GROUND TRUTH - Definition of Earth surface conditions through
direct measurements or visual inspections for calibration or
evaluation of remote sensing observations made from satellites or
HEMATOCRIT - An instrument for determining the relative
amounts of plasma and corpuscles in blood, generally some form of
HEMATOLOGY - A branch of medicine concerning the study of the
blood, the bloodforming tissues, and the diseases of the
HEMOLYSIS - The dissolution of red blood corpuscles with
liberation of their hemoglobin, an iron-containing protein
respiratory pigment occurring in the red blood cells.
HEXOKINASE - An enzyme that catalyzes the transfer of a
high-energy phosphate group of a donor to D-glucose, producing
- Pertaining to fluid or semifluid substances in the body.
IMMUNOLOGY - The medical, bacteriological, and chemical study
of the phenomena and causes of immunity.
IONOSPHERE - Region of ionized gases surrounding the Earth and
extending from about 60 km (33 naut. mi) to distances up to
several hundred kilometers. The ionization in this region is due
to bombardment by ultraviolet radiation and X - rays from the Sun,
and by cosmic rays. The existence of this region makes possible
longrange radio communications through reflection of terrestrial
- Isotopes of an element have the same number of protons in their
nuclei, and hence represent the same element, but they differ in
the number of neutrons and therefore in mass number.
An organic substance (enzyme) which activates other substances to
develop into chemical ferments or enzymes.
LEUKOCYTE - White blood corpuscle.
The outer edge or a portion of the edge of a celestial body such
as the Moon, Venus, or the Sun, as seen from a distance.
LYIDE (Lipid) - A fatty acid insoluble in water.
LYMAN CONTINUUM - The ultraviolet region of the spectrum of the
hydrogen atom immediately adjacent to the Lyman line spectrum with
wavelengths shorter than 912 Å.
LYMPHOCYTE - Lymph cell or white blood corpuscle without
MAGNETOSPHERE - The region around the Earth above about 160 km
(90 naut. mi.) and below the magnetopause (about 15 earth radii in
the solar direction, and at least 40 earth radii in the antisolar
direction ) . Inside the magnetosphere, the Earth's magnetic field
is dominant; outside, the interplanetary magnetic field
MASS SPECTROMETER - An instrument which determines the masses of
atoms and molecules.
METABOLIC CHANGES - The sum of all physical and chemical changes
which take place within an organism; all energy and material
transformations which occur within living cells.
METABOLISM - The interchange of materials between living
organisms and the environment, or within a living organism, by
which energy for maintaining life is secured.
METAPHASE - The stage during cell division in which the
chromosomes are arranged in an equatorial plate.
The light resulting from the transition of a solid particle
(meteoroid) from space through the Earth's atmosphere, commonly
called a "shooting star" or "falling star."
METEORITE - A solid particle from space which enters the
Earth's atmosphere and reaches the surface. Meteorites are
classified as iron meteorites (siderites) and stone meteorites
(aerolites) according to their compositions.
METEOROID - A solid object moving through interplanetary
space of a size considerably smaller than an asteroid and
considerably larger than an atom or molecule. When the object
glows while traveling through the Earth's atmosphere, it is called
a meteor; when it reaches the surface of the Earth, it is called a
METHEMOGLOBIN - A soluble brownish-red, crystalline compound from
which the oxygen cannot be removed in a vacuum. It is formed by
the spontaneous decomposition of blood, and also by the action on
blood of various oxidizing reagents, as oxone, etc.
A unit of length equal to one - millionth of a meter or one -
thousandth of a millimeter, usually called micrometer.
MORPHOLOGY - Branch of biology dealing with the form and
structure of animals and plants. It includes anatomy, histology
and organography, and also the nonphysiological aspects of
cytology and embryology.
MULTISPECTRAL - Utilizing radiation from several discrete bands
of the spectrum simultaneously.
The direction vertically downward (opposite to zenith)
NANOMETER - One - billionth of a meter (10-9 meter).
NOREPINEPHRINE - A hormone secreted by the adrenal medulla in
response to stimulation in the viscera, and stored in granules
that stain strongly with chromium salts. Granules are released
predominantly in response to hypotension (diminished tension or
lower blood pressure).
OCCULTATION - The disappearance of a celestial body behind
another body of equal or larger apparent size, such as the
occultation of the Sun by the Moon as viewed by an Earth observer
during a solar eclipse. Also, the covering of the image of a
celestial body by a disc whose size is equal to or larger than the
size of the image.
OCULOGYRAL ILLUSION - An illusion developed by the movement of the eye
about an axis from the front to the rear of the head.
ORTHOSTATIC - The upright, or erect, position of the human
OSMOLALITY - The property of a liquid to exercise an osmotic
pressure because it contains an electrolyte in solution.
OUTGASSING - The emanation of gas from a material. This
process is usually enhanced in vacuum.
- That point on the trajectory of an orbiting body which is
nearest the Earth when the Earth is the center of attraction.
