RareGlobe

Gravitation is a natural phenomenon and one of the fundamental forces by which all objects with mass attract each other. In everyday life, gravitation is most commonly thought of as the agency that gives objects weight. It is responsible for keeping the Earth and the other planets in their orbits around the Sun; for keeping the Moon in its orbit around the Earth, for the formation of tides; for convection (by which hot fluids rise); for heating the interiors of forming stars and planets to very high temperatures; and for various other phenomena that we observe. Gravitation is also the reason for the very existence of the Earth, the Sun, and most macroscopic objects in the universe; without it, matter would not have coalesced into these large masses and life, as we know it, would not exist.
Modern physics describes gravitation using the general theory of relativity, but the much simpler Newton's law of universal gravitation provides an excellent approximation in most cases.
In scientific usage gravitation and gravity are distinct. "Gravitation" is the overall theory dealing with the attractive influence that all objects exert on each other, while "gravity" or the "law of gravity" specifically refers to a force produced by a massive object (i.e., an object with mass). The terms are mostly interchangeable in everyday use. In general relativity, gravitation is due to spacetime curvatures which causes inertially moving objects to tend to accelerate towards each other. Another (discredited) example is Le Sage's theory of gravitation, in which massive objects are effectively pushed towards each other.

History of gravitational theory

Early history
Efforts to understand gravity began in ancient times. Philosophers in ancient India explained the phenomenon from the 8th century BC. According to Kanada, founder of the Vaisheshika school, "Weight causes falling; it is imperceptible and known by inference."
In the 4th century BC, the Greek philosopher Aristotle believed that there was no effect without a cause, and therefore no motion without a force. He hypothesized that everything tried to move towards its proper place in the crystalline spheres of the heavens, and that physical bodies fell toward the center of the Earth in proportion to their weight.
Brahmagupta, in the Brahmasphuta Siddhanta (628 AD), responded to critics of the heliocentric system of Aryabhata (476-550 AD) stating that "all heavy things are attracted towards the center of the earth" and that "all heavy things fall down to the earth by a law of nature, for it is the nature of the earth to attract and to keep things, as it is the nature of water to flow, that of fire to burn, and that of wind to set in motion... The earth is the only low thing, and seeds always return to it, in whatever direction you may throw them away, and never rise upwards from the earth."
In the 9th century, the eldest Ban? M?s? brother, Muhammad ibn Musa, in his Astral Motion and The Force of Attraction, hypothesized that there was a force of attraction between heavenly bodies, foreshadowing Newton's law of universal gravitation. In the 1000s, the Persian scientist Ibn al-Haytham (Alhacen), in the Mizan al-Hikmah, discussed the theory of attraction between masses, and it seems that he was aware of the magnitude of acceleration due to gravity. In 1121, Al-Khazini, in The Book of the Balance of Wisdom, differentiated between force, mass, and weight, and discovered that gravity varies with the distance from the centre of the Earth, though he believed that the weight of heavy bodies increase as they are farther from the centre of the Earth. All these early attempts at trying to explain the force of gravity were philosophical in nature and it would be Isaac Newton that gave the first correct desciption of gravity.

Scientific revolution
Modern work on gravitational theory began with the work of Galileo Galilei in the late 16th century and early 17th century. In his famous (though probably apocryphal) experiment dropping balls from the Tower of Pisa, and later with careful measurements of balls rolling down inclines, Galileo showed that gravitation accelerates all objects at the same rate. This was a major departure from Aristotle's belief that heavier objects are accelerated faster. (Galileo correctly postulated air resistance as the reason that lighter objects may fall more slowly in an atmosphere.) Galileo's work set the stage for the formulation of Newton's theory of gravity.

Newton's theory of gravitation
In 1687, English mathematician Sir Isaac Newton published Principia, which hypothesizes the inverse-square law of universal gravitation. In his own words, "I deduced that the forces which keep the planets in their orbs must be reciprocally as the squares of their distances from the centers about which they revolve; and thereby compared the force requisite to keep the Moon in her orb with the force of gravity at the surface of the Earth; and found them answer pretty nearly."
Newton's theory enjoyed its greatest success when it was used to predict the existence of Neptune based on motions of Uranus that could not be accounted by the actions of the other planets. Calculations by John Couch Adams and Urbain Le Verrier both predicted the general position of the planet, and Le Verrier's calculations are what led Johann Gottfried Galle to the discovery of Neptune.
Ironically, it was another discrepancy in a planet's orbit that helped to point out flaws in Newton's theory. By the end of the 19th century, it was known that the orbit of Mercury could not be accounted for entirely under Newton's theory, but all searches for another perturbing body (such as a planet orbiting the Sun even closer than Mercury) had been fruitless. The issue was resolved in 1915 by Albert Einstein's new General Theory of Relativity, which accounted for the discrepancy in Mercury's orbit.
Although Newton's theory has been superseded, most modern non-relativistic gravitational calculations are still made using Newton's theory because it is a much simpler theory to work with than General Relativity, and gives sufficiently accurate results for most applications.

General relativity
In general relativity, the effects of gravitation are ascribed to spacetime curvature instead of a force. The starting point for general relativity is the equivalence principle, which equates free fall with inertial motion. The issue that this creates is that free-falling objects can accelerate with respect to each other. In Newtonian physics, no such acceleration can occur unless at least one of the objects is being operated on by a force (and therefore is not moving inertially).
To deal with this difficulty, Einstein proposed that spacetime is curved by matter, and that free-falling objects are moving along locally straight paths in curved spacetime. (This type of path is called a geodesic.) More specifically, Einstein discovered the field equations of general relativity, which relate the presence of matter and the curvature of spacetime and are named after him. The Einstein field equations are a set of 10 simultaneous, non-linear, differential equations. The solutions of the field equations are the components of the metric tensor of spacetime. A metric tensor describes a geometry of spacetime. The geodesic paths for a spacetime are calculated from the metric tensor.
Notable solutions of the Einstein field equations include:
The Schwarzschild solution, which describes spacetime surrounding a spherically symmetric non-rotating uncharged massive object. For compact enough objects, this solution generated a black hole with a central singularity. For radial distances from the center which are much greater than the Schwarzschild radius, the accelerations predicted by the Schwarzschild solution are practically identical to those predicted by Newton's theory of gravity.
The Reissner-Nordstr?m solution, in which the central object has an electrical charge. For charges with a geometrized length which are less than the geometrized length of the mass of the object, this solution produces black holes with two event horizons.
The Kerr solution for rotating massive objects. This solution also produces black holes with multiple event horizons.
The Kerr-Newman solution for charged, rotating massive objects. This solution also produces black holes with multiple event horizons.
The cosmological Robertson-Walker solution, which predicts the expansion of the universe.
General relativity has enjoyed much success because of how its predictions of phenomena which are not called for by the theory of gravity have been regularly confirmed. For example:
General relativity accounts for the anomalous perihelion precession of the planet Mercury.
The prediction that time runs slower at lower potentials has been confirmed by the Pound-Rebka experiment, the Hafele-Keating experiment, and the GPS.
The prediction of the deflection of light was first confirmed by Arthur Eddington in 1919, and has more recently been strongly confirmed through the use of a quasar which passes behind the Sun as seen from the Earth. See also gravitational lensing.
The time delay of light passing close to a massive object was first identified by Irwin Shapiro in 1964 in interplanetary spacecraft signals.
Gravitational radiation has been indirectly confirmed through studies of binary pulsars.
The expansion of the universe (predicted by the Alexander Friedmann) was confirmed by Edwin Hubble in 1929.

