Solar system and orbits

Mercury, Venus, Earth, Mars

Orbital speed increases.

Gravitational attraction provides the centripetal force that continually changes their direction.

Gravity / gravitational force (1).

Any three of: moons/natural satellites, dwarf planets, asteroids, comets (1 for each, max 3 — but capped at marks).

Velocity is a vector with both speed and direction (1). In a circular orbit the direction is constantly changing even though the speed is constant (1), so the velocity is changing and the object is therefore accelerating (towards the centre) (1).

It must move at a higher speed (1) because for a stable orbit at a smaller radius the gravitational (centripetal) force is greater and requires a faster orbital speed (1).

A larger solar mass would increase the gravitational force on the Earth (1). Since gravity provides the centripetal force for the orbit (1), this force would now be larger than that needed to keep Earth moving at its current speed and radius (1). To regain a stable orbit, Earth would have to move faster or move into a smaller orbit so that the required centripetal force matches the gravitational force (1).

The orbital period of a geostationary satellite is 24 hours (1); because the satellite stays above the same point on Earth, a receiving dish can be pointed permanently at the satellite without needing to track it, simplifying installation (1); a single geostationary satellite can cover a very large area of the Earth's surface, allowing broadcasts to millions of receivers simultaneously (1)

Geostationary satellites orbit at about 36 000 km altitude with a 24-hour period; they appear stationary over the same point on Earth, making them ideal for TV broadcasting and weather monitoring because dishes need not track them and a single satellite gives wide coverage (1); LEO satellites orbit at a few hundred to a few thousand kilometres with periods of about 90 minutes; they are not fixed above one point and pass over different parts of the Earth, but their low altitude means less signal delay, stronger signals and better imaging resolution, making them suitable for GPS, Earth observation and the International Space Station (1); geostationary satellites are better for continuous communication with a fixed location, while LEO satellites are better for applications needing global coverage in a network or high-resolution imaging (1)

The ISS is in a circular orbit because gravity acts as the centripetal force, continuously changing its direction but not its speed (1); the station is actually in continuous free fall towards the Earth, but its high orbital speed means it curves around the Earth as fast as the Earth curves away (1); the astronauts inside are also falling freely at exactly the same rate as the station, so there is no contact force between them and the floor — this is experienced as weightlessness (1); gravity is still acting on the astronauts (it is what keeps the ISS in orbit), but because both they and their surroundings accelerate identically they cannot feel it (1)

At aphelion the comet is at its greatest distance from the Sun; gravitational potential energy is at its highest and the comet moves most slowly (1); as the comet falls toward the Sun, gravity acts as the centripetal force directing it inward — the component of gravity along the direction of motion accelerates the comet (1); gravitational potential energy is converted to kinetic energy, so the comet speeds up continuously; by conservation of energy, the loss in gravitational PE equals the gain in KE (assuming no energy dissipation) (1); at perihelion the comet is at its smallest distance, has minimum gravitational PE, maximum kinetic energy and moves fastest; as it swings around and moves away the reverse process occurs (1)