Satellites and orbits

Curriculum links

Quoted word for word from the official documents.

Step by step

The caption is what the class reads on screen; the talking points are in your notes drawer (N) when you present. Steps marked Pupils only appear only in the pupil lesson.

1. 01

   Every working satellite

   Presented and for pupils

   Caption

   Each dot is a working satellite, placed where its latest orbit data puts it right now.

   Talking points

   • The layer is CelesTrak's list of active satellites, each run forward from its element set with the SGP4 orbit model. Debris is not drawn.
   • On 9 October 2026 the viewer's copy of the list held 16,682 satellites. Working out each one's height from its mean motion puts 15,868 of them, about 95%, below 2,000 km.
   • The dots are drawn far bigger than the satellites. NASA's Earth Observatory makes the same point about its own maps of orbiting objects.
   • This view is at true scale: Earth and the orbits are their real sizes, seen from about 20 Earth radii away (farther on a narrow screen).

   Sources 1 3 4Present from this step →

2. 02

   Low Earth orbit

   Presented and for pupils

   Caption

   Low Earth orbit is anywhere below 2,000 km, and that is where most satellites fly, the ISS among them.

   Talking points

   • ESA: low Earth orbit is under 2,000 km, and satellites there travel at about 7.8 km/s, going round in about 90 minutes. Generally satellites do not fly below 180 km, because of the drag from Earth's atmosphere.
   • ESA: most airliners fly no higher than about 12 km, so even the lowest orbit is more than ten times higher.
   • NASA Earth Observatory: drag from the thin upper atmosphere slowly pulls low satellites down, so they need boosts to stay up.

   Ask the class

   Why do satellites generally not orbit below about 180 km?

   1. AThe air there drags on them and brings them downRight answer
   2. BEarth's gravity is too weak there
   3. CThey would have to go round too slowly

   WhyESA: the lowest a satellite can fly is set by Earth's atmosphere. Below about 180 km the air drag is too strong. Gravity is in fact stronger lower down, and a lower orbit is a faster one.

   Sources 3 4Present from this step →

3. 03

   The ISS, live

   Presented and for pupils

   Caption

   This is the International Space Station, where it really is right now.

   Talking points

   • The viewer flies the ISS on its latest element set from CelesTrak (NORAD catalogue number 25544).
   • The element set used for the numbers in this lesson is from 8 October 2026, 20:29 UTC: mean motion 15.48782232 revolutions a day, eccentricity 0.0006778, inclination 51.63 degrees.
   • NASA gives round figures: an orbit about every 90 minutes, and 16 orbits in 24 hours.
   • The orbit changes slowly as drag lowers it and boosts raise it, so a newer element set gives a slightly different period.

   Ask the class

   The ISS's orbital elements give it 15.488 orbits a day. How many minutes does one orbit take?

   Answer93 minutes. Answers from 92.5 to 93.5 are marked right.

   WhyA day is 24 × 60 = 1,440 minutes, and 1,440 ÷ 15.488 = 92.98, so one orbit takes about 93 minutes.

   Sources 2 4 5Present from this step →

4. 04

   How fast?

   Presented and for pupils

   Caption

   One lap is the circumference of the orbit, 2πr, so speed = distance ÷ time gives the ISS's speed.

   Talking points

   • The clock runs at a minute a second, so the ISS laps Earth in about a minute and a half.
   • Where 6,800 km comes from: Kepler's third law with NASA's value of GM for Earth, 398,600 km³/s², gives an orbit radius of 6,798 km for 15.488 orbits a day. That is 420 km above Earth's equatorial radius of 6,378 km, and the small eccentricity keeps it between about 416 and 425 km above that radius.
   • NASA gives the ISS's speed as five miles a second, and ESA about 7.8 km/s for low Earth orbit in general. The element set gives 7.66 km/s.

   Ask the class

   The ISS goes round about 6,800 km from Earth's centre once every 93 minutes. Use speed = distance ÷ time to find its speed in km/s.

   Answer7.66 km/s. Answers from 7.56 to 7.76 are marked right.

   WhyDistance = 2π × 6,800 km = 42,726 km. Time = 93 × 60 = 5,580 s. Speed = 42,726 ÷ 5,580 = 7.66 km/s, more than 27,000 km an hour.

   Sources 2 3 5 6 8Present from this step →

5. 05

   Gravity turns it

   Presented and for pupils

   Caption

   Gravity pulls the ISS towards Earth's centre all the time, so its direction keeps changing while its speed stays the same.

