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Non-atmospheric Flight

5 - 8 Grade Reading

Non-atmospheric Flight: Getting from here to way out there!

To travel from Earth to another planet involves atmospheric and non-atmospheric (space) flight. To fly from here to a distant planet a spacecraft is packaged into a rocket vehicle. This rocket has a streamlined shape because it needs to move efficiently through the Earth's atmosphere (through those molecules of air that cause drag). Once out in space, this spaceship will coast toward its destination planet using the gravitational pull of a nearby massive object (like the Sun) to move it along. Making a few course corrections along the way, it will jettison its rocket-look releasing its inner spacecraft. It will eventually move into orbit around its destination planet. To maintain its orbit, it will fire its boosters periodically. Once its booster rockets run out of fuel, it will be unable to resist the gravitational pull of the planet. It will plummet at an extremely high speed into the planet's surface. To get from here to way out there it will need to fly.

Basically, flight is about moving from one place to another without being in contact with the ground. An aircraft that is not moving (stationary) must generate a force to get moving. It will then keep moving until another force (like weight or drag) makes it slow or stop. This is what Sir Isaac Newton was talking about in the First Law of Motion: a body will remain at rest or continue at constant velocity unless acted upon by an external force.

To reach space for interplanetary flight, a vehicle needs to have enough thrust to move swiftly through the Earth's atmosphere. Rocket-type vehicles do not rely on the lift force to move through the atmosphere. That's why they don't have wings. They do have fins that act much like the tail feathers on an arrow. The fins keep the rocket balanced during flight. Rocket-type vehicles rely on their streamlined shape to reduce drag. They use their powerful rocket engines to generate maximum thrust. This thrust is used to escape Earth's gravitational force. To do this, spacecraft use large rocket engines that accelerate the vehicle as quickly as possible. By reaching a very high speed and following the curvature of the Earth, it can escape the Earth's gravitational force.


A body that moves in a circle is constantly changing direction. That means that its speed (velocity) is changing. Since its velocity does not stay the same it is said to be constant. Since this object's velocity is not constant, that means that some outside (external) force is responsible for the change. To see this in action follow the steps below:

  1. Tie a weight to the end of a 6 foot string.
  2. Stand in an open area with no objects or people in a 40 foot diameter except for observers observing only from behind the demonstrator and a safe distance back.
  3. Slowly swing the weighted string above and around your head.
  4. What do you notice at first? The weight at the end of the string should start to pull on the rope.
  5. Increase the speed (velocity) of the swinging motion.
  6. What do you notice now? The weight at the end of the string should pull more strongly on the rope. The rope will go taut.
  7. Stop moving your hand (that is holding the rope) in a circular motion. Observe what happens. Even with your hand still the rope continues to move in a circular motion.
  8. Increase the velocity slightly of the weighted string.
  9. Release the string suddenly (Safety note: Release the string in a direction away from the observers and any other nearby objects!).
  10. Observe what happens. The weight flies off in a straight line because there is now no force to hold it in its circular motion.

hammerThe Hammer Throw

That force in the string is enough to make the weight at the end of the string circle around you. A little vector math shows that the acceleration of the weight is toward the center of the circle. The magnitude is the square of the velocity divided by circle's radius.

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/Planetary Motion/PlanetaryMotion.htm

As our rocket carries the spacecraft swiftly through the Earth's atmosphere, it is also trying to pull away from the Earth's gravitational force. The Earth's gravitational force pulls at the spacecraft like the tension in the weighted string. The Earth's gravity becomes an external force acting on the spacecraft. The direction of the spacecraft is no longer directly straight up from the Earth's surface. Its trajectory (airborne course) is now following the curvature of the Earth because of the gravitational force. Let's say that the velocity of the spacecraft and the gravitational force of Earth have just the right balance. Then the spacecraft will remain in an orbit around the Earth. If the Earth's gravitational force is too great, then the spacecraft will be pulled down to the planet's surface. To keep this from happening, the spacecraft will have to fire its booster rockets to remain at the same height above the Earth. What if the gravitational force is too low? What if the thrust from the spacecraft's rocket engines are greater than the earth's gravitational force? Then the spacecraft will pull away from the Earth and move into interplanetary flight.

Outside of the atmosphere there is no drag because there are no gas molecules through which to fly. This is great for flying at high speeds using as little fuel as possible. This also means that having a streamlined shape is not really necessary out in space. After the streamlined rocket carries a spacecraft outside the Earth's atmosphere, its smooth outer shell is jettisoned. Solar panels and scientific instruments are unfolded. The Cassini spacecraft shown below is an example of a typical spacecraft shape.


Sometimes finding one's way on Earth in a car with a road map can be difficult. Scientists have to plan a manmade satellite's trip in advance. They also have to be able to change its course during the trip. This type of navigation can be very complex! Every unmanned spacecraft has on board some type of guidance system. Spacecraft t also 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 its location in space. This technique is similar to the Global Positioning System (GPS) used here on Earth.


Learn more about the Global Positioning System (GPS):
JPL Mission and Spacecraft Library
NAVSTAR GPS Operations

When the spacecraft arrives at its destination planet, it must slow its speed to enter an orbit around the planet. The spacecraft will use its rockets this time as brakes. The rockets located at the front of the spacecraft are fired. This will slow the rocket's speed. Rocket scientists talk about "delta V". That's the velocity needed to change the trajectory or course of the spacecraft. The rockets used for orbital maneuvers are typically fired at full throttle. That way the amount of force they deliver is constant (Remember the weighted string?). The longer the rockets burn, the greater the change in velocity will be. To slow the spacecraft's speed a little, the rockets are fired for a short period of time. To slow the spacecraft's speed a lot, the rockets are fired for a longer period of time.

Xenon ion propulsion in Deep Space 1:

Once in orbit around the destination planet, the spacecraft must maintain a safe distance from the planet's surface. It must keep its thrust balanced with the amount of force from the planet's gravitational pull. The planet's gravitational force is constantly pulling the spacecraft closer and closer to its surface. To keep its orbit, the spacecraft will occasionally fire its rockets to boost it away from the planet, but not too far away! If the rockets boost it too far away, it will free fall outside of the planet's gravitational pull. Eventually, the spacecraft's rockets will run out of fuel. The spacecraft will no longer be able to keep the balance between its thrust force and the planet's gravitational force. The gravitational force will pull it through the planet's atmosphere (if the planet has an atmosphere). The spacecraft will encounter the effects of drag. The spacecraft will drop from its orbit and plummet to the planet's surface at an incredible rate of speed. The spacecraft's journey will be over.

Vast amounts of data collected from these space missions give scientists valuable information about the solar system and the destination planet. This information could tell scientists about how the planet was formed. It could tell us what gases are found in the planet's atmosphere. As it orbits the planet it can take photos that show the planet's geographical features. Using special instruments it could send us information about the minerals and rocks found on the planet. Scientists could use the data to examine the planet's potential for life forms. This information could also be valuable for future robotic and human exploration of the planet. To get to way out there from here, the vehicle will need to fly, first through the atmosphere and then through space in non-atmospheric flight.

Planetary Home Page


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