Interplanetary travel
Encyclopedia
Interplanetary spaceflight or interplanetary travel is travel
between planets within a single planetary system
. In practice, spaceflight
s of this type are confined to travel between the planets of the Solar System
.
s have flown by all of the planets of the Solar System from Mercury to Neptune, with the New Horizons
probe currently en route to fly by the dwarf planet Pluto
and the Dawn spacecraft en route to the dwarf planet Ceres. The four most distant spacecraft (Pioneer 10
, Pioneer 11
, Voyager 1
and Voyager 2
) are on course to leave the Solar System.
In general, planetary orbiters and landers return much more detailed and comprehensive information than fly-by missions. Space probes have been placed into orbit around all the five planets known to the ancients: first Mars
(Mariner 9
, 1971), then Venus
(Pioneer Venus Orbiter 1978), Jupiter
(Galileo 1995), Saturn
(Cassini/Huygens 2004), and most recently Mercury
(MESSENGER
, March 2011), and have returned data about these bodies and their natural satellite
s.
The NEAR Shoemaker
mission in 2000 orbited the large near-Earth asteroid 433 Eros
, and was even successfully landed there, though it had not been designed with this maneuver in mind. The Japanese ion-drive spacecraft Hayabusa
in 2005 also orbited the small near-Earth asteroid 25143 Itokawa
, landing on it briefly and returning grains of its surface material to Earth. Another powerful ion-drive mission, Dawn, is in orbit of the large asteroid Vesta
in July 2011 and will later move on to the dwarf planet Ceres in 2015.
Remotely-controlled landers such as Viking
, Pathfinder
and the two Mars Exploration Rover
s have landed on the surface of Mars and several Venera
and Vega
spacecraft have landed on the surface of Venus. The Huygens probe
successfully landed on Saturn's moon, Titan
.
No manned missions have been sent to any other planet of the Solar System. NASA
's Apollo program, however, landed twelve people on the Moon
and returned them to Earth
. The American Vision for Space Exploration
, originally introduced by President George W. Bush
and put into practice through the Constellation program, had as a long term goal to eventually send human astronauts to Mars. However on February 1, 2010, President Barack Obama proposed cancelling the program in Fiscal Year 2011. An earlier project which received some significant planning by NASA included a manned fly-by of Venus in the Manned Venus Flyby
mission, but was cancelled when the Apollo Applications Program
was terminated due to NASA budget cuts in the late 1960s.
, Deep Space 2
and Beagle 2
(the article List of Solar System probes gives a full list).
Many astronomers, geologists and biologists believe that exploration of the Solar System provides knowledge that could not be gained by observations from Earth's surface or from orbit around Earth. But they disagree about whether manned missions make a useful scientific contribution — some think robotic probes are cheaper and safer, while others argue that either astronauts advised by Earth-based scientists, or spacefaring scientists advised by Earth-based scientists, can respond more flexibly and intelligently to new or unexpected features of the region they are exploring.
Those who pay for such missions (primarily in the public sector) are more likely to be interested in benefits for themselves or for the human race as a whole. So far the only benefits of this type have been "spin-off" technologies which were developed for space missions and then were found to be at least as useful in other activities (NASA publicizes spin-offs from its activities).
Other practical motivations for interplanetary travel are more speculative, because our current technologies are not yet advanced enough to support test projects. But science fiction
writers have a fairly good track record in predicting future technologies — for example geosynchronous communications satellites
(Arthur C. Clarke
) and many aspects of computer technology (Mack Reynolds
).
Many science fiction stories (notably Ben Bova
's Grand Tour
stories) feature detailed descriptions of how people could extract minerals from asteroid
s and energy from sources including orbital solar panel
s (unhampered by clouds) and the very strong magnetic field
of Jupiter. Some point out that such techniques may be the only way to provide rising standards of living without being stopped by pollution or by depletion of Earth's resources (for example peak oil
).
Finally, colonizing other parts of the Solar System would prevent the whole human species from being exterminated by any one of a number of possible events (see Human extinction
). One of these possible events is an asteroid impact like the one which may have resulted in the Cretaceous–Tertiary extinction event. Although various Spaceguard
projects monitor the Solar System for objects that might come dangerously close to Earth, current asteroid deflection strategies
are crude and untested. To make the task more difficult, carbonaceous chondrite
s are rather sooty and therefore very hard to detect. Although carbonaceous chondrites are thought to be rare, some are very large and the suspected "dinosaur-killer
" may have been a carbonaceous chondrite.
Some scientists, including members of the Space Studies Institute
, argue that the vast majority of mankind eventually will live in space and will benefit from doing this.
Doing this by brute force – accelerating in the shortest route to the destination and then, if it is farther from the Sun, decelerating to match the planet's speed – would require an extremely large amount of fuel. And the fuel required for deceleration and velocity-matching has to be launched along with the payload, and therefore even more fuel is needed in the acceleration phase.
The change in speed (delta-v
) required to match velocity with another planet is surprisingly large. For example Venus orbits about 5.2 km/s faster than Earth
and Mars orbits about 5.7 km/s slower. To put these figures in perspective, Earth's escape velocity
is about 11.2 km/second. So matching a space shuttle
's velocity with that of Venus or Mars would require a significant percentage of the energy which is used to launch a shuttle from Earth's surface.
For many years economical interplanetary travel meant using the Hohmann transfer orbit
. Hohmann demonstrated that the lowest energy route between any two orbits is an elliptical
"orbit" which forms a tangent
to the starting and destination orbits. Once the spacecraft arrives, a second application of thrust will re-circularize the orbit at the new location. In the case of planetary transfers this means directing the spacecraft, originally in an orbit almost identical to Earth's, so that the aphelion of the transfer orbit is on the far side of the Sun near the orbit of the other planet. A spacecraft traveling from Earth to Mars via this method will arrive near Mars orbit in approximately 18 months, but because the orbital velocity is greater when closer to the center of mass (i.e. the Sun) and slower when farther from the center, the spacecraft will be traveling quite slowly and a small application of thrust is all that is needed to put it into a circular obit around Mars. If the manoeuver is timed properly, Mars will be "arriving" under the spacecraft when this happens.
The Hohmann transfer applies to any two orbits, not just those with planets involved. For instance it is the most common way to transfer satellites into geostationary orbit
, after first being "parked" in low earth orbit
. However the Hohmann transfer takes an amount of time similar to ½ of the orbital period of the outer orbit, so in the case of the outer planets this is many years – too long to wait. It is also based on the assumption that the points at both ends are massless, as in the case when transferring between two orbits around Earth for instance. With a planet at the destination end of the transfer, calculations become considerably more difficult.
The gravitational slingshot technique uses the gravity of planets and moons to change the speed and direction of a spacecraft without using fuel. In typical example, a spacecraft is sent to a distant planet on a path that is much faster than what the Hohmann transfer would call for. This would typically mean that it would arrive at the planet's orbit and continue past it. However, if there is a planet between the departure point and the target, it can be used to bend the path toward the target, and in many cases the overall travel time is greatly reduced. A prime example of this are the two craft of the Voyager program
, which used slingshot effects to change trajectories several times in the outer Solar System. It is difficult to use this method for journeys in the inner part of the Solar System, although it is possible to use other nearby planets such as Venus or even the Moon
as slingshots in journeys to the outer planets.
This maneuver can only change an object's velocity relative to a third, uninvolved object, – possibly the “centre of mass” or the Sun. There is no change in the velocities of the two objects involved in the maneuver relative to each other. The Sun cannot be used in a gravitational slingshot because it is stationary compared to rest of the Solar System, which orbits the Sun. It may be used to send a spaceship or probe into the galaxy because the Sun rotates around the center of the Milky Way.
s were first proposed (1925) and were slow, expensive and unreliable when gravitational slingshot
s were developed (1959). Recent advances in computing
have made it possible to exploit many more features of the gravity fields of astronomical bodies and thus calculate even lower-cost trajectories. Paths have been calculated which link the Lagrange points of the various planets into the so-called Interplanetary Transport Network
. Such "fuzzy orbits" use significantly less energy than Hohmann transfers but are often much slower. They may not offer much advantage for manned missions or for research missions, but may be useful for high-volume transport of low-value commodities
when humanity develops a space-based economy.
