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Micrometeorite

Micrometeorite

A Micrometeoroid (also micrometeorite, micrometeor) is a tiny meteoroid; a small particle of rock from space, usually weighing less than a gram, that poses a threat to space exploration. The risk is especially high for objects in space for long periods of time, such as satellites. They also pose major engineering challenges in theoretical low-cost lift systems such as rotavators, space elevators, and orbital airships. Micrometeoroids are extremely common in space, particularly near the Earth. Their velocities relative to a spacecraft in orbit can be on the order of kilometers per second, and resistance to micrometeoroid impact is a significant design challenge for spacecraft designers. Micrometeoroids are typically small, typically metallic, pieces of rock broken off from larger chunks of rock and debris. They typically date back to the formation of the solar system. Since orbital velocities are so high, and since they can enter an earth orbit from any angle, micrometeoroids in earth orbit constantly intercept the orbits of spacecraft and impact them at high speed. While their tiny size limits the damage incurred, the high velocity constantly degrades the outer casing of spacecraft and, in the long term, can threaten the functionality of systems. Micrometeoroids can also be easily found on earth in places where rainwater can concentrate them (such as a drain spout of roof gutters). Since metallic dust occurs relatively rarely on earth from other sources, metallic micrometeoroids can typically be separated from Earth dust via a strong magnet. Micrometeoroids comprise most of the 30,000 tons of space debris that are deposited on Earth every year. Impacts by small objects with extremely high velocity are a current area of research in terminal ballistics. Accelerating objects up to such velocities is difficult; current techniques include linear motors and shaped charges. In order to understand the micrometeoroid population better, a number of spacecraft (including Lunar Orbiter 1, Luna 3 and Mars 1) include micrometeoroid detectors. Category:Meteoroids

Meteoroid

A meteoroid is a relatively small (sand- to boulder-sized) fragment of debris in the Solar System. When entering a planet's atmosphere, the meteoroid heats up and partially or completely vaporizes. The gas along the path of the meteoroid becomes ionized and glows. The trail of glowing vapor is called a meteor, or shooting star. If any portion of the meteor survives to reach the ground, it is then referred to as a meteorite.

Gram

:For other uses of the words gram or gramme, see gram (disambiguation). The gram or gramme, symbol g, is a unit of mass. It is defined as one one-thousandth of the SI base unit kilogram (i.e., 1×10⁻³ kg). Its name derives from the Greek/Latin root grámma.

History

It was the base unit of mass in the original French metric system and the later centimetre-gram-second (CGS) system of units.

Uses

The gram is today the most widely used unit of measurement for non-liquid ingredients in cooking and grocery shopping worldwide. For food products that are typically sold in quantities far less than 1 kg, the unit price is normally given per 100 g. Most standards and legal requirements for nutrition labels on food products require relative contents to be stated per 100 g of the product, such that the resulting figure can also be read as a percentage.

Conversion factors

- 1 grain = 0.06479891 gram
- 1 ounce (avoirdupois) = 28.349523125 grams
- 1 ounce (troy) = 31.1034768 grams

Satellites

A satellite is any object that orbits another object (which is known as its primary). All masses that are part of the solar system, including the Earth, are satellites either of the Sun, or satellites of those objects, such as the Moon. It is not always a simple matter to decide which is the 'satellite' in a pair of bodies. Because all objects exert gravity, the motion of the primary object is also affected by the satellite. If two objects are sufficiently similar in mass, they are generally referred to as a binary system rather than a primary object and satellite; an extreme example is the 'double asteroid' 90 Antiope. The general criterion for an object to be a satellite is that the center of mass of the two objects is inside the primary object. In popular usage, the term 'satellite' normally refers to an artificial satellite (a man-made object that orbits the Earth or another body). However, scientists may also use the term to refer to natural satellites, or moons. This article is primarily concerned with artificial satellites. See natural satellite for information on moons.

Artificial satellites

History of artificial satellites

natural satellite In May, 1946, the Preliminary Design of an Experimental World-Circling Spaceship stated, "A satellite vehicle with appropriate instrumentation can be expected to be one of the most potent scientific tools of the Twentieth Century. The achievement of a satellite craft would produce repercussions comparable to the explosion of the atomic bomb..." (see: Project RAND) The space age began in 1946, as scientists began using captured German V-2 rockets to make measurements in the upper atmosphere. Before this period, scientists used balloons that went up to 30 km and radio waves to study the ionosphere. From 1946 to 1952, upper-atmosphere research was conducted using V-2s and Aerobee rockets. This allowed measurements of atmospheric pressure, density, and temperature up to 200 km. (see also: magnetosphere, Van Allen radiation belt) The U.S. had been considering launching orbital satellites since 1945 under the Bureau of Aeronautics of the United States Navy. The Air Force's Project RAND eventually released the above report, but did not believe that the satellite was a potential military weapon; rather they considered it to be a tool for science, politics, and propaganda. In 1954, the Secretary of Defense stated, "I know of no American satellite program." Following pressure by the American Rocket Society, the National Science Foundation, and the International Geophysical Year, military interest picked up and in early 1955 the Air Force and Navy were working on Project Orbiter, which involved using a Jupiter C rocket to launch a small satellite called Explorer 1 on January 31, 1958. On July 29, 1955, the White House announced that the U.S. intended to launch satellites by the spring of 1958. This became known as Project Vanguard. On July 31, the Soviets announced that they intended to launch a satellite by the fall of 1957 and on October 4, 1957 Sputnik I was launched into orbit, which triggered the Space Race between the two nations. The largest artificial satellite currently orbiting the earth is the International Space Station, which can sometimes be seen with the unaided human eye.

Types of satellites

Astronomical satellites are satellites used for observation of distant planets, galaxies, and other outer space objects. Communications satellites are artificial satellites stationed in space for the purposes of telecommunications using radio at microwave frequencies. Most communications satellites use geosynchronous orbits or near-geostationary orbits, although some recent systems use low Earth-orbiting satellites. Earth observation satellites are satellites specifically designed to observe Earth from orbit, similar to reconnaissance satellites but intended for non-military uses such as environmental monitoring, meteorology, map making etc. (See especially Earth Observing System.) Navigation satellites are satellites which use radio time signals transmitted to enable mobile receivers on the ground to determine their exact location. The relatively clear line of sight between the satellites and receivers on the ground, combined with ever-improving electronics, allows satellite navigation systems to measure location to accuracies on the order of a few metres in real time. Reconnaissance satellites are Earth observation satellite or communications satellite deployed for military or intelligence applications. Little is known about the full power of these satellites, as governments who operate them usually keep information pertaining to their reconnaissance satellites classified. Solar power satellites are proposed satellites built in high Earth orbit that use microwave power transmission to beam solar power to very large antenna on Earth where it can be used in place of conventional power sources. Space stations are man-made structures that are designed for human beings to live on in outer space. A space station is distinguished from other manned spacecraft by its lack of major propulsion or landing facilities — instead, other vehicles are used as transport to and from the station. Space stations are designed for medium-term living in orbit, for periods of weeks, months, or even years. Weather satellites are satellites that primarily are used to monitor the weather and/or climate of the Earth. Miniaturized satellites are satellites of unusually low weights and small sizes. New classifications are used to categorize these satellites: minisatellite (500–200 kg), microsatellite (below 200 kg), nanosatellite (below 10 kg).

Orbit types

Many times satellites are characterized by their orbit. Although a satellite may orbit at almost any height, satellites are commonly categorized by their altitude:
- Low Earth Orbit (LEO: 200 - 1200km above the Earth's surface)
- Medium Earth Orbit (ICO or MEO: 1200 - 35286 km)
- Geosynchronous Orbit (GEO: 35786 km above Earth's surface)
- Geostationary Orbit (GSO: zero inclination geosynchronous orbit)
- High Earth Orbit (HEO: above 35786 km) The following orbits are special orbits that are also used to categorize satellites:
- Molniya orbits
- Heliosynchronous or sun-synchronous orbit
- Polar orbit
- LTO lunar transfer orbit
- Hohmann transfer orbit For this particular orbit type, it is more common to identify the satellite as a spacecraft.
- Supersynchronous orbit or drift orbit - orbit above GEO. Satellites will drift in a westerly direction.
- (GEO + 235 km + (1000 × CR × A/m) km)
- where CR is the solar pressure radiation coefficient (typically between 1.2 and 1.5) and A/m is the aspect area [m²] to dry mass [kg] ratio
- Subsynchronous orbit or drift orbit - orbits close to but below GEO. Used for satellites undergoing station changes in an eastern direction. Satellites can also orbit libration points.

Countries with satellite launch capability

This list includes counties with an independent capability to place satellites in orbit, including production of the necessary launch vehicle. Many more countries have built satellites that were launched with the aid of others. The French and British capabilities are now subsumed by the European Union under the European Space Agency. In 1998, North Korea claimed to have launched a satellite, but this was never confirmed, and widely believed to be a cover for the test launch of the Taepodong-1 missile over Japan (See Kwangmyongsong).