Gravity and quantum mechanics
Several decades after the discovery of general relativity it was realized that general relativity is incompatible with quantum mechanics. It is possible to describe gravity in the framework of quantum field theory like the other fundamental forces, with the attractive force of gravity arises due to exchange of virtual gravitons, in the same way as the electromagnetic force arises from exchange of virtual photons. This reproduces general relativity in the classical limit. However, this approach fails at short distances of the order of the Planck length, where a more complete theory of quantum gravity (or a new approach to quantum mechanics) is required. Many believe the complete theory to be string theory, or more currently M Theory.

Specifics

Earth's gravity
Every planetary body (including the Earth) is surrounded by its own gravitational field, which exerts an attractive force on all objects. Assuming a spherically symmetrical planet (a reasonable approximation), the strength of this field at any given point is proportional to the planetary body's mass and inversely proportional to the square of the distance from the center of the body.
The strength of the gravitational field is numerically equal to the acceleration of objects under its influence, and its value at the Earth's surface, denoted g, is approximately 9.81 m/s? (32.2 ft/s?) as the standard average. This means that, ignoring air resistance, an object falling freely near the earth's surface increases its velocity with 9.81 m/s (32.2 ft/s or 22 mph) for each second of its descent. Thus, an object starting from rest will attain a velocity of 9.81 m/s (32 ft/s) after one second, 19.6 m/s (64 ft/s) after two seconds, and so on, adding 9.8 m/s to each resulting velocity. According to Newton's 3rd Law, the Earth itself experiences an equal and opposite force to that acting on the falling object, meaning that the Earth also accelerates towards the object. However, because the mass of the Earth is huge, the acceleration of the Earth by this same force is negligible, when measured relative to the system's center of mass.

Equations for a falling body
The kinematical and dynamical equations describing the trajectories of falling bodies are considerably simpler if the gravitational force is assumed constant. This assumption is reasonable for objects falling to Earth over the relatively short vertical distances of our everyday experience, but does not hold over larger distances, such as spacecraft trajectories, since the acceleration due to Earth's gravity is much smaller at large distances.
Under an assumption of constant gravity, Newton's law of gravitation simplifies to F = mg, where m is the mass of the body and g is a constant vector with an average magnitude of 9.81 m/s?. The acceleration due to gravity is equal to this g. An initially-stationary object which is allowed to fall freely under gravity drops a distance which is proportional to the square of the elapsed time. The image on the right, spanning half a second, was captured with a stroboscopic flash at 20 flashes per second. During the first 1/20th of a second the ball drops one unit of distance (here, a unit is about 12 mm); by 2/20ths it has dropped at total of 4 units; by 3/20ths, 9 units and so on.
Under the same constant gravity assumptions, the potential energy, Ep, of a body at height h is given by Ep = mgh (or Ep = Wh, with W meaning weight). This expression is valid only over small distances h from the surface of the Earth. Similarly, the expression h = v2 / 2g for the maximum height reached by a vertically projected body with velocity v is useful for small heights and small initial velocities only. In case of large initial velocities we have to use the principle of conservation of energy to find the maximum height reached. This same expression can be solved for v to determine the velocity of an object dropped from a height h immediately before hitting the ground, , assuming negligible air resistance.

Gravity and astronomy
The discovery and application of Newton's law of gravity accounts for the detailed information we have about the planets in our solar system, the mass of the Sun, the distance to stars, quasars and even the theory of dark matter. Although we have not traveled to all the planets nor to the Sun, we know their masses. These masses are obtained by applying the laws of gravity to the measured characteristics of the orbit. In space an object maintains its orbit because of the force of gravity acting upon it. Planets orbit stars, stars orbit galactic centers, galaxies orbit a center of mass in clusters, and clusters orbit in superclusters. The force of gravity is proportional to the mass of an object and inversely proportional to the square of the distance between the objects.

Gravitational radiation
In general relativity, gravitational radiation is generated in situations where the curvature of spacetime is oscillating, such as is the case with co-orbiting objects. The gravitational radiation emitted by the solar system is far too small to measure. However, gravitational radiation has been indirectly observed as an energy loss over time in binary pulsar systems such as PSR 1913+16. It is believed that neutron star mergers and black hole formation may create detectable amounts of gravitational radiation. Gravitational radiation observatories such as LIGO have been created to study the problem. No confirmed detections have been made of this hypothetical radiation, but as the science behind LIGO is refined and as the instruments themselves are endowed with greater sensitivity over the next decade, this may change.

Alternative theories

Historical alternative theories
Aristotelian theory of gravity
Le Sage's theory of gravitation (1784) also called LeSage gravity, proposed by Georges-Louis Le Sage, based on a fluid-based explanation where a light gas fills the entire universe.
Nordstr?m's theory of gravitation (1912, 1913), an early competitor of general relativity.
Whitehead's theory of gravitation (1922), another early competitor of general relativity.

Recent alternative theories
Brans-Dicke theory of gravity (1961)
Induced gravity (1967), a proposal by Andrei Sakharov according to which general relativity might arise from quantum field theories of matter.
Rosen bi-metric theory of gravity
In the modified Newtonian dynamics (MOND) (1981), Mordehai Milgrom proposes a modification of Newton's Second Law of motion for small accelerations.
The new and highly controversial Process Physics theory attempts to address gravity
The self-creation cosmology theory of gravity (1982) by G.A. Barber in which the Brans-Dicke theory is modified to allow mass creation.
Nonsymmetric gravitational theory (NGT) (1994) by John Moffat
Tensor-vector-scalar gravity (TeVeS) (2004), a relativistic modification of MOND by Jacob Bekenstein

The team at MSSL-UCL helped build ASPERA-4, an instrument to explore one of the differences between Venus and Earth - Venus lacks a magnetic field. Its atmosphere escapes as the solar wind, a gale of charged particles from the Sun, erodes its unprotected atmosphere.

Venus has the hottest planetary surface in the solar system (450 degreeC - hot enough to melt lead), a runaway greenhouse effect, a thick atmosphere (92 times as thick as Earth's), a slow rotation the wrong way round, hurricane force winds and strange vortices near the poles. Other instruments on the mission will look at the thick atmosphere of Venus below its sulphuric acid clouds and how it interacts with the surface. They will also try and understand the runaway greenhouse effect and search for active volcanism.

Venus Express is a wonderful mission of discovery - not only to find out why Earth's twin went wrong and to study its strange atmosphere now, but also to glimpse a possible future for the Earth. The extreme example of Venus will test models of our own atmosphere and humankind's contribution to greenhouse gases here. It will also give a preview of what it may be like in 2 billion years time when the Sun has got brighter in its evolution along the main sequence of stars. At that time Earth will no longer be in the 'Goldilocks' zone and our planet may become more like what Venus is like now.