Aerobraking
uses the atmosphere of the target planet to slow down. It was first used on the Apollo program where the returning spacecraft did not enter Earth orbit
but instead used a series of passes through Earth's atmosphere to reduce its speed until it was safe to land. Aerobraking does not require a thick atmosphere – for example most Mars landers use the technique, and Mars' atmosphere is only about 1% as thick as Earth's.
Aerobraking converts the spacecraft's kinetic energy
into heat, so it requires a heatshield to prevent the craft from burning up. As a result, aerobraking is only helpful in cases where the fuel needed to transport the heatshield to the planet is less than the fuel that would be required to brake an unshielded craft by firing its engines.
Besides making travel faster, such improvements would allow greater design "safety margins" by reducing the imperative to make spacecraft lighter.
In a nuclear thermal rocket
or solar thermal rocket
a working fluid, usually hydrogen
, is heated to a high temperature, and then expands through a rocket nozzle
to create thrust
. The energy replaces the chemical energy of the reactive chemicals in a traditional rocket engine
. Due to the low molecular mass and hence high thermal velocity of hydrogen these engines are at least twice as fuel efficient as chemical engines, even after including the weight of the reactor.
The US Atomic Energy Commission
and NASA tested a few designs from 1959 to 1968 . The NASA designs were conceived as replacements for the upper stages of the Saturn V
launch vehicle, but the tests revealed reliability problems, mainly caused by the vibration and heating involved in running the engines at such high thrust levels. Political and environmental considerations make it unlikely such an engine will be used in the foreseeable future, since nuclear thermal rockets would be most useful at or near the Earth's surface and the consequences of a malfunction could be disastrous. Fission based thermal rocket concepts produce lower exhaust velocities than the electric and plasma concepts described below, and are less suitable except for applications requiring high thrust-to-weight ratio, as in planetary escape.
or solar cell
s to generate electricity
, which is then used to accelerate a chemically inert propellant to speeds far higher than achieved in a chemical rocket. Such drives produce feeble thrust, and are therefore unsuitable for quick maneuvers or for launching from the surface of a planet. But they are so economical in their use of
reaction mass that they can keep firing continuously for days or weeks, while chemical rockets use up reaction mass so quickly that they can only fire for seconds or minutes. Even a trip to the Moon is long enough for an electric propulsion system to outrun a chemical rocket – the Apollo missions took 3 days in each direction.
NASA's Deep Space One
was a very successful test of a prototype ion drive, which fired for a total of 678 days and enabled the probe to run down Comet Borrelly, a feat which would have been impossible for a chemical rocket. Dawn, the first NASA operational (i.e., non-technology demonstration) mission to use an ion drive for its primary propulsion, is currently on track to explore and orbit the large main-belt asteroids 1 Ceres
and 4 Vesta
. A more ambitious, nuclear-powered version was intended for an unmanned Jupiter mission, the Jupiter Icy Moons Orbiter
(JIMO), originally planned for launch sometime in the next decade. Due to a shift in priorities at NASA that favored manned space missions, the project lost funding in 2005. A similar mission is currently under discussion as the US component of a joint NASA/ESA program for the exploration of Europa
and Ganymede
.
A NASA multi-center Technology Applications Assessment Team led from the Johnson Spaceflight Center, has as of January 2011 described "Nautilus-X", a concept study for a multi-mission space exploration vehicle useful for missions beyond low earth orbit
(LEO), of up to 24 months duration for a crew of up to six. Although Nautilus-X
is adaptable to a variety of mission-specific propulsion units of various low-thrust, high specific impulse
(Isp) designs, nuclear ion-electric drive is shown for illustrative purposes. It is intended for integration and checkout at the International Space Station
(ISS), and would be suitable for deep-space missions from the ISS to and beyond the Moon, including Earth/Moon L1
, Sun/Earth L2
, near-Earth asteroidal
, and Mars orbital destinations. It incorporates a reduced-g centrifuge providing artificial gravity for crew health to ameliorate the effects of long-term 0g exposure, and the capability to mitigate the space radiation environment.
s, powered by nuclear fusion
reactions, would "burn" such light element fuels as deuterium, tritium, or 3He. Because fusion yields about 1% of the mass of the nuclear fuel as released energy, it is energetically more favorable than fission, which releases only about 0.1% of the fuel's mass-energy. However, either fission or fusion technologies can in principle achieve velocities far higher than needed for Solar System exploration, and fusion energy still awaits practical demonstration on Earth.
One proposal using a fusion (nuclear bomb) rocket was Project Daedalus
. Another fairly detailed vehicle system, designed and optimized for crewed Solar System exploration, "Discovery II", 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%.
is small and decreases by the square of the distance from the Sun, but unlike rockets, solar sails require no fuel. Although the thrust is small, it continues as long as the Sun shines and the sail is deployed.
The original concept relied only on radiation from the Sun – for example in Arthur C. Clarke
's 1965 story "Sunjammer
". More recent light sail designs propose to boost the thrust by aiming ground-based laser
s or maser
s at the sail. Ground-based laser
s or maser
s can also help a light-sail spacecraft to decelerate: the sail splits into an outer and inner section, the outer section is pushed forward and its shape is changed mechanically to focus reflected radiation on the inner portion, and the radiation focused on the inner section acts as a brake.
Although most articles about light sails focus on interstellar travel
, there have been several proposals for their use within the Solar System.
Currently, the only spacecraft to use a solar sail as the main method of production is IKAROS
which was launched by JAXA on May 21, 2010. It has since been successfully deployed, and shown to be producing acceleration as expected. Many ordinary spacecraft and satellites also use solar collectors, temperature-control panels and Sun shades as light sails, to make minor corrections to their attitude and orbit without using fuel. A few have even had small purpose-built solar sails for this use (for example Eurostar E3000 geostationary communications satellites built by EADS Astrium
).
would synchronously cycle between Mars and Earth, with very little propellant usage to maintain the trajectory. Cyclers are conceptually a good idea, because massive radiation shields, life support and other equipment only need to be put onto the cycler trajectory once. A cycler could combine several roles: habitat (for example it could spin to produce an "artificial gravity" effect); mothership (providing life support for the crews of smaller spacecraft which hitch a ride on it). Cyclers' main limitation would be that they would be slow, because they would rely on gravitational techniques such as Hohmann transfer orbit
s and gravitational slingshot
s.
is a structure designed to transport material from a planet's surface into orbit. The fundamental idea is that, once the expensive job of building the elevator is complete, an indefinite number of loads can be transported into orbit at minimal cost. Even the simplest designs avoid the vicious circle
of rocket launches from the surface, the difficulty that: the fuel needed to travel the last 10% of the distance to orbit must be lifted all the way from the surface; that requires extra fuel; most of the extra fuel must be lifted most of the way before it is burned; that requires more extra fuel; and so on. More sophisticated space elevator designs reduce the energy cost per trip by using counterweight
s, and the most ambitious schemes aim to balance loads going up and down and thus make the energy cost close to zero. Space elevators have also sometimes been referred to as "beanstalks", "space bridges", "space lifts", "space ladders" or "orbital towers".
A terrestrial space elevator is beyond our current technology, although a lunar space elevator
could theoretically be built using existing materials.
Current space vehicles attempt to launch with all their fuel (propellants and energy supplies) on-board that they will need for their entire journey, and current space structures are lifted from the Earth's surface. Non-terrestrial sources of energy and materials
are mostly a lot further away, but most would not require lifting out of a strong gravity field and therefore should be much cheaper to use in space in the long term.
The most important non-terrestrial resource is energy, because it can be used to transform non-terrestrial materials into useful forms (some of which may also produce energy). At least two fundamental non-terrestrial energy sources have been proposed: solar-powered energy generation (unhampered by clouds), either directly by solar cell
s or indirectly by focusing solar radiation on boilers which produce steam to drive generators; and electrodynamic tether
s which generate electricity from the powerful magnetic fields of some planets (Jupiter has a very powerful magnetic field).
