What defines a low Earth orbit (LEO) in terms of orbital period and eccentricity?

A low Earth orbit (LEO) is characterized by an orbital period of 128 minutes or less, meaning a satellite completes at least 11.25 orbits per day. Additionally, it must have an orbital eccentricity of less than 0.25.

What is the typical altitude range for Low Earth Orbit (LEO), and where is the density of artificial objects highest?

The LEO region is generally considered to be the space below an altitude of 2,000 kilometers (about one-third of Earth's radius). The majority of artificial objects in space are found in LEO, with the highest concentration typically occurring around an altitude of 800 kilometers (500 miles).

How does the altitude of an object in LEO relate to the radius of the Earth?

The upper limit of LEO, before transitioning to medium Earth orbit (MEO), is at an altitude of 2,000 kilometers. This altitude is approximately one-third of the Earth's radius.

What is the significance of the LEO region for tracking space objects, even those not strictly in LEO orbits?

The LEO region, defined as the space below 2,000 km altitude, is crucial for tracking. Objects that pass through this zone, even if they have higher apogees or are sub-orbital, must be monitored due to the collision risk they pose to satellites and other objects within LEO.

Which human spaceflights have historically ventured beyond Low Earth Orbit?

Historically, only the lunar missions of the Apollo program, which took place between 1968 and 1972, have traveled beyond LEO. The Artemis II mission is also planned to go beyond LEO in early 2026.

Where have all space stations operated to date been located?

All space stations launched and operated thus far have been situated within Low Earth Orbit (LEO), orbiting the Earth.

While definitions vary, how does the orbital period of 128 minutes relate to the semi-major axis and altitude in LEO?

According to Kepler's third law, an orbital period of 128 minutes corresponds to a semi-major axis of approximately 8,413 kilometers. For circular orbits, this translates to an altitude of about 2,042 kilometers above Earth's mean radius, aligning with some upper altitude limits defined for LEO.

What factors can cause the altitude of an object in orbit to vary, even in seemingly circular orbits?

The altitude of an orbiting object can vary due to several factors. These include the oblateness of Earth's spheroid shape, local topography, and for polar orbits, the variation in altitude above ground can be as much as 30 km.

What is the distinction between a LEO orbit and the LEO region?

A LEO orbit refers to an object's specific path around the Earth that meets the criteria for LEO (period < 128 min, eccentricity < 0.25). The LEO region, however, is a broader zone in space extending up to 2,000 km altitude. This distinction is important because objects not in a LEO orbit (like sub-orbital craft or objects with high apogees) can still enter the LEO region and pose a collision risk.

What is the approximate mean orbital velocity required to maintain a stable Low Earth Orbit?

The mean orbital velocity needed to maintain a stable Low Earth Orbit is approximately 7.8 kilometers per second, which is equivalent to about 28,000 kilometers per hour.

How does the orbital velocity of a satellite change with its altitude within LEO?

Orbital velocity decreases as altitude increases within LEO. For example, at an altitude of 200 km, the velocity is about 7.79 km/s, while at a higher altitude of 1,500 km, it reduces to approximately 7.12 km/s.

What is the typical delta-v requirement for a launch vehicle to achieve Low Earth Orbit?

The delta-v, which represents the change in velocity a rocket must achieve, needed to reach Low Earth Orbit typically starts around 9.4 kilometers per second.

How does the force of gravity in LEO compare to that on Earth's surface, and why do objects remain in orbit?

The pull of gravity in LEO is only slightly less than on the Earth's surface because the altitude is still relatively small compared to the Earth's radius. Objects stay in orbit because they are in a continuous state of free fall, where the gravitational force is balanced by the centrifugal force (a fictitious force in the rotating frame of reference).

What causes weightlessness experienced by astronauts in LEO?

Weightlessness, or microgravity, is experienced in LEO because objects and people within a spacecraft are in a constant state of free fall around the Earth. The gravitational pull and the orbital motion continuously balance each other, preventing objects from falling 'down' relative to the spacecraft.

What atmospheric layers do objects in LEO interact with, and what is the consequence for satellites at lower altitudes?

Objects in LEO encounter atmospheric drag from gases in the thermosphere (roughly 80-600 km) or the exosphere (above 600 km), depending on their altitude. Satellites orbiting below approximately 300 km experience significant atmospheric drag, leading to rapid orbital decay.