Reference

External links

- [http://science.nasa.gov/Realtime/JPass/20/ J-Pass] NASA site for satellite-watching
- [http://www.stoff.pl Orbitron - Satellite Tracking System] Free satellite tracking software
- [http://ilectric.com/glance/Recreation/Radio/Amateur/Satellite_Tracking/ Satellite Tracking in Recreation Radio Amateur] an excellent link to many links
- [http://www.oosa.unvienna.org UN Office for Outer Space Affairs] ensures all countries benefit from satellites
- [http://www.satellite-service-providers.com/ Satellite Service Providers] Compare and review on top satellite tv, radio and internet service providers] Category:Satellites Category:Unmanned vehicles ko:인공 위성 ja:人工衛星

Space elevator

exceeds the total gravity, either by extending the cable or attaching a counterweight, the elevator would stay in place geosynchronously. Once sent far enough, climbers would be accelerated further by the planet's rotation. This diagram is not to scale.]] A space elevator is a hypothetical structure designed to transport material from a planet's surface into space. Many different types of space elevator structures have been proposed. They all share the goal of replacing rocket propulsion with the traversal of a fixed structure, in order to move material into or beyond orbit. Space elevators have also sometimes been referred to as space bridges, beanstalks, space ladders or space lifts. The most common proposal is a tether (usually a cable or ribbon) that spans from the surface to a point beyond geosynchronous orbit. As the planet rotates, the centrifugal force at the end of the tether counteracts gravity and keeps the tether taut. Vehicles can then climb the tether and escape the planet's gravity without the use of rockets. Such a structure could eventually permit delivery of great quantities of cargo and people to orbit, and at costs only a fraction of those associated with current means. Other possible techniques for building a space elevator are space fountains and even very tall compressive structures (i.e. structures that stand on their own). A space fountain would use particles fired up from the ground to form a dynamic, quasi-compressive structure. Compressive structures would be similar to those used for aerial masts. However these structures whilst possibly reaching the agreed altitude for space (100 km), are unlikely to reach geostationary orbit. Due to the difference between sub-orbital and orbital spaceflights, additional rockets or other means of propulsion would be necessary to achieve orbital speed. At this time orbital tethers are the only space elevator concept that is the subject of active research and commercial interest.

Orbital tethers

This concept, also called an orbital space elevator, geosynchronous orbital tether, or a beanstalk (see Jack and the Beanstalk), is one kind of skyhook. Construction would be a vast project: a tether would have to be built of a material that could endure tremendous stress while also being light-weight, cost-effective, and manufacturable. Today's materials technology does not quite meet these requirements. A considerable number of other novel engineering problems would also have to be solved to make a space elevator practical. Not all problems regarding feasibility have yet been addressed. Nevertheless, Brad Edwards says that the necessary technology could be developed by 2008 [http://liftport.com/research2.php] and the first space elevator could be operational by 2018 [http://www.space.com/businesstechnology/technology/space_elevator_020327-1.html] [http://www.isr.us/research_es_se.asp].

Physics and structure

2018 There are a variety of tether designs. Almost every design includes a base station, a cable, climbers, and a counterweight.

Base station

The base station designs typically fall into two categories—mobile and stationary. Mobile stations are typically large oceangoing vessels, though airborne stations have been proposed as well. Stationary platforms are generally located in high-altitude locations, such as on top of high towers. Mobile platforms have the advantage of being able to maneuver to avoid high winds, storms, and space debris. While stationary platforms don't have this, they typically have access to cheaper and more reliable power sources, and require a shorter cable. While the decrease in cable length may seem minimal (typically no more than a few kilometers), that can significantly reduce the minimal width of the cable at the center, and reduce the minimal length of cable reaching beyond geostationary orbit significantly.

Cable

The cable must be made of a material with an extremely high tensile strength/density ratio (the limit to which a material can be stretched without irreversibly deforming divided by its density). A space elevator can be made relatively economically if a cable with a density similar to graphite, with a tensile strength of ~65–120 GPa can be produced in bulk at a reasonable price. By comparison, most steel has a tensile strength of under 1 GPa, and the strongest steels no more than 5 GPa, but steel is heavy. The much lighter material Kevlar has a tensile strength of 2.6–4.1 GPa, while quartz fiber can reach upwards of 20 GPa; the tensile strength of diamond filaments would theoretically be minimally higher. Carbon nanotubes have exceeded all other materials and appear to have a theoretical tensile strength and density that is well within the desired range for space elevator structures and the technology to manufacture bulk quantities [http://www.worldchanging.com/archives/003330.html] and fabricate them into a cable is somewhat developing. While theoretically carbon nanotubes can have tensile strengths beyond 120 GPa, in practice the highest tensile strength ever observed in a single-walled tube is 63 GPa, and such tubes averaged breaking between 30 and 50 GPa. Even the strongest fiber made of nanotubes is likely to have notably less strength than its components. Improving tensile strength depends on further research on purity and different types of nanotubes. Carbon nanotube Most designs call for single-walled carbon nanotubes. While multi-walled nanotubes may attain higher tensile strengths, they have notably higher mass and are consequently poor choices for building the cable. One potential material possibility is to take advantage of the high pressure interlinking properties of carbon nanotubes of a single variety. [http://prola.aps.org/pdf/PRB/v62/i19/p12648_1]. While this would cause the tubes to lose some tensile strength by the trading of sp² bond (graphite, nanotubes) for sp³ (diamond), it will enable them to be held together in a single fiber by more than the usual, weak Van der Waals force (VdW), and allow manufacturing of a fiber of any length. The technology to spin regular VdW-bonded yarn from carbon nanotubes is just in its infancy: the first success to spin a long yarn as opposed to pieces of only a few centimeters has been reported only very recently; but the strength/weight ratio was worse than Kevlar due to inconsistent type construction and short tubes being held together by VdW. (March 2004). Note that as of 2005, carbon nanotubes have an approximate price of $50/gram, and 20 million grams would be necessary to form even a seed elevator. This price is decreasing rapidly, and large-scale production would reduce it further, but the price of suitable carbon nanotube cable is anyone's guess at this time. The cable material is an area of fierce worldwide research because the applications of successful material go much further than space elevators. This is good for space elevators because it is likely to push down the price of the cable material further. Other suggested application areas include suspension bridges, new composite materials, better rockets, lighter aircraft, and so on.

Cable taper

Due to its enormous length a space elevator cable must be carefully designed to carry its own weight as well as the smaller weight of climbers. In an ideal cable, the stress would be constant throughout the whole length such that at any given point, the width would be proportional to the total weight of the cable below. This would imply a tapered design. Using a model that takes into account the Earth's gravitational and centrifugal forces (and neglecting the smaller Sun and Lunar effects), it is possible to show that the cross-sectional area of the cable as a function of height looks like this: :

A(r) = A_ \ 
\exp 
\left[ 
  \frac 
  \left[ 
    \begin\frac\end \omega^ (r_^ - r^2) 
  + g_r_ (1 - \frac) 
  \right] 
\right]

Where

A(r)

is the cross-sectional area as a function of distance

r

from the Earth's center. The constants in the equation are:
-

A_

is the cross-sectional area of the cable on the earth's surface.
-

\rho

is the density of the material the cable is made out of.
-

s

is the tensile strength of the material.
-

\omega

is the rotational frequency of the earth about its axis, 7.292 × 10^-5 radian per second).
-

r_

is the distance between the earth's center and the base of the cable. It is approximately the earth's equatorial radius, 6378 km.
-

g_

is the acceleration due to gravity at the cable's base, 9.780 m/s². This equation gives a shape where the cable thickness initially increases rapidly in an exponential fashion, but slows at an altitude a few times the earth's radius, and then gradually becomes parallel when it finally reaches maximum thickness at geostationary orbit. The cable thickness then decreases again out from geosynchronous orbit. Thus the taper of the cable from base to GEO (r = 42,164 km), :

\frac = \exp \left[ \frac \times 5.294 
\times 10^ \, \mathrm
\right]

Using the density and tensile strength of steel, and assuming a diameter of 1 cm at ground level yields a diameter of several hundred kilometers (!) at geostationary orbit height, showing that steel, and indeed most materials used in present day engineering, are unsuitable for building a space elevator. The equation shows us that there are four ways of achieving a more reasonable thickness at geostationary orbit:
- Using a lower density material. Not much scope for improvement as the range of densities of most solids that come into question is rather narrow, somewhere between 1000 and 5000 kg/m³
- Using a higher strength material. This is the area where most of the research is focused. Carbon nanotubes are tens of times stronger than the strongest types of steel, hugely reducing the cable's cross-sectional area at geostationary orbit.
- Increasing the height of a tip of the base station, where the base of cable is attached. The exponential relationship means a small increase in base height results in a large decrease in thickness at geostationary level. Towers of up to 100 km high have been proposed. Not only would a tower of such height reduce the cable mass, it would also avoid exposure of the cable to atmospheric processes.
- Making the cable as thin as possible at its base. It still has to be thick enough to carry a payload however, so the minimum thickness at base level also depends on tensile strength. A cable made of carbon nanotube would typically be just a millimeter wide at the base.