Water ice would be very useful and is widespread on the moons of Jupiter and Saturn:
Oxygen is a common constituent of the moon
's crust, and is probably abundant in most other bodies in the Solar System. Non-terrestrial oxygen would be valuable as a source of water ice only if an adequate source of hydrogen
can be found. Possible uses include:
Unfortunately hydrogen, along with other volatiles like carbon and nitrogen, are much less abundant than oxygen in the inner Solar System.
Scientists expect to find a vast range of organic compound
s in some of the planets, moons and comets of the outer Solar System, and the range of possible uses is even wider. For example methane
can be used as a fuel (burned with non-terrestrial oxygen), or as a feedstock for petrochemical
processes such as making plastic
s. And ammonia
could be a valuable feedstock for producing fertilizer
s to be used in the vegetable gardens of orbital and planetary bases, reducing the need to lift food to them from Earth.
Even unprocessed rock may be useful as rocket propellant if mass drivers are employed.
article for a discussion of a number of other technologies that could, in the medium to longer term, be the basis of interplanetary missions. Unlike the situation with interstellar travel
, the barriers to fast interplanetary travel involve engineering and economics rather than any basic physics.
and the protection of Earth's magnetosphere, it enters the Van Allen radiation belt
, a region of high radiation. Once through there the radiation drops to lower levels, with a constant background of high energy cosmic rays which pose a health threat
. These are dangerous over periods of years to decades.
Scientists of Russian Academy of Sciences
are searching for methods of reducing the risk of radiation
-induced cancer
in preparation for the mission to Mars. They consider as one of the options a life support system generating drinking water with low content of Deuterium
(stabile isotope of hydrogen
) to be consumed by the crew members. Preliminary investigations have shown that Deuterium-depleted water features certain anti-cancer effects. Hence, Deuterium-free drinking water is considered to have the potential of lowering the risk of cancer caused by extreme radiation exposure of the Martian crew.
In addition, Coronal mass ejections from the Sun
are highly dangerous, and are fatal within a very short timescale to humans unless they are protected by massive shielding
.
mission survived despite an explosion caused by a faulty oxygen tank (1970); the crews of Soyuz 11
(1971), the Space Shuttle
s Challenger
(1986) and Columbia
(2003) were killed by malfunctions of their vessels' components.
reasons, cheap spacecraft travel to other planets is only practical within certain time windows
. Outside these windows the planets are essentially inaccessible from Earth with current technology. This constrains flights and prevents rescue in an emergency.
Travel
Travel is the movement of people or objects between relatively distant geographical locations. 'Travel' can also include relatively short stays between successive movements.-Etymology:...
between planets within a single planetary system
Planetary system
A planetary system consists of the various non-stellar objects orbiting a star such as planets, dwarf planets , asteroids, meteoroids, comets, and cosmic dust...
. In practice, spaceflight
Spaceflight
Spaceflight is the act of travelling into or through outer space. Spaceflight can occur with spacecraft which may, or may not, have humans on board. Examples of human spaceflight include the Russian Soyuz program, the U.S. Space shuttle program, as well as the ongoing International Space Station...
s of this type are confined to travel between the planets of the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
.
Current achievements in interplanetary travel
Remotely guided space probeSpace probe
A robotic spacecraft is a spacecraft with no humans on board, that is usually under telerobotic control. A robotic spacecraft designed to make scientific research measurements is often called a space probe. Many space missions are more suited to telerobotic rather than crewed operation, due to...
s have flown by all of the planets of the Solar System from Mercury to Neptune, with the New Horizons
New Horizons
New Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, Hydra and S/2011 P 1. Its estimated arrival date at the Pluto-Charon system is July 14th, 2015...
probe currently en route to fly by the dwarf planet Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...
and the Dawn spacecraft en route to the dwarf planet Ceres. The four most distant spacecraft (Pioneer 10
Pioneer 10
Pioneer 10 is a 258-kilogram robotic space probe that completed the first interplanetary mission to Jupiter, and became the first spacecraft to achieve escape velocity from the Solar System. The project was managed by the NASA Ames Research Center and the contract for the construction of the...
, Pioneer 11
Pioneer 11
Pioneer 11 is a 259-kilogram robotic space probe launched by NASA on April 6, 1973 to study the asteroid belt, the environment around Jupiter and Saturn, solar wind, cosmic rays, and eventually the far reaches of the solar system and heliosphere...
, Voyager 1
Voyager 1
The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...
and Voyager 2
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...
) are on course to leave the Solar System.
In general, planetary orbiters and landers return much more detailed and comprehensive information than fly-by missions. Space probes have been placed into orbit around all the five planets known to the ancients: first Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
(Mariner 9
Mariner 9
Mariner 9 was a NASA space orbiter that helped in the exploration of Mars and was part of the Mariner program. Mariner 9 was launched toward Mars on May 30, 1971 from Cape Canaveral Air Force Station and reached the planet on November 13 of the same year, becoming the first spacecraft to orbit...
, 1971), then Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
(Pioneer Venus Orbiter 1978), Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
(Galileo 1995), Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
(Cassini/Huygens 2004), and most recently Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
(MESSENGER
MESSENGER
The MErcury Surface, Space ENvironment, GEochemistry and Ranging space probe is a robotic NASA spacecraft in orbit around the planet Mercury. The spacecraft was launched aboard a Delta II rocket in August 2004 to study the chemical composition, geology, and magnetic field of Mercury...
, March 2011), and have returned data about these bodies and their natural satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....
s.
The NEAR Shoemaker
NEAR Shoemaker
The Near Earth Asteroid Rendezvous - Shoemaker , renamed after its 1996 launch in honor of planetary scientist Eugene M. Shoemaker, was a robotic space probe designed by the Johns Hopkins University Applied Physics Laboratory for NASA to study the near-Earth asteroid Eros from close orbit over a...
mission in 2000 orbited the large near-Earth asteroid 433 Eros
433 Eros
433 Eros is a near-Earth asteroid discovered in 1898, and the first asteroid to be orbited by a probe . It is an S-type asteroid approximately 34.4×11.2×11.2 km in size, the second-largest NEA after 1036 Ganymed, and belongs to the Amor group.Eros is a Mars-crosser asteroid, the first known...
, and was even successfully landed there, though it had not been designed with this maneuver in mind. The Japanese ion-drive spacecraft Hayabusa
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....
in 2005 also orbited the small near-Earth asteroid 25143 Itokawa
25143 Itokawa
25143 Itokawa is an Apollo and Mars-crosser asteroid. It was the first asteroid to be the target of a sample return mission, the Japanese space probe Hayabusa.-Discovery and naming:...
, landing on it briefly and returning grains of its surface material to Earth. Another powerful ion-drive mission, Dawn, is in orbit of the large asteroid Vesta
4 Vesta
Vesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta....
in July 2011 and will later move on to the dwarf planet Ceres in 2015.
Remotely-controlled landers such as Viking
Viking program
The Viking program consisted of a pair of American space probes sent to Mars, Viking 1 and Viking 2. Each spacecraft was composed of two main parts, an orbiter designed to photograph the surface of Mars from orbit, and a lander designed to study the planet from the surface...
, Pathfinder
Mars Pathfinder
Mars Pathfinder was an American spacecraft that landed a base station with roving probe on Mars in 1997. It consisted of a lander, renamed the Carl Sagan Memorial Station, and a lightweight wheeled robotic rover named Sojourner.Launched on December 4, 1996 by NASA aboard a Delta II booster a...
and the two Mars Exploration Rover
Mars Exploration Rover
NASA's Mars Exploration Rover Mission is an ongoing robotic space mission involving two rovers, Spirit and Opportunity, exploring the planet Mars...
s have landed on the surface of Mars and several Venera
Venera
The Venera series probes were developed by the Soviet Union between 1961 and 1984 to gather data from Venus, Venera being the Russian name for Venus...
and Vega
Vega program
The Vega program was a series of Venus missions which also took advantage of the appearance of Comet Halley in 1986. Vega 1 and Vega 2 were unmanned spacecraft launched in a cooperative effort among the Soviet Union and Austria, Bulgaria, Hungary, the German Democratic Republic, Poland,...
spacecraft have landed on the surface of Venus. The Huygens probe
Huygens probe
The Huygens probe was an atmospheric entry probe carried to Saturn's moon Titan as part of the Cassini–Huygens mission. The probe was supplied by the European Space Agency and named after the Dutch 17th century astronomer Christiaan Huygens....
successfully landed on Saturn's moon, Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....