In the meantime, says Dr Coates, 'We can't wait to get the data. Generally, the comparison between Earth, Venus and Mars, and why Earth is so special, is scientifically exciting and important for our race. With our instrument, ASPERA, at Venus and Mars, we will compare the two unmagnetized planets and how their atmosphere escapes. Why does escape occur at both but leave very different atmospheric pressures at Venus (almost 100 times Earth's surface pressure) and Mars (a factor 100 less?) We will soon begin to find out.

The rotation of the Earth is not intimately related to the length of the year. It is related to the length of the day. The period of the Earth's orbit around the Sun determines the length of the year, and it is this value that is being synchronized by the leap second program.

The period of the Earth's orbit around the Sun is subject to variations resulting from interactions with all of the gravitational objects of the universe, but most particularly those nearby -- the other planets of the solar system. Because the orbits of the planets around the Sun are not synchronized, the forces upon the Earth due to those planets is different each year, as their arrangement in their repective orbits continually changes. Intercalary or "leap" days are added to the calendar on a regular basis, because the actual length of the year is not an even number of days. This does not imply that the Earth's rotation has slowed down. If it had, it would become necessary to add Intercalary days more and more often, until eventually every year would be a "leap" year.

Leap seconds are similar. The routine addition of 1 second to the year does not indicate that the Earth's rotation is slowing, but rather that the somewhat complicated devices of the leap year tradition are slightly inaccurate. The goal of the leap second program of the IERS is to insure the close synchrony of the UTC and UT1 time scales, as measured by reference to our most accurate reference clocks, so-called "atomic" clocks. As tiny (presumably) gravitational perturbations occur in the orbit of the Earth around the sun, the amount of deviation between UTC (which is how clocks are set around the world) and UT1 (which tracks the time of Solstice at Grenwich) is also perturbed. Without these variations, leap seconds could be scheduled on a regular basis, like leap days, but because these perturbations are sometimes more, sometimes less, and because the goal of the IERS is to keep UTC and UT1 within 1 second at all times, leap seconds are instead reactively scheduled.

The matter of leap seconds has become increasingly prominent in public attention, especially in technical circles, recently, because the LORAN and GPS satellite navigation systems maintain an absolute time measure, which deviates from UTC increasingly as leap seconds are added. As a result, the compromises which resulted in the current system are being reconsidered at the level of the international standards bodies, and in global academic discourse. As the ways in which people use time measurement systems change, the technical means of measuring time and synchronizing clocks change, and the defects in the older models become increasingly visible, it seems likely that some substantial change will occur in the definition of UTC, at least. What form it will take, I will not guess, being an uninformed outsider to the process.

Finally, I will note that the rate of the Earth's rotation is in fact slowing -- but that this slowing will not result in any additional leap second during our conventinally expected lifetimes.

The Van Allen Radiation Belt is a torus of energetic charged particles (plasma) around Earth, held in place by Earth's magnetic field. The Van Allen belts are closely related to the polar aurora where particles strike the upper atmosphere and fluoresce.
The presence of a radiation belt had been proposed by Nicholas Christofilos prior to the Space Age and was confirmed by the Explorer I on January 31, 1958, and Explorer 3 missions, under Dr. James Van Allen at the University of Iowa. The trapped radiation was first mapped out by Sputnik 3, Explorer 4, Pioneer 3 and Luna 1.
Energetic electrons form two distinct radiation belts, while protons form a single belt. Within these belts are particles capable of penetrating about 1 g/cm2 of shielding (e.g., 1 millimetre of lead).
The term Van Allen Belts refers specifically to the radiation belts surrounding Earth; however, similar radiation belts have been discovered around other planets. The Sun does not support long-term radiation belts. The Earth's atmosphere limits the belts' particles to regions above 200-1,000 km, while the belts do not extend past 7 Earth radii RE. The belts are confined to an area which extends about 65 degree from the celestial equator.
An upcoming NASA mission, Radiation Belt Storm Probes will go further and gain scientific understanding (to the point of predictability) of how populations of relativistic electrons and ions in space form or change in response to changes in solar activity and the solar wind.

Outer belt
The large outer radiation belt extends from an altitude of about (3 to 10 Earth radii) above the Earth's surface, and has its greatest intensity is usually around 4-5 RE. Outer electron radiation belt is mostly produced by the inward radial diffusion [e.g. Elkinkington et al., 2001; Shprits and Thorne, 2004] and local acceleration [Horne et al., 2005; Shprits et al., 2006] due to transfer of energy from whistler mode plasma waves to radiation belt electrons. Radiation belt electrons are also constantly removed by collisions with atmospheric neutrals[Thorne et al., 2005], losses to magnetopause, and the outward radial diffusion[Shprits et al., 2006]. The outer belt consists mainly of high energy(0.1-10 MeV) electrons trapped by the Earth's magnetosphere. The gyroradii for energetic protons would be large enough to bring them into contact with the Earth's atmosphere. The electrons here have a high flux and at the outer edge (close to the magnetopause), where geomagnetic field lines open into the geomagnetic "tail", fluxes of energetic electrons can drop to the low interplanetary levels within about 100 km (a decrease by a factor of 1,000).
The trapped particle population of the outer belt is varied, containing electrons and various ions. Most of the ions are in the form of energetic protons, but a certain percentage are alpha particles and O+ oxygen ions, similar to those in the ionosphere but much more energetic. This mixture of ions suggests that ring current particles probably come from more than one source.
The outer belt is larger than the inner belt, and its particle population fluctuates widely. Energetic (radiation) particle fluxes can increase and decrease dramatically as a consequence of geomagnetic storms, which are themselves triggered by magnetic field and plasma disturbances produced by the Sun. The increases are due to storm-related injections and acceleration of particles from the tail of the magnetosphere.
There is debate as to whether the outer belt was discovered by the US Explorer 4 or the USSR Sputnik 2/3.

Inner belt
The inner Van Allen Belt extends from an altitude of 700-10,000 km (0.1 to 1.5 Earth radii) above the Earth's surface, and contains high concentrations of energetic protons with energies exceeding 100 MeV and electrons in the range of hundreds of kiloelectronvolts, trapped by the strong (relative to the outer belts) magnetic fields in the region.
It is believed that protons of energies exceeding 50 MeV in the lower belts at lower altitudes are the result of the beta decay of neutrons created by cosmic ray collisions with nuclei of the upper atmosphere. The source of lower energy protons is believed to be proton diffusion due to changes in the magnetic field during geomagnetic storms.
Due to the slight offset of the belts from Earth's geometric center, the inner Van Allen belt makes its closest approach to the surface at the South Atlantic Anomaly.

