Ask the above title question of anyone in this, our Age of Information, and listen to the answers. Everyone living today has been aware of space adventurism: the first flights, then orbits by Russia and the U.S.; then onward to the moon (the heart-gripping uncertainly of survival of astronauts on Apollo 13); the first steps of a human on the moon ("A giant leap for mankind"); the Space Shuttle exploits (and fiery disastrous explosions); the astonishing assembly of the complex and gigantic Space Station (cooperation between the U.S. and Russia)! Space achievements in the past half-century rank with the history-book adventures of Magellan and Columbus in probing the unknown.
Most everyone knows what gravity is, they have heard the story of Sir Isaac Newton (an apple falling from a tree), his Laws of Motion ("A body at rest will remain at rest; a body in motion will remain in motion, traveling in the same direction - in both cases, until an external force acts upon that body."). Thus, what is it that keeps such vehicles up in the sky: the orbiting Mercury and Apollo vehicles of the past, the Space Shuttle and Space Station of today? Most everyone will tentatively mumble "gravity" (perhaps hopefully - knowing that gravity is somehow involved, but uncertain as to how).
Therefore, the answer to this question: "gravity" is certainly involved (Newton and his apple) - but gravity should actually force all this orbiting hardware (also the moon) to fall to Earth! Why don't they? (If the questioner holds back any further comment until the questionees respond further; probably half will venture uncertainly, "Maybe there is no gravity in space".)
Then you say (not laughing, as the objective is to teach, not disparage), "You're correct in saying gravity is involved, however, gravity force is always active, no matter how far away from our Earth. Gravity is always pulling everything toward Earth's center, (including humans - measured by our weight on a scale). The force of gravity is an attraction between two masses - in orbiting vehicles, it is equal to the product of the masses of Earth and the object (humans, Shuttle, Space Station or moon), divided by the square of the distance between the mass centers). The key question is why these items remain in orbit about Earth; why don't they fall onto Earth? The answer is that they do - they are constantly falling to Earth - they are constantly falling to Earth, but are also traveling so fast around Earth, that gravity is pulling them into an orbit about Earth. In actual fact, space vehicles in orbit (and the moon) are attempting to travel in a straight line, however Earth's gravity pulls them into a circular (or elliptical) orbital path.
To be precise, to put the Space Shuttle or any vehicle into orbit, it must be rocketed with such horizontal speed that the constant pull of gravity forces it into an Earth orbit. Thus, to achieve orbit, the Space Shuttle has to be accelerated and propelled to a horizontal speed of about 18,000 miles per hour - to reach the required velocity for a near-Earth orbit at somewhere over a hundred miles altitude. The faster the speed, the higher the orbit. Since the orbit altitude is much higher than the Earth's atmosphere (air molecules are also affected by gravity), the orbiting vehicles travel in the vacuum of outer space - thus, there is no "drag" force - once the horizontal speed and altitude are in "sync". Therefore, the orbiting vehicle (Mercury, Apollo, Shuttle or moon) will continue to coast along in their orbiting journeys about Earth.
A critical next question: once achieving such high velocity and orbit, how did these vehicles make it back to Earth safely - considering the fiery paths of meteorites plowing through Earth's atmosphere from outer space? How did the space scientists (NASA et al) achieve safe reentry for Astronauts on Mercury, Apollo and the Space Shuttle?
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It is April 12, 1981, the Space Shuttle is on the launching pad at Cape Kennedy after nine years and billions of dollars spent in innovative technologies. A top technical management team of cognizant Engineering managers of all major systems and subsystems - from both NASA and major subcontractors - has been designated and assembled to witness the launch, purpose: so that immediate technical decisions can be made, should any emergency arise. We have been placed as close to the launch site as is considered safe - if an explosion occurs during launch or early lift-off (about three miles) - to visually witness the first moments. We are seated on a rising array of wooden benches as if at a high school athletic field.
Everyone is quiet, apprehensive, each aware of what could go wrong in his (or her) technical area; greeting each other with solemn nods. Only recently, many had participated in an exhaustive briefing to top NASA officials at the final Flight Readiness Review conference - as to the possibilities, probabilities and consequences of failure in any area. Everyone speaks in hushed tones. Three miles away, powerful flood-lights show the Shuttle assembly standing vertically on the launch pad, the Orbiter (astronaut-carrying vehicle), the large center fuel tank and the two solid boosters - all a brilliant white under the concentration of flood-lights, limned against the dark, morning sky.
There is a hush, then the calm, emotionless voice of the NASA announcer counts down the seconds, then, "Lift-off, we have a lift-off!" A thrill of excitement ripples through the crowd. Slowly, the Shuttle assembly rises; all hold their breaths as fingers of fire emanate from the three powerful engines and the solid rocket boosters. The brilliant exhausts gradually lengthen as the Shuttle slowly rises - then the flames lift clear of the launch pad - then the vehicle climbs, gradually increasing speed and diminishing in size - until it is tiny, high in the sky and leveling off. It then rapidly accelerates, growing smaller and smaller until it is out of sight. A cheer resounds, as everyone rises and yells, excitedly pounding each other's back, on everyone's face, a broad smile. Gradually many sit down again, breathing deep breaths.
NASA personnel begin shepherding everyone into the buses, to be taken to Control Center for more congratulations; to watch for awhile on the computer screens; to relax with coffee and sweet-rolls; then to be briefed by NASA regarding the flight to California and Edwards Air Force Base - for the next challenge and uncertainty, a safe re-entry and landing.
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