Solar System: Stability of Orbital Motions; Satellites

Our solar system consists of the Sun at its centre and every object held in orbit around it by the Sun's gravity. AQA expect you to name the main classes of object:

Object	Description	Examples
Star	A massive ball of hot plasma fusing hydrogen into helium	The Sun
Planet	A large body in orbit around a star, has cleared its orbit of debris	Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
Dwarf planet	Orbits the Sun but has NOT cleared its orbital path	Pluto, Eris, Ceres
Natural satellite (moon)	Orbits a planet	The Moon (Earth), Io (Jupiter), Titan (Saturn)
Asteroid	Small rocky body, mostly in the asteroid belt between Mars and Jupiter	Vesta, Ceres
Comet	Small icy body in a highly elliptical orbit; develops a tail when near the Sun	Halley's Comet, Hale-Bopp

The eight planets in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. A common mnemonic is "My Very Easy Method Just Speeds Up Naming". (Pluto was reclassified as a dwarf planet by the International Astronomical Union in 2006.)

Inner vs outer planets.

• Inner ("terrestrial") planets — Mercury, Venus, Earth, Mars. Small, rocky, close to the Sun.
• Outer ("gas/ice giant") planets — Jupiter, Saturn, Uranus, Neptune. Large, gaseous (or icy), much further from the Sun.

The asteroid belt between Mars and Jupiter separates the two groups.

Why are objects in orbit? Every body in the solar system is held by the gravitational attraction of the Sun. Without gravity they would travel in straight lines (Newton's first law). Gravity continuously changes their direction, pulling them into closed (roughly elliptical) orbits.

Scale matters. The Sun contains over 99.8% of the total mass of the solar system. Distances are enormous: Earth is ~150 million km from the Sun (= 1 astronomical unit, AU); Neptune is ~30 AU. Diagrams in textbooks (and in your exam paper) are NEVER drawn to true scale — if they were, on a page-width diagram Neptune would be off the page and the planets would be invisible dots.

Our solar system is part of a much larger structure called the Milky Way galaxy. AQA examiners want you to be clear about the hierarchy of size:

Moon → Planet → Solar system → Galaxy → Universe

A galaxy is a huge collection of stars (and dust, gas, and dark matter) held together by gravity. The Milky Way is a barred spiral galaxy containing roughly 200–400 billion stars. The Sun is just one of those stars, located on the Orion Arm, about two-thirds of the way out from the centre.

The whole Milky Way is around 100,000 light-years across. A light-year is the distance light travels in one year (~9.5 × 10¹⁵ m) — the nearest star to the Sun (Proxima Centauri) is 4.2 light-years away. Even at the speed of light, a signal from us would take 4.2 years to reach the next star.

There are billions of other galaxies in the observable universe — Andromeda, the Triangulum Galaxy and the Magellanic Clouds are some of our nearest neighbours.

Common exam slip. Don't write that "the Sun is the centre of the universe" or "the centre of the galaxy". The Sun is the centre of OUR SOLAR SYSTEM. The galaxy contains hundreds of billions of stars; the universe contains billions of galaxies.

AQA require you to describe the formation of the Sun:

1. About 5 billion years ago, a vast cloud of dust and gas called a nebula existed in space.
2. Gravitational attraction pulled the dust and gas together.
3. As the cloud contracted it heated up, forming a hot, dense core called a protostar.
4. When the core became hot and dense enough, hydrogen nuclei began to fuse into helium — nuclear fusion started, releasing huge amounts of energy.
5. The protostar became a stable main sequence star — our Sun.

The same process also formed the planets: the leftover dust and gas around the young Sun coalesced into the planets, moons, asteroids and comets we see today.

The Sun has been on the main sequence for ~5 billion years and is expected to remain there for another ~5 billion years before evolving into a red giant and finally a white dwarf — covered in the next subtopic (4.8.1.2, life cycle of a star).

Key word check. The energy that powers the Sun comes from nuclear fusion of light nuclei (hydrogen → helium). NOT combustion (the Sun is not burning a fuel like coal) and NOT fission.