   Talking points

   • AQA: velocity is speed in a given direction, and motion in a circle involves constant speed but changing velocity.
   • AQA: gravity provides the force that keeps planets and satellites, natural and artificial, in their circular orbits.
   • The clock runs at a minute a second. The camera rides along behind the station.

   Ask the class

   The ISS keeps a steady speed round its orbit. What happens to its velocity?

   1. AIt stays the same all the way round
   2. BIt keeps changing, because its direction keeps changingRight answer
   3. CIt drops to zero at the top of each orbit

   WhyVelocity is speed in a given direction. Gravity keeps pulling the ISS towards Earth's centre, so its direction, and therefore its velocity, changes all the time while its speed stays the same.

   Sources 8Present from this step →

6. 06

   The geostationary ring

   Presented and for pupils

   Caption

   Geostationary satellites go round once for every turn of Earth, so each one stays over the same spot on the equator.

   Talking points

   • ESA: geostationary satellites fly above the equator, west to east, taking 23 hours 56 minutes and 4 seconds per orbit, the length of a sidereal day (one turn of Earth against the stars). They travel at about 3 km/s at an altitude of 35,786 km.
   • NASA Earth Observatory: at exactly 42,164 km from the centre of the Earth an orbit matches Earth's rotation, and a satellite there over the equator stays over the same place. Weather satellites such as GOES work from there.
   • The clock runs at an hour a second. The low satellites race round while the ring turns with Earth.

   Ask the class

   A geostationary satellite goes round once every 86,164 seconds, 42,164 km from Earth's centre. Find its speed in km/s.

   Answer3.07 km/s. Answers from 3.02 to 3.12 are marked right.

   WhyDistance = 2π × 42,164 km = 264,924 km, and 264,924 ÷ 86,164 s = 3.07 km/s, less than half the ISS's 7.66 km/s.

   Sources 3 4Present from this step →

7. 07

   Why 35,786 km?

   Presented and for pupils

   Caption

   Only one orbit radius gives a period of exactly one turn of Earth, which is why the geostationary satellites form a single ring.

   Talking points

   • Kepler's third law: T²/r³ is the same for everything orbiting Earth, so a period of one sidereal day fixes the radius.
   • NASA gives Earth's sidereal rotation period as 23.9345 hours, which is 86,164 s or 1,436 minutes, matching ESA's 23 hours 56 minutes 4 seconds.
   • A check beyond GCSE: r³ = GM T² ÷ 4π², with NASA's GM of 398,600 km³/s² and T = 86,164 s, gives r = 42,164 km, NASA's figure. Take off Earth's equatorial radius of 6,378 km and the height is 35,786 km, ESA's figure.

   Ask the class

   T²/r³ is the same for every satellite of Earth. The ISS has T = 93 minutes at r = 6,800 km. A geostationary satellite has T = 1,436 minutes. Find its r in km.

   Answer42,165 km. Answers from 41,865 to 42,465 are marked right.

   Whyr = 6,800 × (1,436 ÷ 93)^(2/3) = 6,800 × 15.441^(2/3) = 6,800 × 6.201 = 42,165 km. Take off Earth's equatorial radius, 6,378 km, and the height is about 35,787 km, close to the 35,786 km ESA gives (the inputs were rounded).

   Sources 3 4 6 9Present from this step →

8. 08

   Earth's natural satellite

   Pupils only

   Caption

   Your turn: Earth has one natural satellite. Find it and click on it.

   Talking points

   • NASA: the Moon orbits on average 384,400 km from Earth, once every 27.3217 days measured against the stars, at a mean speed of 1.022 km/s. It is far higher, and far slower, than any of the artificial satellites in this lesson.
   • This view is not to scale.

   Pupil task

   Find the Moon on its orbit round Earth and click on it.

   HintFollow the orbit line round Earth until you reach the small world on it.

   The viewer checks the task as the pupil works and says when it is done.

   Sources 7 8Open this step as a pupil →

Sources

Every fact in this lesson comes from these sources, and each step lists the ones it uses.

1. Active satellites, GP element sets, CelesTrak
2. ISS (ZARYA), NORAD 25544, GP element set, CelesTrak
3. Types of orbits, European Space Agency
4. Catalog of Earth Satellite Orbits, NASA Earth Observatory
5. Space Station Facts and Figures, NASA
6. Earth Fact Sheet, NASA NSSDCA
7. Moon Fact Sheet, NASA NSSDCA
8. AQA GCSE Physics (8463) specification, version 1.1
9. Pearson Edexcel GCSE (9-1) Astronomy (1AS0) specification, issue 3