Keeping Time with the Skies · Lesson 6 of 7
Artificial Satellites and Their Uses
“Explore human-made objects in orbit, their everyday uses, and the challenge of space debris.”
• Distinguish natural satellites from artificial satellites and other space missions. • Explain why some artificial satellites are visible as moving points near twilight. • Connect satellite functions with communication, navigation, observation, and research. • Interpret an example orbital period without applying it to all satellites. • Describe Indian space contributions and student participation. • Explain how debris and collisions make careful space use necessary.
Another Kind of Moving Point
During an evening sky observation, you may notice a point of light moving against the background stars. It may be an artificial satellite reflecting sunlight, although an aircraft or another object can also look like a moving light. A careful observation should describe what is seen before deciding what it is.
An object that revolves around another, larger body. The Moon is Earth’s natural satellite; a human-made object placed in orbit is an artificial satellite.
Artificial satellites are designed and launched for particular tasks. An orbit is the path followed as an object revolves around a body in space. Earth-orbiting satellites continue moving around Earth rather than remaining at the place from which they were launched. Their instruments and communication systems can then observe or serve large regions.
A satellite’s orbital height and purpose affect its path and the time it takes to complete a revolution. The chapter gives an example of a satellite around 800 kilometres above Earth taking about 100 minutes per orbit. Treat those values as an example for that kind of orbit: artificial satellites have many different heights and periods. An Earth-observation satellite, a communications satellite, and a mission travelling toward another world need not have the same path.
The time taken by an orbiting object to complete one revolution.
Problem
In a simplified model, a satellite completes one orbit every 100 minutes. How many complete orbits occur in 500 minutes?
- 1.The orbital period is the duration of one complete revolution: 100 minutes.
- 2.Divide the total duration by the period: 500 ÷ 100 = 5.
- 3.The model satellite completes five orbits. This calculation uses the stated model period, not a universal period for all satellites.
Why Twilight Can Reveal a Satellite
A satellite seen from the ground is usually visible because sunlight reflects from it. After sunset, you may be in Earth’s dark region while a satellite high above you still receives sunlight. The reflected light can stand out against the darkening sky, making twilight a useful observation time.
A similar situation can occur before sunrise. As the satellite travels into Earth’s shadow, it may fade from view even though it continues along its orbit. Its changing angle can also change the reflected brightness. This helps explain why a moving point can appear, brighten, fade, or disappear without switching on a lamp for the observer.
Problem
A moving point is bright after sunset, then fades while following a smooth path. Could this still be a satellite?
- 1.A satellite may reflect sunlight while the ground observer is already in darkness.
- 2.When it enters Earth’s shadow, that source of reflected light is reduced or removed.
- 3.Fading is therefore consistent with a satellite, but the sighting should be checked against a reliable local prediction before identifying a particular object.
An Observation with a Purpose
The satellite-watching investigation asks you to distinguish movement and visibility from an unsupported guess. Choose a clear sky and an open horizon just before sunrise or after sunset, with a teacher or guardian. Begin with your eyes; binoculars may help with some sightings but are not required to notice a bright pass.
Look for points moving steadily relative to surrounding stars. Record the date, place, start and end times, approximate direction, and changes in brightness. Aircraft may show blinking lights, but brightness behaviour alone is not a perfect identification rule. A trusted satellite-pass prediction or astronomy application can help check whether a pass at that place and time matches your record.
The observation is a changing position against the star background. The interpretation, when supported by a matching prediction, is that an artificial satellite is travelling through a visible part of its orbit. If no satellite is seen, possible reasons include clouds, an unsuitable pass, low brightness, or the satellite being in shadow. Lack of a sighting does not mean that there are no satellites above the region.
What Satellites Do for Us
A satellite’s orbit gives it a useful viewpoint or communication route. Its instruments and equipment are chosen for the job: a camera gathers Earth images, while other systems receive and transmit signals or measure radiation from space. We can understand its purpose by linking its equipment and position to the problem it helps solve.
| Use | How the satellite helps | A familiar application |
|---|---|---|
| Communication | Receives and relays signals across large distances | Television, telephone links, and data connections |
| Navigation | Provides precisely timed signals that receivers use to determine position | Finding a location or following a route |
| Weather observation | Monitors clouds and weather systems across broad areas | Forecasting and following a developing storm |
| Disaster management | Supplies repeated images and measurements of affected regions | Assessing flooding, damage, and changing conditions |
| Mapping and resource studies | Records surface features, land use, soils, and vegetation | Planning cities and studying terrain |
| Scientific research | Measures objects and radiation from beyond the atmosphere | Studying stars and other celestial sources |
For example, mapping a large city entirely from street level would take many separate observations. Repeated satellite images show the region together, making changes in roads, construction, or flood-water extent easier to compare. Weather satellites similarly show a cloud system over a large area, rather than just the weather outside one building. The benefit comes from the wide view and repeated observations, not from the claim that every satellite sees every place at every moment.