Impact on space travel
Solar cells, integrated circuits, and sensors can be damaged by radiation. In 1962, the Van Allen belts were temporarily amplified by a high-altitude nuclear explosion (the Starfish Prime test) and several satellites ceased operation. Geomagnetic storms occasionally damage electronic components on spacecraft. Miniaturization and digitization of electronics and logic circuits have made satellites more vulnerable to radiation, as incoming ions may be as large as the circuit's charge. Electronics on satellites must be hardened against radiation to operate reliably. The Hubble Space Telescope, among other satellites, often has its sensors turned off when passing through regions of intense radiation.
During Project Apollo, astronauts traveled through the Van Allen belts on both the outbound and return trips to the moon. The crews spent only limited time in transit in the region, and consequently the radiation exposure was limited. The Apollo 14 crew recorded the highest Van Allen belt exposures during their February 1971 mission, but the crew's short-term exposure was still within acceptable levels. Future manned missions beyond earth orbit must also transit the Van Allen belts, but these missions will be shielded and hardened for much longer-duration exposure to cosmic rays and solar wind.
An object satellite shielded by 3 mm of aluminium will receive about 2,500 rem (25 Sv) per year.

Causes
It is generally understood that the inner and outer Van Allen belts result from different processes. The inner belt, consisting mainly of energetic protons, is the product of the decay of albedo neutrons which are themselves the result of cosmic ray collisions in the upper atmosphere. The outer belt consists mainly of electrons. They are injected from the geomagnetic tail following geomagnetic storms, and are subsequently energized though wave-particle interactions. Particles are trapped in the Earth's magnetic field because it is basically a magnetic mirror. Particles gyrate around field lines and also move along field lines. As particles encounter regions of stronger magnetic field where field lines converge, their "longitudinal" velocity is slowed and can be reversed, reflecting the particle. This causes the particle to bounce back and forth between the earth's poles, where the magnetic field increases.
A gap between the inner and outer Van Allen belts, sometimes called safe zone or safe slot, is caused by the very low frequency (VLF) waves which scatter particles in pitch angle which results in the loss of particles to the atmosphere. Solar outbursts can pump particles into the gap but they drain again in a matter of days. The radio waves were originally thought to be generated by turbulence in the radiation belts, but recent work by James Green of the NASA Goddard Space Flight Center comparing maps of lightning activity collected by the Micro Lab 1 spacecraft with data on radio waves in the radiation-belt gap from the IMAGE spacecraft suggests that they're actually generated by lightning within Earth's atmosphere. The radio waves they generate strike the ionosphere at the right angle to pass through it only at high latitudes, where the lower ends of the gap approach the upper atmosphere. These results are still under scientific debate.
There have been nuclear tests in space that have caused artificial radiation belts. Starfish Prime, a high altitude nuclear test created an artificial radiation belt that damaged or destroyed as many as one third of the satellites in low earth orbit at the time. Thomas Gold has argued that the outer belt is left over from the aurora while Alex Dessler has argued that the belt is a result of volcanic activity.
In another view, the belts could be considered a flow of electric current that is fed by the solar wind. With the protons being positive and the electrons being negative, the area between the belts is sometimes subjected to a current flow, which "drains" away. The belts are also thought to drive auroras, lightning and many other electrical effects.

Removal
The belts are a hazard for artificial satellites and moderately dangerous for human beings, difficult and expensive to shield against.
There is a proposal by the late Robert L. Forward called HiVolt which may be a way to drain at least the inner belt to 1% of its natural level within a year. The proposal involves deploying highly electrically charged tethers in orbit. The idea is that the electrons would be deflected by the large electrostatic fields and intersect the atmosphere and harmlessly dissipate.
Some scientists, however, theorize that the Van Allen belts carry some additional protection against solar wind, meaning that a weakening of the belts could harm electronics and organisms; and that they may influence the Earth's telluric current, so that dissipating the belts could influence the behavior of Earth's magnetic poles.

References in popular culture
In a Halloween special of The Simpsons (The "Attack of the 50-foot Eyesores" section of "Treehouse of Horror VI"), a radio broadcast warns Homer that, "Astronomers from Tacoma to Vladivostok have just reported an ionic disturbance in the vicinity of the Van Allen Belt. Scientists are recommending that necessary precautions be taken." These cause large advertising logos to come to life and cause havoc and destruction across Springfield.
In the 1961 movie Voyage to the Bottom of the Sea, the Van Allen radiation belt catches fire, threatening Earth.
In the cartoon The Adventures of Jimmy Neutron, an episode titled "The 'N' Men" (spoof of the X-Men) had Jimmy and his friends careening out of control through the Van Allen radiation belt, giving them all superpowers based on what each was doing at the moment they went through the radiation belt. This situation parodies the origins of the Fantastic Four.
In Osamu Tezuka's manga series "Astro Boy", Astro Boy uses the Van Allen Belt to destroy parasites from outer space.
In the 1970 Doctor Who episode "Doctor Who and the Silurians", the Silurians plan to attack the Van Allen Belt to cause the Earth to warm, killing all the humans.

The Moon's orbit (its circular path around the Earth) is indeed getting larger, at a rate of about 3.8 centimeters per year. (The Moon's orbit has a radius of 384,000 km.) I wouldn't say that the Moon is getting closer to the Sun, specifically, though--it is getting farther from the Earth, so, when it's in the part of its orbit closest to the Sun, it's closer, but when it's in the part of its orbit farthest from the Sun, it's farther away.

The reason for the increase is that the Moon raises tides on the Earth. Because the side of the Earth that faces the Moon is closer, it feels a stronger pull of gravity than the center of the Earth. Similarly, the part of the Earth facing away from the Moon feels less gravity than the center of the Earth. This effect stretches the Earth a bit, making it a little bit oblong. We call the parts that stick out "tidal bulges." The actual solid body of the Earth is distorted a few centimeters, but the most noticable effect is the tides raised on the ocean.

Now, all mass exerts a gravitational force, and the tidal bulges on the Earth exert a gravitational pull on the Moon. Because the Earth rotates faster (once every 24 hours) than the Moon orbits (once every 27.3 days) the bulge tries to "speed up" the Moon, and pull it ahead in its orbit. The Moon is also pulling back on the tidal bulge of the Earth, slowing the Earth's rotation. Tidal friction, caused by the movement of the tidal bulge around the Earth, takes energy out of the Earth and puts it into the Moon's orbit, making the Moon's orbit bigger (but, a bit pardoxically, the Moon actually moves slower!).

The Earth's rotation is slowing down because of this. One hundred years from now, the day will be 2 milliseconds longer than it is now.

This same process took place billions of years ago--but the Moon was slowed down by the tides raised on it by the Earth. That's why the Moon always keeps the same face pointed toward the Earth. Because the Earth is so much larger than the Moon, this process, called tidal locking, took place very quickly, in a few tens of millions of years.

Many physicists considered the effects of tides on the Earth-Moon system. However, George Howard Darwin (Charles Darwin's son) was the first person to work out, in a mathematical way, how the Moon's orbit would evolve due to tidal friction, in the late 19th century. He is usually credited with the invention of the modern theory of tidal evolution.

So that's where the idea came from, but how was it first measured? The answer is quite complicated, but I've tried to give the best answer I can, based on a little research into the history of the question.

There are three ways for us to actually measure the effects of tidal friction.