.
No manned missions have been sent to any other planet of the Solar System. NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
's Apollo program, however, landed twelve people on the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
and returned them to Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
. The American Vision for Space Exploration
Vision for Space Exploration
The Vision for Space Exploration is the United States space policy which was announced on January 14, 2004 by President George W. Bush. It is seen as a response to the Space Shuttle Columbia disaster, the state of human spaceflight at NASA, and a way to regain public enthusiasm for space...
, originally introduced by President George W. Bush
George W. Bush
George Walker Bush is an American politician who served as the 43rd President of the United States, from 2001 to 2009. Before that, he was the 46th Governor of Texas, having served from 1995 to 2000....
and put into practice through the Constellation program, had as a long term goal to eventually send human astronauts to Mars. However on February 1, 2010, President Barack Obama proposed cancelling the program in Fiscal Year 2011. An earlier project which received some significant planning by NASA included a manned fly-by of Venus in the Manned Venus Flyby
Manned Venus Flyby
A manned Venus flyby was considered by NASA in the mid 1960s as part of the Apollo Applications Program, using hardware derived from the Apollo program...
mission, but was cancelled when the Apollo Applications Program
Apollo Applications program
The Apollo Applications Program was established by NASA headquarters in 1968 to develop science-based manned space missions using surplus material from the Apollo program...
was terminated due to NASA budget cuts in the late 1960s.
Reasons for interplanetary travel
The costs and risk of interplanetary travel receive a lot of publicity — spectacular examples include the malfunctions or complete failures of unmanned probes such as Mars 96Mars 96
Mars 96 was a failed Mars mission launched in 1996 to investigate Mars by the Russian Space Forces and not directly related to the Soviet Mars probe program of the same name. After failure of the second fourth-stage burn, the probe assembly re-entered the Earth's atmosphere, breaking up over a...
, Deep Space 2
Deep Space 2
Deep Space 2 was a NASA probe which was part of the New Millennium Program. It included two highly advanced miniature space probes which were sent to Mars aboard the Mars Polar Lander in January 1999. The probes were named "Scott" and "Amundsen", in honor of Robert Falcon Scott and Roald Amundsen,...
and Beagle 2
Beagle 2
Beagle 2 was an unsuccessful British landing spacecraft that formed part of the European Space Agency's 2003 Mars Express mission. All contact with it was lost upon its separation from the Mars Express six days before its scheduled entry into the atmosphere...
(the article List of Solar System probes gives a full list).
Many astronomers, geologists and biologists believe that exploration of the Solar System provides knowledge that could not be gained by observations from Earth's surface or from orbit around Earth. But they disagree about whether manned missions make a useful scientific contribution — some think robotic probes are cheaper and safer, while others argue that either astronauts advised by Earth-based scientists, or spacefaring scientists advised by Earth-based scientists, can respond more flexibly and intelligently to new or unexpected features of the region they are exploring.
Those who pay for such missions (primarily in the public sector) are more likely to be interested in benefits for themselves or for the human race as a whole. So far the only benefits of this type have been "spin-off" technologies which were developed for space missions and then were found to be at least as useful in other activities (NASA publicizes spin-offs from its activities).
Other practical motivations for interplanetary travel are more speculative, because our current technologies are not yet advanced enough to support test projects. But science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...
writers have a fairly good track record in predicting future technologies — for example geosynchronous communications satellites
Geosynchronous orbit
A geosynchronous orbit is an orbit around the Earth with an orbital period that matches the Earth's sidereal rotation period...
(Arthur C. Clarke
Arthur C. Clarke
Sir Arthur Charles Clarke, CBE, FRAS was a British science fiction author, inventor, and futurist, famous for his short stories and novels, among them 2001: A Space Odyssey, and as a host and commentator in the British television series Mysterious World. For many years, Robert A. Heinlein,...
) and many aspects of computer technology (Mack Reynolds
Mack Reynolds
Dallas McCord "Mack" Reynolds was an American science fiction writer. His pen names included Clark Collins, Mark Mallory, Guy McCord, Dallas Ross and Maxine Reynolds. Many of his stories were published in Galaxy Magazine and Worlds of If Magazine...
).
Many science fiction stories (notably Ben Bova
Ben Bova
Benjamin William Bova is an American science-fiction author and editor. He is the recipient of six Hugo Awards for Best Professional Editor for his work at Analog Science Fiction in the 1970's.-Personal life:...
's Grand Tour
Grand Tour (novel series)
-Summary:The novels present a theme of exploration and colonization of the solar system by humans in the late 21st century. Most of the books focus on the exploration of one particular planet or planetary moon....
stories) feature detailed descriptions of how people could extract minerals from asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
s and energy from sources including orbital solar panel
Photovoltaic module
A solar panel is a packaged, connected assembly of solar cells, also known as photovoltaic cells...
s (unhampered by clouds) and the very strong magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
of Jupiter. Some point out that such techniques may be the only way to provide rising standards of living without being stopped by pollution or by depletion of Earth's resources (for example peak oil
Peak oil
Peak oil is the point in time when the maximum rate of global petroleum extraction is reached, after which the rate of production enters terminal decline. This concept is based on the observed production rates of individual oil wells, projected reserves and the combined production rate of a field...
).
Finally, colonizing other parts of the Solar System would prevent the whole human species from being exterminated by any one of a number of possible events (see Human extinction
Human extinction
Human extinction is the end of the human species. Various scenarios have been discussed in science, popular culture, and religion . The scope of this article is existential risks. Humans are very widespread on the Earth, and live in communities which are capable of some kind of basic survival in...
). One of these possible events is an asteroid impact like the one which may have resulted in the Cretaceous–Tertiary extinction event. Although various Spaceguard
Spaceguard
The term Spaceguard loosely refers to a number of efforts to discover and study near-Earth objects . Asteroids are discovered by telescopes which repeatedly survey large areas of sky. Efforts which concentrate on discovering NEOs are considered part of the "Spaceguard Survey," regardless of which...
projects monitor the Solar System for objects that might come dangerously close to Earth, current asteroid deflection strategies
Asteroid deflection strategies
Asteroid mitigation strategies are "planetary defense" methods by which near-Earth objects could be diverted, preventing potentially catastrophic impact events. A sufficiently large impact would cause massive tsunamis or an impact winter, or both...
are crude and untested. To make the task more difficult, carbonaceous chondrite
Carbonaceous chondrite
Carbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 7 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites...
s are rather sooty and therefore very hard to detect. Although carbonaceous chondrites are thought to be rare, some are very large and the suspected "dinosaur-killer
Chicxulub Crater
The Chicxulub crater is an ancient impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is located near the town of Chicxulub, after which the crater is named...
" may have been a carbonaceous chondrite.
Some scientists, including members of the Space Studies Institute
Space Studies Institute
Space Studies Institute is a non-profit organization that was founded in 1977 by the late Princeton University Professor Dr. Gerard K. O'Neill. The stated mission is to "open the energy and material resources of space for human benefit within our lifetime"...
, argue that the vast majority of mankind eventually will live in space and will benefit from doing this.
Economical travel techniques
Interplanetary travel has to solve two problems, other than escaping from the planet of origin:- The planet from which the spaceship starts is moving round the Sun at a different speed than the planet to which the spaceship is traveling, because the two planets are at different distances from the Sun. So as it approaches its destination, the spaceship must increase its speed if the destination is closer to the Sun, or decrease its speed if the destination is further away (assuming a Hohmann transfer orbit).
- If the destination is farther away, the spaceship must lift itself "up" against the force of the Sun's gravity.
Doing this by brute force – accelerating in the shortest route to the destination and then, if it is farther from the Sun, decelerating to match the planet's speed – would require an extremely large amount of fuel. And the fuel required for deceleration and velocity-matching has to be launched along with the payload, and therefore even more fuel is needed in the acceleration phase.
