Colombo studied the planet Mercury, and it was his calculations which showed how to get a spacecraft into a solar orbit which would encounter Mercury multiple times, using a gravity assist manoeuvre with Venus.
Due to this idea, NASA was able to have the Mariner 10 accomplish three fly-bys of Mercury instead of one.
Colombo also explained the spin-orbit resonance in Mercury's orbit, showing that it rotates three times for every two orbits around the Sun.
Colombo also made significant contributions to the study of Saturn's rings, mostly using ground-based observations in the era before space exploration reached the outer Solar System.
Several astronomical objects and spaceships are named after to honour him: