Orbital Mechanics

Orbital mechanics at its heart is the application of physics to understand and predict the motion of objects in space, particularly those following orbits around celestial bodies, whether that be the Moon, Earth, or any other object with enough mass to have gravity.

Instead of travelling in straight lines, objects in space follow curved paths called orbits, which are determined by the gravitational forces acting upon them. With the right speed and direction, and object can keep "falling" around a planet without it hitting the surface, creating a stable orbit.

By understanding these principles, we can design spacecraft trajectories, predict the positions of planets and satellites, and plan journeys to other bodies.

Gravity is the fundamental force that governs the motion of objects in space. It is the attractive force between two masses, and it is what keeps planets in orbit around the Sun and moons in orbit around planets. It is the tether between celestial bodies.

Celestial bodies with sufficient mass generate a gravitational field that influences the motion of nearby objects, determining their orbital paths and stability. Bodies with a heavier mass have a stronger gravity pull, which affects everything around them.

There is a balance between speed and distance from the central body that determines whether an object will orbit, fall back, or escape into space. An object too slow will fall back, while an object too fast will escape into space.

An orbit is the curved path that an object follows around a celestial body due to the balance between the gravitational pull of the central body and the object's forward velocity. Not every orbit is perfectly circular either, most orbits are elliptical, with an apoapsis (highest point) and periapsis (lowest point).

The idea of elliptical orbits was first proposed by Johannes Kepler, who showed that planets don't move in perfect circles. Orbits are regular, repeatable, and shaped by gravity.

What creates an orbit?

Forward motion and gravity are the 2 main principles that create an orbit. Instead of flying in a straight line, the object continuously curves, or "falls" around the body its orbiting. The result is a stable path that repeats over time.

Not all orbits are the same. Depending on how fast an object moves and how far it is from the body its orbiting, its path can take on different shapes and behaviours. Around Earth, we tend togroup orbits into key types based on their height and purpose.

Low Earth Orbit (LEO): This is the region closest to Earth. Satellites in LEO orbit Earth in about 90 minutes and are commonly used for communication, Earth observation, and scientific research.

Medium Earth Orbit (MEO): This region is higher than LEO and is often used for navigation satellites, like those in the GPS constellation. Satellites in MEO have longer orbital periods, typically around 12 hours.

Geostationary Orbit (GEO): This is a special type of orbit located about 35,786 kilometers above the equator. Satellites in GEO orbit Earth at the same rate that Earth rotates, allowing them to remain fixed over a specific point on the surface. This makes GEO ideal for communication and weather satellites.

Highly Elliptical Orbit (HEO): These orbits have a very elongated shape, with one end much closer to Earth than the other. They are often used for scientific missions that require long observation times over specific regions of Earth.

In orbit, speed is everything. It decides whether an object will fall, stay in orbit, or escape into space. The key idea is that orbit isn't about "not falling"; it's about falling in a controlled way while moving forward fast enough to miss the surface.

There's a narrow middle ground where an object can maintain a stable orbit without falling or escaping. At this speed, the object is in a state of continuous free fall around the celestial body it's orbiting. That's a stable orbit.

Too slow: Falls back down.

Just right: Stable orbit.

Too fast: Escapes into space.

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Once an object is in orbit, it doesn't stay ther forver by accident; it can be deliberately moved. In spaceflight, changing an orbit is called an orbital manoeuvre, and its done using small, carefully timed engine burns.

Unlike flying in atmosphere, you don't "turn" in space. Instead you adjust your speed, and that changes the shape and size of your orbit over time.

Prograde Burn: Burning the engine prograde (forward) will increase speed, raising the opposite side of the orbit.

Retrograde Burn: Burning the engine retrograde (backward) will decrease speed, lowering the opposite side of the orbit.

Radial-In: Burning while facing the centre of the body will slightly tilt the orbital axis and eccentricity (shape).

Radial-Out: Burning while facing away from the centre of the body will do the opposite of radial-in.

Normal: Burning while pointing above the orbital plane changes the orbit inclination and tilts orbit relative to the equator.

Anti-Normal: Burning while pointing below the orbital plane will do the opposite of normal.

Each direction is relative to your orbit, and are all used in manoeuvring transfers.