The change in speed (delta-v
Delta-v
In astrodynamics a Δv or delta-v is a scalar which takes units of speed. It is a measure of the amount of "effort" that is needed to change from one trajectory to another by making an orbital maneuver....
) required to match velocity with another planet is surprisingly large. For example Venus orbits about 5.2 km/s faster than Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
and Mars orbits about 5.7 km/s slower. To put these figures in perspective, Earth's escape velocity
Escape velocity
In physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero gravitational potential energy is negative since gravity is an attractive force and the potential is defined to be zero at infinity...
is about 11.2 km/second. So matching a space shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...
's velocity with that of Venus or Mars would require a significant percentage of the energy which is used to launch a shuttle from Earth's surface.
Hohmann transfers
Hohmann transfer orbit
In orbital mechanics, the Hohmann transfer orbit is an elliptical orbit used to transfer between two circular orbits, typically both in the same plane....
. Hohmann demonstrated that the lowest energy route between any two orbits is an elliptical
Ellipse
In geometry, an ellipse is a plane curve that results from the intersection of a cone by a plane in a way that produces a closed curve. Circles are special cases of ellipses, obtained when the cutting plane is orthogonal to the cone's axis...
"orbit" which forms a tangent
Tangent
In geometry, the tangent line to a plane curve at a given point is the straight line that "just touches" the curve at that point. More precisely, a straight line is said to be a tangent of a curve at a point on the curve if the line passes through the point on the curve and has slope where f...
to the starting and destination orbits. Once the spacecraft arrives, a second application of thrust will re-circularize the orbit at the new location. In the case of planetary transfers this means directing the spacecraft, originally in an orbit almost identical to Earth's, so that the aphelion of the transfer orbit is on the far side of the Sun near the orbit of the other planet. A spacecraft traveling from Earth to Mars via this method will arrive near Mars orbit in approximately 18 months, but because the orbital velocity is greater when closer to the center of mass (i.e. the Sun) and slower when farther from the center, the spacecraft will be traveling quite slowly and a small application of thrust is all that is needed to put it into a circular obit around Mars. If the manoeuver is timed properly, Mars will be "arriving" under the spacecraft when this happens.
The Hohmann transfer applies to any two orbits, not just those with planets involved. For instance it is the most common way to transfer satellites into geostationary orbit
Geostationary orbit
A geostationary orbit is a geosynchronous orbit directly above the Earth's equator , with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers...
, after first being "parked" in low earth orbit
Low Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...
. However the Hohmann transfer takes an amount of time similar to ½ of the orbital period of the outer orbit, so in the case of the outer planets this is many years – too long to wait. It is also based on the assumption that the points at both ends are massless, as in the case when transferring between two orbits around Earth for instance. With a planet at the destination end of the transfer, calculations become considerably more difficult.
Gravitational slingshot
Voyager program
The Voyager program is a U.S program that launched two unmanned space missions, scientific probes Voyager 1 and Voyager 2. They were launched in 1977 to take advantage of a favorable planetary alignment of the late 1970s...
, which used slingshot effects to change trajectories several times in the outer Solar System. It is difficult to use this method for journeys in the inner part of the Solar System, although it is possible to use other nearby planets such as Venus or even the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
as slingshots in journeys to the outer planets.
This maneuver can only change an object's velocity relative to a third, uninvolved object, – possibly the “centre of mass” or the Sun. There is no change in the velocities of the two objects involved in the maneuver relative to each other. The Sun cannot be used in a gravitational slingshot because it is stationary compared to rest of the Solar System, which orbits the Sun. It may be used to send a spaceship or probe into the galaxy because the Sun rotates around the center of the Milky Way.
Powered slingshot
A powered slingshot is the use of a rocket engine at or around closest approach to a body (periapsis). The use at this point multiplies up the effect of the delta-v, and gives a bigger effect than at other times.Fuzzy orbits
Computers did not exist when Hohmann transfer orbitHohmann transfer orbit
In orbital mechanics, the Hohmann transfer orbit is an elliptical orbit used to transfer between two circular orbits, typically both in the same plane....
s were first proposed (1925) and were slow, expensive and unreliable when gravitational slingshot
Gravitational slingshot
In orbital mechanics and aerospace engineering, a gravitational slingshot, gravity assist maneuver, or swing-by is the use of the relative movement and gravity of a planet or other celestial body to alter the path and speed of a spacecraft, typically in order to save propellant, time, and expense...
s were developed (1959). Recent advances in computing
Computing
Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...
have made it possible to exploit many more features of the gravity fields of astronomical bodies and thus calculate even lower-cost trajectories. Paths have been calculated which link the Lagrange points of the various planets into the so-called Interplanetary Transport Network
Interplanetary Transport Network
The Interplanetary Transport Network is a collection of gravitationally determined pathways through the solar system that require very little energy for an object to follow. The ITN makes particular use of Lagrange points as locations where trajectories through space are redirected using little...
. Such "fuzzy orbits" use significantly less energy than Hohmann transfers but are often much slower. They may not offer much advantage for manned missions or for research missions, but may be useful for high-volume transport of low-value commodities
Commodity
In economics, a commodity is the generic term for any marketable item produced to satisfy wants or needs. Economic commodities comprise goods and services....
when humanity develops a space-based economy.
Aerobraking
Aerobraking
Aerobraking is a spaceflight maneuver that reduces the high point of an elliptical orbit by flying the vehicle through the atmosphere at the low point of the orbit . The resulting drag slows the spacecraft...
uses the atmosphere of the target planet to slow down. It was first used on the Apollo program where the returning spacecraft did not enter Earth orbit
but instead used a series of passes through Earth's atmosphere to reduce its speed until it was safe to land. Aerobraking does not require a thick atmosphere – for example most Mars landers use the technique, and Mars' atmosphere is only about 1% as thick as Earth's.
Aerobraking converts the spacecraft's kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
into heat, so it requires a heatshield to prevent the craft from burning up. As a result, aerobraking is only helpful in cases where the fuel needed to transport the heatshield to the planet is less than the fuel that would be required to brake an unshielded craft by firing its engines.
Improved travel technologies
Several technologies have been proposed which both save fuel and provide significantly faster travel than Hohmann transfers. Most are still just theoretical, but the Deep Space One mission was a very successful test of an ion drive. These improved technologies focus on one or more of:- Space propulsion systems with much better fuel economy. Such systems would make it possible to travel much faster while keeping the fuel cost within acceptable limits.
- Using solar energy and In-Situ Resource UtilizationIn-Situ Resource UtilizationIn space exploration, in-situ resource utilization describes the proposed use of resources found or manufactured on other astronomical objects to further the goals of a space mission....
to avoid or minimize the expensive task of shipping components and fuel up from the Earth's surface, against the Earth's gravity (see "Using non-terrestrial resources", below).
Besides making travel faster, such improvements would allow greater design "safety margins" by reducing the imperative to make spacecraft lighter.
Improved 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, of initial (M0, including fuel) to final (M1, fuel depleted) mass. The main consequence is that mission velocities of more than a few times the velocity of the rocket motor exhaust (with respect to the vehicle) rapidly become impractical.Nuclear thermal and solar thermal rockets
Nuclear thermal rocket
In a nuclear thermal rocket a working fluid, usually liquid hydrogen, is heated to a high temperature in a nuclear reactor, and then expands through a rocket nozzle to create thrust. In this kind of thermal rocket, the nuclear reactor's energy replaces the chemical energy of the propellant's...
or solar thermal rocket
Solar thermal rocket
Solar thermal propulsion is a form of spacecraft propulsion that makes use of solar power to directly heat reaction mass, and therefore does not require an electrical generator as most other forms of solar-powered propulsion do. A solar thermal rocket only has to carry the means of capturing solar...
a working fluid, usually hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
, is heated to a high temperature, and then expands through a rocket nozzle
Nozzle
A nozzle is a device designed to control the direction or characteristics of a fluid flow as it exits an enclosed chamber or pipe via an orifice....
to create thrust
Thrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....
