question: How does moving horizontally prevent satellites from falling towards earth?
comment: Stand on a mountain and throw a ball. It moves horizontally to the surface of the Earth, but gravity pulls out down so it eventually his the ground. Now imagine a really tall mountain and you throw the ball really fast. It moves away from the Earth because you are on the high mountain, but gravity pulls it back towards the Earth as well. There is a specific velocity where these forces balance out and the ball "falls" all the way around the Earth and just keeps falling. That's a satellite. It's not just the horizontal motion. The horizontal motion is a result of the upward motion and the pull of gravity.
passages: BULLET::::- Earth observation satellites and spy satellites use LEO as they are able to see the surface of the Earth clearly by being close to it. They are also able to traverse the surface of the Earth. A majority of artificial satellites are placed in LEO, making one complete revolution around the Earth in about 90 minutes.
  
  BULLET::::- The International Space Station is in a LEO about to above Earth's surface, and needs reboosting a few times a year due to orbital decay.
  
  BULLET::::- Iridium satellites orbit at about .
   BULLET::::- Geostationary satellites have a "geostationary orbit" (GEO), which is from Earth's surface. This orbit has the special characteristic that the apparent position of the satellite in the sky when viewed by a ground observer does not change, the satellite appears to "stand still" in the sky. This is because the satellite's orbital period is the same as the rotation rate of the Earth. The advantage of this orbit is that ground antennas do not have to track the satellite across the sky, they can be fixed to point at the location in the sky the satellite appears.
   BULLET::::- At any given location, the total gravitational force is not quite constant over time, because the objects that generate the gravity are moving. For instance, on Earth the horizontal plane at a given point (as determined by a pair of spirit levels) changes with the position of the Moon (air, sea and land tides).
   There are some advantages of geosynchronous satellites:
  
  BULLET::::- Get high temporal resolution data.
  
  BULLET::::- Tracking of the satellite by its earth stations is simplified.
  
  BULLET::::- Satellite always in same position.
   BULLET::::- In reality, the gravity field of a heterogeneous planet such as Earth is deformed due to the inhomogeneous spatial distribution of materials with different densities. Actual horizontal planes are thus not even parallel even if their reference points are along the same vertical line, since a vertical line is slightly curved.
   BULLET::::- A Rotovator is a rotating tether where the lower tip is moving in the opposite direction to the tether's orbital velocity, reducing the difference in velocity relative to the ground, and hence reducing the velocity of rendezvous; the upper tip is likewise moving at greater than orbital velocity, allowing propellantless transfer between orbits. Around an airless world, such as the moon, the lower tip can actually touch the ground with zero horizontal velocity. As with any momentum exchange tether, orbital energy is gained or lost in the transfer.
  
  Section::::Fictional.
   BULLET::::- A line drawn from the planet to the satellite sweeps out "equal areas in equal times" no matter which portion of the orbit is measured.
  
  BULLET::::- The square of a satellite's orbital period is proportional to the cube of its average distance from the planet.
  
  BULLET::::- Without applying force (such as firing a rocket engine), the period and shape of the satellite's orbit won't change.
   BULLET::::5. In Ariane 4 flights using the same type of inertial reference system there had been no such failure because the trajectory during the first 40 seconds of flight is such that the particular variable related to horizontal velocity cannot reach, with an adequate operational margin, a value beyond the limit present in the software.
   This type of attitude control tether has a small mass on one end, and a satellite on the other. Tidal forces stretch the tether between the two masses. There are two ways of explaining tidal forces. In one, the upper end mass of the system is moving faster than orbital velocity for its altitude, so centrifugal force makes it want to move further away from the planet it is orbiting. At the same time, the lower end mass of the system is moving at less than orbital speed for its altitude, so it wants to move closer to the planet. The end result is that the tether is under constant tension and wants to hang in a vertical orientation. Simple satellites have often been stabilized this way; either with tethers, or with how the mass is distributed within the satellite.
   BULLET::::6. Ariane 5 has a high initial acceleration and trajectory, which leads to a build-up of horizontal velocity five times more rapid than for Ariane 4. The higher horizontal velocity of Ariane 5 generated, within the 40-second timeframe, the excessive value that caused the inertial system computers to cease operation.
   In typical free-fall, the acceleration of gravity acts along the direction of an object's velocity, linearly increasing its speed as it falls toward the Earth, or slowing it down if it is moving away from the Earth. In the case of an orbiting spacecraft, which has a velocity vector largely "perpendicular" to the force of gravity, gravitational acceleration does not produce a net change in the object's speed, but instead acts centripetally, to constantly "turn" the spacecraft's velocity as it moves around the Earth. Because the acceleration vector turns along with the velocity vector, they remain perpendicular to each other. Without this change in the direction of its velocity vector, the spacecraft would move in a straight line, leaving the Earth altogether.
   BULLET::::- Atmosphere — In order to minimize gravity drag the vehicle should begin gaining horizontal speed as soon as possible. On an airless body such as the Moon this presents no problem, however on a planet with a dense atmosphere this is not possible. A trade-off exists between flying higher before starting downrange acceleration, thus increasing gravity drag losses; or starting downrange acceleration earlier, reducing gravity drag but increasing the aerodynamic drag experienced during launch.
   As satellites in MEO and LEO orbit the Earth faster, they do not remain visible in the sky to a fixed point on Earth continually like a geostationary satellite, but appear to a ground observer to cross the sky and "set" when they go behind the Earth. Therefore, to provide continuous communications capability with these lower orbits requires a larger number of satellites, so one will always be in the sky for transmission of communication signals. However, due to their relatively small distance to the Earth their signals are stronger.
  
