Further, they are not smooth. Just like the students' samples, comets are rough and have vents where the gasses escape. Note that density is never measured directly, but inferred based on mass estimates derived from non-gravitational forces caused by sublimation. Ultimately, all of a comet's light comes from the sun, either from the scattering of radiated sunlight by dust particles or the reemission of absorbed sunlight by gas molecules as fluorescence. The corn syrup in the recipe is representative of organic compounds found in comets.
Organics are carbon based molecules, molecules which are common to all known forms of life.
The coma of bright comets extends well over 100,000 km. The Hydrogen coma extends well over 1,000,000 km from the nucleus. The coma can be approximated as spherical. For Halley, each orbit, it sheds about 1 meter of its surface. NOAO is operated by the Association of Universities for Research in Astronomy, Inc., under agreement with the National Science Foundation.