Gerasimenko in 2014 as part of the broader Rosetta mission.

Hayabusa was an unmanned spacecraft developed by the Japan Aerospace Exploration Agency to return a sample of material from a small near-Earth asteroid named 25143 Itokawa to Earth for further analysis. Hayabusa was launched on 9 May 2003 and rendezvoused with Itokawa in mid-September 2005. After arriving at Itokawa, Hayabusa studied the asteroid's shape, spin, topography, colour, composition, density, and history. In November 2005, it landed on the asteroid to collect samples. The spacecraft returned to Earth on 13 June 2010.

Deep space exploration

Chandra, Hubble, and Spitzer image NGC 1952

Star cluster Pismis 24 and NGC 6357

Whirlpool Galaxy (Messier 51)

Future of space exploration

Concept art for a NASA Vision mission

In the 2000s, several plans for space exploration were announced; both government entities and the private sector have space exploration objectives. China has announced plans to have a 60-ton multi-module space station in orbit by 2020.

The NASA Authorization Act of 2010 provided a re-prioritized list of objectives for the American space program, as well as funding for the first priorities. NASA proposes to move forward with the development of the Space Launch System (SLS), which will be designed to carry the Orion Multi-Purpose Crew Vehicle, as well as important cargo, equipment, and science experiments to Earth's orbit and destinations beyond. Additionally, the SLS will serve as a back up for commercial and international partner transportation services to the International Space Station. The SLS rocket will incorporate technological investments from the Space Shuttle program and the Constellation program in order to take advantage of proven hardware and reduce development and operations costs. The first developmental flight is targeted for the end of 2017.^[33]

AI in Space Exploration

The idea of using high level automated systems for space missions has become a desirable goal to space agencies all around the world. Such systems are believed to yield benefits such as lower cost, less human oversight, and ability to explore deeper in space which is usually restricted by long communications with human controllers.^[34]

Autonomous System

Autonomy is defined by 3 requirements:^[34]

1. Being able to sense the world and their state, make decisions, and carry them out on their own
2. Can interpret the given goal as a list of actions to take
3. Fail flexibly

Benefits

Autonomed technologies would be able to perform beyond predetermined actions. It would analyze all possible states and events happening around them and come up with a safe response. In addition, such technologies can reduce launch cost and ground involvement. Performance would increase as well. Autonomy would be able to quickly respond upon encountering an unforeseen event, especially in deep space exploration where communication back to Earth would take too long.^[34]

NASA’s Autonomous Science Experiment

NASA began its autonomous science experiment (ASE) on the Earth Observing 1 (EO-1) which is NASA’s first satellite in the new millennium program Earth observing series launched on 21 November 2000. The autonomy of ASE is capable of on-board science analysis, replanning, robust execution, and later the addition of model-based diagnostic. Images obtained by the EO-1 are analyzed on-board and downlinked when a change or an interesting event occur. The ASE software has successfully provided over 10,000 science images.^[34]

Rationales

Astronaut Buzz Aldrin, had a personal Communion service when he first arrived on the surface of the Moon.

The research that is conducted by national space exploration agencies, such as NASA and Roscosmos, is one of the reasons supporters cite to justify government expenses. Economic analyses of the NASA programs often showed ongoing economic benefits (such as NASA spin-offs), generating many times the revenue of the cost of the program.^[35] It is also argued that space exploration would lead to the extraction of resources on other planets and especially asteroids, which contain billions of dollars worth of minerals and metals. The revenue generated from such expeditions could generate a lot of revenue.^[36] As well, it has been argued that space exploration programs help inspire youth to study in science and engineering.^[37]

Another claim is that space exploration is a necessity to mankind and that staying on Earth will lead to extinction. Some of the reasons are lack of natural resources, comets, nuclear war, and worldwide epidemic. Stephen Hawking, renowned British theoretical physicist, said that "I don't think the human race will survive the next thousand years, unless we spread into space. There are too many accidents that can befall life on a single planet. But I'm an optimist. We will reach out to the stars."^[38]