Also, used in connection with orbits of celestial bodies around
other celestial bodies.
PHOTOMETER - An instrument for measuring the intensity of
light by comparing it with a standard.
PHOTOMULTIPLIER TUBE - An electron tube that produces electrical signals
in response to light. In the tube, the electric signal is
amplified to produce a measurable output signal even from very
small quantities of light.
PHOTOSPHERE - Intensely luminous surface layer of the Sun in
which the sunspots and several other solar phenomena occur.
Bright regions in the chromosphere of the Sun, usually near
sunspots, indicating areas of enhanced magnetic field strength (10
to 100 Gauss*) and
increased solar activity. Plages are best observed in
monochromatic light of hydrogen or calcium.
A gas composed of ions, electrons, neutral atoms, and molecules.
The interactions between particles of a plasma are mainly
electromagnetic. Although many of the individual particles are
electrically positive or negative, the plasma as a whole is
PLASMA RENIN - Plasma is the fluid portion of the blood in which
the corpuscles are suspended. Renin is an enzyme involved in
changing proteins into other products.
PLETHYSMOGRAPH - An instrument for determining and registering
variations in the size of an organ part or limb, and in the amount
of blood present or passing through it.
POLARIMETER - An instrument for determining the degree of
polarization of electromagnetic radiation, specifically the
polarization of light.
POLARIZATION - The state of electromagnetic radiation when the
transverse oscillations take place in some regular manner, e.g.,
all in one plane.
PRECESSION - The angular motion (tilting) of the axis of a
spinning or rotating body caused by a torque whose axis is not
parallel with the axis of rotation.
PRINCIPAL INVESTIGATOR - The researcher responsible for defining an
investigation or experiment as part of a NASA science or
technology project, and for ensuring that the investigation of the
experiment meets with its specific objectives.
PROMINENCE - A plasma protuberance above the surface of the
Sun, originating in the chromosphere and extending sometimes to a
height of several hundred thousand kilometers.
Positively-charged nuclear particle which forms a significant part
of all atomic nuclei. The nucleus of a normal hydrogen atom is a
proton. It is 1,837 times heavier than the electron.
RADIOMETER - An instrument for detecting and measuring radiant
A geometric pattern followed by the sending element of a detector
system or by the electron beam of a television transmitter or
RATE GYRO - A gyroscope with one free gimbal axis (
precession axis ) . Motion around this axis is constrained by a
spring. The second gimbal axis is tied to the spacecraft. An
angular motion of the spacecraft around this axis produces an
angular excursion of the precession axis against the spring force;
the angle of excursion is proportional to the rate of angular
rotation in radians per second.
REDUCTASE - An enzyme that has a reducing action on chemical
REDUNDANCY - Originally "exceeding what is necessary or normal
;" technically a back - up system which takes over when the prime
RESOLVING POWER (LIGHT) - The ability of an optical system to separate
adjacent points and lines in the image and to show fine detail of
Ribonucleic acid, a nucleoprotein found in the cell's cytoplasm.
It probably controls protein synthesis under the regulatory
influence of DNA. See DNA.
S - BAND
- A range of frequencies used in radar and communications that
extends from 1,550 to 5,200 megahertz (wavelength region from 0.06
m to 0.2 m).
SCINTILLATION - A flash of light produced in a phosphor by an
ionizing particle or photon.
SINGLE CRYSTAL - A crystal having a homogeneous, undisturbed
lattice structure throughout.
To change the direction of an antenna or telescope in order to
follow a moving target, or to change from one target to
SOLAR WIND - Streams of particles (mostly ions of hydrogen and
helium, and electrons ) emanating from the Sun and flowing
approximately radially outward at velocities from 300 to 800 km
SPECTROSCOPE - An optical instrument which spreads a beam of
electromagnetic radiation into a spectrum of different wavelengths
for visual inspection.
SPECTROGRAPH - Modification of a spectroscope in which the
spectrum is photographed or recorded electronically.
SPECTROHELIOGRAPH - A modification of the spectrograph which permits
taking pictures of the complete solar disk in monochromatic
SPIROMETER - An instrument to measure the breathing
- Dark, irregular areas on the solar surface with strong magnetic
fields (1000 to 2000 Gauss) surrounded by gray zones (penumbra).
Sunspots occur mostly in pairs, with opposite magnetic polarity;
they persist for periods of days, weeks, or months. Their
temperatures are about 2000 K lower than the temperature of the
surrounding photosphere (5780° K). The frequency of sunspots
follows an eleven - year cycle.
SUPERGRANULATION - A net - like pattern of irregular cells, 15,000
km (800 naut. ml.) to 40,000 km (22,000 naut. ml.) in diameter,
superimposed upon the photospheric granulation network, and caused
by plasma convection within the chromosphere. Cell lifetime is
about 20 hours. Supergranulation is best observed in the
monochromatic light of a calcium spectral line.