How do left over materials in space pose a threat to people on the ground?

What is the period of difference between the lunar and solar year called? What energy you get from the sun? Does every planet have a latitude and longitude? Which country sun on the 12PM night? What mythological figure is the moon name after? If you were to send a bottle rocket 15 kilometers up into the air which layer of the atmosphere would it reach? What would happen if earth stopped rotating and if its axis were perpendicular to its orbit? Who is the biggest terrestrial planet? What are storms on the surface of the sun called? How does water move from the atmosphere to the hydro sphere? How far did Apollo 11 travel? Does an atomic bomb effect the atmosphere? How do you drive space shuttle? How does high pressure probably affect the size of a nebula? Names of research stations in antarctica? How long does it take for all 9 planet to orbit the sun? The large amount of money used for space programmes should be use help poor people and developing countries? What goes the word cirrus mean? How is the full moon lit up? How did Yuri Gagarin impact the world?

How do left over materials in space pose a threat to people on the ground?