Climbers

geostationary orbit A space elevator cannot be an elevator in the typical sense (with moving cables) due to the need for the cable to be significantly wider at the center than the tips at all times. While designs employing smaller, segmented moving cables along the length of the main cable have been proposed, most cable designs call for the "elevator" to climb up the cable. Climbers cover a wide range of designs. On elevator designs whose cables are planar ribbons, some have proposed to use pairs of rollers to hold the cable with friction. Other climber designs involve moving arms containing pads of hooks, rollers with retracting hooks, magnetic levitation (unlikely due to the bulky track required on the cable), and numerous other possibilities. Power is a significant obstacle for climbers. Energy storage densities, barring significant advances in compact nuclear power, are unlikely to ever be able to store the energy for an entire climb in a single climber without making it weigh too much. Some potential solutions have involved laser or microwave power beaming. Others have gained part of their energy through regenerative braking of down-climbers passing energy to up-climbers as they pass, magnetospheric braking of the cable to dampen oscillations, tropospheric heat differentials in the cable, ionospheric discharge through the cable, and other concepts. The primary power methods (laser and microwave power beaming) have significant problems with both efficiency and heat dissipation on both sides, although with optimistic numbers for future technologies, they are feasible. Climbers must be paced at optimal timings so as to minimize cable stress, oscillations, and maximize throughput. The weakest point of the cable is near its planetary connection; new climbers can typically be launched so long as there are not multiple climbers in this area at once. An only-up elevator can handle a higher throughput, but has the disadvantage of not allowing energy recapture through regenerative down-climbers. Additionally, as one cannot "fall out of orbit", an only-up elevator would require another method to let payloads/people get rid of their orbital energy, such as conventional rockets. Finally, only-up climbers that don't return to earth must be disposable; if used, they should be modular so that their components can be used for other purposes in geosynchronous orbit. In any case, smaller climbers have the advantage over larger climbers of giving better options for how to pace trips up the cable, but may impose technological limitations.

Counterweight

There have been two dominant methods proposed for dealing with the counterweight need: a heavy object, such as a captured asteroid, positioned past geosynchronous orbit; and extending the cable itself well past geosynchronous orbit. The latter idea has gained more support in recent years due to the simplicity of the task and the ability of a payload that travels to the end of the counterweight-cable to be flung off as far as Saturn (and farther using a gravitational slingshot).

Launching into outer space

As a payload is lifted up a space elevator, it gains not only altitude but angular momentum as well. This angular momentum is taken from Earth's own rotation. As the payload climbs it "drags" on the cable, causing it to tilt very slightly to the west (lagging behind slightly on the Earth's rotation). The horizontal component of the tension in the cable applies a tangential pull on the payload, accelerating it eastward. Conversely, the cable pulls westward on Earth's surface, insignificantly slowing it. The opposite process occurs for payloads descending the elevator, tilting the cable eastwards and very slightly increasing Earth's rotation speed. In both cases the centrifugal force acting on the cable's counterweight causes it to return to a vertical orientation, transferring momentum between Earth and payload in the process. We can determine the velocities that might be attained at the end of Pearson's 144,000 km tower (or cable). At the end of the tower, the tangential velocity is 10.93 kilometers per second which is more than enough to escape Earth's gravitational field and send probes as far out as Saturn. If an object were allowed to slide freely along the upper part of the tower, a velocity high enough to escape the solar system entirely would be attained. This is accomplished by trading off overall angular momentum of the tower (and the Earth) for velocity of the launched object, in much the same way one snaps a towel or throws a lacrosse ball. For higher velocities, the cargo can be electromagnetically accelerated, or the cable could be extended, although that would require additional strength in the cable.

Extraterrestrial elevators

A space elevator could also be constructed on some of the other planets, asteroids and moons. A Martian tether could be much shorter than one on Earth. Mars' surface gravity is 38% of Earth's, while it rotates around its axis in about the same time as Earth. Because of this, Martian areostationary orbit is much closer to the surface, and hence the elevator would be much shorter. Exotic materials might not be required to construct such an elevator. However, building a Martian elevator would be a unique challenge because the Martian moon Phobos is in a low orbit, and intersects the equator regularly (twice every orbital period of 11 h 6 min). A collision between the elevator and the 22.2 km diameter moon would have to be avoided through active steering of the elevator, or perhaps by moving the moon itself out of the area. Conversely, a Venusian space elevator would need to be much longer. Although a tether placed at the stationary orbit of the slowly rotating Venus would intersect the sun, one could be constructed that rotated with the fast-moving cloud decks of the planet which take only four earth days to make a complete cycle. The cable would need to exceed 100 thousand kilometres long, still possible using nanotubes. Such an elevator could support aerostats or floating cities in the benign regions of the atmosphere. A lunar space elevator would need to be very long—more than twice the length of an Earth elevator, but due to the low gravity of the moon, can be made of existing engineering materials. Alternatively, due to the lack of atmosphere on the moon, a rotating tether could be used with its center of mass in orbit around the moon with a counterweight at the short end and a payload at the long end. The path of the payload would be an epicycloid around the moon, touching down at some integer number of times per orbit. Thus, payloads are lifted off the surface of the moon, and flung away at the high point of the orbit. Rapidly spinning asteroids or moons could use cables to eject materials in order to move the materials to convenient points, such as Earth orbits; or conversely, to eject materials in order to send the bulk of the mass of the asteroid or moon to Earth orbit or a Lagrangian point. This was suggested by Russell Johnston in the 1980s. Freeman Dyson has suggested using such smaller systems as power generators at points distant from the Sun where solar power is uneconomical.

Construction

The construction of a space elevator would be a vast project, requiring advances in engineering and physical technology. NASA has identified "Five Key Technologies for Future Space Elevator Development": # Material for cable (e.g. carbon nanotube and nanotechnology) and tower # Tether deployment and control # Tall tower construction # Electromagnetic propulsion (e.g. magnetic levitation) # Space infrastructure and the development of space industry and economy Two different ways to deploy a space elevator have been proposed.

Traditional way

One early plan involved lifting the entire mass of the elevator into geosynchronous orbit, and simultaneously lowering one cable downwards towards the Earth's surface while another cable is deployed upwards directly away from the Earth's surface. Tidal forces (gravity and centrifugal force) would naturally pull the cables directly towards and directly away from the Earth and keeps the elevator balanced around geosynchronous orbit. However, this approach requires lifting hundreds or even thousands of tons on conventional rockets. This would be very expensive.

Brad Edwards' proposal

Bradley C. Edwards, Director of Research for the Institute for Scientific Research (ISR), based in Fairmont, West Virginia, is a leading authority on the space elevator concept. His designs contrast with previous designs by presenting a plausible scheme showing how a space elevator could be built in little more than a decade, rather than the far future. He proposes that a single hairlike 20 short ton (18 metric ton) 'seed' cable be deployed in the traditional way, giving a very lightweight elevator with very little lifting capacity. Then, progressively heavier cables would be pulled up from the ground along it, repeatedly strengthening it until the elevator reaches the required mass and strength. This is much the same technique used to build suspension bridges. Although 20 short tons for a seed cable may sound like a lot, it would actually be very lightweight — the proposed average mass is about 0.2 kilogram per kilometer. Conventional copper telephone wires running to consumer homes weigh about 4 kg/km.

Other designs

These are far less well developed, and will be mentioned here only in passing. If the cable provides a useful tensile strength of about 62.5 GPa or above, then it turns out that a constant width cable can reach beyond Geosynchronous orbit without breaking under its own weight. The far end can then be turned around and passed back down to the Earth forming a constant width loop. The two sides of the loop are naturally kept apart by coriolis forces due to the rotation of the Earth and the cable. By exponentially increasing the thickness of the cable from the ground a very quick buildup of a new elevator may be performed (it helps that no active climbers are needed, and power is applied mechanically.) However, because the loop runs at constant speed, joining and leaving the loop may be somewhat challenging, and the strength of the loop is lower than a conventional tapered design, reducing the maximum payload that can be carried without snapping the cable [http://www.mit.edu/people/gassend/publications/ExponentialTethers.pdf] Other structures such as mechanically-linked multiple looped designs hanging off of a central exponential tether might also be practical, and would seem to avoid the laser power beaming; this design has higher capacity than a single loop, but still requires perhaps twice as much tether material.

Failure modes and safety issues

As with any structure, there are a number of ways in which things could go wrong. A space elevator would present a considerable navigational hazard, both to aircraft and spacecraft. Aircraft could be dealt with by means of simple air-traffic control restrictions, but impacts by space objects (in particular, by meteoroids and micrometeorites) pose a more difficult problem.