Every star — including our Sun — begins life in the same way:

1. Nebula. A vast cloud of dust and gas (mostly hydrogen) drifts in space.
2. Gravitational collapse. Gravity pulls the cloud together. As it contracts, gravitational potential energy is transferred to thermal energy and the core heats up.
3. Protostar. The dense, hot core is called a protostar. It is not yet stable — it is still contracting.
4. Main sequence star. When the core becomes hot enough (~10 million °C) and dense enough, hydrogen nuclei fuse into helium — nuclear fusion begins. The outward "push" from radiation and hot gas balances the inward "pull" of gravity. The star becomes stable.

A star spends most of its life on the main sequence. Our Sun has been on the main sequence for ~5 billion years and will stay there for another ~5 billion years.

Why is the main sequence star stable?

The outward force from radiation/hot gas (from fusion) = the inward force of gravity.

If gravity were stronger the star would collapse; if radiation pressure were stronger the star would explode. The balance is called hydrostatic equilibrium (you do not need that exact phrase for AQA — but you do need to describe the balance).

Stars with a mass similar to the Sun follow this path:

Nebula → Protostar → Main sequence → Red giant → White dwarf → (eventually) Black dwarf

Red giant. When the core's hydrogen runs out, gravity pulls the core inward and heats it further. The outer layers expand and cool, glowing red. Helium begins to fuse into carbon and oxygen.

White dwarf. Once all the fuel is used up, the outer layers drift away (often forming a beautiful "planetary nebula"). What remains is a small, hot, very dense core called a white dwarf. There is no more fusion — the white dwarf just radiates away its stored heat.

Black dwarf. Over many billions of years the white dwarf cools and fades to a cold black dwarf. (Note: the universe is not yet old enough for any black dwarfs to actually exist — it's a theoretical end stage.)

Our Sun will follow this path in about 5 billion years.

Stars much more massive than the Sun follow a more dramatic path:

Nebula → Protostar → Main sequence → Red super giant → Supernova → Neutron star (or Black hole if very massive)

Red super giant. Bigger than a red giant. Once hydrogen runs out, fusion continues with progressively heavier elements (He → C → O → … → iron). Iron is the heaviest element that can be formed by ordinary stellar fusion.

Supernova. When fusion can no longer support the star (iron does not yield net energy when fused), the core collapses catastrophically and the outer layers are blasted into space in a colossal explosion — a supernova. The shock wave and extreme conditions force lighter nuclei to fuse into elements heavier than iron (gold, uranium, lead). Without supernovae, none of these elements would exist.

Neutron star or black hole. What is left depends on the mass of the original star:

• Neutron star — a tiny, incredibly dense remnant (~20 km across, but more massive than the Sun) made almost entirely of neutrons. Some are observed as pulsars.
• Black hole — if the star was very large indeed, the remnant collapses further into a black hole, where gravity is so strong not even light can escape.

Why this matters: where atoms come from. Every atom of carbon in your body, oxygen in the air and iron in your blood was made by fusion inside a star. Atoms heavier than iron (the gold in jewellery, the uranium in nuclear reactors) only exist because earlier stars exploded as supernovae. You are, quite literally, made of stardust.

Nuclear fusion is the process where light nuclei join (fuse) to form a heavier nucleus, releasing energy. In stars:

• Main sequence: hydrogen → helium.
• Red giant / super giant: helium → carbon → oxygen → … → silicon → iron.
• Supernova: fusion runs away during the explosion to create elements heavier than iron (gold, lead, uranium, etc.).

Fusion of nuclei lighter than iron releases energy. Fusion of iron or anything heavier does NOT release net energy — this is why iron marks the limit of normal stellar fusion and why high-mass stars cannot stop their core collapsing once it has fused to iron.

The supernova explosion then distributes these elements (light and heavy) across space, where they eventually become incorporated into new nebulas, new stars and new planets. Earth and everything on it formed from material recycled in this way.

