SCIENCE • Form 5 • Chapter 9: Space Technology
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.
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 \(27\,500\ \text{km/h}\). A satellite in MEO at 20 200 km travels at \(13\,900\ \text{km/h}\). A satellite in HEO at 35 780 km travels at \(11\,100\ \text{km/h}\).
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.
Satellite orbit types and their orbital heights.
| Orbit type |
Orbital height |
| Low Earth Orbit (LEO) |
\(180\ \text{km} - 2\,000\ \text{km}\) |
| Medium Earth Orbit (MEO) |
\(2\,000\ \text{km} - 35\,780\ \text{km}\) |
| High Earth Orbit (HEO) |
\(\geq 35\,780\ \text{km}\) |
| Geosynchronous Orbit (GSO) |
\(35\,786\ \text{km}\), inclined orbital plane |
| Geostationary Orbit (GEO) |
\(35\,786\ \text{km}\), 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.
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.
Examples of orbital altitude and required satellite speed.
| Orbit |
Altitude |
Speed |
| LEO |
\(705\ \text{km}\) |
\(27\,500\ \text{km/h}\) |
| MEO |
\(20\,200\ \text{km}\) |
\(13\,900\ \text{km/h}\) |
| HEO |
\(35\,780\ \text{km}\) |
\(11\,100\ \text{km/h}\) |
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 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 (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:
\[\text{Period of orbit, } T = \frac{\text{Length of orbit}}{\text{Speed of satellite}} = \frac{2\pi \times (\text{Orbital height} + \text{Radius of Earth})}{\text{Speed of satellite}}\]
Earth's radius \(\approx 6.37 \times 10^6\ \text{m}\).
\[\text{Frequency of ISS orbiting Earth in a day} = \frac{86\,400\ \text{s}}{\text{Period of orbit, } T\ \text{(in seconds)}}\]
Mobile smartphone applications can track real-time positions and passes of the ISS relative to an observer's location.
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.
1
What is the key difference between a Geostationary Orbit (GEO) and a Geosynchronous Orbit (GSO)?
Answer: Both have an altitude of \(35\,786\ \text{km}\), but GEO's orbital plane is exactly along the equatorial plane, whereas GSO's orbital plane is inclined at an angle to the equatorial plane.
2
Why does a satellite in Low Earth Orbit (LEO) travel at a much higher speed than a satellite in High Earth Orbit (HEO)?
Answer: Gravitational force on a satellite is stronger at lower altitudes, requiring a higher speed to counterbalance gravity and remain in a stable orbit.
3
What is the difference between apogee and perigee in an elliptical orbit?
Answer: Apogee is the point in the orbit furthest from Earth, while perigee is the point closest to Earth.
- !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 \(35\,786\ \text{km}\).
- !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.
Misconception
Satellites stay in orbit because there is no gravity in space.
Correct Concept
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.
Misconception
An expendable launch vehicle (ELV) can be refuelled and launched again.
Correct Concept
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.
- Satellites are objects orbiting planets or stars, grouped into 5 orbit types: LEO (\(180 - 2\,000\ \text{km}\)), MEO (\(2\,000 - 35\,780\ \text{km}\)), HEO (\(\geq 35\,780\ \text{km}\)), GSO (\(35\,786\ \text{km}\), inclined), and GEO (\(35\,786\ \text{km}\), 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.
A. Objective Quiz
B. Fill in the Blanks
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.
Concept / Orbit
LEO
Perigee
Hohmann transfer orbit
Space junk
Description
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
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
Satellite
LEO
MEO
HEO
GEO
GSO
Apogee
Perigee
Hohmann Transfer Orbit
ELV
RLV
Zero Gravity
Space Junk
