Interplanetary spaceflight

Interplanetary spaceflight or interplanetary travel is travel between planets within a single planetary system. In practice, spaceflights of this type are confined to travel between the planets of the Solar System.

Current achievements in interplanetary travel

Remotely guided space probes have flown by all of the planets of the Solar system from Mercury to Neptune. The four most distant spacecraft (Pioneer 10, Pioneer 11, Voyager 1 and Voyager 2) are on course to leave the Solar system.

Space probes have also been inserted into orbit around the planets Venus, Mars, Jupiter, and Saturn, and have returned data about these bodies and their natural satellites. Further probes are currently "en route" to orbit Mercury, the dwarf planet Ceres and the large asteroid Vesta.

Remotely controlled landers such as Viking, Pathfinder and the two Mars Exploration Rovers have landed on the surface of Mars and several Venera and Vega spacecraft have landed on the surface of Venus. The NEAR Shoemaker orbiter successfully landed on the asteroid 433 Eros, even though it was not designed with this maneuver in mind. 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.

Reasons for interplanetary travel

The costs and risks of interplanetary travel receive a lot of publicity — spectacular examples include the malfunctions or complete failures of unmanned probes such as Mars 96, 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 round 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 astronauts, can respond more flexibly and intelligently to new or unexpected features of the region they are exploring. [cite web | url=http://zuserver2.star.ucl.ac.uk/~iac/spaceflight.html | title=The Scientific Case for Human Spaceflight | last=Crawford | first=I.A. | date=1998 | journal=Astronomy and Geophysics | pages=14-17 ]

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 ( [http://www.sti.nasa.gov/tto/ 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 asteroids and energy from sources including orbital solar panels (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 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 chondrites 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. [cite web | url=http://ssi.org/?page_id=2 | title=A Space Road
last=Valentine | first=L | date=2002 | publisher=Space Studies Institute, Princeton]

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 decrease its speed if the destination is closer to the sun, or increase its speed if the destination is further away (assuming a Hohmann transfer orbit).
* If the destination is further 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 further 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. [cite web | url=http://www.windows.ucar.edu/tour/link=/the_universe/uts/orbits_data.html | title=Orbital Data for the Planets] 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 (it varies slightly depending on the launch direction). 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.

Hohmann transfers

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 apogee 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 round 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.

Gravitational slingshot

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 manoeuvre can only change an objects 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 object involved in the manoeuvre relative to each other.

Fuzzy orbits

Computers did not exist when Hohmann transfer orbits were first proposed (1925) and were slow, expensive and unreliable when gravitational slingshots 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. [cite web | url=http://discovermagazine.com/1994/sep/gravitysrim419/article_view?b_start:int=1&-C= | title= Gravity's Rim | publisher=discovermagazine.com ] [Belbruno, E. 2004. [http://www.pupress.princeton.edu/titles/7687.html Capture Dynamics and Chaotic Motions in Celestial Mechanics: With the Construction of Low Energy Transfers] , Princeton University Press] 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 if humanity develops a space-based economy.

Aerobraking

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.

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 Utilization 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.

Electric propulsion

Electric propulsion systems use an external source such as a nuclear reactor or solar cells 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. A more ambitious, nuclear-powered version was intended for an unmanned Jupiter mission, the Jupiter Icy Moons Orbiter, 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, effectively canceling the JIMO mission.

olar Sails

Solar sails rely on the fact that light reflected from a surface exerts pressure on the surface. The radiation pressure 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. [cite web | url=http://www.ugcs.caltech.edu/~diedrich/cgi/search.cgi?solar+sail | title=Abstracts of NASA articles on solar sails]

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 lasers or masers at the sail. Ground-based lasers or masers 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.

No spacecraft powered only or mainly by light sails have been built. But ordinary spacecraft and satellites sometimes 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).

Nuclear thermal and solar thermal rockets

In a nuclear thermal rocket or solar thermal rocket a working fluid, usually hydrogen, is heated in 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.Fact|date=April 2007

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 5 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 horrific.

