The gap between Mars and Jupiter is large - and filled with mini planets called asteroids (or Near Earth Asteroids - NEAs). The largest of the known asteroids is 1036 Ganymed. These NEAs are the result of the break-up of a planet. Asteroids are important as they can tell us what the interior of the planets are like.
The asteroids are small and mostly irregularly shaped. As the planets orbit the sun, gravitational pull causes asteroids from the main belt to cross the orbits of Mars, Earth, Venus and Mercury.
Source: IAU: Near Earth Asteroids (NEAs)
NASA / public domain