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satellite
Any object, man-made or natural, that orbits another body.

saturation
Condition in which a surface becomes completely covered with craters, such that the addition of new craters does not increase the overall crater density.

Saturn
Saturn is the sixth planet from the sun. It is second in size only to Jupiter. It is named for the Roman god of agriculture.
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scarp
A line of cliffs produced by faulting or erosion. Also known as rupes.

scopulus
Irregular line of cliffs (see scarp).

seafloor spreading
The movement of two oceanic plates away from each other, resulting in the construction of a mid-ocean ridge.

semimajor axis
The semimajor axis of an ellipse (e.g. a planetary orbit) is 1/2 the length of the major axis which is a segment of a line passing thru the foci of the ellipse with endpoints on the ellipse itself. The semimajor axis of a planetary orbit is also the average distance from the planet to its primary. The periapsis and apoapsis distances can be calculated from the semimajor axis and the eccentricity by rp = a(1-e) and ra = a(1+e).

shepherd satellite
(Also 'shepherd moon') A satellite which constrains the extent of a planetary ring through gravitational forces.

Shoemaker-Levy 9 (S-L9)
The comet that broke up and fell into Jupiter in June 1994.

shooting star
A meteor.

sidereal
Of, relating to, or concerned with the stars. Sidereal rotation is that measured with respect to the stars rather than with respect to the Sun or the primary of a satellite.

sidereal month
The average period of revolution of the moon around the earth in reference to a fixed star, equal to 27 days, 7 hours, 43 minutes in units of mean solar time.

silicate
A compound containing silicon and oxygen (e.g. olivine). A group of minerals constituting about 95% of the Earth's crust, and containing silicon and oxygen combined with one or more other elements.

sinus
Literally "bay"; really a small plain.

solar cycle
The approximately 11-year quasi-periodic variation in frequency or number of solar active events.

solar nebula
The cloud of gas and dust that began to collapse about 5 billion years ago to form the solar system.

solar system
A system of planets and other bodies orbiting a star. Our solar system consists of the sun, nine planets, more than 100 moons, comets, asteroids and lots of other interesting things.
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solar wind
The wind from the Sun. More specifically, particles, usually electrons and protons, continually streaming away from the corona of the Sun. The solar wind is extremely sparse, containing only a few fast moving particles per cubic centimeter at the Earth.

spacecraft
A vehicle made to travel in the space. A spacecraft can either fly by, orbit or land on the object it is studying. (Image: Diagram of the Cassini spacecraft, which is currently in orbit around Saturn.)

spectrograph
A device that separates light by wavelengths to produce a spectrum.

spectrometer
An instrument that measures the wavelength of electromagnetic radiation.

speed
The measure of how fast an object is moving; the rate of change of position per change in time.

speed of light
= 299,792,458 meters/second (186,000 miles/second). Einstein's Theory of Relativity implies that nothing can go faster than the speed of light.

spicules
Grass-like patterns of gas seen in the solar atmosphere.

SSI
Solid State Imaging.

star
A ball of mostly hydrogen and helium gas that shines extremely brightly. Our Sun is a star. A star is so massive that its core is extremely dense and hot. At the high stellar core temperatures, atoms move so fast that they sometimes stick to other atoms when they collide with them, forming more massive atoms and releasing a great amount of energy. This process is known as nuclear fusion.

stellar classification
Stars given a designation consisting of a letter and a number according to the nature of their spectral lines which corresponds roughly to surface temperature. The classes are: O, B, A, F, G, K, and M; O stars are the hottest; M the coolest. The numbers are simply subdivisions of the major classes. The classes are oddly sequenced because they were assigned long ago before we understood their relationship to temperature. O and B stars are rare but very bright; M stars are numerous but dim. The Sun is designated G2.

stratigraphy
The layering of rock or ice strata, from which information on succession, age relations, and origin can be deduced.

stratosphere
The layer of the atmosphere above the troposphere and below the mesosphere where the temperature increases with height.

subduction zone
A place on the surface of the Earth where two plates move toward each other, and the oceanic plate plunges beneath the other tectonic plate.

sublime or sublimate
To change directly from a solid to a gas without becoming liquid

subsolar
The point on a body that is directly beneath the Sun. The Sun's rays will hit this point at 90 degrees to the surface.

sulcus
Subparallel furrows and ridges.

sulfuric acid
An oily, colorless liquid made of hydrogen, sulfur and oxygen.

Sun
The star closest to Earth. The center of our solar system. A ball of hot, glowing gases which gives Earth heat and light.
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sunspot
An area seen as a dark spot on the photosphere of the Sun; sunspots are concentrations of magnetic flux, typically occurring in bipolar clusters or groups; they appear dark because they are cooler than the surrounding photosphere.

superior planets
The planets Mars, Jupiter, Saturn, Uranus, Neptune and Pluto are called superior planets because their orbits are farther from the Sun than Earth's orbit. (Mercury and Venus are called "inferior" planets.)

surface
the outermost solid or liquid layer of an object. The surface of the Earth is composed of land and oceans.

synchronous orbit radius
The orbital radius at which the satellite's orbital period is equal to the rotational period of the planet. A synchronous satellite with an orbital inclination of zero (same plane as the planet's equator) stays fixed in the sky from the perspective of an observer on the planet's surface (such orbits are commonly used for communications satellites).

synchronous rotation
Said of a satellite if the period of its rotation about its axis is the same as the period of its orbit around its primary. This implies that the satellite always keeps the same hemisphere facing its primary (e.g. the Moon). It also implies that one hemisphere (the leading hemisphere) always faces in the direction of the satellite's motion while the other (trailing) one always faces backward. Most of the satellites in the solar system rotate synchronously.