NASA has produced a series of public service announcement videos supporting the concept of space exploration.^[39]

Overall, the public remains largely supportive of both manned and unmanned space exploration. According to an Associated Press Poll conducted in July 2003, 71% of U.S. citizens agreed with the statement that the space program is "a good investment", compared to 21% who did not.^[40]

Arthur C. Clarke (1950) presented a summary of motivations for the human exploration of space in his non-fiction semi-technical monograph Interplanetary Flight.^[41] He argued that humanity's choice is essentially between expansion off the Earth into space, versus cultural (and eventually biological) stagnation and death.

Topics

Delta-v's in km/s for various orbital maneuvers

Spaceflight

Spaceflight is the use of space technology to achieve the flight of spacecraft into and through outer space.
Spaceflight is used in space exploration, and also in commercial activities like space tourism and satellite telecommunications. Additional non-commercial uses of spaceflight include space observatories, reconnaissance satellites and other earth observation satellites.

A spaceflight typically begins with a rocket launch, which provides the initial thrust to overcome the force of gravity and propels the spacecraft from the surface of the Earth. Once in space, the motion of a spacecraft—both when unpropelled and when under propulsion—is covered by the area of study called astrodynamics. Some spacecraft remain in space indefinitely, some disintegrate during atmospheric reentry, and others reach a planetary or lunar surface for landing or impact.

Satellites

Satellites are used for a large number of purposes. Common types include military (spy) and civilian Earth observation satellites, communication satellites, navigation satellites, weather satellites, and research satellites. Space stations and human spacecraft in orbit are also satellites.

Commercialization of space

Current examples of the commercial use of space include satellite navigation systems, satellite television and satellite radio. Space tourism is the recent phenomenon of space travel by individuals for the purpose of personal pleasure.

Alien life

Astrobiology is the interdisciplinary study of life in the universe, combining aspects of astronomy, biology and geology.^[42] It is focused primarily on the study of the origin, distribution and evolution of life. It is also known as exobiology (from Greek: έξω, exo, "outside").^[43][44][45] The term "Xenobiology" has been used as well, but this is technically incorrect because its terminology means "biology of the foreigners".^[46] Astrobiologists must also consider the possibility of life that is chemically entirely distinct from any life found on earth.^[47] In the Solar System some of the prime locations for current or past astrobiology are on Enceladus, Europa, Mars, and Titan.^[48]

Living in space

The European Space Agency's Columbus Module at the International Space Station, launched into space on the U.S. Space Shuttle mission STS-122 in 2008

Space colonization, also called space settlement and space humanization, would be the permanent autonomous (self-sufficient) human habitation of locations outside Earth, especially of natural satellites or planets such as the Moon or Mars, using significant amounts of in-situ resource utilization.

To date, the longest human occupation of space is the International Space Station which has been in continuous use for 14 years, 69 days. Valeri Polyakov's record single spaceflight of almost 438 days aboard the Mir space station has not been surpassed. Long-term stays in space reveal issues with bone and muscle loss in low gravity, immune system suppression, and radiation exposure.

Many past and current concepts for the continued exploration and colonization of space focus on a return to the Moon as a "stepping stone" to the other planets, especially Mars. At the end of 2006 NASA announced they were planning to build a permanent Moon base with continual presence by 2024.^[49]

Beyond the technical factors that could make living in space more widespread, it has been suggested that the lack of private property, the inability or difficulty in establishing property rights in space, has been an impediment to the development of space for human habitation. Since the advent of space technology in the latter half of the twentieth century, the ownership of property in space has been murky, with strong arguments both for and against. In particular, the making of national territorial claims in outer space and on celestial bodies has been specifically proscribed by the Outer Space Treaty, which had been, as of 2012, ratified by all spacefaring nations.^[50]