Satellites

If nothing were done, essentially all satellites with perigees below the top of the elevator will eventually collide. Twice per day, each orbital plane intersects the elevator, as the rotation of the Earth swings the cable around the equator. Usually the satellite and the cable will not line up. However, except for synchronized orbits, the elevator and satellite will eventually occupy the same place at the same time, almost certainly leading to structural failure of the space elevator and destruction of the satellite. Most active satellites are capable of some degree of orbital maneuvering and could avoid these predictable collisions, but inactive satellites and other orbiting debris would need to be either preemptively removed from orbit by "garbage collectors" or would need to be closely watched and nudged whenever their orbit approaches the elevator. The impulses required would be small, and need be applied only very infrequently; a laser broom system may be sufficient to this task. In addition, Brad Edward's design actually allows the elevator to move out of the way, because the fixing point is at sea and mobile. Further, transverse oscillations of the cable could be controlled so as to ensure that the cable avoids satellites on known paths -- the required amplitudes are modest, relative to the cable length.

Meteoroids and micrometeorites

Meteoroids present a more difficult problem, since they would not be predictable and much less time would be available to detect and track them as they approach Earth. It is likely that a space elevator would still suffer impacts of some kind, no matter how carefully it is guarded. However, most space elevator designs call for the use of multiple parallel cables separated from each other by struts, with sufficient margin of safety that severing just one or two strands still allows the surviving strands to hold the elevator's entire weight while repairs are performed. If the strands are properly arranged, no single impact would be able to sever enough of them to overwhelm the surviving strands. Far worse than meteoroids are micrometeorites; tiny high-speed particles found in high concentrations at certain altitudes. Avoiding micrometeorites is essentially impossible, and they will ensure that strands of the elevator are continuously being cut. Most methods designed to deal with this involve a design similar to a hoytether or to a network of strands in a cylindrical or planar arrangement with two or more helical strands. Creating the cable as a mesh instead of a ribbon helps prevent collateral damage from each micrometeorite impact. It is not enough, however, that other fibers be able to take over the load of a failed strand — the system must also survive the immediate, dynamical effects of fiber failure, which generates projectiles aimed at the cable itself. For example, if the cable has a working stress of 50 GPa and a Young's modulus of 1000 GPa, its strain will be 0.05 and its stored elastic energy will be 1/2 × 0.05 × 50 GPa = 1.25×10⁹ joules per cubic meter. Breaking a fiber will result in a pair of de-tensioning waves moving apart at the speed of sound in the fiber, with the fiber segments behind each wave moving at over 1,000 m/s (more than the muzzle velocity of an M16 rifle). Unless these fast-moving projectiles can be stopped safely, they will break yet other fibers, initiating a failure cascade capable of severing the cable. The challenge of preventing fiber breakage from initiating a catastrophic failure cascade seems to be unaddressed in the current (January, 2005) literature on terrestrial space elevators. Problems of this sort would be easier to solve in lower-tension applications (e.g., lunar elevators).

Corrosion

Corrosion is a major risk to any thinly built tether (which most designs call for). In the upper atmosphere, atomic oxygen steadily eats away at most materials. A tether will consequently need to either be made from a corrosion-resistant material or have a corrosion-resistant coating, adding to weight. Gold and platinum have been shown to be practically immune to atomic oxygen; several far more common materials such as aluminum are damaged very slowly and could be repaired as needed.

Weather

In the atmosphere, the risk factors of wind and lightning come into play. The basic mitigation is location. As long as the tether's anchor remains within two degrees of the equator, it will remain in the quiet zone between the Earth's Hadley cells, where there is relatively little violent weather. Remaining storms could be avoided by moving a floating anchor platform. The lightning risk can be minimized by using a nonconductive fiber with a water-resistant coating to help prevent a conductive buildup from forming. The wind risk can be minimized by use of a fiber with a small cross-sectional area that can rotate with the wind to reduce resistance. (NOTE: Ice forming on the cable also presents a problem. It can add significantly to the cable's weight and affect the passage of elevator cars. Also, ice falling from the cable could damage elevator cars or possibly the cable itself.)

Sabotage

Sabotage is a relatively unquantifiable problem. Elevators are probably less susceptible than suspension bridges carrying mass vehicular traffic, of which there are many worldwide. Nonetheless there are few more spectacular possible targets: no terrorist act in history has approached the potential destruction caused by the carefully-targeted sabotage of a space elevator. Concern over sabotage may have an effect on location, since what would be required would be not only an equatorial site but also one outside the range of unstable territories.

Vibrational harmonics

A final risk of structural failure comes from the possibility of vibrational harmonics within the cable. Like the shorter and more familiar strings of stringed instruments, the cable of a space elevator has a natural resonant frequency. If the cable is excited at this frequency, for example by the travel of elevators up and down it, the vibrational energy could build up to dangerous levels and exceed the cable's tensile strength. This can be avoided by the use of intelligent damping systems within the cable, and by scheduling travel up and down the cable keeping its resonant frequency in mind. It may be possible to do damping against Earth's magnetosphere, which would additionally generate electricity that could be passed to the climbers.

In the event of failure

If despite all these precautions the elevator is severed anyway, the resulting scenario depends on where exactly the break occurred.

Cut near the anchor point

If the elevator is cut at its anchor point on Earth's surface, the outward force exerted by the counterweight would cause the entire elevator to rise upward into a stable orbit. This is because a space elevator must be kept in tension, with greater centrifugal force pulling outward than gravitational force pulling inward, or any additional payload added at the elevator's bottom end would pull the entire structure down. The ultimate altitude of the severed lower end of the cable would depend on the details of the elevator's mass distribution. In theory, the loose end might be secured and fastened down again. This would be an extremely tricky operation, however, requiring careful adjustment of the cable's center of gravity to bring the cable back down to the surface again at just the right location. It may prove to be easier to build a new system in such a situation.

Cut at about 25,000 km

If the break occurred at higher altitude, up to about 25,000 km, the lower portion of the elevator would descend to Earth and drape itself along the equator east of the anchor point, while the now unbalanced upper portion would rise to a higher orbit. Some authors have suggested that such a failure would be catastrophic, with the thousands of kilometers of falling cable creating a swath of meteoric destruction along Earth's surface. However, in most cable designs, the upper portion of any cable that fell to Earth would burn up in the atmosphere. Additionally because proposed initial cables (the only ones likely to be broken) have very low mass (roughly 1 kg per kilometer) and are flat, the bottom portion would likely settle to Earth with less force than a sheet of paper due to air resistance on the way down. If the break occurred at the counterweight side of the elevator the lower portion, now including the "central station" of the elevator would entirely fall down if not prevented by an early self-destruct of the cable shortly below it. Depending on the size however it would burn up on reentry anyway.

Elevator pods

Any elevator pods on the falling section would also reenter Earth's atmosphere, but it is likely that the elevator pods will already have been designed to withstand such an event as an emergency measure anyway. It is almost inevitable that some objects - elevator pods, structural members, repair crews, etc.—will accidentally fall off the elevator at some point. Their subsequent fate would depend upon their initial altitude. Except at geosynchronous altitude, an object on a space elevator is not in a stable orbit and so its trajectory will not remain parallel to it. The object will instead enter an elliptical orbit, the characteristics of which depend on where the object was on the elevator when it was released. If the initial height of the object falling off of the elevator is less than 23,000 km, its orbit will have an apogee at the altitude where it was released from the elevator and a perigee within Earth's atmosphere—it will intersect the atmosphere within a few hours, and not complete an entire orbit. Above this critical altitude, the perigee is above the atmosphere and the object will be able to complete a full orbit to return to the altitude it started from. By then the elevator would be somewhere else, but a spacecraft could be dispatched to retrieve the object or otherwise remove it. The lower the altitude at which the object falls off, the greater the eccentricity of its orbit. If the object falls off at the geostationary altitude itself, it will remain nearly motionless relative to the elevator just as in conventional orbital flight. At higher altitudes the object would again wind up in an elliptical orbit, this time with a perigee at the altitude the object was released from and an apogee somewhere higher than that. The eccentricity of the orbit would increase with the altitude from which the object is released. Above 47,000 km, however, an object that falls off of the elevator would have a velocity greater than the local escape velocity of Earth. The object would head out into interplanetary space, and if there were any people present on board it might prove impossible to rescue them. All of these altitudes are given for an Earth-based space elevator; a space elevator serving a different planet or moon would have different critical altitudes where each of these scenarios would occur.

Van Allen Belts

The space elevator runs through the Van Allen Belts. This is not a problem for most freight, but the amount of time a climber spends in this region would cause radiation sickness to any unshielded human or other living things. Some people speculate that passengers and other living things will continue to travel by high-speed rocket, while the space elevator hauls bulk cargo. Research into lightweight shielding and techniques for clearing out the belts is underway. An elevator could carry passenger cars with heavy lead or other shielding. For the thin cable of an initial elevator, this would reduce its overall capacity; weight would become less of a problem later, when the cable has been thickened. However, the shielding itself can in some cases consist of useful payload, for example food, water, supplies, fuel or construction/maintenance materials, and no additional shielding costs are then incurred on the way up. More conventional and faster atmospheric reentry techniques such as aerobraking might be employed on the way down to minimize radiation exposure. De-orbit burns use relatively little fuel, and so can be cheap. To shield cargo and passengers from the radiation in the Van Allen Belt, perhaps counterintuitively, material composed of light elements should be used, as opposed to lead shielding. In fact, high energy protons and electrons in the Van Allen Belts produce dangerous X-rays when they strike atoms of heavy elements. Materials containing great amounts of hydrogen, such as water or (lightweight) plastics such as polyethylene are also effective and lighter metals such as aluminium are better than heavier ones such as steel (iron). These shields do not protect against X-rays however, which reach the Earth in dangerously high levels during a solar flare. To prevent lethal exposure elevator cars would require additional (lead) shielding or employ a system to escape behind the Earth or into the atmosphere. Advance warning of solar flares can be deployed, as in a solar weather report, since the solar particles travel more slowly than the light that provides a visual indication of the flares. (Note: Lightweight shielding does offer some protection from x-rays. Although x-rays can be released during Solar Particle Events (Coronal mass ejection), the greatest danger comes from charged particles. Any crewed elevator car that had enough lightweight shielding to protect against charged particles in the Van Allen belts during a solar flare should have more than adequate protection against x-rays, so there should be no need for additional lead shielding.)