* Measure the change in the length of the lunar month over time.

This can be accomplished by examining the thickness of tidal deposits preserved in rocks, called tidal rhythmites, which can be billions of years old, although measurements only exist for rhythmites that are 900 million years old. As far as I can find (I am not a geologist!) these measurements have only been done since the early 90's.

* Measure the change in the distance between the Earth and the Moon.

This is accomplished in modern times by bouncing lasers off reflectors left on the surface of the Moon by the Apollo astronauts. Less accurate measurements were obtained in the early 70's.

* Measure the change in the rotational period of the Earth over time.

Nowadays, the rotation of the Earth is measured using the Very Long Baseline Interferometry, a technique using many radio telescopes a great distance apart. With VLBI, the positions of quasars (tiny, distant, radio-bright objects) can be measured very accuarately. Since the rotating Earth carries the antennas along, these measurements can tell us the rotation speed of the Earth very accurately.

However, the change in the Earth's rotational period was first measured using eclipses, of all things. Astronomers who studied the timing of eclipses over many centuries found that the Moon seemed to be accelerating in its orbit, but what was actually happening was the the Earth's rotation was slowing down. The effect was first noticed by Edmund Halley in 1695, and first measured by Richard Dunthorne in 1748--though neither one really understood what they were seeing. I think this is the earliest discovery of the effect.

At the point on the ocean's surface closest to the moon (point A in the illustration), the lunar gravitational attractive force is strongest and it pulls the ocean toward itself. On the opposite side of Earth (point B), its attractive force is weakest, which allows the ocean to bulge outward again, in this case away from the moon. As the planet rotates from west to east the two bulges tend to stay on the Earth-moon line. (The moon also revolves around Earth in the same direction as Earth's rotation but at a much slower rate.) For an observer stationed on the surface and revolving with it, the bulges would appear as a giant wave, which follows the apparent motion of the moon to the west and has two crests per lunar day.

Real ocean tides are of course complicated by the water?s uneven depth and the presence of land. But Laplace's theory is perfectly applicable to the atmosphere if ocean depth in the tidal equation is replaced by a quantity called equivalent depth, characterizing the extent of the atmosphere above the surface. Just as our weight puts pressure on the ground beneath our feet, the weight of the atmosphere above us exerts pressure on the planet's surface and everything located on it (recall that pressure is defined as force per unit surface). This is the usual atmospheric surface pressure that we hear about in weather forecasts. It is clear then that Laplace's theory predicts two pressure maxima per lunar day corresponding to the two ocean bulges [see illustration]. One occurs approximately when the moon is directly overhead, the other half-a-day later. The dominant lunar tide in the atmosphere is therefore semidiurnal (half-daily).

Theory predicts stronger lunar pressure oscillations in the tropics but their amplitude rarely exceeds 100 microbars or 0.01 percent of the average surface pressure. Detection of such a tiny signal masked by much larger pressure variations associated with weather phenomena required the development of special statistical techniques and the accumulation of a long series of regular observations.

Surprisingly, such observations show that the sun also causes semidiurnal tides in the atmosphere, which are more than 20 times stronger, although the solar gravitational forcing is less than half that of the moon. After all, it is the moon that causes the dominant tides in the ocean, not the sun. (The average lunar day is about 51 minutes longer than the solar day because of the moon?s rotation around Earth and this allows scientists to reliably separate the two tides in long observational records.) Apparently, Laplace had suspected this, suggesting that the strong solar tide was primarily generated by solar heating and not by solar gravity. Scientists finally confirmed this hypothesis in the 1960s when it became possible to develop adequate models of solar atmospheric heating. As with the gravitational pull of a celestial body, the uneven solar heating on Earth's dayside distorts the spherical symmetry of the atmosphere, but in a more complex way. The thermal solar tide therefore consists of several dominant waves, the most prominent being the diurnal and semidiurnal ones.

Pressure variations cause tidal oscillations in other atmospheric characteristics as well. It is common for atmospheric waves to grow in amplitude with height as the air becomes thinner. The lunar tide, however, remains weak compared to the solar tide in the upper atmosphere. Still, at altitudes above roughly 80 kilometers (50 miles) lunar tides have been detected in winds, temperature, airglow emissions and a number of ionospheric parameters. Almost two centuries after atmospheric lunar tides were predicted and first observed, they are still studied. They represent a unique type of atmospheric motion whose forcing mechanism is known with great precision, allowing us to test our numerical models and theoretical predictions.

The Chandler wobble is a small motion in the Earth's axis of rotation relative to the Earth's surface, which was discovered by American astronomer Seth Carlo Chandler in 1891. It amounts to 0.7 arcseconds (about 15 meters on the Earth's surface) and has a period of 433 days. This wobble combines with another wobble with a period of one year so that the total polar motion varies with a period of about 7 years. The Chandler wobble is an example of the kind of motion that can occur for a spinning object that is not a sphere; this is called a free nutation. Somewhat confusingly, the direction of the Earth's spin axis relative to the stars also varies with different periods, and these motions (caused by the tidal attraction of the Moon and Sun) are also called nutations, except for the slowest, which is the precession of the equinoxes.
The existence of a free nutation of the Earth was predicted by Leonhard Euler in 1755 as part of his studies of the dynamics of rotating bodies. Based on the known flattening of the Earth he predicted that it would have a period of 355 days. Several astronomers searched for motions with this period, but none were found. Chandler's contribution was to look for motions at any possible period; once the Chandler wobble was observed, the difference between its period and the one predicted by Euler was explained (by Simon Newcomb) as being caused by the non-rigidity of the Earth. The full explanation for the period also involves the fluid nature of the Earth's core and oceans: the wobble in fact produces a very small ocean tide, the pole tide, which is the only tide not caused by bodies outside the Earth.
To measure the wobble, the International Latitude Observatories were established in 1899. (The wobble is also called the variation of latitude). These provided data on the Chandler and annual wobble for most of the 20th century, though they were eventually superseded by other methods of measurement. Monitoring of the polar motion is now done by the International Earth Rotation Service.
The wobble's amplitude has varied since its discovery, reaching its largest size in 1910 and fluctuating noticeably from one decade to another. While it has to be maintained by changes in the mass distribution or angular momentum of the Earth's outer core, atmosphere, oceans, or crust (from earthquakes), for a long time the actual source was unclear, since no available motions seemed to be coherent with what was driving the wobble. On 18 July 2000, however, the Jet Propulsion Laboratory announced that "the principal cause of the Chandler wobble is fluctuating pressure on the bottom of the ocean, caused by temperature and salinity changes and wind-driven changes in the circulation of the oceans."

The electric field is a measure of the force that is exerted per coulomb of charge. Its measure is defined as kq/r2 where k is the electrostatic constant constant, q is the amount of charge, and r is the distance between charges. The presence of an electric field is identified using a test charge.

The earth's atmosphere has an electric field that is directed radially inward. Most of my sources show that knowing the electric field of the earth can lead to the calculation of the charge on the earth's surface. Though some of the figures obtained are for the earth's atmosphere, it is true that the magnitude of the electric field outside a uniformly charged sphere is the same as if all the charge were concentrated at the center.