Imagine a ball on the end of a string being swung in a horizontal circle. The tension in the string pulls the ball towards the centre of the circle. Let go of the string and the ball flies off tangentially in a straight line (Newton's first law).

A planet orbiting the Sun is exactly the same idea — except there is no string. The gravitational attraction of the Sun pulls the planet towards it. This pull continually bends the planet's path away from a straight line and into a (nearly) circular orbit.

Gravity provides the centripetal force — the force needed to keep an object moving in a circle.

The same applies to:

• The Moon orbiting Earth — Earth's gravity pulls on the Moon.
• The ISS orbiting Earth — Earth's gravity pulls on the ISS.
• Planets orbiting the Sun — the Sun's gravity pulls on them.
• Moons orbiting planets — the planet's gravity pulls on the moon.

A common student confusion: "Why don't satellites just fall down?"

They are falling — continuously! But they are also moving sideways so fast that as they fall, the curve of the Earth falls away beneath them at the same rate. The result is a closed orbit. (You may have heard the phrase "a satellite is falling, but it keeps missing the Earth".)

In a stable circular orbit the speed of the satellite is constant. Yet its velocity is constantly changing — because velocity is a vector and the direction keeps changing as it goes round.

Because velocity is changing, the satellite is accelerating (acceleration is the rate of change of velocity).

Newton's second law (F = ma) then says there must be a resultant force acting on the satellite — and that force is gravity. The force always points towards the centre of the orbit (the Earth, or the Sun).

Speed → unchanged. Velocity → changes (direction changes). Acceleration → towards the centre. Resultant force → gravity, towards the centre.

Common exam mistake. Writing "the satellite has no acceleration because its speed is constant". That's wrong — acceleration is change in velocity, not change in speed. A simple sketch of two velocity arrows at different points of the orbit, both with the same length but different directions, is enough to show the change.

AQA require a qualitative understanding: for a stable circular orbit, changing the radius forces a change in orbital speed.

The closer a satellite is to the body it orbits, the stronger the gravitational pull on it. To stay in a stable circular orbit it must move faster so that the centripetal force needed for that speed and radius matches the available gravitational pull.

Smaller orbital radius → higher orbital speed (and shorter period). Larger orbital radius → lower orbital speed (and longer period).

Examples.

Orbit	Radius (from Earth's centre)	Speed	Period
Low Earth orbit (e.g. ISS)	~6700 km	~7.7 km/s	~90 min
Geostationary orbit	~42 000 km	~3.1 km/s	24 h
Moon	~384 000 km	~1.0 km/s	~27 days

Notice the pattern: bigger orbits = slower speeds = longer periods. This is consistent with planet orbits too — Mercury (closer to the Sun) takes 88 days to orbit, but Neptune (much further out) takes 165 years.

Putting a satellite at a different altitude. If you tried to slow down a satellite without moving it, gravity would still be the same, but the required centripetal force for that radius would be less than gravity is supplying — the satellite would spiral inwards (lower altitude). To stay in a stable orbit at the new (lower) altitude it must speed up.

A satellite is any object that orbits another body. AQA distinguishes:

• Natural satellites — formed naturally. Examples: the Moon (orbits Earth), Io / Europa / Ganymede / Callisto (Jupiter's four large Galilean moons), Titan (Saturn).
• Artificial satellites — human-made and launched into orbit.

Common uses of artificial satellites:

Type of orbit	Use	Example
Low Earth orbit (LEO)	Earth observation, the International Space Station, Hubble Space Telescope	ISS at ~400 km
Geostationary orbit	TV/communications, weather monitoring of one region	Sky satellites, Meteosat
Polar orbit	Whole-Earth imaging (passes over every point as Earth rotates beneath)	Sentinel weather satellites
Medium Earth orbit	Navigation (GPS, Galileo)	GPS satellites (~20 000 km)

Geostationary orbits. A geostationary satellite orbits directly above the equator at a height where its orbital period equals 24 hours — it travels east at the same rate Earth spins, so it appears to stay over the same point on Earth's surface. Perfect for fixed-dish satellite TV.