Gravity and Motion

Motion in Space
If you look up at the night sky, you might guess that all of the objects in space are motionless, just hanging there. But really, these celestial bodies are constantly in motion. Objects in space, such as the planets in our solar system, follow the same laws of physics that govern the motion of objects on Earth. The solar system is the Sun together with the group of celestial bodies that are held by its attraction and revolve around it. We live on Earth, the third closest planet to the Sun. A planet is an object that orbits a star; is large enough to be nearly spherical in shape, and has no other large object in its orbital path.

Orbits
Have you ever hit a tetherball and watched it spin around a pole? The motion of the tetherball around the pole is in some ways like that of a planet orbiting the Sun. An orbit is the path an object follows as it moves around another object. Objects in orbit can be natural like the Moon, or human-made like the International Space Station. An object in orbit around a larger object is called a satellite. Objects in orbit move along an imaginary flat, disk-like surface called the orbital plane. The orbital plane connects the centers of the object being orbited ‘with the orbiting object.

The Shape of Orbits
You might think that objects in space, such as the planets, orbit the Sun in a perfect circle. Instead, an object's orbit is an ellipse—a stretched-out circle. Ellipses can vary in shape. Some can be nearly circular, and some can look like a circle that has been flattened. The measure of how far an object's orbit is from being circular is its Eccentricity.

Orbital Eccentricity
Scientists assign eccentricity values to the planets to quantitatively describe how circular or elliptic their orbits are. A planet with an eccentricity value of zero has a perfectly circular orbit. The higher the eccentricity value, the more elliptical the Orbit

A perfectly circular orbit is a special situation. Most orbits are elliptical even if they appear to be circular. Two fixed points, or foci, determine the shape of an ellipse. A circular orbit occurs when the two foci overlap with each other.

Gravity and Motion in Space
Imagine that you are bouncing on a trampoline. No matter how high you jump, you are always brought back to Earth's surface. Why do you return to the trampoline instead of bouncing off into space? The force that brings you back down to Earth is called gravity. Gravity is an attractive force that exists between all objects that have Mass.

Gravity
Gravity keeps us from floating off into space when we bounce on a trampoline, but have you ever thought about how gravity affects objects in our solar system? Gravity is the force that keeps the planets in orbit around the Sun. It also keeps the Moon and human-made satellites in orbit around Earth.

Effects of Gravity
The strength of the force of gravity depends on mass and distance. More massive objects exert a greater gravitational attraction than less massive objects. For example, the Sun exerts a greater force of gravity on an object than Earth has on the same object at the same distance. As distance between objects increases, the force of gravity decreases.

Gravity and Momentum
Objects in orbit have forward momentum, a measure of how hard it is to stop a moving object. Momentum is a product of an object's mass and velocity. Without the presence of the force of gravity, the momentum of the planets orbiting the Sun would launch them into space.

Objects in motion have inertia, the tendency for objects in motion to resist changes to their direction and speed. Inertia keeps the planets in our solar system in nearly steady orbits. The only force in space strong enough to affect the forward motion of the planets are the gravitational forces that they exert on each other.

Gravity and Orbital Shape
The shape of an abject's orbit is a balance between gravity and s forward motion. Remember the shape of an object’s orbit is typically elliptical, like the planets in our solar system. An object in orbit has just enough momentum to slightly pull away from the object it s orbiting. When it gets far enough away from the object itis. orbiting, the orbital speed reduces. At this reduced speed, the larger object will be able to pul the smaller orbiting object toward it. This fluctuation causes the orbit to have an elliptical shape.

Did you know that over time, Earth's orbital shape changes? Gravity from Jupiter and Saturn causes the shape of Earth's orbit to vary from nearly circular to slightly elliptical in a cycle that repeats every 100,000 years,

The Effects of the Sun’s Gravity
The largest object in our solar system is the Sun, a star. Its diameter is about 1.4 million km—ten times the diameter of the largest planet, Jupiter. Its mass is about 99 percent of the solar system’s mass. Because of its mass, the Sun applies gravitational forces on objects in the solar system. These gravitational forces cause the planets to orbit the Sun. Different types of objects orbit the Sun. These objects include planets, moons, dwarf planets, meteors, asteroids, and comets. You might be surprised to find out that Earth's gravitational pull, while very weak, manages to make the Sun wobble a bit.

Revolution and Rotation
The planets that orbit our Sun complete a full orbit in different lengths of time. The time it takes an object to travel once around the Sun is its period of revolution. It takes Earth 365.24 days to complete one revolution. A planet's speed changes as it orbits the Sun based on the elliptical shape of its orbit. The closer the planet is to the Sun, the faster it moves. The farther the planet is from the Sun, the slower it moves.

A planet farther from the Sun has a longer period of revolution than a closer planet, and travels slower since the Sun's gravitational pull is weaker. Planets also spin, or rotate, as they orbit the Sun. The time it takes an object to complete one rotation is its period of rotation. Earth's period of rotation is about 24 hours.

The Effects of Planetary Gravity
Have you ever thought about how the gravity of the planets in our solar system affect each other? Planets exert a force of gravity on each other. This can cause a slight shift to the orbit of a planet. The orbits of the planets are affected by three factors: the distance from the Sun, the distance to other nearby objects, and the mass of the planet.

In 1846, astronomers noticed that Uranus was not moving along its orbit as expected. This could not be explained by the gravitational attraction of the Sun and known planets at that time. They proposed that the gravitational pull of another planet was tugging on Uranus, causing the irregularities in its orbit. This led astronomers to the discovery of Neptune.

Gravity Slingshot
Imagine riding a bike up a hill and then down that hill. Gravity slows you down as you go up and speeds you up as you go down. Similarly, spacecrafts in motion can harness gravity to slow down or speed up. The gravity- assist method uses the gravity of a planet or moon to alter the path and speed of a spacecraft, sort of acting as a slingshot. This technique is commonly used by The National Aeronautics and Space Administration (NASA) because it is beneficial to conserving the spacecraft's fuel.

How It Works

By passing through the gravitational fields of the objects in our solar system, a spacecraft can increase or decrease its energy through adding or subtracting momentum. To speed up, the spacecraft glides with the movement of the planet. By doing so, its able to pick up some of the planet’s orbital energy to push itself along. To slow down, the spacecraft lies against the movement of the planet, like swimming against the current in a stream. Larger planets have stronger gravitational energy, giving the spacecraft a greater push or pull. In addition, the closer the spacecraft i to the planet, the stronger the push or pull

The Voyager Missions
Both launched in 1977, Voyager 1 and Voyager 2 were the first spacecrafts launched by NASA to use the gravity-assist method to explore the outer planets. A rare alignment of the four outer planets which happens every 175 years allowed for the spacecrafts to fly by the planets using the optimal amount of fuel. Without gravity assist, the Voyager spacecrafts would have only been able to reach Jupiter. The unique alignment of the outer planets allowed for the spacecrafts to slingshot from one planet to the next with ease.