What are Equatorial Low Earth Orbits (ELEO), and what are their advantages?

Equatorial Low Earth Orbits (ELEO) are a subset of LEO with very low orbital inclination relative to the equator. They offer advantages such as rapid revisit times over low-latitude regions and lower launch energy requirements due to leveraging Earth's rotation.

How do polar and Sun-synchronous orbits differ from ELEO in terms of coverage?

Polar orbits and Sun-synchronous orbits, unlike ELEO, have higher inclinations to the equator. This allows them to provide coverage over higher latitudes on Earth, whereas ELEO is optimized for low-latitude regions.

What are the potential risks associated with orbits higher than LEO compared to LEO itself?

Orbits situated higher than Low Earth Orbit can expose electronic components to more intense radiation. This increased radiation can lead to component failure and charge accumulation issues over time.

What are Very Low Earth Orbits (VLEO), and what technological challenges do they present?

Very Low Earth Orbits (VLEO) are orbits below approximately 450 kilometers (280 miles). They present a challenge because they operate in a region where atmospheric drag would normally cause satellites to decay too quickly for practical use, necessitating advanced technologies for orbit raising and maintenance.

What are the primary advantages of using Low Earth Orbit for satellites and communication?

LEO requires the least amount of energy for satellite placement and offers high bandwidth with low communication latency. Satellites in LEO are also more accessible for crewed servicing missions. These factors make LEO suitable for many communication applications, such as the Iridium phone system.

What is a significant disadvantage of LEO satellites concerning continuous global coverage?

Satellites in LEO have a relatively small field of view, meaning they can only observe or communicate with a fraction of the Earth at any given time. Consequently, providing continuous global coverage requires a large network or constellation of many satellites.

What is the primary challenge for satellites in lower LEO altitudes, and how is it addressed?

Satellites in lower LEO altitudes experience significant atmospheric drag, causing their orbits to decay rapidly. To counteract this, they require periodic re-boosting maneuvers to maintain their altitude or must be replaced when they re-enter the atmosphere.

What is the approximate altitude of the International Space Station (ISS) and how does its orbit decay?

The International Space Station (ISS) orbits at an altitude of approximately 400 to 420 kilometers (250 to 260 miles) above the Earth's surface. Its orbit decays by about 2 kilometers per month, necessitating re-boosting maneuvers a few times each year to maintain its altitude.

At what altitude do the Iridium telecom satellites typically orbit?

The Iridium satellite constellation operates in Low Earth Orbit at an altitude of approximately 780 kilometers (480 miles).

Why are Earth observation and remote sensing satellites often placed in LEO?

Earth observation and remote sensing satellites, including spy and imaging satellites, are frequently placed in LEO because their proximity to the Earth's surface allows them to capture clearer images and data of the planet.

What is the altitude of the Hubble Space Telescope?

The Hubble Space Telescope orbits the Earth at an altitude of approximately 540 kilometers (340 miles).

Name two examples of satellite internet constellations that utilize Low Earth Orbit.

Examples of satellite internet constellations that use Low Earth Orbit include Starlink and OneWeb (though OneWeb is not explicitly mentioned in the text, Starlink is).

What is the altitude range of the Chinese Tiangong space station?

The Chinese Tiangong space station orbits the Earth at an altitude between 340 and 450 kilometers (210 and 280 miles).

What was the orbital altitude of the former ESA gravimetry mission GOCE?

The European Space Agency's (ESA) gravimetry mission GOCE orbited at a very low altitude of approximately 255 kilometers (158 miles) during its operational period from 2009 to 2013.

What was the record-setting low altitude achieved by the Japanese satellite Tsubame?

The Japanese Super Low Altitude Test Satellite, nicknamed Tsubame, orbited at an extremely low altitude of 167.4 kilometers (104.0 miles), setting a record for the lowest altitude achieved by an Earth observation satellite.

How was Low Earth Orbit depicted in the film *2001: A Space Odyssey*?

In the film *2001: A Space Odyssey*, the fictional Earth transit station, referred to as 'Space Station V', was depicted as orbiting approximately 300 kilometers above the Earth.

What is the primary concern regarding the increasing congestion of Low Earth Orbit?

The increasing congestion of LEO with space debris is a major concern because collisions between objects at orbital velocities can be extremely dangerous and potentially catastrophic, leading to the creation of even more debris.

