We begin with the fundamental element, or you may say, the first step of traveling to space: orbiting around Earth or another celestial body. Consider an object orbiting the Earth; unless there are factors such interaction with the upper atmosphere, solar winds, and inertial energy losses, the object will orbit indefinitely. The reason is that all objects in orbit are essentially falling around the body they are orbiting. This is relatively simple to illustrate. The acceleration of gravity at the surface of the Earth is 32.1741 ft/s2 (9.8067 m/s2) and that means, from Newton's Laws, in one second an object will fall 16.087 feet or 4.9033 meters from rest.
The radius of the Earth at the equator is 3,963.19 statute miles (6,378.14km). If the Earth were a smooth sphere with the radius of the Earth's equator, then the distance traversed along the surface from a point A to a point B 16.087 feet lower than point A is 25,947 feet (7,908.7 meters). So if an object were one foot above the surface of this perfect sphere, and traveling at a speed of 25,947 ft/s (7,908.7 m/s) parallel to the surface, then it would fall the same distance as the surface of the Earth curves and falls away from the starting point. That is, it would continuously fall "around the sphere'' at an altitude of one foot, without ever striking the surface. It would in fact be in orbit around the sphere. So an object in orbit around a body is falling around that body at sufficient speed that it does not move closer to the surface. Occupants in that orbiting body are not experiencing zero gravity, they are experiencing zero net force.
To show that, consider the acceleration of a body moving along a curved path that is at constant speed V, but with a constantly varying flight path angle. The acceleration perpendicular to the flight path that is necessary to maintain the curved path is:
Using the equatorial radius of the Earth, with the magnitude of the speed V = 25,947ft/s (7,908.7m/s), the normal (perpendicular) acceleration is equal to the acceleration to gravity in magnitude but acting in the opposite direction. So an object in orbit around a body is free falling around that object and there are no net forces on the object or on anything on that object. That is often described mistakenly but colorfully by the popular press as a condition of "zero-gravity"; instead it is the difference between two essentially equal and opposite forces. Micro-gravity would instead be a more appropriate term, for there is always a minute residual difference between gravity and normal acceleration. The balance is so delicate that on an orbital station an occupant that sneezes can ruin a microgravity experiment. Technically, such disturbances go by the name of microgravity "jitters".
So in order to go to space, we first need a transportation system from the surface to Earth orbit and return. To go to the Moon and beyond, for instance to Mars, we need a propulsion system that can leave Earth's orbit and then establish an orbit around its destination object. We are able to do this to the Moon relatively easily with the currently operational propulsion systems. That is because to reach the Moon an elliptical orbit containing the Earth and Moon at its foci is sufficient. To reach Mars instead we must reach and exceed escape speed. Mars requires a round trip of two years with current propulsion systems. So for Mars a propulsion system that ensures minimum radiation damage to human travelers is still in the laboratory. In order to go Pluto and beyond, we need propulsion systems not yet built, but envisioned by people that seek to travel beyond our solar system. However, to travel much farther beyond Pluto remains for the time being only an expectation.
If you were to ask the question, ''What is Space Propulsion ?'' probably the most common answer would be rockets. Beginning in 1957 with Sputnik, chemical rockets have propelled payloads and satellites into Earth orbit, to Mercury, Venus, Mars and Titan, one of Saturn's satellites, and have propelled two Pioneer spacecraft (Pioneer 10 and 11) to the boundary between our solar system and interstellar space. Pioneer 10's last telemetry transmission to the NASA Deep Space Network (DSN) was 22 April 2002, having been launched on 2 March 1972. On 22 January 2003 the DSN recorded Pioneer 10's last weak radio signal at a distance of 7.6 billion miles (7.6 x 109 miles) from Earth. That signal took 11 hours and 20 minutes to reach DSN [AW&ST, 3/2003]. Pioneer 11's last telemetry transmission was in 1995. Its journey has taken nearly 31 years, and it is now beginning to cross the boundary between our solar system and interstellar space (the so-called Heliopause). This is the problem we face with chemical rocket propulsion, the extremely long times to cover large distances, because the speed possible with chemical rockets is severely limited by how long the rocket motors can function. Had an operational Pioneer spacecraft reached a distance from Earth that is 100 times the distance the Earth is from the Sun (i.e., of the order of the Heliopause) it would take light 14 hours to traverse the one-way distance, so a two-way communication requires 28 hours, four hours longer than one day! That is to say that, at light speed, Pioneer 10 would have the reached the Heliopause some 32 years ago! Pioneer 10 is on its way to the red star Aldebaran, but it will not arrive there for more than another 2 million years [AW&ST, 3/2003]. The Pioneer spacecraft team that was present when the Pioneer spacecraft passed by Jupiter, Saturn, Neptune or Uranus is no longer the group listening for the sporadic-distant signals being received from the Pioneer spacecraft. In reality the Pioneer spacecraft moves so slowly that following its progress is beyond the practical ground-based tracking team's functional duration. To move faster requires high accelerations, but those are limited by the rocket propulsion systems available and by human physiological and spacecraft hardware tolerance to acceleration ("g" tolerance). To approach light speed or faster than light (FTL) speed what is needed is not anti-gravity but anti-mass/inertia. A question is, Is FTL possible? A conclusion [Goff and Siegel, 2004] is:
Current warp drive investigations [Goldin and Svetlichny, 1994] apply general relativity to try to produce spacetime curvature that propagates at superlight speeds. Special relativity is preserved inside the warp field, but the contents are perceived to move at FTL speeds from the external frames. Such a classical warp drive cannot avoid the temporal paradox (i.e., time travel). If quantum systems are the only system that permits backward-in-time causality without temporal paradox, then any rational warp drive will need to be based on quantum principles. This means that until we have a workable theory of quantum gravity, research into warp drives based on General Relativity is probably doomed to failure.
A second example of our chemical rocket speed limitations is a Pluto mission. The planet Pluto has a distance from the Sun varying from 2.78 x 109 to 4.57 x 109 statute miles, for an average of 3.67 x 109 statute miles. Depending on its distance, a one-way radio signal takes between 4 hours, 10 minutes and 6 hours, 48 minutes to reach Pluto from Earth. So the two-way transmission from Earth and return takes from about 8 hours to 13 hours. That is a considerable time to consider communicating with and controlling a spacecraft. If a correction to its flight path, or a correction to its software programming, or remedying a problem is necessary, it will be between 16 and 26 hours before a return signal can confirm whether or not the action was successful. In that period of time a great deal can happen to harm, injure or destroy the spacecraft. So these spacecraft that are operating at the fringe of practical control because of the propulsion system's performance must essentially be robots, capable of diagnosing and correcting problems without human intervention.
The question is: ''What propulsion performance is necessary to significantly change this chemical rocket paradigm?'' The performance of a rocket is measured by its ability to change the magnitude of its speed in a given direction (velocity) by the ejection of mass at a characteristic velocity. That change in the magnitude of the speed, DV, can be expressed in the simplest way as: (1) where:
c * = gIsp = Characteristic velocity
DV DV Initial mass WR = exp-
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