Khan Academy
Khan Academy: Activity: Scale of the Universe on a String
This activity will help your students comprehend the huge scales of time and space by using familiar objects. You can explore the massive distances of our Solar System using scale models made out of string. Included is a scale model of...
NASA
Nasa: Space Place: Messenger: Planet Mass Comparison
Compare the masses of planets in this interactive animation. Simply pile a number of Earths on one side of the scale, to balance with the mass of one Jupiter. Then try this with all the planets.
Curated OER
Nasa: Space Place: Solar System Comparative Sizes
This picture compares the sizes of the moons of the different planets in the solar system.