Global Challenges in Physics
The space physics in this article (the solar system, orbits, stars, the Big Bang and red shift) is not examined on Combined Science. The practice questions and knowledge check for this lesson test the mains electricity, national grid, transformer and insulation content from topic P6, which is taught in the domestic electricity lesson and in the energy resources lesson.
The Solar System and Beyond
Our solar system consists of the Sun (a star), eight planets, dwarf planets (such as Pluto), moons, asteroids and comets. The four inner planets (Mercury, Venus, Earth, Mars) are small and rocky. The four outer planets (Jupiter, Saturn, Uranus, Neptune) are much larger gas or ice giants (P8.3a).
The solar system is part of the Milky Way galaxy, which contains billions of stars. The universe contains billions of galaxies, each with billions of stars. Distances in space are so vast that we measure them in light-years — the distance light travels in one year (about \(9.46 \times 10^{12}\) km).
The solar system has eight planets: four rocky inner planets and four gas/ice outer giants. It sits within the Milky Way galaxy. Distances are measured in light-years.
Satellite Orbits
Satellites orbit planets due to gravitational attraction. Artificial satellites are placed in two main types of orbit (P8.3b):
• Geostationary orbit — the satellite orbits once every 24 hours, staying above the same point on Earth. Used for communications (TV, phone) and weather monitoring. These orbits are high (~36,000 km above Earth) and positioned above the equator.
• Polar orbit (low Earth orbit) — the satellite passes over both poles, orbiting much closer to Earth (~200–2000 km). As the Earth rotates beneath it, the satellite can scan the entire surface over time. Used for mapping, military surveillance and environmental monitoring.
Humans have explored space using telescopes (ground-based and space-based like the Hubble and James Webb), robotic probes (e.g. Mars rovers like Perseverance), and crewed missions (e.g. the International Space Station). Space exploration has provided valuable knowledge: understanding the origins of the universe, discovering exoplanets, and developing technologies with everyday applications (satellite navigation, weather forecasting, water purification) (P8.3c).
Challenges of space exploration include the extreme distances involved (even light takes ~4 years to reach the nearest star), the hostile environment (radiation, vacuum, microgravity effects on the body), and the enormous cost. Despite these challenges, international collaboration (e.g. NASA, ESA) continues to push the boundaries of exploration.
Gravity is the force responsible for keeping planets in orbit around the Sun, and moons in orbit around planets. The gravitational force depends on the masses of the two objects and the distance between them. Closer objects and more massive objects experience stronger gravitational attraction. This is why the inner planets orbit faster than the outer planets — they are closer to the Sun’s gravitational pull.
The orbital speed of a satellite depends on its altitude. Lower orbits require higher speeds (the ISS orbits at ~7.7 km/s). Higher orbits require lower speeds (geostationary satellites orbit at ~3 km/s). If a satellite’s speed is too low for its orbit, it will spiral inward; too high and it will move to a higher orbit or escape entirely.
Geostationary satellites orbit once per 24 hours above the equator (for communications). Polar orbit satellites are lower and scan the whole Earth (for mapping and monitoring).
The Life Cycle of Stars
Stars form from nebulae — clouds of gas and dust. Gravity pulls the material together, and as it compresses, it heats up. When the core is hot enough, nuclear fusion begins (hydrogen fuses into helium), and a star is born. The star enters its main sequence phase, where it remains stable for billions of years as the outward pressure from fusion balances the inward pull of gravity (P8.3d).
When a star runs out of hydrogen fuel, its fate depends on its mass:
• Sun-sized stars: expand into a red giant, then shed outer layers to form a planetary nebula, leaving behind a white dwarf which gradually cools.
• Massive stars: expand into a red supergiant, then explode in a supernova. The core collapses to form either a neutron star or, if massive enough, a black hole. Supernovae scatter heavy elements into space, which form new stars and planets.
The Big Bang theory states that the universe began from an extremely hot, dense point approximately 13.8 billion years ago and has been expanding ever since. Evidence for this includes:
• Red shift — light from distant galaxies is shifted towards the red end of the spectrum. This shows galaxies are moving away from us. The further away a galaxy, the greater its red shift, meaning it is moving away faster.
• Cosmic microwave background radiation (CMBR) — low-level microwave radiation detected from all directions in space. This is the remnant heat from the Big Bang, predicted by the theory and discovered in 1965.
These observations support the conclusion that the universe is expanding. Scientists continue to study dark matter and dark energy, which appear to make up most of the universe but remain poorly understood.
Stars form from nebulae. Sun-sized stars become red giants, then white dwarfs. Massive stars go supernova, forming neutron stars or black holes. Red shift and CMBR are evidence for the Big Bang and expanding universe.