Space Physics

Space Physics is Topic 6 of the Cambridge IGCSE Physics (0625) syllabus. It covers the Solar System, orbital motion, the life cycle of stars, and the expanding Universe. It is the newest major topic in the syllabus, added in the 2023 update, so many older revision guides skip it entirely.

What does Space Physics cover in 0625?

The topic splits cleanly by tier. Core students need the Solar System and basic orbits: the eight planets, the accretion model, day-night and the seasons, and the orbital speed equation. Orbital speed equals $2\pi r$ divided by $T$, where $r$ is orbital radius and $T$ is orbital period. Extended students add two Supplement-only chapters: the life cycle of stars and the Universe, redshift and the Big Bang. These include the Hubble constant, with a syllabus value of $2.2 \times 10^{-18}$ per second.

Why do students lose marks here?

Students lose marks here in three predictable ways. First, the $v = \dfrac{2\pi r}{T}$ calculation. Orbital periods arrive in days or years, and radii in kilometres or AU. Skip the unit conversion and the answer is wrong by factors of thousands. Second, the star life cycle. Examiners want the exact sequence: nebula, protostar, main sequence, then red giant to white dwarf for Sun-sized stars, or supernova to neutron star or black hole for massive ones. Mixing the two branches costs the whole question. Third, redshift explanations. “Galaxies are moving away, so wavelengths stretch towards the red end” earns marks. “The Universe is expanding so light goes red” does not.

How should you revise Space Physics?

Treat it as two short jobs, not one long one. Learn the descriptive content as sequences and diagrams: planet order, the star life cycle flowchart, the Big Bang evidence chain. Then drill the two calculations (orbital speed and Hubble’s law) with five questions each until unit conversion is automatic. Most students can secure this topic in under a week, which makes it the best marks-per-hour topic in 0625. It appears in every paper series, usually as one structured question worth 6 to 10 marks.

Is Space Physics hard? No. It has the fewest equations of any 0625 topic and almost no apparatus to memorise. The danger is neglect, not difficulty. Because it sits last in the syllabus, schools often rush it in the final term. If your school has not covered stars or redshift by March, self-study the two Supplement chapters using the subtopic guides below.

How Space Physics Is Assessed Across the Papers

Space Physics is short, yet it earns a place on every paper series since the 2023 update. Papers 1 (Core) and 2 (Extended) carry one or two multiple-choice items: planet order, why a year differs from a day, or a redshift idea. Papers 3 (Core) and 4 (Extended) usually set one structured question worth 6 to 10 marks, blending a description with a calculation. The Core and Extended split is the cleanest of any topic. Core candidates need the Solar System, orbits and the orbital speed equation $\left(v = \dfrac{2\pi r}{T}\right)$. The Supplement tier adds two whole chapters: the life cycle of stars, and the Universe with redshift, the Big Bang and the Hubble constant (syllabus value $2.2 \times 10^{-18}\ \text{per second}$). There is no practical paper content for this topic, so Papers 5 and 6 leave it alone, which is one reason it stays compact.

A Worked Example That Shows the Method

A satellite orbits the Earth at a radius of $8.0 \times 10^{6}\ \text{m}$ with an orbital period of $7200\ \text{s}$. Calculate its orbital speed. [3]

Worked solution:

$v = \dfrac{2\pi r}{T}$

$v = \dfrac{2\pi \times 8.0 \times 10^{6}}{7200} = \dfrac{5.027 \times 10^{7}}{7200}$

$v = 6981\ \text{m/s} \approx 7.0 \times 10^{3}\ \text{m/s}$

So the orbital speed is about $7.0 \times 10^{3}\ \text{m/s}$ (2 significant figures), which is roughly $7\ \text{km/s}$. The method is simple, but the marks live in the unit handling. Here the radius is already in metres and the period in seconds, so no conversion is needed, yet many real questions give the period in days or years and the radius in kilometres or astronomical units. Convert every quantity to metres and seconds before you substitute, or the answer drifts out by factors of thousands. The circumference $2\pi r$ is the distance for one orbit, and dividing by the period gives the average speed.

Mark scheme:

• M1: $v = \dfrac{2\pi r}{T}$ with correct substitution
• A1: $v = 6981\ \text{m/s}$ (allow $6980$ to $6990$)
• B1: answer rounded sensibly, $7.0 \times 10^{3}\ \text{m/s}$, with the unit

Work through each subtopic guide below, then check every formula against the physics equations list and practise the method in the calculation questions guide.