  Section::::Satellite orbits.:Low Earth orbit (LEO).
   BULLET::::- Center of gravity — A body's center of gravity is the point around which the resultant torque due to gravity forces vanishes. Where a gravity field can be considered to be uniform, the mass-center and the center-of-gravity will be the same. However, for satellites in orbit around a planet, in the absence of other torques being applied to a satellite, the slight variation (gradient) in gravitational field between closer-to (stronger) and further-from (weaker) the planet can lead to a torque that will tend to align the satellite such that its long axis is vertical. In such a case, it is important to make the distinction between the center-of-gravity and the mass-center. Any horizontal offset between the two will result in an applied torque.
   Eventually the satellite comes around to point D where Earth's gravity is now reducing the satellite's orbital velocity. This causes it to fall into a lower orbit, which actually increases the angular speed of the satellite around the Sun. This continues until point E where the satellite's orbit is now lower and faster than Earth's orbit, and it begins moving out ahead of Earth. Over the next few centuries it completes its journey back to point A.
   Short tether systems are commonly used on satellites and robotic space probes. Most notably, tethers are used in the "yo-yo de-spin" mechanism, often used in systems where a probe set spinning during a solid rocket injection motor firing, but needs the spin removed during flight. In this mechanism, weights on the end of long cables are deployed away from the body of the spinning satellite. When the cables are cut, much or all of the angular momentum of the spin is transferred to the discarded weights. As an example, the Dawn Mission utilized two 3 kg of weights deployed on 12 meter cables.
   The idea is to use the Earth's gravitational field and tidal forces to keep the spacecraft aligned in the desired orientation. The gravity of the Earth decreases according to the inverse-square law, and by extending the long axis perpendicular to the orbit, the "lower" part of the orbiting structure will be more attracted to the Earth. The effect is that the satellite will tend to align its axis of minimum moment of inertia vertically.
   Since it requires less energy to place a satellite into a LEO, and a satellite there needs less powerful amplifiers for successful transmission, LEO is used for many communication applications, such as the Iridium phone system. Some communication satellites use much higher geostationary orbits, and move at the same angular velocity as the Earth as to appear stationary above one location on the planet.
  
  Section::::Use of LEO.:Disadvantages.
   When the satellite gets to point B, it is traveling at the same speed as Earth. Earth's gravity is still accelerating the satellite along the orbital path, and continues to pull the satellite into a higher orbit. Eventually, at Point C, the satellite reaches a high and slow enough orbit such that it starts to lag behind Earth. It then spends the next century or more appearing to drift 'backwards' around the orbit when viewed relative to the Earth. Its orbit around the Sun still takes only slightly more than one Earth year. Given enough time, the Earth and the satellite will be on opposite sides of the Sun.
   BULLET::::- Areocentric orbit: An orbit around the planet Mars, such as by moons or artificial satellites.
  
  The general structure of a satellite is that it is connected to the earth stations that are present on the ground and connected through terrestrial links.
  
  Section::::Orbit types.:Altitude classifications.
  
  BULLET::::- Low Earth orbit (LEO): Geocentric orbits ranging in altitude from 180 km –
  
  BULLET::::- Medium Earth orbit (MEO): Geocentric orbits ranging in altitude from – . Also known as an intermediate circular orbit.
   In some cases momentum exchange systems are intended to run as balanced transportation schemes where an arriving spacecraft or payload is exchanged with one leaving with the same speed and mass, and then no net change in momentum or angular momentum occurs.
  
  Section::::Tether systems.
  
  Section::::Tether systems.:Tidal stabilization.
  
  Gravity-gradient stabilization, also called "gravity stabilization" and "tidal stabilization", is a simple and reliable method for controlling the attitude of a satellite that requires no electronic control systems, rocket motors or propellant.
   BULLET::::- Subsynchronous orbit: A drift orbit close to but below GSO/GEO. Satellites will drift east.
  
  BULLET::::- Graveyard orbit: An orbit a few hundred kilometers above geosynchronous that satellites are moved into at the end of their operation.
  
  BULLET::::- Disposal orbit: A synonym for graveyard orbit.
  
  BULLET::::- Junk orbit: A synonym for disposal orbit.
  
  BULLET::::- Areosynchronous orbit: A synchronous orbit around the planet Mars with an orbital period equal in length to Mars' sidereal day, 24.6229 hours.
   BULLET::::- "Medium Earth orbit" (MEO) satellites are closer to Earth. Orbital altitudes range from above Earth.
  
  BULLET::::- The region below medium orbits is referred to as "low Earth orbit" (LEO), and is about above Earth.
   Stationary orbit
  
  In celestial mechanics, the term stationary orbit refers to an orbit around a planet or moon where the orbiting satellite or spacecraft remains orbiting over the same spot on the surface. From the ground, the satellite would appear to be standing still, hovering above the surface in the same spot, day after day.
   Molniya orbits can be an appealing alternative in such cases. The Molniya orbit is highly inclined, guaranteeing good elevation over selected positions during the northern portion of the orbit. (Elevation is the extent of the satellite's position above the horizon. Thus, a satellite at the horizon has zero elevation and a satellite directly overhead has elevation of 90 degrees.)
  
raw_presuppositions: A satellite moving horizontally should fall towards earth.
raw_corrections: The horizontal motion of a satellite in relation to the earth is a result of upward motion and the pull of gravity.