. The energy replaces the chemical energy of the reactive chemicals in a traditional rocket engine
Rocket engine
A rocket engine, or simply "rocket", is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...
. Due to the low molecular mass and hence high thermal velocity of hydrogen these engines are at least twice as fuel efficient as chemical engines, even after including the weight of the reactor.
The US Atomic Energy Commission
United States Atomic Energy Commission
The United States Atomic Energy Commission was an agency of the United States government established after World War II by Congress to foster and control the peace time development of atomic science and technology. President Harry S...
and NASA tested a few designs from 1959 to 1968 . The NASA designs were conceived as replacements for the upper stages of the Saturn V
Saturn V
The Saturn V was an American human-rated expendable rocket used by NASA's Apollo and Skylab programs from 1967 until 1973. A multistage liquid-fueled launch vehicle, NASA launched 13 Saturn Vs from the Kennedy Space Center, Florida with no loss of crew or payload...
launch vehicle, but the tests revealed reliability problems, mainly caused by the vibration and heating involved in running the engines at such high thrust levels. Political and environmental considerations make it unlikely such an engine will be used in the foreseeable future, since nuclear thermal rockets would be most useful at or near the Earth's surface and the consequences of a malfunction could be disastrous. Fission based thermal rocket concepts produce lower exhaust velocities than the electric and plasma concepts described below, and are less suitable except for applications requiring high thrust-to-weight ratio, as in planetary escape.
Electric propulsion
Electric propulsion systems use an external source such as a nuclear reactorNuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
or solar cell
Solar cell
A solar cell is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect....
s to generate electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
, which is then used to accelerate a chemically inert propellant to speeds far higher than achieved in a chemical rocket. Such drives produce feeble thrust, and are therefore unsuitable for quick maneuvers or for launching from the surface of a planet. But they are so economical in their use of
reaction mass that they can keep firing continuously for days or weeks, while chemical rockets use up reaction mass so quickly that they can only fire for seconds or minutes. Even a trip to the Moon is long enough for an electric propulsion system to outrun a chemical rocket – the Apollo missions took 3 days in each direction.
NASA's Deep Space One
Deep Space 1
Deep Space 1 is a spacecraft of the NASA New Millennium Program dedicated to testing a payload of advanced, high risk technologies....
was a very successful test of a prototype ion drive, which fired for a total of 678 days and enabled the probe to run down Comet Borrelly, a feat which would have been impossible for a chemical rocket. Dawn, the first NASA operational (i.e., non-technology demonstration) mission to use an ion drive for its primary propulsion, is currently on track to explore and orbit the large main-belt asteroids 1 Ceres
1 Ceres
Ceres, formally 1 Ceres, is the smallest identified dwarf planet in the Solar System and the only one in the asteroid belt. With a diameter of about 950 km, Ceres is by far the largest and most-massive asteroid, comprising about a third of the mass of the asteroid belt. Discovered on 1 January 1801...
and 4 Vesta
4 Vesta
Vesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta....
. A more ambitious, nuclear-powered version was intended for an unmanned Jupiter mission, the Jupiter Icy Moons Orbiter
Jupiter Icy Moons Orbiter
The Jupiter Icy Moons Orbiter was a proposed spacecraft designed to explore the icy moons of Jupiter. The main target was Europa, the suspected ocean of which is one of the places where simple alien life is a possibility in our solar system...
(JIMO), originally planned for launch sometime in the next decade. Due to a shift in priorities at NASA that favored manned space missions, the project lost funding in 2005. A similar mission is currently under discussion as the US component of a joint NASA/ESA program for the exploration of Europa
Europa (moon)
Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...
and Ganymede
Ganymede (moon)
Ganymede is a satellite of Jupiter and the largest moon in the Solar System. It is the seventh moon and third Galilean satellite outward from Jupiter. Completing an orbit in roughly seven days, Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively...
.
A NASA multi-center Technology Applications Assessment Team led from the Johnson Spaceflight Center, has as of January 2011 described "Nautilus-X", a concept study for a multi-mission space exploration vehicle useful for missions beyond low earth orbit
Low Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...
(LEO), of up to 24 months duration for a crew of up to six. Although Nautilus-X
Nautilus-X
Nautilus-X is a multi-mission space exploration vehicle imagined by the Technology Applications Assessment Team by NASA....
is adaptable to a variety of mission-specific propulsion units of various low-thrust, high specific impulse
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...
(Isp) designs, nuclear ion-electric drive is shown for illustrative purposes. It is intended for integration and checkout at the International Space Station
International Space Station
The International Space Station is a habitable, artificial satellite in low Earth orbit. The ISS follows the Salyut, Almaz, Cosmos, Skylab, and Mir space stations, as the 11th space station launched, not including the Genesis I and II prototypes...
(ISS), and would be suitable for deep-space missions from the ISS to and beyond the Moon, including Earth/Moon L1
Lagrangian point
The Lagrangian points are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects...
, Sun/Earth L2
Lagrangian point
The Lagrangian points are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects...
, near-Earth asteroidal
Near-Earth object
A near-Earth object is a Solar System object whose orbit brings it into close proximity with the Earth. All NEOs have a perihelion distance less than 1.3 AU. They include a few thousand near-Earth asteroids , near-Earth comets, a number of solar-orbiting spacecraft, and meteoroids large enough to...
, and Mars orbital destinations. It incorporates a reduced-g centrifuge providing artificial gravity for crew health to ameliorate the effects of long-term 0g exposure, and the capability to mitigate the space radiation environment.
Fission powered rockets
The electric propulsion missions already flown, or currently scheduled, have used solar electric power, limiting their capability to operate far from the Sun, and also limiting their peak acceleration due to the mass of the electric power source. Nuclear-electric or plasma engines, operating for long periods at low thrust and powered by fission reactors, can reach speeds much greater than chemically powered vehicles. Such vehicles probably have the potential to power Solar System exploration with trip times quite comparable to the great 16th through 18th century explorations, within the current century.Fusion rockets
Fusion rocketFusion rocket
A fusion rocket is a theoretical design for a rocket driven by fusion power which could provide efficient and long-term acceleration in space without the need to carry a large fuel supply. The design relies on the development of fusion power technology beyond current capabilities, and the...
s, powered by nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
reactions, would "burn" such light element fuels as deuterium, tritium, or 3He. Because fusion yields about 1% of the mass of the nuclear fuel as released energy, it is energetically more favorable than fission, which releases only about 0.1% of the fuel's mass-energy. However, either fission or fusion technologies can in principle achieve velocities far higher than needed for Solar System exploration, and fusion energy still awaits practical demonstration on Earth.
One proposal using a fusion (nuclear bomb) rocket was Project Daedalus
Project Daedalus
Project Daedalus was a study conducted between 1973 and 1978 by the British Interplanetary Society to design a plausible unmanned interstellar spacecraft. Intended mainly as a scientific probe, the design criteria specified that the spacecraft had to use current or near-future technology and had to...
. Another fairly detailed vehicle system, designed and optimized for crewed Solar System exploration, "Discovery II", based on the D3He reaction but using hydrogen as reaction mass, has been described by a team from NASA's Glenn Research Center
Glenn Research Center
NASA John H. Glenn Research Center at Lewis Field is a NASA center, located within the cities of Brook Park, Cleveland and Fairview Park, Ohio between Cleveland Hopkins International Airport and the Cleveland Metroparks's Rocky River Reservation, and has other subsidiary facilities in Ohio...
. 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%.
Solar sails
Solar sails rely on the fact that light reflected from a surface exerts pressure on the surface. The radiation pressureRadiation pressure
Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...
is small and decreases by the square of the distance from the Sun, but unlike rockets, solar sails require no fuel. Although the thrust is small, it continues as long as the Sun shines and the sail is deployed.
The original concept relied only on radiation from the Sun – for example in Arthur C. Clarke
Arthur C. Clarke
Sir Arthur Charles Clarke, CBE, FRAS was a British science fiction author, inventor, and futurist, famous for his short stories and novels, among them 2001: A Space Odyssey, and as a host and commentator in the British television series Mysterious World. For many years, Robert A. Heinlein,...
