Satellite
A satellite is a celestial or artificial object that orbits a more massive body under the influence of gravity. The term encompasses two very different categories: natural satellites, such as the Moon around Earth, and artificial satellites, created by humans and placed in orbit since 1957.
Natural satellites
All the planets in the solar system except Mercury and Venus have natural satellites. Earth has only one: the Moon. Mars has two, Phobos and Deimos. Jupiter and Saturn have dozens, the most famous of which are Io, Europa, Ganymede, and Callisto for Jupiter, and Titan and Enceladus for Saturn. Dwarf planets like Pluto also have their own moons—Charon being the best known. In total, more than 400 confirmed natural moons orbit within the solar system.
Artificial satellites
The first artificial satellite, Sputnik 1, was launched by theSoviet Union on October 4, 1957, marking the beginning of the space age. Today, tens of thousands of man-made objects orbit the Earth, ranging from space telescopes like Hubble to constellations of communications satellites like Starlink, as well as weather satellites, GPS, scientific, and military satellites.
Types of Orbits
Artificial satellites occupy different orbits depending on their function. The low Earth orbit (LEO, 200–2,000 km) is home to the International Space Station, Hubble, and the Starlink constellations. The medium Earth orbit (MEO, 2,000–35,000 km) hosts GPS and Galileo satellites. Geostationary orbit (GEO, 35,786 km) allows satellites to remain stationary above a specific point on Earth— ideal for telecommunications.
Importance in Astronomy
Natural satellites are fascinating planetary laboratories: Europa may harbor a liquid ocean beneath its icy crust, Titan has lakes and rivers of liquid methane, and Enceladus ejects water plumes from its subglacial vents. Artificial observation satellites, for their part, have revolutionized astronomy by breaking free from Earth’s atmosphere— Hubble, Spitzer, Kepler, James Webb, Planck, and many others have each opened new windows onto the universe.
Did you know?
The largest natural satellite in the solar system is Ganymede, a moon of Jupiter, with a diameter of 5,268 km—larger than the planet Mercury. Earth’s Moon, at 3,474 km, is the fifth-largest moon in the solar system but one of the largest relative to its host planet.
Frequently Asked Questions
How many artificial satellites are orbiting Earth?
More than 10,000 active satellites are currently in orbit, along with more than 30,000 pieces of space debris large enough to be tracked. Constellations like Starlink are adding several thousand satellites per year, raising concerns about traffic management and light pollution for astronomers.
Why don’t satellites fall to Earth?
They are constantly falling, but they move fast enough that the Earth curves beneath them at every moment. This is the principle of orbit: a balance between free fall and horizontal velocity. Newton had already understood this principle in the 17th century.
Do artificial satellites affect astronomy?
Yes, increasingly so. Constellations like Starlink leave light trails on astronomical images, and their radio emissions interfere with radio telescopes. Operators are developing technical solutions to mitigate these effects, but the problem remains a concern for the astronomical community.