Astronomy Homework Help?

A very crude model of a typical comet nucleus is a cube of ice (density 1000 kg/m^3) 10 km on a side (a) What is the mass of this nucleus (b) Suppose 1% of the mass of the nucleus evaporates away to form the comet's tail. Suppose further that this tail is 10^8 km long and 10^6 km wide. Estimate the average density of the tail (in kg/m^3). For comparison, the density of the air you breathe is about 1.2 kg/m3. (c) In 1910 Earth actually passed through the tail of Comet Halley. At the time there was some concern among the general public that this could have deleterious effects on human health. Was this concern justified? Why or why not?