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Non-atmospheric Flight: Getting from here to way out there!Flight is fundamentally about moving from one place to another without being in contact with the ground. A vehicle that is initially stationary must supply force to get moving. It will then continue moving until another force makes it slow down or stop. This is in accordance with Newton's 1st Law: a body will remain at rest or at constant velocity unless acted upon by an external force. In the atmosphere, drag acts to slow down moving bodies. Outside the atmosphere, there is no drag because there is no gas to fly through. Streamlined shapes are not required for vehicles flying outside an atmosphere: after a streamlined rocket carries a spacecraft outside the atmosphere, the smooth outer surface is discarded and solar panels and scientific instruments are unfolded. The Cassini vehicle shown in Figure 1 is a typical example of a utilitarian spacecraft shape.
Although the lack of drag in space is rather convenient, there is also no lift, so some other force must be available to avoid being pulled down to the surface by gravity. To leave the earth, spacecraft rely on thrust: large rocket engines are designed to accelerate the vehicle as quickly as possible. If the rocket reaches a very high speed it can avoid falling back to Earth, even without a lift force, by moving in a circle around the planet. With even higher speed, the vehicle can escape the influence of Earth's gravity and fly in interplanetary space. We will consider the simple case of a circular orbit to demonstrate that the vehicle can overcome gravity without continuous lift or thrust.
 The Hammer
Throw
A body that moves in a circle is constantly changing direction, which means that the velocity is changing (velocity is a vector, so it has magnitude and direction). If velocity is not constant, some external force is responsible for the change. You can feel that force if you tie a weight to the end of a string and whirl it around your head. The string gets taut, and pulls at your hand. That force in the string is enough to make the weight circle around you. A little vector mathematics shows that the acceleration of the body is toward the center of the circle, with magnitude being the square of the velocity divided by the circle's radius. If the string is broken, the weight flies off in a straight line because there is no force to make it go in circles. Written as an equation it looks like this:
Acceleration = (Velocity)2 / radius A body that flies close to a planet is affected by the gravitational force of that planet. The planet's gravitational force pulls at body like the tension in the weighted string. With the external force from gravity, the direction of the body is changed. If the velocity and gravitational force have just the right balance, the body can move into orbit around the planet. If the gravitational force is too great, the body will be pulled down to the planet's surface. If the gravitational force is too low, the body will escape from the pull and move away from the planet. Look at an intersting example of circular motion. Gravity acts like a string to hold satellites in a circle. http://plabpc.csustan.edu/general/tutorials/Planeta$ Motion/PlanetaryMotion.htm When spacecraft arrive at their destination planet they generally must slow down to enter orbit. They use rockets to brake: they fire in front of the vehicle to slow down instead of firing behind the vehicle to speed up. Rocket scientists talk about "delta V": the change in velocity required to change the trajectory. The rockets used for orbital maneuvers are typically fired at full throttle, so the amount of force they deliver is constant. The amount of velocity change is then determined by the duration of the burn: a longer period of acceleration produces a greater change in velocity. Navigation for unmanned spacecraft is very difficult, because there are not many references available to pinpoint the vehicle's position. Inertial guidance systems integrate the accelerations experienced by the vehicle, but small errors in measurement cause increasingly large errors in estimated position as the length of time increases. Spacecraft typically carry some sort of optical system for viewing stars. By measuring the angle between the spacecraft and several distant stars, trigonometry can be used to determine location. This technique is similar to that used in the Global Positioning System (GPS), which measures the distance between the unit on Earth and several orbiting satellites that are in well-specified positions. The figure shows the optical system and associated computer for calculating spacecraft position relative to the stars.
Learn more about the Global Positioning System (GPS): Once in orbit around the destination planet, the man-made satellite must maintain a safe distance from the planet's surface. The satellite must now counter the effect of the planet's gravitational force which is constantly pulling it into an ever increasing lower orbit. To do so, the orbiter will occasionally fire its rockets boosting it away from the planet, but not so far away that it free falls outside the planet's gravitational force. The satellite uses its booster rockets to maintain its prescribed distance above the planet's surface until its rockets no longer have the fuel necessary to function. Without the occasional rocket boost the satellite will suffer from the drag effects of the planet's atmosphere (if there is one) and the planet's unrelenting gravitational force. It will drop from its orbit and plummet into the planet's surface at an incredible speed. The valuable and vast amounts of data collected from these man-made satellites provides scientists with information about how the planet was formed, the planet's atmosphere, geography, geology and potential for life. This information could also be valuable for future robotic and human exploration of the planet.
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