's 1965 story "Sunjammer
Sunjammer
"Sunjammer" is a science fiction short story by English author Arthur C. Clarke. It was originally published in 1963. It has also been published under the title "The Wind from the Sun" and has been included into Clarke's 1972 collection of short stories with this title.-Plot summary:John...
". More recent light sail designs propose to boost the thrust by aiming ground-based laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
s or maser
Maser
A maser is a device that produces coherent electromagnetic waves through amplification by stimulated emission. Historically, “maser” derives from the original, upper-case acronym MASER, which stands for "Microwave Amplification by Stimulated Emission of Radiation"...
s at the sail. Ground-based laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
s or maser
Maser
A maser is a device that produces coherent electromagnetic waves through amplification by stimulated emission. Historically, “maser” derives from the original, upper-case acronym MASER, which stands for "Microwave Amplification by Stimulated Emission of Radiation"...
s can also help a light-sail spacecraft to decelerate: the sail splits into an outer and inner section, the outer section is pushed forward and its shape is changed mechanically to focus reflected radiation on the inner portion, and the radiation focused on the inner section acts as a brake.
Although most articles about light sails focus on interstellar travel
Interstellar travel
Interstellar space travel is manned or unmanned travel between stars. The concept of interstellar travel in starships is a staple of science fiction. Interstellar travel is much more difficult than interplanetary travel. Intergalactic travel, or travel between different galaxies, is even more...
, there have been several proposals for their use within the Solar System.
Currently, the only spacecraft to use a solar sail as the main method of production is IKAROS
IKAROS
IKAROS is a Japan Aerospace Exploration Agency experimental spacecraft. The spacecraft was launched on 21 May, 2010, aboard an H-IIA rocket, together with the Akatsuki probe and four other small spacecraft...
which was launched by JAXA on May 21, 2010. It has since been successfully deployed, and shown to be producing acceleration as expected. Many ordinary spacecraft and satellites also use solar collectors, temperature-control panels and Sun shades as light sails, to make minor corrections to their attitude and orbit without using fuel. A few have even had small purpose-built solar sails for this use (for example Eurostar E3000 geostationary communications satellites built by EADS Astrium
EADS Astrium
Astrium Satellites is one of the three business units of Astrium, a subsidiary of EADS. It is a European space manufacturer involved in the manufacture of spacecraft used for science, Earth observation and telecommunication, as well as the equipment and subsystems used therein and related ground...
).
Cyclers
It is possible to put stations or spacecraft on orbits that cycle between different planets, for example a Mars cyclerMars cycler
A Mars cycler is a special kind of spacecraft trajectory that encounters Earth and Mars on a regular basis. The term Mars cycler may also refer to a spacecraft on a Mars cycler trajectory...
would synchronously cycle between Mars and Earth, with very little propellant usage to maintain the trajectory. Cyclers are conceptually a good idea, because massive radiation shields, life support and other equipment only need to be put onto the cycler trajectory once. A cycler could combine several roles: habitat (for example it could spin to produce an "artificial gravity" effect); mothership (providing life support for the crews of smaller spacecraft which hitch a ride on it). Cyclers' main limitation would be that they would be slow, because they would rely on gravitational techniques such as Hohmann transfer orbit
Hohmann transfer orbit
In orbital mechanics, the Hohmann transfer orbit is an elliptical orbit used to transfer between two circular orbits, typically both in the same plane....
s and gravitational slingshot
Gravitational slingshot
In orbital mechanics and aerospace engineering, a gravitational slingshot, gravity assist maneuver, or swing-by is the use of the relative movement and gravity of a planet or other celestial body to alter the path and speed of a spacecraft, typically in order to save propellant, time, and expense...
s.
Space elevator
A space elevatorSpace elevator
A space elevator, also known as a geostationary orbital tether or a beanstalk, is a proposed non-rocket spacelaunch structure...
is a structure designed to transport material from a planet's surface into orbit. The fundamental idea is that, once the expensive job of building the elevator is complete, an indefinite number of loads can be transported into orbit at minimal cost. Even the simplest designs avoid the vicious circle
Vicious Circle
Vicious Circle is an album released in 1994 by L.A. Guns. Most of the songs have Phil Lewis on lead vocals, but the track "Nothing Better to Do" features Kelly Nickels on lead vocals, and "Tarantula" is instrumental. MC Bones drums on several songs. Lewis and Bones also played together in the band...
of rocket launches from the surface, the difficulty that: the fuel needed to travel the last 10% of the distance to orbit must be lifted all the way from the surface; that requires extra fuel; most of the extra fuel must be lifted most of the way before it is burned; that requires more extra fuel; and so on. More sophisticated space elevator designs reduce the energy cost per trip by using counterweight
Counterweight
A counterweight is an equivalent counterbalancing weight that balances a load.-Uses:A counterweight is often used in traction lifts , cranes and funfair rides...
s, and the most ambitious schemes aim to balance loads going up and down and thus make the energy cost close to zero. Space elevators have also sometimes been referred to as "beanstalks", "space bridges", "space lifts", "space ladders" or "orbital towers".
A terrestrial space elevator is beyond our current technology, although a lunar space elevator
Lunar space elevator
A lunar space elevator is a proposed cable running from the surface of the Moon into space.It is similar in concept to the better known Earth space elevator idea...
could theoretically be built using existing materials.
Using non-terrestrial resources
- See main article In-situ resource utilizationIn-Situ Resource UtilizationIn space exploration, in-situ resource utilization describes the proposed use of resources found or manufactured on other astronomical objects to further the goals of a space mission....
Current space vehicles attempt to launch with all their fuel (propellants and energy supplies) on-board that they will need for their entire journey, and current space structures are lifted from the Earth's surface. Non-terrestrial sources of energy and materials
In-Situ Resource Utilization
In space exploration, in-situ resource utilization describes the proposed use of resources found or manufactured on other astronomical objects to further the goals of a space mission....
are mostly a lot further away, but most would not require lifting out of a strong gravity field and therefore should be much cheaper to use in space in the long term.
The most important non-terrestrial resource is energy, because it can be used to transform non-terrestrial materials into useful forms (some of which may also produce energy). At least two fundamental non-terrestrial energy sources have been proposed: solar-powered energy generation (unhampered by clouds), either directly by solar cell
Solar cell
A solar cell is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect....
s or indirectly by focusing solar radiation on boilers which produce steam to drive generators; and electrodynamic tether
Electrodynamic tether
Electrodynamic tethers are long conducting wires, such as one deployed from a tether satellite, which can operate on electromagnetic principles as generators, by converting their kinetic energy to electrical energy, or as motors, converting electrical energy to kinetic energy...
s which generate electricity from the powerful magnetic fields of some planets (Jupiter has a very powerful magnetic field).
Water ice would be very useful and is widespread on the moons of Jupiter and Saturn:
- The low gravity of these moons would make them a cheaper source of water for space stations and planetary bases than lifting it up from Earth's surface.
- Non-terrestrial power supplies could be used to electrolyseElectrolysisIn chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...
water ice into oxygen and hydrogen for use in bipropellant rocket engines. - Nuclear thermal rocketNuclear thermal rocketIn a nuclear thermal rocket a working fluid, usually liquid hydrogen, is heated to a high temperature in a nuclear reactor, and then expands through a rocket nozzle to create thrust. In this kind of thermal rocket, the nuclear reactor's energy replaces the chemical energy of the propellant's...
s or Solar thermal rocketSolar thermal rocketSolar thermal propulsion is a form of spacecraft propulsion that makes use of solar power to directly heat reaction mass, and therefore does not require an electrical generator as most other forms of solar-powered propulsion do. A solar thermal rocket only has to carry the means of capturing solar...
s could use it as reaction mass. Hydrogen has also been proposed for use in these engines and would provide much greater specific impulseSpecific impulseSpecific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...
(thrust per kilogram of reaction mass), but it has been claimed that water will beat hydrogen in cost/performance terms despite its much lower specific impulse by orders of magnitude.