Economics

Main article: space elevator economics With a space elevator, materials could be sent into orbit at a fraction of the current cost. Modern rocketry gives prices that are on the order of thousands of U.S. dollars per kilogram for transfer to low earth orbit, and roughly 20 thousand dollars per kilogram for transfer to geosynchronous orbit. For a space elevator, the price could be on the order of a few hundreds of dollars per kilogram, or possibly much less. Space elevators have high capital cost but low operating expenses, so they make the most economic sense in a situation where it would be used over a long period of time to handle very large amounts of payload. The current launch market may not be large enough to make a compelling case for a space elevator, but a dramatic drop in the price of launching material to orbit would likely result in new types of space activities becoming economically feasible. In this regard they share similarities with other transportation infrastructure projects such as highways or railroads. Development costs might be roughly equivalent, in modern dollars, to the cost of developing the shuttle system. A question subject to speculation is whether a space elevator would return the investment, or if it would be more beneficial to instead spend the money on developing rocketry further.

Political issues

One potential problem with a space elevator would be the issue of ownership and control. Such an elevator would require significant investment (estimates start at about US$5 billion for a very primitive tether), and it could take at least a decade to recoup such expenses. At present, only governments are able to spend in the space industry at that magnitude. Assuming a multi-national governmental effort was able to produce a working space elevator, many delicate political issues would remain to be solved. Which countries would use the elevator and how often? Who would be responsible for its defense from terrorists or enemy states? A space elevator would allow for easy deployment of satellites into orbit, and it is becoming ever more obvious that space is likely to be militarised. A space elevator could potentially cause numerous rifts between states over the military applications of the elevator. Furthermore, establishment of a space elevator would require knowledge of the positions and paths of all existing satellites in Earth orbit and their removal if they cannot adequately avoid the elevator. An initial elevator could be used in relatively short order to lift the materials to build more such elevators, but whether this is done and in what fashion the resulting additional elevators are utilized depends on whether the owners of the first elevator are willing to give up any monopoly they may have gained on space access. However, once the technologies are in place, any country with the appropriate resources would most likely be able to create their own elevator. As space elevators (regardless of the design) are inherently fragile but militarily valuable structures, they would likely be targeted immediately in any major conflict with a state that controls one. Consequently, most militaries would elect to continue development of conventional rockets (or other similar launch technologies) to provide effective backup methods to access space. The cost of the space elevator is not excessive compared to other projects and it is conceivable that several countries or an international consortium could pursue the space elevator. Indeed, there are companies and agencies in a number of countries that have expressed interest in the concept. Generally, megaprojects need to be either joint public-private partnership ventures or government ventures and they also need multiple partners. It is also possible that a private entity (risks notwithstanding) could provide the financing — several large investment firms have stated interest in construction of the space elevator as a private endeavor. The political motivation for a collaborative effort comes from the potential destabilizing nature of the space elevator. The space elevator clearly has military applications, but more critically it would give a strong economic advantage for the controlling entity. Information flowing through satellites, future energy from space, planets full of real estate and associated minerals, and basic military advantage could all potentially be controlled by the entity that controls access to space through the space elevator. An international collaboration could result in multiple ribbons at various locations around the globe, since subsequent ribbons would be significantly cheaper, thus allowing general access to space and consequently eliminating any instabilities a single system might cause. While few ordinary citizens might profit directly from space elevator applications, the general public would probably reap benefits through cheap, environmentally-friendly solar power, enhanced satellite navigation and communication services, and even through improved health, education and social services made possible by the savings made by governments in accessing space. Clarke compared the space elevator project to Cyrus Field's efforts to build the first transatlantic telegraph cable, "the Apollo Project of its age" [http://www.spaceelevator.com/docs/acclarke.092079.se.2.html].

History

The concept of the space elevator first appeared in 1895 when a Russian scientist Konstantin Tsiolkovsky was inspired by the Eiffel Tower in Paris to consider a tower that reached all the way into space. He imagined placing a "celestial castle" at the end of a spindle-shaped cable, with the "castle" orbiting Earth in a geosynchronous orbit (i.e. the castle would remain over the same spot on Earth's surface). The tower would be built from the ground up to an altitude of 35,790 kilometers above mean sea level (geostationary orbit). Comments from Nikola Tesla suggest that he may have also conceived such a tower. Tsiolkovsky's notes were sent behind the Iron Curtain after his death. Tsiolkovsky's tower would be able to launch objects into orbit without a rocket. Since the elevator would attain orbital velocity as it rode up the cable, an object released at the tower's top would also have the orbital velocity necessary to remain in geosynchronous orbit. Building from the ground up, however, proved an impossible task; there was no material in existence with enough compressive strength to support its own weight under such conditions. It took until 1957 for another Russian scientist, Yuri N. Artsutanov, to conceive of a more feasible scheme for building a space tower. Artsutanov suggested using a geosynchronous satellite as the base from which to construct the tower. By using a counterweight, a cable would be lowered from geosynchronous orbit to the surface of Earth while the counterweight was extended from the satellite away from Earth, keeping the center of gravity of the cable motionless relative to Earth. Artsutanov published his idea in the Sunday supplement of Komsomolskaya Pravda in 1960. He also proposed tapering the cable thickness so that the tension in the cable was constant—this gives a thin cable at ground level, thickening up towards GEO.[http://www.liftport.com/files/Artsutanov_Pravda_SE.pdf] Making a cable over 35,000 kilometers long is a difficult task. In 1966, four American engineers decided to determine what type of material would be required to build a space elevator, assuming it would be a straight cable with no variations in its cross section. They found that the strength required would be twice that of any existing material including graphite, quartz, and diamond. In 1975 an American scientist, Jerome Pearson, designed a tapered cross section that would be better suited to building the tower. The completed cable would be thickest at the geosynchronous orbit, where the tension was greatest, and would be narrowest at the tips to reduce the amount of weight per unit area of cross section that any point on the cable would have to bear. He suggested using a counterweight that would be slowly extended out to 144,000 kilometers (almost half the distance to the Moon) as the lower section of the tower was built. Without a large counterweight, the upper portion of the tower would have to be longer than the lower due to the way gravitational and centrifugal forces change with distance from Earth. His analysis included disturbances such as the gravitation of the Moon, wind and moving payloads up and down the cable. The weight of the material needed to build the tower would have required thousands of Space Shuttle trips, although part of the material could be transported up the tower when a minimum strength strand reached the ground or be manufactured in space from asteroidal or lunar ore. Arthur C. Clarke introduced the concept of a space elevator to a broader audience in his 1978 novel, The Fountains of Paradise, in which engineers construct a space elevator on top of a mountain peak (Adam's Peak in Sri Lanka) in the equatorial island of Taprobane (the Discoveries era name for Sri Lanka) . David Smitherman of NASA/Marshall's Advanced Projects Office has compiled plans for such an elevator that could turn science fiction into reality. His publication, "Space Elevators: An Advanced Earth-Space Infrastructure for the New Millennium" [http://flightprojects.msfc.nasa.gov/fd02_elev.html], is based on findings from a space infrastructure conference held at the Marshall Space Flight Center in 1999. Another American scientist, Bradley C. Edwards, suggests creating a 100,000 km long paper-thin ribbon, which would stand a greater chance of surviving impacts by meteors. The work of Edwards has expanded to cover: the deployment scenario, climber design, power delivery system, orbital debris avoidance, anchor system, surviving atomic oxygen, avoiding lightning and hurricanes by locating the anchor in the western equatorial pacific, construction costs, construction schedule, and environmental hazards. Plans are currently being made to complete engineering developments, material development and begin construction of the first elevator. Funding to date has been through a grant from NASA Institute for Advanced Concepts. Future funding is sought through NASA, the United States Department of Defense, private, and public sources. The largest holdup to Edwards' proposed design is the technological limits of the tether material. His calculations call for a fiber composed of epoxy-bonded carbon nanotubes with a minimal tensile strength of 130 GPa; however, tests in 2000 of individual single-walled carbon nanotubes (SWCNTs), which should be notably stronger than an epoxy-bonded rope, indicated the strongest measured as 63 GPa [http://bucky-central.mech.nwu.edu/RuoffsPDFs/91.pdf]. Space elevator proponents are planning competitions for space elevator technologies [http://msnbc.msn.com/id/5792719/], similar to the Ansari X Prize. [http://www.elevator2010.org/ Elevator:2010] will organize annual competitions for climbers, ribbons and power-beaming systems. The Robolympics Space Elevator Ribbon Climbing [http://robolympics.net/rules/climbing.shtml] organizes climber-robot building competitions. In March of 2005 NASA's Centennial Challenges program announced a partnership with the Spaceward Foundation (the operator of Elevator:2010), raising the total value of prizes to US$400,000 [http://www.nasa.gov/home/hqnews/2005/mar/HQ_m05083_Centennial_prizes.html][http://www.space.com/news/050323_centennial_challenge.html]. On April 27, 2005 "the LiftPort Group of space elevator companies has announced that it will be building a carbon nanotubes manufacturing plant in Millville, New Jersey, to supply various glass, plastic and metal companies with these strong materials. Although LiftPort hopes to eventually use carbon nanotubes in the construction of a 100,000 km (62,000 mile) space elevator, this move will allow it to make money in the short term and conduct research and development into new production methods." [http://www.universetoday.com/am/publish/liftport_manufacture_nanotubes.html?2742005] On September 9 the group announced that they had obtained permission from the Federal Aviation Administration to use airspace to conduct preliminary tests of its high altitude robotic lifters. The experiment was successful.

