Why Orbits Happen
Discover how gravity and forward motion work together to keep planets, moons, and satellites traveling in curved paths through space.
Earth & Space Science
What Is an Orbit?
An orbit is the curved path one object follows around another object in space. The Moon orbits Earth, Earth orbits the Sun, and satellites orbit Earth: all for the same reason.
Two Key Ingredients
Gravity
A pull between any two objects with mass. The bigger the mass, the stronger the pull. Gravity provides the inward force that bends a straight path into a curve.
Forward Motion
Also called inertia: an object in motion stays in motion in a straight line unless a force acts on it. This "sideways speed" keeps the orbiting object from crashing inward.
Falling Around the Earth
Newton's Law of Gravity
Every object with mass attracts every other object. The force follows:
\( F \)
Gravitational force (Newtons)
\( G \)
Gravitational constant
\( m_1, m_2 \)
Masses of the two objects
\( r \)
Distance between centers
What the Formula Tells Us
1
More mass means more pull. If either \( m_1 \) or \( m_2 \) increases, the gravitational force \( F \) increases.
2
Farther apart means weaker pull. Because distance \( r \) is squared in the denominator, doubling the distance cuts the force to one-quarter.
3
Gravity never turns off. It reaches across all of space, although it gets weaker with distance. That is why the Sun can hold distant planets in orbit.
Orbit vs. Falling Straight Down
| Orbit | Straight Fall | |
|---|---|---|
| Forward speed | High: object moves sideways fast enough to keep missing the surface | Zero or too slow: object moves straight toward the center |
| Gravity | Continuously pulls inward, bending the straight path into a curve | Pulls the object directly inward: it hits the surface |
| Result | Endless curved path around the body | Collision with the surface |
Newton's Cannonball
Newton imagined firing a cannon from a very tall mountain. Fire too slowly and the ball crashes. Fire faster and it travels farther before landing. Fire at just the right speed, and the ball falls around Earth forever: an orbit.
Circular vs. Elliptical Orbits
What Determines Orbital Speed?
For a roughly circular orbit, the speed needed is:
1
Closer orbit = faster speed. A satellite 300 km above Earth travels at about 7.7 km/s. The Moon, much farther away, travels at about 1.0 km/s.
2
Bigger central body = faster speed. A planet orbiting a massive star must travel faster than one orbiting a small star at the same distance.
3
Mass of the orbiting object does not matter. A feather and a bowling ball in the same orbit travel at the same speed.
How to Model an Orbit
1
Identify the two objects. Which body has more mass (the central body)? Which is orbiting?
2
Draw the gravity arrow. It always points from the orbiting object toward the center of the larger body.
3
Draw the velocity arrow. It is tangent to the orbit path (perpendicular to the gravity arrow for a circular orbit).
4
Check the balance. If speed is right, the object curves continuously without spiraling in or flying away.
5
Repeat at another point. The gravity arrow now points in a different direction, but it still points inward. The velocity arrow is still tangent.
Example 1: The Moon Orbits Earth
Example 2: The ISS in Low Orbit
What If Speed Changes?
Too Slow
Gravity wins. The object spirals inward and crashes.
Just Right
Gravity and speed balance. A stable, continuous orbit.
Too Fast
Speed wins. The object reaches escape velocity and flies away.
Common Mistakes
✕ "There is no gravity in space"
Gravity extends infinitely. Astronauts experience about 89% of surface gravity in low orbit. They feel weightless because they are in free fall.
✕ "Orbits need a pushing force"
No engine is needed to maintain an orbit. Once at the right speed, inertia keeps the object moving forward while gravity curves the path. There is no air resistance in space to slow it down.
✕ "Heavier objects orbit slower"
Orbital speed depends only on the central body's mass and the distance, not on the orbiting object's mass. A bowling ball and a marble at the same altitude orbit at the same speed.
Orbit Summary
| Factor | Effect on Orbit | Example |
|---|---|---|
| Increase central mass | Need faster speed to orbit | Jupiter's moons orbit faster than Mars's |
| Increase distance | Need slower speed to orbit | Neptune orbits slower than Earth |
| Speed too low | Object falls inward | De-orbiting satellite re-enters atmosphere |
| Speed too high | Object escapes gravity | Voyager probes leaving the solar system |
| Speed just right | Stable orbit | ISS at 7.66 km/s, Moon at 1.0 km/s |
Question 1 of 5
What two things must combine for an object to maintain a stable orbit?
An orbit requires gravity pulling the object inward and forward motion (inertia) carrying it sideways. No engine or air is needed once the orbit is established.
Question 2 of 5
According to Newton's law of universal gravitation, if you double the distance between two objects, what happens to the gravitational force?
Because distance is squared in \( F = G\,\frac{m_1 m_2}{r^2} \), doubling \( r \) means dividing by \( 2^2 = 4 \). The force becomes one-quarter as strong.
Question 3 of 5
Astronauts on the ISS appear to float. Why?
Gravity at ISS altitude is still about 89% of surface gravity. Astronauts appear weightless because they and the station are falling together at the same rate while moving forward fast enough to orbit.
Question 4 of 5
A satellite in a low orbit travels at 7.7 km/s, while the Moon orbits at about 1.0 km/s. Why is the satellite so much faster?
From \( v = \sqrt{\frac{GM}{r}} \), a smaller orbital radius means the object must travel faster. The satellite is much closer to Earth than the Moon is, so it needs much greater speed to maintain its orbit.
Question 5 of 5
In Newton's cannonball thought experiment, what happens if the cannonball is fired at exactly the right speed from a very tall mountain?
At just the right speed, the cannonball falls toward Earth due to gravity, but its forward speed is high enough that the ground curves away beneath it at the same rate. The result is a continuous orbit: the cannonball keeps "missing" the Earth.