I obtained values for the magnitude of the electric field at the Earth's surface. These were in the range of 66 N/C to 150 N/C. These values are close enough to assume that each source received their data from a different primary source and each may be accurate in their own right. I am convinced that the figure is closer to 150 N/C than to 66 N/C because of the sources themselves.

Though the electric field is reported as being constant by some of my sources, The Handbook of Physics and Chemistry proves that the electric field intensity varies measurements taken at different altitudes above sea level. Its intensity decreases as you move farther away from the earth's surface.

Cosmic dust is a type of dust composed of particles in space which are a few molecules to 0.1 mm in size. Cosmic dust can be further distinguished by its astronomical location; for example: intergalactic dust, interstellar dust, circumplanetary dust, dust clouds around other stars, and the major interplanetary dust components to our own zodiacal dust complex (seen in visible light as the zodiacal light): Comet dust, asteroidal dust plus some of the less significant contributors: Kuiper belt dust, interstellar dust passing through our solar system, and beta meteoroids.
Cosmic dust was once solely an annoyance to astronomers, as it obscures objects they wish to observe. When infrared astronomy began, those so-called annoying dust particles were observed to be significant and vital components of astrophysical processes.
For example, Cosmic Dust can drive the mass loss when a star is nearing the end of its life, play a part in the early stages of star formation, and form planets. In our own solar system, dust plays a major role in the zodiacal light, Saturn's B Ring spokes, the outer diffuse planetary rings at Jupiter, Saturn, Uranus and Neptune, the resonant dust ring at the Earth, and comets.
The study of dust is a many-faceted research topic that brings together different scientific fields: physics (solid-state, electromagnetic theory, surface physics, statistical physics, thermal physics), (fractal mathematics), chemistry (chemical reactions on grain surfaces), meteoritics, as well as every branch of astronomy and astrophysics. These disparate research areas can be linked by the following theme: the cosmic dust particles evolve cyclically; chemically, physically and dynamically. The evolution of dust traces out paths in which the universe recycles material, in processes analogous to the daily recycling steps with which many people are familiar: production, storage, processing, collection, consumption, and discarding. Observations and measurements of cosmic dust in different regions provide an important insight into the universe's recycling processes; in the clouds of the diffuse interstellar medium, in molecular clouds, in the circumstellar dust of young stellar objects, and in planetary systems such as our own solar system, where astronomers consider dust as in its most recycled state. The astronomers accumulate observational 'snapshots' of dust at different stages of its life and, over time, form a more complete movie of the universe's complicated recycling steps.
The detection of cosmic dust points to another facet of cosmic dust research: dust acting as photons. Once cosmic dust is detected, the scientific problem to be solved is an inverse problem to determine what processes brought that encoded photon-like object (dust) to the detector. Parameters such the particle's initial motion, material properties, intervening plasma and magnetic field determined the dust particle's arrival at the dust detector. Slightly changing any of these parameters can give significantly different dust dynamical behavior. Therefore one can learn about where that object came from, and what is (in) the intervening medium.

Detection methods
Cosmic dust can be detected by indirect methods utilizing the radiative properties of cosmic dust.
Cosmic dust can also be detected directly ('in-situ') using a variety of collection methods and from a variety of collection locations. At the Earth, generally, an average of 40 tons per day of extraterrestrial material falls to the Earth. The Earth-falling dust particles are collected in the Earth's atmosphere using plate collectors under the wings of stratospheric-flying NASA airplanes and collected from surface deposits on the large Earth ice-masses (Antarctica and Greenland / the Arctic) and in deep-sea sediments. Don Brownlee at the University of Washington in Seattle first reliably identified the extraterrestrial nature of collected dust particles in the later 1970s. Another source is the meteorites, which contain stardust extracted from them (see below). Stardust grains are solid refractory pieces of individual presolar stars. They are recognized by their extreme isotopic compositions, which can only be isotopic compositions within evolved stars, prior to any mixing with the interstellar medium. These grains condensed from the stellar matter as it cooled while leaving the star.
In interplanetary space, dust detectors on planetary spacecraft have been built and flown , some are presently flying, and more are presently being built to fly. The large orbital velocities of dust particles in interplanetary space (typically 10-40 km/s) make intact particle capture problematic. Instead, in-situ dust detectors are generally devised to measure parameters associated with the high-velocity impact of dust particles on the instrument, and then derive physical properties of the particles (usually mass and velocity) through laboratory calibration (i.e. impacting accelerated particles with known properties onto a laboratory replica of the dust detector). Over the years dust detectors have measured, among others, the impact light flash, acoustic signal and impact ionisation. Recently the dust instrument on Stardust captured particles intact in low-density aerogel.
Dust detectors in the past flew on the HEOS-2, Helios, Pioneer 10, Pioneer 11, Giotto, and Galileo space missions, on the Earth-orbiting LDEF, Eureca, and Gorid satellites, and some scientists have utilized the Voyager 1,2 spacecraft as giant Langmuir probes to directly sample the cosmic dust. Presently dust detectors are flying on the Ulysses, Cassini, Proba, Rosetta, Stardust, and the New Horizons spacecraft. The collected dust at Earth or collected further in space and returned by sample-return space missions is then analyzed by dust scientists in their respective laboratories all over the world. One large storage facility for cosmic dust exists at the NASA Houston JSC.
Infrared light can penetrate the cosmic dust clouds, allowing us to peer into regions of star formation and the centers of galaxies. NASA's Spitzer Space Telescope is the largest infrared telescope ever launched into space. The Spitzer Space Telescope (formerly SIRTF, the Space Infrared Telescope Facility) was launched into space by a Delta rocket from Cape Canaveral, Florida on 25 August 2003. During its mission, Spitzer will obtain images and spectra by detecting the infrared energy, or heat, radiated by objects in space between wavelengths of 3 and 180 microns (1 micron is one-millionth of a meter). Most of this infrared radiation is blocked by the Earth's atmosphere and cannot be observed from the ground. The findings from the Spitzer already revitalized the studies of cosmic dust. A recent report from a Spitzer team shows some evidence that cosmic dust is formed near a supermassive black hole.

Radiative properties of cosmic dust
A dust particle interacts with electromagnetic radiation in a way that depends on its cross section, the wavelength of the electromagnetic radiation, and on the nature of the grain: its refractive index, size, etc. The radiation process for an individual grain is called its emissivity, dependent on the grain's efficiency factor. Furthermore, we have to specify whether the emissivity process is extinction, scattering, or absorption. In the radiation emission curves, several important signatures identify the composition of the emitting or absorbing dust particles.
Dust particles can scatter light nonuniformly. Forward-scattered light means that light is redirected slightly by diffraction off its path from the star/sunlight, and back-scattered light is reflected light.
The scattering and extinction ("dimming") of the radiation gives useful information about the dust grain sizes. For example, if the object(s) in one's data is many times brighter in forward-scattered visible light than in back-scattered visible light, then we know that a significant fraction of the particles are about a micrometer in diameter.
The scattering of light from dust grains in long exposure visible photographs is quite noticeable in reflection nebulas, and gives clues about the individual particle's light-scattering properties. In x-ray wavelengths, many scientists are investigating the scattering of x-rays by interstellar dust, and some have suggested that astronomical x-ray sources would possess diffuse haloes, due to the dust.

