9.1 Satellite
A satellite is an object that orbits a planet or star. While the Moon is Earth's natural satellite, numerous man-made satellites have been launched into orbit for various purposes such as communication, weather forecasting, and scientific research.
Learning Objectives
- Explain the five types of satellite orbits based on orbital height: LEO, MEO, HEO, GSO, and GEO.
- Describe the two orbital shapes and define apogee and perigee in an elliptical orbit.
- Relate satellite orbital height to orbital velocity and gravitational force.
- Describe the two methods of placing satellites into orbit and compare ELV with RLV.
- State the function of the ISS and explain the concept of zero gravity.
- Calculate the orbital period and daily orbital frequency of a space station.
- Discuss the impacts of rapid space technology development.
Types and Altitudes of Satellite Orbits
Satellite orbits are grouped by their height above Earth, from LEO close to the surface to HEO far beyond.
Short Explanation
Definition
A satellite is an object that orbits a planet or star.
Satellites orbit Earth at specific altitudes depending on their operational mission. Lower orbits experience stronger gravitational pull and require extremely high orbital velocities to avoid falling back to Earth. As altitude increases, the required orbital speed decreases.
Orbital height and speed. A satellite in LEO at 705 km travels at 27500 km/h. A satellite in MEO at 20 200 km travels at 13900 km/h. A satellite in HEO at 35 780 km travels at 11100 km/h.
Launch and placement. Launch vehicles place satellites either directly into orbit or via a Hohmann transfer orbit. Launch vehicles may be ELV or RLV.
ISS and space junk. The ISS advances human scientific knowledge. However, rapid orbital missions have increased space junk, raising collision risks in crowded orbits.
Five Types of Satellite Orbits
Orbits are grouped according to their height above Earth.
| Orbit type | Orbital height |
|---|---|
| Low Earth Orbit (LEO) | 180 km - 2000 km |
| Medium Earth Orbit (MEO) | 2000 km - 35780 km |
| High Earth Orbit (HEO) | geq 35780 km |
| Geosynchronous Orbit (GSO) | 35786 km, inclined orbital plane |
| Geostationary Orbit (GEO) | 35786 km, equatorial orbital plane |
Orbital Shapes
Satellites follow one of two orbital shapes:
- Perfectly circular orbit: Example: GEO.
- Elliptical orbit: Examples: MEO and HEO.
LEO and GSO can be either perfectly circular or elliptical.
Apogee and Perigee
For satellites in an elliptical orbit, there are two distinct positions relative to Earth:
- Apogee: the position furthest from the planet or star.
- Perigee: the position closest to the planet or star.
Orbital Height and Satellite Velocity
Higher orbits need lower speed because gravity is weaker.
The higher the orbital height of a satellite, the lower the satellite speed required to remain in orbit. This is because the gravitational force exerted by Earth on a satellite decreases as orbital height increases.
| Orbit | Altitude | Speed |
|---|---|---|
| LEO | 705 km | 27500 km/h |
| MEO | 20200 km | 13900 km/h |
| HEO | 35780 km | 11100 km/h |
If a satellite's speed decreases too much, gravity pulls it down toward Earth. If its speed increases too much, it escapes into a higher orbit or out of orbit.
Launch and Placement of Satellites
Rockets send satellites into space using two primary methods.
Launch vehicles, made of one or more rockets, send satellites into space via two primary methods:
Directly into orbit
The rocket follows a trajectory directly placing the payload into its operational orbit.
Hohmann transfer orbit
An elliptical trajectory used to move a spacecraft from a low circular orbit to a higher orbit. It involves engine burning at perigee to enter the transfer orbit, and burning at apogee to enter the final larger orbit.
Types of Launch Vehicles
Expendable Launch Vehicle (ELV)
A launch vehicle used only once; parts fall away or are expended.
Reusable Launch Vehicle (RLV)
A launch vehicle designed to be recovered and reused for multiple launches.
The International Space Station and Zero Gravity
The ISS is an international microgravity research platform.
The International Space Station (ISS) was built through the joint efforts of five space agencies: NASA (USA), Roscosmos (Russia), JAXA (Japan), ESA (Europe), and CSA (Canada).
Dato’ Dr. Sheikh Muszaphar Shukor Al Masrie bin Sheikh Mustapha was the first Malaysian to reach the ISS.