Fiction

Titles in alphabetical order. Note: Some depictions were made before the space elevator concept became known.

in Novels and Fairy tales

- 3001: The Final Odyssey, novel by Arthur C. Clarke
- Assassin Gambit, novel by William Forstchen.
- Chasm City, a novel by Alastair Reynolds
- Feersum Endjinn, novel by Iain M. Banks
- Foreigner, novel by Robert J. Sawyer
- Friday, novel by Robert A. Heinlein
- Hothouse, novel by Brian Aldiss.
- Jack and the Beanstalk, fairy tale. Due to this story, another name for a space elevator is a 'beanstalk'.
- Jumping Off the Planet, novel by David Gerrold
- The Mars Trilogy, a series of novels (Red Mars, Green Mars, and Blue Mars) by Kim Stanley Robinson, depicts space elevators on Earth and on Mars whose cables are made of carbon nanotube which are manufactured on an asteroid, and lowered into the atmosphere, using the asteroid as a counterweight. Red Mars depicts what happens when the cable is cut at the asteroid anchor point.
- Mercury, a novel by Ben Bova about a space elevator sabotage that gets an innocent man exiled from Earth
- Old Man's War, a novel by John Scalzi
- Rainbow Mars, novel by Larry Niven with a "beanstalk" on Mars and Earth
- Songs of Distant Earth, novel by Arthur C. Clarke
- Strata, one of Terry Pratchett's two solely science fiction novels
- Sundiver, novel by David Brin
- The Descent of Anansi, novel by Steven Barnes and Larry Niven (ISBN 0812512928)
- The End of the Empire, novel by Alexis A. Gilliland
- The Fountains of Paradise, novel by Arthur C. Clarke
- The Night's Dawn Trilogy, novels by Peter F. Hamilton.
- The Web Between the Worlds, novel by Charles Sheffield
- The Science of Discworld, by Terry Pratchett, Jack Cohen and Ian Stewart, in which Roundworld humanity escapes to the stars via an elevator.
- Zavtra Nastupit Vechnost (Tomorrow The Eternity Will Come), novel by Russian sci-fi writer Alexander Gromov
- Zastryat v Lifte (To Struck in The Elevator), short-story by Russian sci-fi writer Dmitry Tarabanov. In this technical love-story the sabotage on the first space elevator, built by LiftPort Group and named after Yuri Artsutanov, is described.

in Anime, Comics, and Manga

- 21st Century Fox, web comic by Scott Kellogg, depicts attempt to build a proof of concept space elevator.
- Bubblegum Crisis Tokyo 2040, anime series, contains a skyhook throughout the series.
- Gunnm, a manga by Yukito Kishiro, depicts a space elevator and cities built on its bottom and its Lagrangian point.
- Kiddy Grade, anime series, in which a space elevator is used on every planet and works two ways: 1) guide space ships and help with their launch, 2) Transport people & materials through elevator services.
- Kurau: Phantom Memory, anime series.
- Nemesis the Warlock, comic strip by

Spacecraft

, 2004.]] A spacecraft is a vehicle that travels through space. Spacecraft include robotic or unmanned space probes as well as manned vehicles. The term is sometimes also used to describe artificial satellites, which have similar design criteria.

Overview

The term spaceship is generally applied only to spacecraft capable of transporting people. A space suit has at times been called a miniature spacecraft or spaceship, emphasizing its purpose of keeping its wearer alive while traveling in the vacuum of outer space. The spacecraft is one of the primal elements in science fiction. Numerous short stories and novels are built up around various ideas for spacecraft. Some hard science fiction books focus on the technical details of the craft, while others treat the spacecraft as a given and delve little into its actual implementation.

Examples of past or existing spacecraft

Manned
- Apollo Spacecraft
- Gemini Spacecraft
- International Space Station
- Mir
- Mercury Spacecraft
- Shuttle Buran
- Shenzhou Spacecraft
- Space Shuttle
- Soyuz Spacecraft
- SpaceShipOne
- Voskhod Spacecraft
- Vostok Spacecraft Unmanned
- Cassini-Huygens
- Cluster
- Deep Space 1
- Genesis
- Mars Exploration Rover
- Mars Global Surveyor
- Mars Pathfinder
- Pioneer 10
- Pioneer 11
- Progress
- SOHO
- Stardust
- Viking 1
- Viking 2
- Voyager 1
- Voyager 2
- WMAP

Spacecraft under development

- Crew Exploration Vehicle
- Kliper
- Automated Transfer Vehicle
- H-II Transfer Vehicle
- Ansari X Prize (incl. a list of spacecraft in various stages of completion as of 2005) The US Space Command, according to its "Long Range Plan", is currently planning to develop a weaponized spaceship, which has yet to be announced.[http://www.fas.org/spp/military/docops/usspac/]

External links

- [http://science.hq.nasa.gov/missions/phase.html NASA: Space Science Spacecraft Missions]
- [http://www.skyrocket.de/space/ Gunter's Space Page - Complete information on spacecraft]
- [http://www.cinespaceships.net/ Cinespaceships - Database on spaceships in movie]
- ja:宇宙船

Rain

: For other uses see Rain (disambiguation). Rain is a form of precipitation, other forms of which include snow, sleet, hail, and dew. Rain forms when separate drops of water fall to the Earth's surface from clouds. Not all rain reaches the surface, however; some evaporates while falling through dry air. When none of it reaches the ground, it is a precipitation called virga.

Rain in nature

Rain plays a major role in the hydrologic cycle in which [http://wiktionary.org/wiki/moisture moisture] from the oceans evaporates, condenses into clouds, precipitates back to earth, and eventually returns to the ocean via streams and rivers to repeat the cycle again. There is also a small amount of water vapor that respires from plants and evaporates to join other water molecules in condensing into clouds. The amount of rainfall is measured using a rain gauge. It is expressed as the depth of water that collects on a flat surface, and can be measured to the nearest 0.25 mm or 0.01 in. It is sometimes expressed in litres per square metre (1 L/m² = 1 mm). Falling raindrops are often depicted in cartoons or anime as "tear-shaped", round at the bottom and narrowing towards the top, but this is incorrect (only drops of water dripping from some sources are tear-shaped at the moment of formation). Small raindrops are nearly spherical. Larger ones become increasingly flattened, like hamburger buns; very large ones are shaped like parachutes. [http://www.ems.psu.edu/~fraser/Bad/BadRain.html] On average, raindrops are 1 to 2 mm in diameter. The biggest raindrops on Earth were recorded over Brazil and the Marshall Islands in 2004 - some of them were as large as 10 mm. The large size is explained by condensation on large smoke particles or by collisions between drops in small regions with particularly high content of liquid water. Generally, rain has a pH slightly under 6. This is because atmospheric carbon dioxide dissolves in the droplet to form minute quantities of carbonic acid, which then partially dissociates, lowering the pH. In some desert areas, airborne dust contains enough calcium carbonate to counter the natural acidity of precipitation, and rainfall can be neutral or even alkaline. Rain below pH 5.6 is considered acid rain. Rain is said to be heavier immediately after a bolt of lightning. The cause of this phenomenon is traceable to the bipolar aspect of the water molecule. The intense electric and magnetic field generated by a lightning bolt forces many of the water molecules in the air surrounding the stroke to line up. These molecules then spontaneously create localized chains of water (similar to nylon or other 'poly' molecules). These chains then form water droplets when the electric/magnetic field is removed. These drops then fall as intensified rain.

