How Orbits Work

Orbits

The answer lies in the delicate physics of orbits. Orbits are the reason why satellites stay in space and why the Earth has a moon. 

Copernicus probably said it best. Imagine a canon sitting atop a mountain. The first cannonball is shot at a slow velocity and only lands a few hundred feet away. Another cannonball is shot at a faster velocity and lands a mile away. The faster the cannonball, the further it will travel. Eventually–ignoring air resistance–a cannonball will travel so fast that it will circle the Earth and end up behind the canon itself. This is the concept of orbiting.

This can be applied to missiles and spacecraft. A missile is fired at a velocity that isn’t fast enough to loop around the Earth but might make it halfway. Eventually, the missile will fall back down to Earth. However, a rocket carrying a spacecraft needs to travel so fast that the spacecraft keeps looping around the Earth, again and again, never falling back down.

This is a balance between velocity and gravity. Orbiting is basically constantly falling down to Earth but constantly missing at the same time. However, orbits are not only a perfect circle; they can be ellipses or require faster or slower velocities depending on the planet. 

Speeds of Orbits

As mentioned before, orbits come in many different shapes and sizes.

Using the equation; v = √(GM/r)

We can see that the velocity required to orbit a planet depends on two factors. The mass, M, and the radius or distance of the orbit from the planet, r. G is a constant. 

The greater the mass, the greater the velocity. The greater the radius, the lower the velocity.

For example, a planet with a large mass and a low orbital altitude will require a high orbital velocity. A planet with a small mass and a high orbital altitude will require a low orbital velocity. 

Here are some examples. Jupiter, the largest planet in our solar system, requires a high orbital velocity as it has a mass 318 times greater than that of the Earth. Pluto, on the other hand, has a very low orbital velocity as it has a mass 2% that of the Earth. 

The further a spacecraft or object is from a planet, the slower it will orbit the planet. For example, for the Space Shuttle to orbit the Earth, it had to travel at 17,500 miles per hour. The Space Shuttle had a low orbital altitude. On the other hand, the Moon, which is located about 240,000 miles away from the Earth, only needs to travel at 2,300 miles per hour to maintain its altitude. 

Shapes of Orbits

Although these are good examples for comparing speed, these orbits are all roughly circular. However, when orbits begin to take on weird shapes, for example, ovals. Then, the velocity fluctuates at different points in the orbit. To understand this, there are two crucial terms:

Apoapsis: Highest distance from the spacecraft to the planet

Periapsis: Lowest distance from the spacecraft to the planet

At apoapsis, a spacecraft is traveling the slowest. At periapsis, the spacecraft is traveling the fastest. This can be explained using the equation, again, as r is a variable that changes in these elliptical orbits. At apoapsis, a spacecraft has the greatest r, which correlates to the slowest v. At periapsis, a spacecraft has the lowest r, which correlates to the highest v. 

Different Types of Orbits

• ‍Low Earth Orbit (LEO):
  • Altitude: 160 to 2,000 km.
  • Examples: Earth observation, communication, and scientific research.

• Medium Earth Orbit (MEO):
  • Altitude: 2,000 to 35,786 km.
  • Examples: GPS and navigation satellites.

• Geostationary Orbit (GEO):
  • Altitude: 35,786 km - r must be constant to maintain a constant v
  • Type: Satellites are perpendicular to a specific point on Earth’s surface
  • Examples: Weather forecasting and telecommunications

• Polar Orbit:
  • Type: Passes over the Earth’s North and South poles
  • Examples: Global mapping and reconnaissance