What is Kessler syndrome?

Kessler syndrome is a theoretical scenario where the density of objects in Low Earth Orbit becomes so high that collisions between objects generate cascading debris. This debris then increases the probability of further collisions, potentially rendering LEO unusable for extended periods.

According to NASA's Orbital Debris Program, how many objects larger than 10 cm are tracked in LEO?

NASA's Orbital Debris Program tracks over 25,000 objects larger than 10 centimeters in diameter within Low Earth Orbit.

What are the estimated numbers of space debris particles in LEO between 1 and 10 cm, and larger than 1 mm?

Beyond the tracked objects larger than 10 cm, it is estimated that there are 500,000 particles of space debris in LEO between 1 and 10 cm in size. The number of particles larger than 1 mm is estimated to exceed 100 million.

Why are even small impacts from space debris dangerous in LEO?

Even small impacts from space debris are dangerous in LEO because the debris travels at extremely high speeds, up to 7.8 km/s (approximately 28,000 km/h or 17,500 mph). At these velocities, even a tiny particle can possess enough kinetic energy to cause severe damage to a spacecraft.

What is the significance of the boundary between Earth and outer space in relation to LEO?

The boundary between Earth and outer space, often considered to be around 100 km altitude (the Kármán line), lies within the lower reaches of Low Earth Orbit. Objects flying at orbital velocities within LEO are essentially in a continuous state of free fall around the Earth.

What is the definition of 'free fall' as it applies to objects in orbit?

In the context of orbital mechanics, 'free fall' means that gravity is the only significant force acting on an object. While an object in orbit is constantly being pulled by gravity, its sideways velocity prevents it from hitting the Earth, resulting in a continuous state of falling around the planet.

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Defining LEO

Orbital Period

A Low Earth Orbit (LEO) is characterized by an orbital period of 128 minutes or less, enabling a satellite to complete at least 11.25 orbits per day. The orbital eccentricity must be less than 0.25.

Altitude Range

The LEO region is generally defined as the space extending from Earth's surface up to an altitude of 2,000 kilometers (approximately 1,200 miles). Most artificial objects in space currently reside within this zone, with peak density around 800 km (500 miles).

Human Spaceflight Boundary

LEO represents the primary domain for human space exploration beyond Earth's atmosphere. With the exception of the Apollo lunar missions and the planned Artemis II, all human spaceflights have occurred within LEO. All space stations to date have operated within this orbital region.

Orbital Characteristics

Velocity and Energy

Maintaining a stable LEO requires a mean orbital velocity of approximately 7.8 km/s (about 17,500 mph). This speed is crucial for balancing Earth's gravitational pull. Consequently, LEO requires the least amount of energy for satellite deployment compared to higher orbits.

Gravity and Weightlessness

Despite the lower altitude, the force of gravity in LEO is only slightly diminished compared to Earth's surface. Objects in orbit are in a continuous state of free fall, where gravitational force is balanced by the orbital velocity, resulting in the phenomenon of weightlessness experienced by astronauts and equipment.

Atmospheric Drag

Satellites in LEO, particularly those below 300 km (190 miles), encounter atmospheric drag from the upper layers of Earth's atmosphere (thermosphere and exosphere). This drag causes orbital decay, necessitating periodic re-boosting maneuvers to maintain altitude or eventual re-entry.

LEO Orbit Types

Equatorial Orbits

Equatorial Low Earth Orbits (ELEO) are characterized by a low inclination relative to the Earth's equator. These orbits facilitate rapid revisit times over equatorial regions and benefit from lower launch energy requirements due to Earth's rotation.

Polar and Sun-Synchronous Orbits

Orbits with higher inclinations, such as polar orbits, provide comprehensive coverage of the entire Earth's surface, including higher latitudes. Sun-synchronous orbits, a specific type of polar orbit typically around 800 km, ensure that a satellite passes over any given point on Earth at the same local solar time, ideal for consistent imaging.

Very Low Earth Orbits (VLEO)

Emerging orbits below approximately 450 km (280 miles) are termed Very Low Earth Orbits (VLEO). These orbits experience significant atmospheric drag, requiring advanced technologies for orbit-raising and station-keeping to counteract rapid decay and maintain operational viability.

Applications of LEO

Communications

LEO is utilized for various communication applications, including satellite phone systems like Iridium. The proximity of LEO satellites offers high bandwidth and low communication latency, making them suitable for real-time data transmission.

