2.1 - The Solar System & the Universe

The Solar System and the objects in it

• The Solar System is the Sun together with everything that orbits it.
• At its centre is the Sun, our nearest star: a hot sphere of gas.

Objects that orbit the Sun

• Planets - Large bodies that orbit a star. A planet that orbits a star outside our Solar System is called an exoplanet.
• Dwarf planets - Planet-like bodies that orbit the Sun but aren't big enough to count as planets, such as Pluto.
• Moons - Natural satellites: bodies that orbit a planet, the way planets orbit the Sun.
• Asteroids - Lumps of rock and metal that orbit the Sun, mostly in the asteroid belt between Mars and Jupiter.

A satellite is any object that orbits a larger one, so both moons (natural satellites) and artificial satellites are types of satellite.

The eight planets in order from the Sun

• There are eight planets in the Solar System.
• In order of distance from the Sun, moving outwards: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
• The four inner planets (Mercury, Venus, Earth, Mars) orbit closest to the Sun.
• The four outer planets (Jupiter, Saturn, Uranus, Neptune) lie beyond the asteroid belt.
• All eight orbit the Sun in roughly the same plane.

Galaxies and the scale of the Universe

• A galaxy is a huge collection of billions of stars, held together by gravity.
• Our galaxy is the Milky Way. The Sun is just one of its many billions of stars, lying about halfway along one of its spiral arms.
• Other stars in the Milky Way are vastly further from Earth than the Sun is, which is why they appear as tiny points of light while the Sun looks large and bright.
• Distances between neighbouring stars are typically hundreds of thousands of times greater than the distance from the Earth to the Sun.

The Universe

• The Milky Way is just one of billions of galaxies. Together, all of these galaxies make up the Universe.
• Galaxies are separated by even larger distances than the stars within them, so the Universe is mostly empty space.
• The ladder of scale:
  • The Universe contains billions of galaxies.
  • Each galaxy contains billions of stars.
  • Around a star like the Sun lies a planetary system.

Units of astronomical distance

• Distances between stars and galaxies are so vast that measuring them in kilometres is impractical.
• A light-year is the distance that light travels through space in one year. It's used for the large gaps between stars and galaxies.
• The astronomical unit (AU) is the mean distance from the Earth to the Sun. It's used for distances within a planetary system.

Worked example - Distance of one light-year

Light travels at 3 × 10⁸ m/s. Calculate the distance, in metres, of one light-year (the distance light travels in one year).

Risks to astronauts during space travel

Launch

Rockets carry enormous amounts of highly flammable fuel, so a fault during take-off risks an explosion.

Radiation

• The Earth's atmosphere shields us from harmful ionising radiation called cosmic rays, which come from the Sun and deep space.
• Beyond the atmosphere, astronauts receive a much higher dose, which damages body cells.
• Spacecraft use shielding to reduce this radiation dose.

Life support

• Oxygen - There is no oxygen in space, so life support systems must supply it.
• Temperature - Without an atmosphere to regulate temperature, extremes are severe (about 120 °C in sunlight to −160 °C in shadow), requiring active thermal control.
• Pressure - Space is an approximate vacuum. Spacecraft and suits must be pressurised close to atmospheric pressure on Earth; otherwise body fluids would boil.

Re-entry

Returning through the atmosphere exposes the spacecraft to extreme friction and heating.

Powering spacecraft with photovoltaic cells

• Spacecraft rely on electricity to power equipment and life support systems.
• Photovoltaic cells (solar cells) convert light from the Sun directly into electricity.

The effect of distance from the Sun

• Travelling towards the Sun - Sunlight is more intense, so cells risk overheating and may need to be angled away from the Sun.
• Travelling away from the Sun - Sunlight is weaker, requiring a larger surface area of cells to generate sufficient power.

The International Space Station

• Photovoltaic arrays rotate to face the Sun to absorb maximum light.
• When passing through the Earth's shadow, the station runs on backup batteries that recharge in sunlight.