In the physical sci-ences, a particle is a small localized object to which can be ascribed several physical properties such as volume or mass. The word is rather general in meaning, and is refined as needed by various scientific fields.

Whether objects can be considered particles depends on the scale of the con-text; if an object’s own size is small

or negligible, or if geometrical proper-ties and structure are irrelevant, then it can be consid-ered a particle. For example, grains of sand on a beach can be considered particles because the size of one grain of sand (c. 1 mm) is negligible compared to the beach, and the features of individual grains of sand are usually irrelevant to the problem at hand.

The term “particle” is usually applied

differently to three class

of sizes. The term macroscopic parti-

cle, usually refers to particles much larger

than atoms and molecules.

These are usually abstracted as point-

like particles, even thoughthey

have volumes, shapes, structures,

etc. Examples of macroscopic par-

ticles would include dust, sand, pieces of debris during a

car accident or even objects as big as the

stars of a galaxy.

Another type, micro-scopic particles usu-ally refers to particles of sizes ranging from atoms to molecules, such as carbon di-oxide, nanoparticles, and colloidal particles. The small-est of particles are the subatomic particles, which refer to particles smaller than atoms. These would include par-ticles such as the constituents of atoms

– protons, neutrons, and electron – as well as other types of par-ticles which can only be produced in particle accelerators or cos-mic rays.

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The particles are the first pieces of a comet to have ever been plucked

from outer space and returned to Earth. The collection was part of the

National Aeronautics and Space Administration’s (NASA) Stardust sam-

ple return mission which launched in February 1999. The primary goal

of Stardust was to collect dust and carbon-based samples during its

closest encounter with Comet Wild 2. The Stardust sample-return canis-

ter parachuted onto the desert salt flats of Utah on Jan. 15, following

a journey of nearly three million miles, bringing with it thousands of

particles from the edge of the solar system.

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The overarch-ing goal, which is pursued in several distinct ways, is to find and understand what physics may lie beyond the stan-dard model. There are several powerful experimental reasons to expect new phys-ics, including dark matter and neutrino mass. There are also theoretical hints that this new phys-ics should be found at accessible energy scales. Furthermore, there may be unex-pected and unpre-dicted surprises that will give us the most opportunity to learn about nature.

In nuclear physics and nuclear chem-istry, nuclear fission refers to either a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into smaller parts (lighter nuclei), often producing free neutrons and pho-tons (in the form of gamma rays), and re-

nuclei produced are most often of comparable but slightly different sizes, typically with a mass ratio of products of about 3 to 2, for common fis-sile isotopes. Most fissions are binary fis-sions (produc-ing two charged fragments), but occasionally (2 to 4 times per 1000 events), three posi-tively charged fragments are produced, in a ternary fission. The smallest of these fragments in ternary pro-cesses ranges in

Nuclear Physics

leasing a very large amount of energy, even by the energetic standards of radioac-tive decay. The two

First Beam Collision

Cosmic dust is a type of dust composed of particles in space which are a few mol-ecules to 0.1 µm in size. Cosmic dust can be further distin-guished by its astro-nomical location; for example: intergalac-tic dust, interstellar dust, interplanetary dust (such as in the zodiacal cloud) and circumplanetary dust (such as in a plan-etary ring).In our own Solar System, interplan-etary dust causes the zodiacal light. Sourc-es include comet dust, asteroidal dust, dust from the Kuiper belt, and interstellar dust passing through our solar system.The terminology has no specific applica-

tion for describing materials found on the planet Earth, other than in the most general sense that all elements with an atomic number higher than helium are believed to be formed in the core of stars via stellar nucleosyn-thesis and superno-va nucleosynthesis events. As such all elements that exist can be indiscrimi-nately considered to be a form of “cos-mic dust.” In Octo-ber 2011, scientists reported that cosmic dust contains com-plex organic mat-ter (“amorphous organic solids with a mixed aromatic-aliphatic structure”) that could be cre-ated naturally, and rapidly, by stars.