please explain

Physics 1101 Group Problem 1 You have been given a job working on a team searching for asteroids that might collide with the Earth. This search is important because current theories indicate that dinosaurs and other organisms became extinct when the Earth was struck by a large asteroid. If such a collision happens again it could cause the extinction of the current dominant species on Earth, us. The reason given for the extinction of dinosaurs is that 65 million years ago dust from an asteroid impact went into the upper atmosphere all around the globe, where it stayed for several years blocking the sunlight from reaching Earth's surface. To scan space for this danger, you need to know how large an asteroid you are looking for. A dangerous asteroid size is roughly the same as the one that wiped out the dinosaurs. You are told that about 20% of that asteroid's mass ended up as dust that was spread uniformly over Earth after eventually settling out of the upper atmosphere. Geological exploration indicates that a layer of 0.020 grams of dust, which is chemically different than the Earth's rock, covers each square centimeter of the Earth's surface on the average. Typical asteroids have a density of about 2.0 g/cm3 Possibly Useful Information: Earth's Radius = 6380 km Volume of sphere = 3 X R3 1 km = 105 cm Surface area of sphere = 4 x R2 mass mass Volume density = volume Surface density = area