Space

One astronomical unit is the distance from the Earth to the sun, which is 150 million km.  (AU) A light-year is the distance light travels in a year. 

Mass is a measure of how much matter something is made up of. All objects with mass attract each other due to gravity. So the more mass, the greater attraction. Gravity on the moon is 1/6 of that on Earth. Weight is dependant on mass and gravity. It is measured in Newtons.

Some are made of ice rock. The gas you see is actually the tail. The tails is bits of the ice rock that's been swept up by the Sun's atmospheric pressure. It is a possible source of water, and it's orbit is elliptic. An asteroid is a ball of rock that goes in a circular orbit.

Orbit

Planets orbit because the gravitational force is pulling the planet into orbit, thus stopping it from escaping and going in a straight line. If f=mxa,…