What is the 'subject' of a satellite?

When we talk about satellites, we often focus on what they do — communicate, observe, measure, or navigate. But a lesser-known and equally important concept is the “subject” of a satellite. So, what exactly is the subject?

🎯 The Subject: What the Satellite Interacts With

In simple terms, the subject is whatever the satellite is sensing, observing, or communicating with. It’s the entity that the satellite’s payload is designed to serve, study, or engage.

Let’s explore a few examples to understand this better:

📺 For Communication Satellites

If your TV works through a dish installed on your rooftop, then your dish is the subject. The satellite is in geostationary orbit, continuously beaming signals down, and your dish is tuned to receive them.

Similarly, for internet satellites, the subjects might be:

• 🛩️ An antenna mounted on an airplane providing in-flight Wi-Fi.

• 🚢 A large dome-shaped antenna on a ship offering internet access in the middle of the ocean.

• 📶 A rural broadband receiver installed on a home far from city infrastructure.

🌦️ For Weather Satellites

Satellites used in weather forecasting might have multiple subjects:

• Temperature of different atmospheric layers.

• Humidity or moisture content.

• Wind patterns or cyclone formation.

These satellites continuously monitor Earth’s dynamic weather systems, helping us forecast conditions that affect millions.

🌍 For Earth Observation Satellites

Earth observation satellites have a wide range of subjects:

• Air quality – pollutants like NO₂ or PM2.5.

• Fire incidents – identifying and tracking wildfires in real time.

• Soil moisture – vital for agriculture and drought monitoring.

• Water levels – satellites can detect surface water and even underground aquifers!

Government agencies and private companies now use satellite data for environmental monitoring, agriculture, urban planning, and more.

🌌 For Deep Space Missions

Subjects aren't always on Earth. Many satellites are designed to study other celestial bodies.

• The Moon has been orbited by over 60 spacecrafts to study its surface and composition.

• Mars, Venus, and Jupiter have also had dedicated orbiters collecting atmospheric and geological data.

• Even small bodies like asteroids and comets have been studied — with missions like Rosetta and OSIRIS-REx.

In these cases, the subject is the planet or object the spacecraft is orbiting or flying past.

🧭 Why This Concept Matters

Understanding the subject helps clarify the purpose of the satellite and what kind of data it generates. It also shows how satellite design depends heavily on who or what the satellite is “looking at.”

A satellite supporting maritime internet needs to account for moving ships with special antennas, while a satellite imaging forest health needs high-resolution optical sensors. Everything — from orbit design to power needs — is influenced by the nature of the subject.

Final Thought

The subject gives meaning to a satellite’s existence. It’s the reason the satellite was built in the first place. Whether it’s your TV dish, a wildfire in California, or the mysterious surface of Europa — the subject is the silent focus of every space mission.

✅ TL;DR (Too Long; Didn’t Read)

The subject of a satellite is the entity it interacts with or observes.

• For communication satellites: it could be your TV dish or an airplane antenna.

• For weather satellites: temperature, humidity, or storms.

• For Earth observation: air quality, soil moisture, or fire events.

• For deep space missions: planets, moons, or asteroids.

Understanding the subject helps define what the satellite does and how it’s designed.

About the image: Cooper Creek, as seen by the International Space Station

Image Credit: Earth Science and Remote Sensing Unit, NASA Johnson Space Center

Link to the image: https://eol.jsc.nasa.gov/SearchPhotos/photo.pl?mission=ISS062&roll=E&frame=136862