Origins of the Solar System and Types of Orbit

- Overview

Planets, moons, and other celestial bodies formed from the accretion of dust, gas, and ice within a large disk surrounding the young Sun, a process driven by gravity. 

Gravity pulled particles together, causing them to clump and form larger objects, which explains why all planets orbit the Sun in the same direction and roughly the same plane. This process is known as the nebular hypothesis, and it also explains how artificial satellites stay in orbit around Earth today. 

1. Formation of planets and other bodies: 

• Nebular hypothesis: The prevailing theory is that the solar system formed from a giant cloud of gas and dust (a nebula) that collapsed under its own gravity.
• Flattening into a disk: As the cloud collapsed, it began to spin and flattened into a disk, with the Sun forming at the center and the remaining material forming a surrounding disk of gas and dust.
• Accretion: Within this disk, dust and gas particles began to collide and stick together in a process called accretion.
• Growth: These clumps grew larger and larger, eventually becoming planetesimals and then protoplanets, which finally became the planets, moons, and asteroids we see today.

2. Orbits and their characteristics:

• Heliocentric orbits: All planets, asteroids, and comets orbit the Sun in a heliocentric orbit because of the Sun's massive gravitational pull.
• Elliptical paths: Most of these bodies follow elliptical (oval-shaped) paths around the Sun, with the Sun at one focus of the ellipse.
• Common orbital plane: The planets' orbits are all roughly on the same flat plane, which is a result of how the solar system formed from the spinning, flattened disk of gas and dust.
• Satellite orbits: The same principles apply to artificial satellites, which are kept in orbit around Earth by Earth's gravity.

Please refer to the following for more information:

• Wikipedia: Earth's Orbit
• Wikipedia: Low Earth Orbit

- Earth's Orbit 

Earth orbits the Sun in an elliptical path, meaning it's not a perfect circle, due to the force of gravity, completing one orbit around the Sun in approximately 365.256 days (a year), while traveling a distance of 940 million kilometers at an average speed of 29.78 kilometers per second; when viewed from the Northern Hemisphere, Earth's orbit appears counterclockwise. 

Key characteristics about Earth's orbit:

• Shape: Earth's orbit is not a perfect circle but an ellipse, with the Sun at one of the foci of the ellipse.
• Duration: One complete Earth orbit around the Sun takes 365.256 days, which is considered a year.
• Distance: The distance between the Earth and the Sun varies throughout the year due to the elliptical nature of the orbit, with the closest point called perihelion and the farthest point called aphelion.
• Perspective: When viewed from above the Northern Hemisphere, Earth's orbit appears to be counterclockwise.
• Eccentricity: Earth's orbit has a small eccentricity of 0.0167, meaning it's very close to a perfect circle.
• Barycenter: The point around which Earth and the Sun orbit is called the barycenter, and it's not exactly located at the center of the Sun, but slightly closer to it.

- Geostationary Orbits and Applications

A geostationary orbit is a circular orbit over the Earth's equator where satellites match the planet's rotation, appearing stationary from the ground. 

This orbit is located at an altitude of 35,786 km above the equator and is used for weather and communication satellites because it allows for continuous observation of a specific region on Earth. 

1. Key characteristics:

• Altitude and distance: 35,786 kilometers above the equator, or 42,164 kilometers from the Earth's center.
• Orbital period: Matches Earth's rotation, completing one orbit in 23 hours, 56 minutes, and 4 seconds.
• Appears stationary: Because the satellite moves at the same speed as Earth, it remains fixed over a single point on the equator.

2. Applications:

• Communication: Satellites are used for television, radio, and internet services, as ground-based antennas can remain fixed on the satellite's position.
• Weather observation: Satellites like the Geostationary Operational Environmental Satellites (GOES) provide continuous coverage for weather monitoring and can aid in search and rescue operations.
• Navigation: They can enhance regional navigation systems and provide a constant calibration point.

3. Historical context:

• Concept: The idea was popularized by author Arthur C. Clarke in a 1945 paper.
• First satellite: The first satellite to enter this orbit was Syncom 3, launched on August 19, 1964.

- Low Earth Orbit (LEO) and LEO Satellites

Low Earth orbit (LEO) is the closest orbital range to Earth, with an altitude of 1,200 miles (2,000 km) or less. LEO orbits have an eccentricity of less than 0.25 and a period of 128 minutes or less. Satellites in LEO typically take 90 minutes to 2 hours to complete one orbit around Earth. 

LEO is the most common type of orbit and is considered close enough to Earth for transportation, communication, observation, and resupply. It's also the easiest orbit to reach in terms of rocket power and energy. However, LEO orbits have some disadvantages, including susceptibility to link failures and electromagnetic interference. 

LEO satellites orbit at altitudes ranging from 700 to 3,000 km, but most orbits are between 160 and 2,000 km due to atmospheric drag. The altitude of a LEO constellation is usually determined by the cost of placing satellites at a certain altitude, as well as other factors like: Radiation environment, Space debris, and Intended mission or market. 

Key features of LEO include:

• Orbital Period: Satellites in LEO typically complete one orbit in 90 minutes to 2 hours.
• Common Use: LEO is the most common type of orbit, used for transportation, communication, observation, and resupply.
• Advantages: It is the easiest orbit to reach in terms of rocket power and energy.
• Disadvantages: A major drawback is its susceptibility to link failures and electromagnetic interference.
• Altitude Factors: The specific altitude for a LEO constellation is determined by factors like cost, the radiation environment, the amount of space debris, and the mission's purpose.