Earth Observation

Satellites in LEO, such as Earth observation, remote sensing, and imaging satellites (e.g., Hubble Space Telescope, GRACE-FO), benefit from their closeness to the planet, enabling clearer views and detailed data collection of Earth's surface and atmosphere.

Space Stations

LEO provides a stable environment for long-duration human presence in space. Major space stations, including the International Space Station (ISS) and the Tiangong space station, are situated in LEO, facilitating crewed missions and scientific research.

Challenges in LEO

Orbital Decay

The persistent atmospheric drag in lower LEO altitudes causes gradual orbital decay. Satellites require regular re-boosting maneuvers, consuming valuable propellant, or must be replaced when they eventually re-enter Earth's atmosphere. The long-term effects of vaporized metals from decaying satellites on the stratosphere are still under investigation.

Limited Coverage

Unlike geostationary satellites, LEO satellites have a relatively small field of view. This necessitates the deployment of large constellations of satellites to provide continuous global coverage for communication or observation services.

Space Debris

The increasing number of object launches has led to significant congestion in LEO. This space debris poses a severe collision risk to operational satellites and spacecraft. NASA tracks tens of thousands of objects larger than 10 cm, with millions more smaller particles. Collisions at orbital velocities can generate further debris, potentially triggering a cascading effect known as Kessler Syndrome.

Notable LEO Examples

Space Stations

International Space Station (ISS): Orbits at approximately 400-420 km altitude, requiring periodic re-boosting.
Tiangong Space Station: Operates between 340-450 km altitude.

Satellite Constellations

Iridium: A constellation of communication satellites orbiting at about 780 km.
Starlink: Utilizes numerous satellites in LEO for internet services, with varying orbital inclinations.
Earth Observation Satellites: Including remote sensing and imaging platforms like Hubble Space Telescope (540 km) and GRACE-FO (500 km).

Past Missions

GOCE: An ESA gravimetry mission that orbited at a very low altitude of about 255 km.
Super Low Altitude Test Satellite (Tsubame): Held the record for the lowest Earth observation satellite orbit at approximately 167 km.

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References

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This content has been generated by an Artificial Intelligence and is intended for informational and educational purposes only. It is based on data derived from publicly available sources, including Wikipedia, and may not reflect the most current information or nuances of the subject matter.

This is not professional advice. The information provided herein does not constitute expert consultation in fields such as aerospace engineering, physics, or astronomy. Users should consult official documentation and qualified professionals for specific applications or inquiries related to orbital mechanics, spaceflight, or satellite operations. Reliance on any information provided here is solely at your own risk.

The creators of this page are not responsible for any inaccuracies, omissions, or consequences arising from the use of this information.

LEO: The Cosmic Threshold

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Defining LEO 📍

⏱️ Orbital Period

⬆️ Altitude Range

🧑‍🚀 Human Spaceflight Boundary

Orbital Characteristics ⚙️

⚡ Velocity and Energy

⚖️ Gravity and Weightlessness

💨 Atmospheric Drag

LEO Orbit Types 🌌

🌍 Equatorial Orbits

🧭 Polar and Sun-Synchronous Orbits

🔬 Very Low Earth Orbits (VLEO)

Applications of LEO 🛰️

📡 Communications

📸 Earth Observation

🏠 Space Stations

Challenges in LEO ⚠️

📉 Orbital Decay

🔭 Limited Coverage

💥 Space Debris

Notable LEO Examples 🌟

🏢 Space Stations

📡 Satellite Constellations

⏳ Past Missions

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ℹ️ Important Notice

Dive in with Flashcard Learning!

Defining LEO

Orbital Period

Altitude Range

Human Spaceflight Boundary

Orbital Characteristics

Velocity and Energy

Gravity and Weightlessness

Atmospheric Drag

LEO Orbit Types

Equatorial Orbits

Polar and Sun-Synchronous Orbits

Very Low Earth Orbits (VLEO)

Applications of LEO

Communications

Earth Observation

Space Stations

Challenges in LEO

Orbital Decay

Limited Coverage

Space Debris

Notable LEO Examples

Space Stations

Satellite Constellations

Past Missions

Teacher's Corner

True or False?

Test Your Knowledge!

Gamer's Corner

Discover other topics to study!

References

Feedback & Support

Disclaimer

Important Notice