Culture

lightning Cultural attitudes towards rain differ across the world. In the largely temperate Western world, rain traditionally has a sad and negative connotation — reflected in children's rhymes like Rain Rain Go Away — in contrast to the bright and happy sun. In dry places such as India and the Middle East, the rain is greeted with euphoria. Several cultures have developed means of dealing with rain and have developed numerous protection devices such as umbrellas and raincoats, and diversion devices such as gutters and storm drains that lead rains to sewers. Many people also prefer to stay inside on rainy days, especially in tropical climates where rain is usually accompanied by thunderstorms or rain is extremely heavy (monsoon). Rain may be collected for drinking water since rainwater is pure, or used as greywater. Excessive rain, particularly after a dry period has hardened the soil so that it cannot absorb water, can cause floods. Many people find the scent smelt during and immediately after rain especially pleasant or distinctive. The source of this smell is petrichor, an oil produced by plants, then absorbed by rocks and soil, and later released into the air during rainfall.

Terminal ballistics

Terminal ballistics, a sub-field of ballistics, is the study of the behavior of a projectile when it hits its target. It is often referred to as stopping power when dealing with human or other living targets. Terminal ballistics is as relevant for both small caliber projectiles as for large caliber projectiles (fired from artillery). The study of extremely high velocity impacts is still very new and is as yet mostly applied to spacecraft design.

Small caliber terminal ballistics

Classes of bullet

There are three basic classes of bullet: those designed for maximum accuracy at varying ranges, those designed to inflict maximal damage to a target by optimizing the depth to which the bullet penetrates, and those designed to maximize damage to a target by penetrating as deeply as possible.

Bullets for target shooting

For short range target shooting on ranges up to 50 meters (55 yd) aerodynamics is relatively unimportant and velocities are low. As long as the bullet is balanced so it doesn't tumble, the aerodynamics are unimportant. For shooting at paper targets, the best bullet is one that will punch a perfect hole through the target. These bullets are called wadcutters, and they have a very flat front, often with a relatively sharp edge along the perimeter. The flat front punches out a large hole in the paper, close to if not equal to the full diameter of the bullet. This allows for easy, unambiguous scoring of the target. Since cutting the edge of a target ring will result in scoring the higher score, fractions of an inch are important. In magazine fed pistols, the square shape of a wadcutter will often not feed reliably. To address this, the semiwadcutter was developed. The semiwadcutter consists of a conical section that comes to a smaller flat, and a thin sharp shoulder at the base of the cone. The flat point punches a clean hole, and the shoulder opens the hole up cleanly. For steel targets, the concern is to provide enough force to knock over the target, but to minimize the damage to the target. A soft lead bullet, or a jacketed hollow point bullet or soft point bullet will flatten out on impact (if the velocity at impact is sufficient to make it deform), spreading the force over a larger area of the target, allowing more total force to be applied without damaging the steel target. There are also specialized bullets designed specifically for use in long-range precision target shooting with high-powered rifles; the designs vary somewhat from manufacturer to manufacturer, but all are based on the MatchKing bullets introduced by the Sierra Bullet Company around 1963. Based on research done in the 1950s by the US Air Force, in which it was discovered that bullets are more stable in flight for longer distances and more resistant to crosswinds if the center of gravity is somewhat to the rear of the center of pressure, the MatchKing bullet (which is still in wide use and holds many records) is a hollowpoint design with a tiny aperture in the jacket at the point of the bullet and a hollow air space under the point of the bullet, where previous conventional bullets had had a lead core that went all the way up to the point. Other designs from other manufacturers may be anything from close copies of the MatchKing design to hollowpoint bullets with a deep, wide cavity containing a long, slender, pointed plastic or aluminum plug. In all these cases, the bullet is designed to have its center of gravity to the rear of its center of pressure. MatchKing type hollowpoint bullets, as contrasted with hollowpoint bullets intended for hunting or police use, are not designed to flatten out on impact; this makes them a relatively poor choice for hunting, as they tend to perform erratically and unpredictably upon entering an animal's body--they may tumble, or break apart, thought most often they punch straight through making a narrow wound that usually does not cause death quickly (as full metal jacket ammunition normally does). The US military now issues ammunition to snipers that use bullets of this type. In 7.62 x 51 mm NATO, M852 Match and M118LR ammunition are issued, both of which use Sierra MatchKing bullets; in 5.56 x 45 mm NATO, those US Navy and US Marine snipers who use accurized M16 type rifles are issued the Mk 262 Mod 0 cartridge developed jointly by Black Hills Ammunition and Crane Naval Special Warfare Center, using a bullet manufactured by the Nosler company that is very similar to a Sierra MatchKing bullet. In the mid 1990s, the US military Adjutant General's Office issued a legal opinion holding that the Sierra MatchKing bullet, despite being a hollowpoint design, is not designed specifically to cause greater damage or suffering in a human target, and in fact normally does not create a wound readily distinguishable from wounds caused by conventional full metal jacket bullets, and is therefore in their opinion legal under the Hague Convention for use in war.

Bullets for maximum penetration

For use against armored targets, or large, tough game animals, penetration is the most important consideration. Focusing the largest amount of momentum on the smallest possible area of the target provides the greatest penetration. Bullets for maximum penetration are designed to resist deformation upon impact, and usually are made of lead that is covered in a copper, brass, or mild steel jacket (some are even solid copper or bronze alloy). The jacket completely covers the front of the bullet, although often the rear is left with exposed lead (this is manufacturing consideration, the jacket is formed first, and the lead is swaged in from the rear). For penetrating substances significantly harder than jacketed lead, the lead core is supplemented with or replaced with a harder material, such as hardened steel. Military armor piercing small arms ammunition is made from a copper jacketed steel core; the steel resists deformation better than the usual soft lead core leading to greater penetration. The current NATO 5.56 mm SS109 bullet uses a steel tipped lead core to improve penetration, the steel tip providing resistance to deformation for armor piercing, and the heavier lead core (25% heavier than the previous bullet) providing increased sectional density for better penetration in soft targets. For larger, higher velocity calibers, such as tank guns, hardness is of secondary importance to density, and are normally sub-caliber projectiles made from depleted uranium fired in a light aluminum or magnesium alloy sabot. Oddly, many modern tank guns are smoothbore, not rifled. This is because practical rifling twists can only stabilize projectiles with a length to diameter ratio of up to about 5:1, and also because the rifling adds friction and reduces the velocity it is possible to achieve. To get the maximum force on the smallest area, anti-tank rounds have aspect ratios of 10:1 or more. Since these cannot be stabilized by rifling, they are built instead like large darts, with fins providing the stabilizing force, negating the need for rifling. These subcaliber rounds are held in place in the bore by sabots. The sabot is a light material that transfers the pressure of the charge to the penetrator, then is discarded when the round leaves the barrel.

Bullets for controlled penetration

The final category of bullets are those intended to maximize damage to living targets. These are used primarily for hunting and civilian antipersonnel use; they are not generally used by the military, since the use of expanding bullets in international conflicts is prohibited by the Hague Convention. These bullets are designed to increase their surface area upon impact, thus creating greater drag and limiting the travel through the target. A desirable side effect is that the expanded bullet makes a larger hole, increasing tissue disruption and speeding incapacitation. In some applications, preventing exit from the rear of the target is also desirable. A bullet which penetrates through-and-through tends to cause more profuse bleeding, allowing a game animal to be bloodtrailed more easily. On the other hand, a perforating bullet can then continue on (likely not coaxial to the original trajectory due to target deflection) and might cause unintended damage or injury. Frangible bullets, made of tiny fragments held together by a weak binding, are often sold as an "ultimate" expanding bullet, as they will increase their effective diameter by an order of magnitude. When they work, they work extremely well, causing massive trauma to the target. On the other hand, when they fail, it is due to underpenetration, and the damage to the target is superficial and leads to very slow incapacitation.

Flat point bullets

The simplest maximum disruption bullet is one with a wide, flat tip. This increases the effective surface area, as rounded bullets can allow tissues to "flow" around the edges. It also increases drag, which decreases the depth to which the bullet penetrates. Flat point bullets, with fronts of up to 90% of the overall bullet diameter, are usually designed for use against large or dangerous game. They are often made of unusually hard alloys, are longer and heavier than normal for their caliber, and even include exotic materials such as tungsten to increase their sectional density. These bullets are designed to penetrate deeply through muscle and bone, while causing a wound channel of nearly the full diameter of the bullet. These bullets are designed to penetrate deeply enough to reach vital organs from any shooting angle. One of the common hunting applications of the flat point bullet is large game such as bear hunted with a handgun in a .44 Magnum or larger caliber. The disadvantage of flat point bullets is the reduction in aerodynamic performance; the flat point induces much drag, leading to significantly reduced velocities at long range.

Expanding bullets

More effective on lighter targets are the expanding bullets, the hollow point bullet and the soft point bullet. These are designed to use the hydraulic pressure of muscle tissue to expand the bullet. This process is called mushrooming, as the ideal result is a shape that resembles a mushroom—a cylindrical base, topped with a wide surface where the tip of the bullet has peeled back to expose more area. A copper-plated hollowpoint loaded in a .44 Magnum, for example, with an original weight of 240 grains (16 g) and a diameter of 0.43 inch (11 mm) might mushroom on impact to form a rough circle with a diameter of 0.70 inch (18 mm) and a final weight of 239 grains (15 g). This is excellent performance; almost the entire weight is retained, and the frontal surface area increased to over 265% of its original size. Penetration of the hollowpoint would be less than half that of a similar nonexpanding bullet, and the resulting wound cavity would be much wider.

