How to make asteroids bounce off eachother

Hi, i am new to unity, and have finished the space shooter tutorial. i want to add a tweak to it, in which i can make the asteroids bounce off eachother and not destroy. and one more tweak in which i can give the ship the ability to shoot more than one bolt after specific score (like 500, 1000). but i am completely clueless on how to do it, i have tried to google about it, but couldn't find any solution.... so any help will be much appreciated Thank you.