Oxygen is a common constituent of the moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
's crust, and is probably abundant in most other bodies in the Solar System. Non-terrestrial oxygen would be valuable as a source of water ice only if an adequate source of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
can be found. Possible uses include:
- In the life support systemLife support systemIn human spaceflight, a life support system is a group of devices that allow a human being to survive in space.US government space agency NASA,and private spaceflight companies...
s of space ships, space stations and planetary bases. - In rocket engines. Even if the other propellant has to be lifted from Earth, using non-terrestrial oxygen could reduce propellant launch costs by up to 2/3 for hydrocarbon fuel, or 85% for hydrogen. The savings are so high because oxygen accounts for the majority of the mass in most rocket propellantRocket propellantRocket propellant is mass that is stored in some form of propellant tank, prior to being used as the propulsive mass that is ejected from a rocket engine in the form of a fluid jet to produce thrust. A fuel propellant is often burned with an oxidizer propellant to produce large volumes of very hot...
combinations.
Unfortunately hydrogen, along with other volatiles like carbon and nitrogen, are much less abundant than oxygen in the inner Solar System.
Scientists expect to find a vast range of organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
s in some of the planets, moons and comets of the outer Solar System, and the range of possible uses is even wider. For example methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
can be used as a fuel (burned with non-terrestrial oxygen), or as a feedstock for petrochemical
Petrochemical
Petrochemicals are chemical products derived from petroleum. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewable sources such as corn or sugar cane....
processes such as making plastic
Plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...
s. And ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
could be a valuable feedstock for producing fertilizer
Fertilizer
Fertilizer is any organic or inorganic material of natural or synthetic origin that is added to a soil to supply one or more plant nutrients essential to the growth of plants. A recent assessment found that about 40 to 60% of crop yields are attributable to commercial fertilizer use...
s to be used in the vegetable gardens of orbital and planetary bases, reducing the need to lift food to them from Earth.
Even unprocessed rock may be useful as rocket propellant if mass drivers are employed.
Exotic propulsion
See the spacecraft propulsionSpacecraft propulsion
Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the...
article for a discussion of a number of other technologies that could, in the medium to longer term, be the basis of interplanetary missions. Unlike the situation with interstellar travel
Interstellar travel
Interstellar space travel is manned or unmanned travel between stars. The concept of interstellar travel in starships is a staple of science fiction. Interstellar travel is much more difficult than interplanetary travel. Intergalactic travel, or travel between different galaxies, is even more...
, the barriers to fast interplanetary travel involve engineering and economics rather than any basic physics.
Life support
Life support systems must be capable of supporting human life for weeks, months or even years. A breathable atmosphere of at least 35 kPa (5psi) must be maintained, with adequate amounts of oxygen, nitrogen, and controlled levels of carbon dioxide, trace gases and water vapor.Radiation
Once a vehicle leaves low earth orbitLow Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...
and the protection of Earth's magnetosphere, it enters the Van Allen radiation belt
Van Allen radiation belt
The Van Allen radiation belt is a torus of energetic charged particles around Earth, which is held in place by Earth's magnetic field. It is believed that most of the particles that form the belts come from solar wind, and other particles by cosmic rays. It is named after its discoverer, James...
, a region of high radiation. Once through there the radiation drops to lower levels, with a constant background of high energy cosmic rays which pose a health threat
Health threat from cosmic rays
The health threat from cosmic rays is the danger posed by galactic cosmic rays and solar energetic particles to astronauts on interplanetary missions.Galactic cosmic rays consist of high energy protons and other nuclei with extrasolar origin...
. These are dangerous over periods of years to decades.
Scientists of Russian Academy of Sciences
Russian Academy of Sciences
The Russian Academy of Sciences consists of the national academy of Russia and a network of scientific research institutes from across the Russian Federation as well as auxiliary scientific and social units like libraries, publishers and hospitals....
are searching for methods of reducing the risk of radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
-induced cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
in preparation for the mission to Mars. They consider as one of the options a life support system generating drinking water with low content of Deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
(stabile isotope of hydrogen
Isotopes of hydrogen
Hydrogen has three naturally occurring isotopes, sometimes denoted 1H, 2H, and 3H. Other, highly unstable nuclei have been synthesized in the laboratory but not observed in nature. The most stable radioisotope is tritium, with a half-life of 12.32 years...
) to be consumed by the crew members. Preliminary investigations have shown that Deuterium-depleted water features certain anti-cancer effects. Hence, Deuterium-free drinking water is considered to have the potential of lowering the risk of cancer caused by extreme radiation exposure of the Martian crew.
In addition, Coronal mass ejections from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
are highly dangerous, and are fatal within a very short timescale to humans unless they are protected by massive shielding
.
Reliability
Any major failure to a spacecraft en route is likely to be fatal, and even a minor one could have dangerous results if not repaired quickly, something difficult to accomplish in open space. The crew of the Apollo 13Apollo 13
Apollo 13 was the seventh manned mission in the American Apollo space program and the third intended to land on the Moon. The craft was launched on April 11, 1970, at 13:13 CST. The landing was aborted after an oxygen tank exploded two days later, crippling the service module upon which the Command...
mission survived despite an explosion caused by a faulty oxygen tank (1970); the crews of Soyuz 11
Soyuz 11
Soyuz 11 was the first manned mission to arrive at the world's first space station, Salyut 1. The mission arrived at the space station on June 7, 1971 and departed on June 30, 1971. The mission ended in disaster when the crew capsule depressurized during preparations for re-entry, killing the...
(1971), the Space Shuttle
Space Shuttle program
NASA's Space Shuttle program, officially called Space Transportation System , was the United States government's manned launch vehicle program from 1981 to 2011...
s Challenger
Space Shuttle Challenger
Space Shuttle Challenger was NASA's second Space Shuttle orbiter to be put into service, Columbia having been the first. The shuttle was built by Rockwell International's Space Transportation Systems Division in Downey, California...
(1986) and Columbia
Space Shuttle Columbia
Space Shuttle Columbia was the first spaceworthy Space Shuttle in NASA's orbital fleet. First launched on the STS-1 mission, the first of the Space Shuttle program, it completed 27 missions before being destroyed during re-entry on February 1, 2003 near the end of its 28th, STS-107. All seven crew...
(2003) were killed by malfunctions of their vessels' components.
Launch windows
For astrodynamicsAstrodynamics
Orbital mechanics or astrodynamics is the application of ballistics and celestial mechanics to the practical problems concerning the motion of rockets and other spacecraft. The motion of these objects is usually calculated from Newton's laws of motion and Newton's law of universal gravitation. It...
reasons, cheap spacecraft travel to other planets is only practical within certain time windows
Launch window
Launch window is a term used in spaceflight to describe a time period in which a particular launch vehicle must be launched. If the rocket does not launch within the "window", it has to wait for the next window....
. Outside these windows the planets are essentially inaccessible from Earth with current technology. This constrains flights and prevents rescue in an emergency.
See also
- Spacecraft propulsionSpacecraft propulsionSpacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the...
- delta-vDelta-vIn astrodynamics a Δv or delta-v is a scalar which takes units of speed. It is a measure of the amount of "effort" that is needed to change from one trajectory to another by making an orbital maneuver....
- List of interplanetary voyages
- Manned mission to MarsManned mission to MarsA manned mission to Mars has been the subject of science fiction, engineering, and scientific proposals throughout the 20th century and into the 21st century...
- Interstellar travelInterstellar travelInterstellar space travel is manned or unmanned travel between stars. The concept of interstellar travel in starships is a staple of science fiction. Interstellar travel is much more difficult than interplanetary travel. Intergalactic travel, or travel between different galaxies, is even more...
- Health threat from cosmic raysHealth threat from cosmic raysThe health threat from cosmic rays is the danger posed by galactic cosmic rays and solar energetic particles to astronauts on interplanetary missions.Galactic cosmic rays consist of high energy protons and other nuclei with extrasolar origin...
- Mars to StayMars to StayMars to Stay missions propose astronauts sent to Mars for the first time should stay there indefinitely, both to reduce cost and to ensure permanent settlement of Mars. Among many notable Mars to Stay advocates, former Apollo astronaut Buzz Aldrin has been particularly outspoken, suggesting in...