Stardust
Stardust grains are contained within meteorites, from which they are extracted in terrestrial laboratories. So-called carbonaceous chondrites are especially fertile reservoirs of stardust. Each stardust grain existed before the earth was formed. The meteorites have preserved the previously interstellar stardust grains since that time. Stardust is a scientific term rather than a poetic one, referring to refractory dust grains that condensed from cooling ejected gases from individual presolar stars. Many different types of stardust have been identified by laboratory measurements of the highly unusual isotopic composition of the chemical elements that comprise each stardust grain. Many new aspects of nucleosynthesis have been discovered from those isotopic ratios . The following website http://www.dtm.ciw.edu/lrn/psg_main.html contains an excellent introduction to, and photographs of, many differing types of stardust. An important property of stardust is the hard, refractory, high-temperature nature of the grains. Prominent are silicon carbide, graphite, aluminum oxide, aluminum spinel, and other such grains that would condense at high temperature from a cooling gas, such as in stellar winds or in the decompression of the inside of a supernova. They differ greatly from the solids formed at low temperature within the interstellar medium. Also important are their extreme isotopic compositions, which are expected to exist nowhere in the interstellar medium. This also suggests that the stardust condensed from the gases of individual stars before the isotopes could be diluted by mixing with the interstellar medium. These allow the source stars to be identified. For example, the heavy elements within the SiC grains are almost pure s process isotopes, fitting their condensation within AGB star red giant winds inasmuch as the AGB stars are the main source of s process nucleosynthesis and have atmospheres observed by astronomers to be highly enriched in dredged-up s process elements. Another dramatic example comes from the supernova condensates, usually shortened by acronym to SUNOCON to distinguish them from other stardust condensed within stellar atmospheres. SUNOCONs show evidence that they condensed containing abundant radioactive 44Ti, which has a 65 yr halflife. It was thus still alive when the SUNOCON condensed within the expanding supernova interior but would have been extinct after mixing with the interstellar gas. Its discovery proved the prediction from 1975 to identify SUNOCONs in this way. But SiC SUNOCONs are only about 1% as numerous as are SiC stardust.
Exciting as stardust is, it is but a modest fraction of the condensed cosmic dust. It seems that stardust is less than 0.1% of the mass of total interstellar solids. Its interest lies in the new information that it has brought to the sciences of stellar evolution and nucleosynthesis.
A fascinating aspect to human culture is the study within terrestrial laboratories of solids that existed before the earth existed. This was once thought impossible, especially in the decades when cosmochemists were confident that the solar system began as a hot gas virtually devoid of any remaining solids, which would have been vaporized by high temperature. The very existence of stardust shows that that historic picture was incorrect.

Some bulk properties of cosmic dust
Cosmic dust is made of dust grains and aggregates of dust grains. These particles are irregularly-shaped with porosity ranging from fluffy to compact. The composition, size, and other properties depends on where the dust is found, and conversely, a compositional analysis of a dust particle can reveal the much about the dust particle's origin. General diffuse interstellar medium dust, dust grains in dense clouds, planetary rings dust, and circumstellar dust, are each different in their characteristics. For example, grains in dense clouds have acquired a mantle of ice and on average are larger than dust particles in the diffuse interstellar medium. Interplanetary dust particles (IDPs) are generally larger still.
Most of the influx of extraterrestrial matter that falls onto the Earth is dominated by meteoroids with diameters in the range 50 to 500 micrometers, of average density 2.0 g/cm? (with porosity about 40%). The densities of most stratospheric-captured IDPs range between 1 and 3 g/cm?, with an average density at about 2.0 g/cm?.
Other specific dust properties:
In circumstellar dust, astronomers have found molecular signatures of CO, silicon carbide, amorphous silicate, polycyclic aromatic hydrocarbons, water ice, and polyformaldehyde, among others. (In the diffuse interstellar medium, there is evidence for silicate and carbon grains.)
Cometary dust is generally different (with overlap) from asteroidal dust. Asteroidal dust resembles carbonaceous chondritic meteorites, and cometary dust resembles interstellar grains, which can include the elements: silicates, polycyclic aromatic hydrocarbons, and water ice.

Dust grain formation
The large grains start with the silicate particles forming in the atmospheres of cool stars, and carbon grains in the atmospheres of cool carbon stars. Stars, which have evolved off the main sequence, and which have entered the giant phase of their evolution, are a major source of dust grains in galaxies. Star dust, sung and written in the popular media, is a colloquial term referring to the birthplace of most dust grains in the Universe. If one indeed traces the origin of the elements out of which our bodies are made, we are star dust.
Astronomers know that the dust is formed in the envelopes of late-evolved stars from specific observational signatures. An (infrared) 9.7 micrometre emission silicate signature is observed for cool evolved (oxygen-rich giant) stars. And an (infrared) 11.5 micrometre emission silicon carbide signature is observed for cool evolved (carbon-rich giant) stars. These help provide evidence that the small silicate particles in space came from the outer envelopes (ejecta) of these stars.label
It is believed that conditions in interstellar space are generally not suitable for the formation of silicate cores. The arguments are that: given an observed typical grain diameter a, the time for a grain to attain a, and given the temperature of interstellar gas, it would take considerably longer than the age of the universe for interstellar grains to form label. Furthermore, grains are seen to form in the vicinity of nearby stars in real-time, meaning in a) nova and supernova ejecta, and b) R Coronae Borealis, which seem to eject discrete clouds containing both gas and dust.
Most dust in our solar system is highly processed dust, recycled from the material out of which our solar system formed and subsequently collected in the planetesimals, and leftover solid material (for example: comets and asteroids), and reformed in each of those bodies' collisional lifetimes. During our solar system's formation history, the most abundant element was (and still is) H2. The metallic elements: magnesium, silicon, and iron, which are the principal ingredients of rocky planets, condensed into solids at the highest temperatures. The range of elements of the solar nebula between H2 and (Mg, Si, Fe) is not known well (Wood, J., 1999). Some molecules such as CO, N2, NH3, and free oxygen, existed in a gas phase. Some molecules, for example, graphite (C) and SiC condensed into solid grains. Some molecules also formed complex organic compounds and some molecules formed frozen ice mantles, of which either could coat the "refractory" (Mg, Si, Fe) grain cores.
The formation of these molecules was determined, in large part, by the temperature of the solar nebula. Since the temperature of the solar nebula decreased with heliocentric distance, scientists can infer a dust grain's origin(s) with knowledge of the grain's materials. Some materials could only have been formed at high temperatures, while other grain materials could only have been formed at much lower temperatures. The materials in a single interplanetary dust particle often show that the grain elements formed in different locations and at different times in the solar nebula. Most of the matter present in the original solar nebula has since disappeared; drawn into the Sun, expelled into interstellar space, or reprocessed, for example, as part of the planets, asteroids or comets.
Due to their highly-processed nature, IDPs are fine-grained mixtures of thousands to millions of mineral grains and amorphous components. We can picture an IDP as a "matrix" of material with embedded elements which were formed at different times and places in the solar nebula and before our solar nebula's formation. Examples of embedded elements in cosmic dust are GEMS, chondrules, and CAIs.