Zero Gravity
Zero gravity is a condition where no significant gravitational force effect is felt. In indoor skydiving, floatation occurs because the upward air thrust equals body weight in the opposite direction, cancelling the sensation of gravity. Similarly, astronauts inside the orbiting ISS experience apparent weightlessness, a free-fall condition.
Tracking Space Stations
The orbital period T and frequency can be calculated:
Period of orbit, T = Length of orbit / Speed of satellite = 2π × (Orbital height + Radius of Earth) / Speed of satellite
Earth's radius ≈ 6.37 × 10⁶ m.
Frequency of ISS orbiting Earth in a day = 86 400 s / Period of orbit, T (in seconds)
Mobile smartphone applications can track real-time positions and passes of the ISS relative to an observer's location.
Impact of Rapid Development in Space Technology
Rapid space development brings both challenges and opportunities.
Increased space waste
Space junk includes non-functioning satellites, discarded ELV stages, rocket casings, and collision fragments. NASA reports that 95% of thousands of man-made objects in LEO are space junk. Active satellites like the GOES weather satellite must periodically adjust their orbits to avoid catastrophic collisions.
Growth in R&D activities
Technological advancements have spurred research in human health, climate change monitoring, disaster response, telecommunications, global education, and the space economy.
Try to Answer First
Answer in your mind, then press “Check Answer”.
What is the key difference between a Geostationary Orbit (GEO) and a Geosynchronous Orbit (GSO)?
Why does a satellite in Low Earth Orbit (LEO) travel at a much higher speed than a satellite in High Earth Orbit (HEO)?
What is the difference between apogee and perigee in an elliptical orbit?
Common Mistakes
- Confusing GEO with GSO. GEO is strictly on the equatorial plane, while GSO is inclined at an angle, even though both share the exact altitude of 35786 km.
- Inverting the relationship between altitude and orbital speed. Satellites higher up move slower, not faster, because Earth's gravitational pull is weaker at higher altitudes.
- Mixing up apogee and perigee. Apogee is the furthest position; perigee is the closest position.
- Misunderstanding zero gravity. It does not mean gravity does not exist in orbit; it means no significant gravitational force effect is felt, a free-fall state.
Concept Misunderstandings
Satellites stay in orbit because there is no gravity in space.
Gravity is present and significant in space. Satellites remain in orbit because their forward horizontal velocity creates a centripetal balance where they constantly fall around Earth's curve without hitting the ground.
An expendable launch vehicle (ELV) can be refuelled and launched again.
ELVs are used only once; their stages are discarded, burned up, or left in orbit. Only reusable launch vehicles (RLVs) can be recovered and flown again.
Summary
- Satellites are objects orbiting planets or stars, grouped into 5 orbit types: LEO (180 - 2000 km), MEO (2000 - 35780 km), HEO (geq 35780 km), GSO (35786 km, inclined), and GEO (35786 km, equatorial).
- Orbits are circular (GEO) or elliptical (MEO, HEO; LEO/GSO can be either). Elliptical orbits feature an apogee (furthest point) and a perigee (closest point).
- Orbital velocity decreases as orbital height increases due to the reduction in gravitational force.
- Satellite placement is done directly or via a Hohmann transfer orbit using ELV (single-use) or RLV (reusable) launch vehicles.
- The ISS serves as an international microgravity research platform; its orbital period and daily passes can be mathematically determined.
- Space junk poses collision hazards to operational satellites, necessitating orbital avoidance maneuvers.
Short Activity
A. Objective Quiz
1 Which satellite orbit has an orbital height of 35786 km with an orbital plane lying directly on the equatorial plane?
2 What happens to the required speed of a satellite as its orbital altitude increases?
B. Fill in the Blanks
3 In an elliptical orbit, the position of a satellite that is furthest from Earth is known as the .
4 A launch vehicle that can be recovered and reused for multiple space missions is called a launch vehicle.
C. Drag and Drop: Match the Concept to Its Description
Drag each concept to the correct description. On a phone, tap an answer first, then tap a matching box. Tap the Answer Choices area to return a selected item.
C. Match the Concept to Its Description
Write the correct concept next to each description.
| Description | Concept / Orbit |
|---|---|
| 1. Orbit with an altitude of 180 km to 2 000 km | |
| 2. Position in an elliptical orbit closest to Earth | |
| 3. Elliptical orbit used to transfer a satellite from a low orbit to a higher orbit | |
| 4. Non-functioning satellites, exhausted rockets, and collision fragments |
Answer choices: LEO • Perigee • Hohmann transfer orbit • Space junk
