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Moment of truth
The fastest man-made object that ever leave the Earth is the New Horizon spacecraft, launched in January 2006, and attained a speed of more than 36000 mph.

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ascorbic acid Food Sci.
See vitamin C

astrometry Astron.
A branch of astronomy that measures and analyses the motions and positions of celestial objects.

astronomical unit Metro.
A unit distance measurement that is used in astronomy, equal to the average distance between the Earth and the Sun (1.496 x 108 km). It has a symbol AU and is commonly used as a unit of distance within the Solar System. 1 light year = 63 240 AU or 1 AU = 8.3 light minute.

asymptotic freedom Phys.
A phenomenon that describes the fact that the force between quarks become stronger rather than weaker as they move apart. This is because the field particle, gluons, that carry the strong force interact with each other and quarks. Quarks that constantly emit and reabsorb gluons therefore weakens the strong interactions at short distances. The asymptotic freedom also explains why free quarks are never seen (confinement).

atmospheric pressure Metro.
The pressure due to the atmosphere at the surface of the Earth. Its standard value is 101325 N/m2 (standard atmosphere) and is equal to the pressure that exerts a column of mercury 760 mm high at 0C (760 mmHg).

atom Chem.
The fundamental unit of a substance. An atom consists of a number of protons and neutrons in the nucleus of the atom, with a number of electrons (equal to the number of protons) surround the nucleus. The extension of electrons into space defines the size of the atom. Different types of atoms are uniquely identified by the number of protons in the nuclei and the atoms are destroyed if they are further split.

atomic clock Phys.
A precision time-keeping device that uses an atomic resonance frequency standard to feed its counter. A standard atomic clock is usually based on cesium-133 (Cs-133) atomic resonance. Basically it works by probing a beam of cesium-133 atoms by microwaves with a frequency adjusted to a value of exactly 9192631770 Hz. This corresponds to the microwave photon energy equal to the energy difference between two very closely spaced (hyperfine) energy levels that make up the ground state of the cesium atoms. The atoms absorb this energy and a signal is fed back to the microwave source, which stops it drifting from this specific frequency. Such stability allows the atomic clock as a standard reference to define the second as the duration of 9192631770 periods of the radiation corresponding to the transition between these two levels of the ground state of the cesium-133 atom.

The accuracy of an atomic clock therefore depends on the stability of the operating frequency of microwave. One of the most accurate clock, the NIST-F1, has attained an accuracy to better than one part in 1015. Such precision time-keeping is important to the Global Positioning System and the Internet.

atomic force microscopy (AFM) Phys.
A probe microscope for imaging the surface structure of a sample in atomic resolutions. The probe consists of a fine tip (made of Si or Si3N4) mounted on a cantilever (usually about 500 mm long and made of silicone). The microscope operates by raster-scanning the surface with the probe tip over the sample. The response of the cantilever (vertical movement) as a result of tip interaction with the surface is monitored with a detector (such as laser) and converts the local sample height to the topographic image. AFM can also study mechanical phenomena such as adhesion, friction, lubrication and so on, by measuring the force (at nano-newton scale) experiences by the tip.

atomic mass unit (amu) Chem.
It is a unit of mass used to describe relative atomic mass. It is equal to 1/12 of the mass of one carbon-12 isotope. This is equal to 1.66033 x 10-27 kg. Normal hydrogen isotope with only one proton has the mass unit of 1.0 amu. It is also called the unified mass unit or dalton.

atomic number Chem.
The number of protons in the nucleus of an atom. It determines the uniqueness of an atom type (element) and has a symbol Z. It is used to determine classification of atoms in the periodic table of the elements. The atomic number also equal to the number of electrons surrounding an atom in its neutral state.

atomic weight Chem.
See relative atomic mass.

autophagy Biol.
The process of self-digestion by a living cell through the action of lysosomes. Also called autophagocytosis, it derives from the Greek word auto, meaning self and phagy, meaning eating. It is a tightly regulated process that is responsible to maintain the overall health state (cell growth and development) and general constant conditions (homeostasis) of a living cell.

autosome Biol.
Any chromosome that is not one of the sex-determining chromosomes. Humans have 22 pairs of autosomes.

autotroph Biol., Bot.
A living organism that synthesizes all the complex molecules necessary to sustain life using only simple, inorganic compounds such as carbon dioxide. Examples are plants, algae and certain bacteria. The name is derived from the Greek autos, means 'self' and trophos means 'feeder'.

Avogadro constant Chem.
It is the number of atoms or molecules in one mole of substance. It has a value of 6.0221 x 1023 and is equal to the number of carbon-12 atoms in 12.0 g of carbon-12. It is also known as the Avogadro's number and is named after Count Amadeo Avogadro (1776-1856), an Italian chemist.

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