Basic Concepts Of Orbital Spaceflight
The basic concepts of orbital spaceflight are easy to understand, and this knowledge will give you a better understanding of space-related news, history and current events.
The common conception of orbital flight is that of an object flying through space, following a circular or elliptical path around another object. That description is accurate, but incomplete. It leaves out the most important element of orbital flight: gravity.
An orbit is the path of an object in space as it moves around another object due to the force of gravity. Orbital flight is not powered flight, it is the result of a careful balance between gravity and momentum. A rocket is used to carry a spacecraft into space, but once the spacecraft has achieved sufficient altitude and velocity, the rocket engine is turned off, and often discarded. At this point, the spacecraft will be held in orbit by gravity - the same force that holds the moon in orbit and makes the planets revolve around the sun.
In 1687, in a book titled Principia Mathematica, the physicist Isaac Newton presented the first explanation of orbital motion. It is still one of the simplest and clearest explanations available.
Newton imagined a canon on top of a very high mountain. A cannonball is shot out, which travels for a distance, but eventually gravity pulls it down and it strikes the ground. On a second shot, more gunpowder is used, and the cannonball travels further before striking the ground. In each case, the cannonball follows a curved path to the ground.
The surface of the earth is also curved. If enough gunpowder were used, Newton suggested, the curvature of the cannonball's trajectory would be the same as the curvature of the earth. The cannonball would be falling, but it would never reach the ground. The cannonball would "fall" all the way around the world and strike the cannoneer in the back of the head.
Momentum is the tendency of a moving object to stay in motion. When you are in a moving vehicle that stops suddenly, it is your own momentum that throws you forward.
Momentum is related to velocity. The greater velocity an object has, the more momentum it has.
This is the definition of an orbit. The momentum of the cannonball and the pull of gravity are balanced. The cannonball is in a continuous state of free fall, and will remain so until affected by another force (Newton's theoretical example assumes the cannon is high enough that the resistance of earth's atmosphere can be ignored).
NASA has an online app that shows Newton's cannon in action. You can plug in different amounts of gunpowder and see the resulting trajectory of the cannonball: Shoot a Cannonball into Orbit!
The velocity needed to maintain a given orbit is called orbital velocity. The velocity required depends on the altitude of the orbit. The closer an orbiting spacecraft is to earth, the higher the velocity required to remain in that orbit. To give you an idea of the speeds required, maintaining a circular orbit at an altitude of 100 miles requires a velocity of 17,478 miles per hour.
Getting Into Orbit
Rockets are used to give a spacecraft sufficient altitude and velocity to achieve orbit. A rocket is launched vertically, but as it ascends it adjusts its trajectory to become more horizontal. This puts the spacecraft in the proper position for orbit.
When the desired altitude and velocity have been reached, the rocket engine is cut off. If altitude and velocity are correct, the spacecraft will be in orbit. The path of the orbit is determined by the spacecraft's velocity.
Once in the desired orbit, the spacecraft's velocity must be carefully monitored. Changes in velocity will change the path of the orbit. In fact, this is how a spacecraft in orbit returns to earth. Small rockets, known as retrorockets, fire briefly against the direction of flight, changing the velocity of the spacecraft. This changes the path of the spacecraft's orbit to one that will bring the spacecraft back into the atmosphere.
Weightlessness is an interesting side effect of being in orbit. A common misconception is that objects in space float because there is no gravity in space. The term "weightlessness" reinforces this idea, but the word is a misnomer. In fact, the pull of gravity on objects in orbit is diminished only by a small amount. For example, an astronaut that weights 160 pounds on earth will weigh about 140 pounds while in orbit.
The astronaut feels weightless because he is in a constant state of free fall. Few of us have ever been in free fall long enough to notice, but a person in free fall doesn't feel his own weight. Only when something is opposing the pull of gravity do we experience the feeling of weight. On earth, gravity pulls us into something - the ground or the floor, for example - and this opposition to gravity is why we feel weight.
Everything in orbit with the astronaut floats around him because it all is continuously falling at the same rate as he is. Although travelling at thousands of miles per hour, these objects are not moving at all relative to each other, and they appear to be floating in space, or "weightless".
Apogee and Perigee
Not all orbits are circular. Many take the shape of an ellipse. The point of highest altitude in an elliptical orbit is called the apogee, and the lowest point is the perigee. A circular orbit is just a special case of an elliptical orbit where the apogee and perigee are the same.
In a circular orbit, the velocity of an object is constant, while velocity varies throughout an elliptical orbit. In such an orbit, velocity is greatest at the perigee, and lowest at apogee.
Earth's atmosphere doesn't end suddenly at the edge of space. It gradually thins out. A spacecraft in low-earth orbit will experience drag, or resistance, from the thin atmosphere it encounters. This drag will cause an orbit to deteriorate, or decay, over time. All current manned space activity takes place in low-earth orbit, and must take drag into account. The International Space Station, for example, must periodically be boosted to its proper altitude and velocity to compensate for the effects of atmospheric drag.
The first two manned launches of America's space program, by Alan Shepard and Gus Grissom in 1961, were not orbital flights. The rockets available at the time were powerful enough to carry astronauts into space, but could not provide the velocity needed to achieve orbit. These flights were known as suborbital flights.
By 1962, more powerful rockets were available, and there were no more manned suborbital flights for several decades. In 2004, SpaceShipOne, the first privately funded spacecraft to take a human into space, made three suborbital flights. The plan is for a similar spacecraft, SpaceShipTwo, to begin taking tourists on suborbital space flights, possibly beginning in 2011.
Space tourism has renewed interest in suborbital spaceflight, which is now being considered for other commercial applications, such as transportation, experimentation in weightlessness, and even extreme sports. The Suborbital Institute is a good place to learn more about the potential benefits of suborbital spaceflight.