Gravity, weight and orbits

Gravity holds the Solar System together and decides where meteorites end up. In this lesson you will calculate weight on different worlds and see how Jupiter sends asteroid fragments towards Earth.

Mass and weight

Mass is the amount of matter in an object, measured in kilograms. It is the same everywhere. Weight is the force of gravity pulling on that mass, measured in newtons (N). It depends on where you are.

Weight = mass × gravitational field strength, or W = m × g. On Earth, g is about 10 N/kg (more precisely 9.8 N/kg). On the Moon it is about 1.6 N/kg, on Mars about 3.7 N/kg, and on the large asteroid Vesta only about 0.25 N/kg. The smaller the world, the weaker its gravity.

Why weak gravity matters for meteorites

Weak gravity makes it easier for rock to escape. A large impact on Vesta, the Moon or Mars can throw some rock fast enough to leave completely, which is how meteorites from those bodies reach space. Escaping Earth needs over 11 km/s, while escaping Vesta needs only about 0.36 km/s.

Gravity also explains orbits. The Sun's gravity pulls every asteroid towards it, but each asteroid is moving sideways so fast that it keeps missing the Sun and travels around it instead.

Jupiter the shepherd

Jupiter is the largest planet, and its gravity tugs on every asteroid. At certain distances, an asteroid goes around the Sun exactly three times for every one orbit of Jupiter, for example. There the tugs always come at the same point in the orbit and add up, stretching the asteroid's path until it crosses the orbits of Mars and Earth. These zones, called resonances, show up as gaps in the asteroid belt. They act like escape hatches, delivering a steady supply of fragments to the inner Solar System.

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