Final answer:
At 100 AU from the Sun, a comet only presents its nucleus, without a visible coma or tails. However, at 1 AU, comparable to Earth's distance from the Sun, the comet develops a coma and long tails due to solar heating and the solar wind.
Step-by-step explanation:
What a comet looks like when it is 100 AU from the Sun versus 1 AU from the Sun can be quite different due to solar influence on the comet's structure. At 100 AU, a distance well beyond Neptune which is at 30.1 AU, the comet is too far from the Sun to have substantial solar heating, implying that its appearance is likely to be subdued. In this far-off state, only its nucleus is present, which consists of a small, solid core made up of ice and dust. There are no visible coma or tails because these features are produced when the comet's surface volatiles sublimate due to the Sun's heat, and at 100 AU this heat is insufficient.
In stark contrast, when a comet is at a distance of 1 AU, comparable to the Earth's distance from the Sun, it is experiencing much greater solar radiation. This heat causes sublimation of ices on the comet's nucleus, creating a surrounding cloud of gas and dust called the coma. Moreover, the solar wind and radiation pressure shape this material into two distinct tails pointing away from the Sun: the dust tail and the ion (or gas) tail, which can extend millions of kilometers into space. Hence, at 1 AU, a comet is typically a spectacular sight with a vivid coma and long, streaming tails.