A Dusty Trail from the Solar Nebula to Earth
The arrows in the adjacent diagram show one possible path from a collected interplanetary dust particle back to the early stages of the solar nebula.
We can follow the trail to the right in the diagram to the IDPs that contain the most volatile and primitive elements. The trail takes us first from interplanetary dust particles to chondritic interplanetary dust particles. Planetary scientists classify chondritic IDPs in terms of their diminishing degree of oxidation so that they fall into three major groups: the carbonaneous, the ordinary, and the enstatite chondrites. As the name implies, the carbonaceous chondrites are rich in carbon, and many have anomalies in the isotopic abundances of H, C, N, and O (Jessberger, 2000). From the carbonaceous chondrites, we follow the trail to the most primitive materials. They are almost completely oxidized and contain the most low condensation temperature elements ("volatile" elements) and the largest amount of organic compounds. Therefore, dust particles with these elements are thought to be formed in the early life of our solar system. Why? The volatile elements have never seen temperatures above about 500 K, therefore, one can conclude that the IDP grain "matrix" consists of some very primitive solar system material. Such a scenario is true in the case of comet dust.
We can learn more about these particles' origin, by examining their surfaces. If we examine, in the laboratory, dust particles' density of solar flare tracks, their amorphous rims, and the spallogenic isotopes from cosmic rays (Flynn, 1996), then we have good clues for how long a particle has been travelling in space. Nuclear damage tracks are caused by the ion flux from solar flares. Solar wind ions impacting on the particle's surface produce amorphous radiation damaged rims on the particle's surface. And spallogenic nuclei are produced by galactic and solar cosmic rays. A dust particle that originates in the Kuiper Belt at 40AU would have many more times the density of tracks, thicker amorphous rims and higher integrated doses than a dust particle originating in the main-asteroid belt.

Dust grain destruction
How are the interstellar grains destroyed? There are several ultraviolet processes which lead to grain "explosions". In addition, evaporation, sputtering (when an atom or ion strikes the surface of a solid with enough momentum to eject atoms from it), and grain-grain collisions have a major influence on the grain size distribution. label
These destructive processes happen in a variety of places. Some grains are destroyed in the supernovae/novae explosion (and others are formed afterwards). Some of the dust is ejected out of the protostellar disk in the strong stellar winds that occur during a protostar's active T Tauri phase and may be destroyed when passing through shocks, e.g. in Herbig-Haro objects. Plus there are some gas-phase processes in a dense cloud where ultraviolet photons eject energetic electrons from the grains into the gas.
Dust grains incorporated into stars are also destroyed, but only a relatively small fraction of the mass of a star-forming cloud actually ends up in stars. This means a typical grain goes through many molecular clouds and has mantles added and removed many times before the grain core is destroyed.

Some "dusty" clouds in the universe
Our solar system has its own interplanetary dust cloud; extrasolar systems too.
There are different types of nebulae with different physical causes and processes. One might see these classifications:
diffuse nebula
infrared (IR) reflection nebula
supernova remnant
molecular cloud
HII regions
photodissociation regions
Distinctions between those types of nebula are that different radiation processes are at work. For example, H II regions, like the Orion Nebula, where a lot of star-formation is taking place, are characterized as thermal emission nebulae. Supernova remnants, on the other hand, like the Crab Nebula, are characterized as nonthermal emission (synchrotron radiation).
Some of the better known dusty regions in the universe are the diffuse nebula in the Messier catalog, for example: M1, M8, M16, M17, M20, M42, M43 Messier Catalog
Some larger 'dusty' catalogs that you can access from the NSSDC, CDS, and perhaps other places are:
Sharpless (1959) A Catalogue of HII Regions
Lynds (1965) Catalogue of Bright Nebulae
Lunds (1962) Catalogue of Dark Nebulae
van den Bergh (1966) Catalogue of Reflection Nebulae
Green (1988) Rev. Reference Cat. of Galactic SNRs
at
The National Space Sciences Data Center (NSSDC)
CDS Online Catalogs

Images

The poles of astronomical bodies are determined based on their axis of rotation in relation to the celestial poles of the celestial sphere.

Geographic poles
The International Astronomical Union defines the geographic north pole of a planet or other object in the solar system as the planetary pole that is in the same ecliptic hemisphere as the Earth's North Pole. More accurately, "The north pole is that pole of rotation that lies on the north side of the invariable plane of the solar system". This definition means that an object's axial tilt is always 90 degree or less, but its rotation period may be negative (retrograde rotation) - in other words, it rotates clockwise when viewed from above its north pole, rather than the "normal" counterclockwise direction exhibited by the Earth.
Another common definition uses the right-hand rule to define an object's north pole: it is then the pole around which the object rotates counterclockwise. With this definition, axial tilts may be greater than 90 degree but rotation periods are always positive.
The projection of a planet's geographic north pole onto the celestial sphere gives its north celestial pole.
Some bodies in the solar system, including Saturn's moon Hyperion and the asteroid 4179 Toutatis, lack a stable geographic north pole. They rotate chaotically because of their irregular shape and gravitational influences from nearby planets and moons, and as a result the instantaneous pole wanders over their surface, and may momentarily vanish altogether (when the object comes to a standstill with respect to the distant stars).

Magnetic poles
Planetary magnetic poles are defined analogously to the Earth's magnetic poles: they are the locations on the planet's surface at which the planet's magnetic field lines are vertical. The direction of the field determines whether the pole is a magnetic north or south pole, exactly as on Earth. The Earth's magnetic axis is orientated in approximately the same direction as its rotational axis, meaning that the magnetic poles are reasonably close to the geographic poles. However, this is not necessarily the case for other planets; the magnetic axis of Uranus, for example, is inclined by as much as 60 degree.

Near, far, leading and trailing poles
In the particular (but frequent) case of synchronous satellites, four more poles can be defined. They are the near, far, leading, and trailing poles. Take Io for example; this moon of Jupiter rotates synchronously, so its orientation with respect to Jupiter stays constant. There will be a single, unmoving point of its surface where Jupiter is at the zenith, exactly overhead - this is the near pole, also called the sub- or pro-Jovian point. At the antipode of this point is the far pole, where Jupiter lies at the nadir; it is also called the anti-Jovian point. There will also be a single unmoving point which is furthest along Io's orbit (best defined as the point most removed from the plane formed by the north-south and near-far axes, on the leading side) -this is the leading pole. At its antipode lies the trailing pole. Io can thus be divided into north and south hemispheres, into pro- and anti-Jovian hemispheres, and into leading and trailing hemispheres. Note that these poles are mean poles because the points are not, strictly speaking, unmoving: there is constant jiggling about the mean orientation, because Io's orbit is slightly eccentric and the gravity of the other moons disturbs it regularly.