If we throw an object straight up, it will rise until the negative acceleration of gravity stops it, then it returns to Earth. Gravity force diminishes as distance from the center of the Earth increases. However, if we threw the object with high initial upward velocity, the decreasing gravity's force can never bring it to a complete stop. Its decreasing velocity can always be high enough to overcome gravity's pull. The initial velocity needed to achieve that condition is called escape velocity. From the surface of the Earth, escape velocity (ignoring air friction) is about 11.2Km/s. Given that initial speed, an object needs no additional force applied to escape Earth's gravity completely. Escape velocity is defined to be the minimum velocity an object must have in order to escape the gravitational field of the earth, that is, to escape the earth without ever falling back. The range of the gravitational force is infinite. Therefore, it is technically never correct to say something "leaves" the gravitational field, even if it is launched with escape velocity. Escape velocity simply means that the object never stops moving, i.e., never falls back towards the Earth, in this case.That is the total energy of the body always remains positive but it never becomes zero. Therefore, an object never leaves the gravitational field of the earth, no matter what initial velocity it has. What happens, if a rocket does not reach escape velocity? A rocket can leave the earth at a much slower "speed" by simply overcoming the force of gravity at the location and moment of its climb. If you had a ladder tall enough (and a ridiculous supply chain) you could very slowly climb up away from the earth under your own power. Similarly, it will continue to move up and away from the earth at any velocity if it has so long as it maintains a thrust sufficient to overcome the diminishing gravitational attraction between it and the earth--eventually escaping our planet.
A space elevator is a proposed non-rocket-space-launch structure (a structure designed to transport material from earth to space). There are many elevator variants which involve travelling along a fixed structure instead of using rocket powered space launch have been suggested. These space elevators have also sometimes been referred to as beanstalks, space bridges, space lifts, ladders, skyhooks, orbital towers, or orbital elevators etc. Early key concepts of the space elevator appeared in 1895 when Russian scientist Konstantin Tsiolkovsky was inspired by the Eiffel Tower in Paris
A geostationary orbit, therefore, appears to be hovering in the same spot in the sky directly over the same patch of ground at all times. This sounds that the cable space elevator concept is technologically feasible. But current technology is not capable of manufacturing practical engineering materials that are sufficiently strong and light to build an Earth-based space elevator of the geostationary orbital tether type. Most recent discussions focus on tensile structures (specifically, tethers) reaching from the ground to geostationary orbit. Since the measured strength of microscopic carbon nano-tubes appears to have a great tensile strength to make this possible, the recent conceptualizations for the cable elevator are notable in their plans to use carbon nano-tube or boron nitride nano-tube based materials as the tensile element in the tether design. Carbon nano-tubes have a great tensile strength because of hexagonally shaped arrangements of carbon atoms that have been rolled into tubes. These tiny straw-like cylinders of pure carbon have useful properties.
To meet the end of the space elevator the construction would be a large project. The Earth-based cable space elevator’s minimum length must be over 38,000 km (24,000 mi). The tether would have to be made of a material that could endure tremendous stress while also being light-weight, cost-effective, and manufacturable in great quantities. Materials currently available do not meet these requirements, although carbon nano-tube technology shows great promise. Since the elevator would attain orbital velocity as it rode up the cable, an object released at the top would also have the orbital velocity necessary to remain in geostationary orbit. The estimated calculations for fiber show that the tether should have a minimal tensile strength of 130 Giga Pascal (GPa). A space elevator cable must carry its own weight as well as the (smaller) weight of climbers. Carbon nano-tubes' theoretical tensile strength has been estimated between 140 and 177 GPa depending on their geometry and its measured tensile strength varies in the range 11–150 GPa, however, only on a microscopic scale. The current technology allows growing tubes up to a few tens of centimeters. This limit can be mitigated by spinning nano-tubes into a yarn. By comparison, most steel has a tensile strength of less than 2 GPa, and the strongest steel resists no more than 5.5 GPa. The much lighter material Kevlar has a tensile strength of 2.6–4.1 GPa, while quartz fibers can reach 20 GPa. Quartz fibers have an advantage that they can be drawn to a length of hundreds of kilometers (270 km—168 mi) even with the present-day technology. While various designs employing moving cables have been proposed, most cable designs call for the "elevator" to climb up a stationary cable.
Some works were expanded to cover the deployment scenario, climber design, power delivery system, orbital debris avoidance, anchor, surviving atomic oxygen, avoiding lightning and hurricanes by locating the anchor. For gearing up our interest to space, it is necessary to invite many scientists and engineers to discuss concepts and compile plans for an elevator to turn the concept into a reality. In the twenty first century, the space elevator technology would be able to launch objects into orbit without a rocket.
References:
1. Wikipedia, the free encyclopedia.
2. The Spaceward Foundation.
3.The orbital tower: a space craft launcher using the Earth's rotational energy
JEROME PEARSON, U.S. Air Force Flight Dynamics Laboratory, Wright-Patterson Air Force Base, OH 45433, U.S.A. (Received 17 September 1974; revised 27 January 1975).
4. Yu, Min-Feng; Lourie, O; Dyer, MJ; Moloni, K; Kelly, TF; Ruoff, RS (2000). "Strength and Breaking Mechanism of Multiwalled Carbon Nanotubes Under Tensile Load". Science 287 (5453): 637–640.