- Medium Earth Orbit (MEO) and MEO Satellites

Medium Earth Orbit (MEO) is a region in space between 2,000 and 36,000 kilometers (1,243 ~ 22,300 miles) above Earth, used by satellites for navigation and communication. 

MEO satellites, also known as Intermediate Circular Orbit (ICO) satellites, are commonly used for GPS and other global navigation satellite systems (GNSS). They offer a balance between the higher coverage of geostationary orbits and the lower latency of low Earth orbits. 

1. Medium Earth Orbit (MEO):

• Altitude: Situated between low Earth orbit (LEO) and geostationary orbit (GEO), at an altitude of roughly 2,000 ~ 36,000 km.
• Orbital Period: Satellites in MEO orbit the Earth at least twice a day.
• Advantages: It requires fewer satellites for global coverage compared to LEO, and it is easier and less fuel-intensive to maintain than LEO. It also offers a balance between coverage and latency.
• Challenges: The MEO region includes parts of the Van Allen radiation belts, which pose a radiation risk to astronauts and satellites.

2. MEO Satellites:

• Primary use: Navigation systems like the Global Positioning System (GPS), Galileo, GLONASS, and BeiDou rely on MEO constellations for global positioning and timing services.
• Other uses: MEO is also used for high-bandwidth, low-latency telecommunications services, environmental monitoring, and data-intensive applications for governments.
• Protection: Satellites operating in MEO are equipped with special shielding to protect them from high radiation levels.

- Polar Orbits, Sun-synchronous Orbit (SSO), and Satellites

A polar orbit is an orbit where a satellite passes over or near the Earth's North and South Poles, while a sun-synchronous orbit (SSO) is a specific type of polar orbit that ensures the satellite passes over the same spot at the same local solar time each day. 

SSOs achieve this consistency by matching the Earth's rotation around the Sun, which is useful for consistent lighting and is used for applications like imaging, reconnaissance, and weather monitoring. 

1. Polar Orbit: An orbit with a high inclination (close to 90^∘) raised to the composed with power where a spacecraft travels from pole to pole on each revolution.

• Characteristics: Satellites in polar orbits can see the entire Earth over a period of time.
• Altitude: Typically in low Earth orbit (LEO), between 200 and 800 km.
• Uses: Weather tracking, atmospheric measurements, long-term Earth observation, and reconnaissance.

2. Sun-Synchronous Orbit (SSO): A specialized type of polar orbit where the satellite's orbital plane precesses (rotates) at the same rate as the Earth orbits the Sun.

• Characteristics: The satellite crosses the equator at the same local solar time each pass, providing consistent lighting for imaging.
• Altitude: Also typically in LEO, ranging from 200–800 km.
• Uses: Earth-observing satellites, spy satellites, and imaging satellites that require consistent solar illumination.

[Earth Orbit - Niko Lang]

- Transfer Orbits and Geostationary Transfer Orbit (GTO)

A transfer orbit is a special type of orbit that allows a satellite to move from one orbit to another. A geostationary transfer orbit (GTO) is a type of transfer orbit that is used to reach geosynchronous or geostationary orbit. 

When satellites are launched from Earth and carried to space with launch vehicles such as Ariane 5, the satellites are not always placed directly on their final orbit.

A GTO is a Hohmann transfer orbit, which is an elliptical orbit that transfers between two circular orbits of different radiuses in the same plane. The perigee (closest point to Earth) of a GTO is typically as high as low Earth orbit (LEO), while its apogee (furthest point from Earth) is as high as geostationary orbit.

Rockets often drop off their payloads in transfer orbits as halfway points en route to a satellite's final position. From transfer orbit, a satellite can use engine burns to change its inclination and circularize its orbit.

- Lagrange Points

Lagrange Points are positions in space where the gravitational forces of a two-body system like the Sun and Earth produce enhanced regions of attraction and repulsion. These can be used by spacecraft as "parking spots" in space to remain in a fixed position with minimal fuel consumption.

Lagrange points, also known as L-points or libration points, are positions in space where the gravitational forces of two large masses balance out the centripetal force of a smaller object. This means that small objects can remain in a fixed position at these points with minimal fuel consumption.

The five Lagrange points are L1, L2, L3, L4, and L5, and they are all located in the orbital plane of the two larger bodies. L1 and L2 are located on the Sun-Earth axis, L3 is located behind the Sun, opposite Earth, and L4 and L5 are located 60° ahead and behind Earth's orbit, respectively. 

Lagrange points are named after Joseph-Louis Lagrange, an 18th century Italian astronomer and mathematician who discovered them while studying the restricted three-body problem. The term "restricted" refers to the condition that two of the masses are much heavier than the third.

Spacecraft can use Lagrange points as "parking spots" in space, and space agencies often send satellites to L1 and L2 for scientific missions. For example, the James Webb Space Telescope (JWST) has been orbiting at L2 since January 2022, where it has a clear view of deep space.

L4 and L5 are permanently stable Lagrange points, so any objects that exist there will remain. Astronomers call these objects Trojan asteroids, and Jupiter has the most of them. NASA's Lucy mission launched in 2021 to explore Jupiter's Trojan asteroids and is expected to arrive at its first one in 2027.

[More to come ...]