Problem
A team needs to assess which parts of a city are flooded and how water coverage changes over several days. Which satellite function is most useful?
- 1.The team needs repeated observations of Earth’s surface over a large area.
- 2.Earth-observation and mapping images can show the extent of flooding and changes between dates.
- 3.Navigation could locate a rescue team and communication could connect teams, but neither alone replaces the requested surface images. Several satellite functions can support the same disaster response.
Indian Space Work and Different Missions
India’s space programme has developed satellite applications and scientific missions for many purposes. Vikram Sarabhai, who lived from 1919 to 1971, helped establish that programme and is widely called its father. The Vikram Sarabhai Space Centre in Thiruvananthapuram bears his name and works on launch-vehicle technology.
Cartosat satellites provide detailed Earth images useful for mapping, city planning, and disaster studies. Bhuvan is a platform through which users can explore Indian geospatial information, including terrain, land use, soil, and vegetation information. Bhuvan is a way of accessing and using data; it is not itself a satellite orbiting Earth.
AstroSat is a space observatory used to study stars and other celestial objects. Other Indian mission examples include Chandrayaan-1, Chandrayaan-2, and Chandrayaan-3 for exploring the Moon; Aditya-L1 for studying the Sun; and Mangalyaan for exploring Mars. They demonstrate different destinations and tasks. Do not classify all these missions as artificial satellites currently revolving around Earth: some include orbiters elsewhere, landers, or other kinds of spacecraft.
Students have also contributed to space projects, including projects associated with AzaadiSat, INSPIREsat-1, and Jugnu. Such participation can involve designing instruments, building parts, analysing signals, and working with scientific teams. Mentioning a project does not imply that every launch attempt succeeded or that all projects had identical designs. Their educational significance is that students can contribute to real technical investigations.
A natural Moon, an Earth-orbiting artificial satellite, a lunar lander, and a mission exploring Mars have different roles and paths. Identify what revolves around what before using the word satellite.
When Useful Objects Become Debris
Satellites and rockets cannot be treated as if space were an unlimited rubbish bin. Some inactive satellites, spent rocket stages, and fragments remain in orbit. These objects may no longer serve a purpose, yet they still move and can threaten working spacecraft.
Human-made objects or fragments in space that no longer perform a useful function, including inactive satellites and discarded rocket parts.
A collision can damage a working satellite and create further fragments. That makes debris a shared problem for future missions as well as present ones. When objects re-enter the atmosphere, many small pieces burn up, but not every object is guaranteed to disappear completely; some larger pieces can survive to lower levels or the surface.
Reducing debris requires responsible mission planning, tracking objects, avoiding collisions where possible, and arranging safe disposal at the end of a mission. International cooperation matters because objects in orbit are not confined above the country that launched them. The same sky that supplies natural time references also contains human technology whose long-term use requires care.
Quiz
Which object is a natural satellite of Earth?
Why can a satellite be visible just after sunset?
Which is the most direct use of an Earth-observation satellite after a flood?
Which statement correctly describes Bhuvan?
What is space debris?
What should you infer from the example of an 800-kilometre orbit with a roughly 100-minute period?
Practice Problems
- Distinguish the Moon, an Earth-observation satellite, and a lunar lander by origin, role, and path.
- Explain how a satellite can reflect sunlight after the observer’s local sunset and why it may later fade.
- For the simplified 100-minute orbital period, calculate the time for three complete orbits and explain the limitation of the model.
- Describe the satellite-sighting record you would keep. Explain how you would check an identification.
- Choose a communication, navigation, weather, mapping, or research task and explain how a satellite helps.
- Compare the purposes of Cartosat and AstroSat, and explain what Bhuvan provides.
- Identify the destinations of Chandrayaan, Aditya-L1, and Mangalyaan without calling every mission an Earth satellite.
- Explain how a debris collision can create additional hazards and why end-of-mission planning and cooperation matter.
Key Takeaways
• The Moon is natural; artificial satellites are human-made objects placed in orbit. • Different satellite missions have different orbital heights, periods, equipment, and purposes. • A sun-lit satellite can be visible near twilight while an observer is already in darkness. • Satellite applications include communication, navigation, weather, mapping, disaster management, and research. • Cartosat supports Earth mapping, AstroSat supports astronomy, and Bhuvan provides access to geospatial information. • Indian missions explore Earth, the Moon, the Sun, and Mars; their spacecraft do not all follow Earth orbits. • Inactive objects and fragments form space debris, creating collision and re-entry concerns that need responsible planning.