Frangible bullets

The last category of expanding bullets are the frangible bullets. These bullets are designed to break up on impact, which results in a huge increase in surface area. The most common of these bullets are made of small diameter lead pellets, placed in a thin copper shell and held in place by an epoxy or similar binding agent. Upon impact, the epoxy shatters and the copper shell opens up, much like a hollowpoint. The individual lead balls then spread out in a wide pattern, and due to their low mass to surface area ratio, stop very quickly. Similar bullets are made out of sintered metals, which turn to powder upon impact. These bullets are usually restricted to pistol cartridges, as the nonhomogenous cores tend to cause inaccuracies that, while acceptable at short pistol ranges, are not acceptable for the typical range at which rifles are used. One interesting use of the sintered metal rounds is in shotguns in hostage rescue situations; the sintered metal round is used at near-contact range to shoot the lock mechanism out of doors. The resulting metal powder will immediately disperse after knocking out the door lock, and cause little or no damage to occupants of the room. Frangible rounds are also rumored to be used by armed security agents on aircraft. The concern is not depressurization (a bullet hole will not depressurise an airliner) but over penetration and damage to vital electrical or hydraulic lines, or injury to an innocent bystander by a bullet that travels through a target's body completely instead of stopping in the body. Also used are bullets similar to hollowpoint bullets or soft point bullets whose cores and/or jackets are deliberately weakened to cause deformation or fragmentation upon impact. The Warsaw Pact 5.45 x 39 mm M74 assault rifle round exemplifies a trend that is becoming common in the era of high velocity, small caliber military rounds. The 5.45 x 39 mm uses a steel jacketed bullet with a 2 part core, the rear being steel and the front being lead. Upon impact, the lead deforms, bending the bullet into a slight "L" shape. This causes the bullet to tumble in the tissue, thus increasing its effective frontal surface area by traveling sideways more often than not. This does not violate the Hague Convention, as it specifically mentions bullets that expand or flatten in the body. The NATO SS109 also tends to bend at the steel/lead junction, but with its weaker jacket, it fragments into many dozens of pieces. NATO 7.62 mm ball manufactured by some countries, such as Germany and Sweden, are also known to fragment due to jacket construction. Other bullets in use by militaries are quite back heavy, due to a long, sharp point created in an attempt to get the maximum ballistic coefficient (see external ballistics). These bullets will flip over after impact, then settle into a stable, back first orientation before stopping. The Swiss military actually redesigned their 5.56 mm assault rifle bullet to prevent this, to more fully comply with the spirit of the Hague Convention, though according to some sources the present GP90 5.56x45mm Swiss assault rifle ammunition was actually designed as an armor-piercing bullet, because in the 1980s it was perceived that the Soviets and their Warsaw Pact allies were going to issue soft body armor to infantry units on a wide basis, but after the end of the Cold War, the Bofors corporation, having spent a great deal of money on developing the new bullet, changed the sales pitch in order to sell it to the Swiss government. It might seem that if the whole purpose of a maximum disruption round is to expand to a larger diameter, it would make more sense to start out with the desired diameter rather than relying on the somewhat inconsistent results of expansion upon impact. While there is merit to this (there is a strong following of the .45 ACP, as compared to the 0.355 in diameter 9 x 19 mm, for just this reason) there are also significant downsides. A larger diameter bullet is going to have significantly more drag than a smaller diameter bullet of the same mass, which means long range performance will be significantly degraded. A larger diameter bullet also means more space is required to store the ammunition, which means either bulkier guns or smaller magazine capacities. The common trade-off when comparing .45 ACP and 9 x 19 mm pistols is a 7 or 8 round capacity in the .45 ACP vs. a 13 to 15 round capacity in the 9x19 mm. Although several .45-caliber pistols are available with high-capacity magazines (Para Ordnance being one of the first in the late 1980s) many people find the wide grip required uncomfortable and difficult to use. Especially where the military requirement of a nonexpanding round is concerned, there is fierce debate over whether it is better to have fewer, larger bullets for enhanced terminal effects, or more, smaller bullets for increased number of potential target hits.

Selecting bullets for terminal performance

The standard medium for testing bullets for performance on tissue is ballistic gelatin. Tests have shown that properly prepared and calibrated 10% (by mass) gelatin at 4 degrees Celsius correlates very closely to observed performance in the muscle tissue of living, anesthetised swine. Performance is generally graded with two factors, the maximum depth of penetration and the size of the cavity formed in the gelatin by the bullet impact. The size of the cavity represents the distance which tissue is thrown radially outward due to "splash." The penetration represents how far into the tissue the bullet will ultimately penetrate. Unfortunately, gelatin is a poor medium for evaluating actual effectiveness. The observed "tissue splash," usually referred to as "temporary cavitation," is not an indication of terminal performance in an animal, as gelatin has a much, much lower elastic limit than most living tissues; a force which tears a gelatin block in half may result in nothing more than slight bruising if applied to living flesh. Penetration figures may not be accurate. Many testers do not calibrate their gelatin. The standard calibration is 85 mm of penetration when shot by a standard .177 caliber steel bb traveling at 180 m/s (590 ft/s). Uncalibrated gelatin may show a variance of up to + or - 50% from calibrated gelatin. Further, animals' skin resists penetration much more than the muscle tissue which gelatin simulates. Human skin tissue on the torso resists penetration as much as 50 mm (2 in) of muscle, and horses' skin is the equivalent of approximately 200 mm (7.9 in). For a quick incapacitation, a hit to a vital, blood-bearing organ or the central nervous system is needed, so a bullet that will penetrate to the depth required for such a hit should be chosen. When hunting groundhogs, for example, a bullet that expands quickly to form a large cavity with minimum penetration would be the best choice. When hunting deer, a bullet which penetrates deeper is required; this can be accomplished by either limiting expansion (2 times the original width is often regarded as ideal), or by using a more powerful cartridge. For hunting bear, yet more penetration is required. The pattern is, of course, that the larger the animal, the deeper its vital organs will be located, and therefore a firearm, cartridge, and bullet type should be chosen that will be able to reach the vital organs and kill humanely. For dangerous game especially, deep penetration depth is critical; the reason for this is that the shooter cannot always choose their shots. If a hunter finds himself staring at a deer's hindquarters, it is very unlikely that he or she will choose to fire at that deer anyway, in the hopes that their bullet will be able to reach a vital organ through several layers of muscle and gut. The better choice in that scenario would be to wait until the deer decides to turn around. A lion, however, may decide to charge at a person other than the shooter, presenting a much less than optimal shooting angle. To hit the vital organs on a large game animal requires penetrating the thick fat and muscle tissue surrounding the chest cavity, and quite often bone as well. A hard, nondeforming bullet is often chosen, though many modern rifle calibers are quite capable of killing 1,000 lb (450 kg) elk and similar-sized animals with a deforming bullet; even the venerable .30-06 is up to the task, with a powerful enough load. Elephant hunters normally attempt to shoot for the brain, which is much smaller than the size of the elephant's head, and so must be targeted quite precisely, and require a firearm and bullet capable of punching through a foot (300 mm) or more of tough, albeit hollow, bone and reaching the brain.

Terminal ballistics for non-military defensive purposes

The rules of engagement for non-military use of firearms usually require that a life be in immediate danger for shots to be fired. Under such circumstances, the goal is to incapacitate the target as quickly as possible, to prevent the harm from being done. In most cases, the shots are fired from a handgun, which is, compared to a rifle, very much underpowered. Humans are in roughly the same class as deer sized game, and in most places, the minimum cartridge power required to hunt deer is more than twice that of the average police sidearm. Handguns are also very inaccurate in the hands of all but the best shooters, and the average defensive shooter is not an expert, and is under a great deal of stress, which further degrades accuracy. These factors combine to require extremely effective terminal ballistics to provide swift incapacitation of the target under far less than ideal circumstances. Humans walk upright and present relatively unprotected vital organ targets from some angles, and have substantially thinner skin, so the bare minimum penetration is lower than for deer. Cross-torso shots and shots which must first penetrate an arm are relatively common in defensive shooting scenarios, however. Bullets for use on humans are usually designed to comply with the FBI's penetration requirement of 12 to 18 inches (30 to 46 cm), which is based on the IWBA's requirement of 12.5 to 14 inches (32 to 36 cm). This is to ensure that the bullet can reach a vital blood-bearing organ or central nervous system structure from most angles. Frangible rounds, while they are sold for defensive purposes, are not well suited for the role, as they generally penetrate less than 10 inches (25 cm), and are therefore prone to failure when they must pass through nonvital tissues, such as a hand or arm. When they work, they work very well, but when they fail, they tend to fail badly. Hollowpoint bullets normally expand most when at their highes

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