Cyclers

It is possible to put stations or spacecraft on orbits that cycle between different planets, for example a Mars_cycler 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). [cite web | url=http://www.popularmechanics.com/science/air_space/2076326.html?page=1 | title=Buzz Aldrin's Roadmap To Mars | date=2005 | last=Aldrin | first=B | last2= Noland | first2=D | publisher=Popular Mechanics] Cyclers' main limitation would be that they would be slow, because they would rely on gravitational techniques such as Hohmann transfer orbits and gravitational slingshots.

pace elevator

A space elevator is a structure designed to transport material from a planet's surface into orbit. [cite web | url=http://www.space.com/businesstechnology/technology/space_elevator_020327-1.html | title=The Space Elevator Comes Closer to Reality | last=David | first=D | publisher=space.com | date=2002] 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 counterweights, 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.

Using non-terrestrial resources

:"See main article In-situ resource utilization"

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 cells or indirectly by focusing solar radiation on boilers which produce steam to drive generators; and electrodynamic tethers which generate electricity from the powerful magnetic fields of some planets (Jupiter has a very powerful magnetic field).

Water ice would be very useful if it can be found on the moons of Jupiter or 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 electrolyse water ice into oxygen and hydrogen for use in bipropellant rocket engines.
* Nuclear thermal rockets or Solar thermal rockets could use it as reaction mass. Hydrogen has also been proposed for use in these engines and would provide much greater specific impulse (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. [ [http://www.neofuel.com/moonice1000/ Origin of How Steam Rockets can Reduce Space Transport Cost by Orders of Magnitude] ] [ [http://www.neofuel.com "Neofuel" -interplanetary travel using off-earth resources] ]

Oxygen is a common constituent of the moons crust, and is probably abundant in most other bodies in the solar system. Non-terrestrial oxygen would be about as valuable as water ice, for example:
* In the life support systems 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 propellant combinations.

Scientists expect to find a vast range of organic compounds 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 plastics. And ammonia could be a valuable feedstock for producing fertilizers 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 propulsion 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. New physics for space propulsion will need to be invented and old physics concepts of the forties will need to be put on the shelf.

Difficulties of manned interplanetary travel

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.

In practice on the International Space Station the Elektron oxygen generator unit has been temperamental.

Radiation

Once a vehicle leaves low earth orbit 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. These are dangerous over periods of years to decades.

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 ( [http://www.popularmechanics.com/science/air_space/2076326.html?page=6] [http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TVS-3XP0HBD-J&_user=2160112&_coverDate=06%2F30%2F1999&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000056461&_version=1&_urlVersion=0&_userid=2160112&md5=f51dd3ab45a2b265af1643f07fe0790c] [http://www.nature.com/embor/journal/v4/n11/full/embor7400016.html] [http://aoss.engin.umich.edu/Radiation/presentations/Biblography.pdf] [http://www.islandone.org/Settlements/MagShield.html] [http://iss.jaxa.jp/iss/kibo/develop_status_09_e.html] [http://yarchive.net/space/spacecraft/debris_shield.html] [http://uplink.space.com/showflat.php?Board=sciastro&Number=44199] ).

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 13 mission survived despite an explosion caused by a faulty oxygen tank (1970); the crews of the Space Shuttles Challenger (1986) and Columbia (2003) were killed by malfunctions of their vessels' components.

Launch windows

For astrodynamics 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.

Feasibility of manned interplanetary travel

While manned interplanetary travel (with the arguable exception of the Apollo program) has not yet been achieved, a trip to Mars is probably feasible, even with chemical rocket propulsion, and could probably be achieved within a decade (at most two) if the funds were made available. NASA's "Design Reference Mission" proposes a Mars exploration program costing $50 billion, but others have made detailed proposals with projected costs much less (see Mars Direct).

ee also

*Spacecraft propulsion
*delta-v
*List of interplanetary voyages
*Interstellar travel

References