Originally posted by eriktheawful
Edit: Almost forgot: another factor that makes these objects so hard to find is because of how far they are from the sun, they hardly reflect any light at all. When we get to Pluto's orbit, our sun looks like a bright star in the sky. The Oort cloud is much further than Pluto, so the amount of sun light is even smaller.
Just as important, when comets are far from the Sun, they are frozen solid, but as they get closer they start to "out-gas" as water sublimates. This makes the coma, and gives them a tail (or tails), making them much easier to see.
To the OP (and anyone else who is interested), you might like to listen to this recent radio program about comets:
In Our Time(BBC Radio 4)
I'm not sure if it's available to listeners outside the UK, but I think it's worth posting the link anyway.
Also, to the OP, there may be large gaps in our knowledge of comets, but we have also learned a lot about them, especially in recent years. We have had probes visit comets, and even return samples of a comets coma. Lets not forget that meteor showers (which are usually due to comets), and there are many of them (over 500 known showers), also give us a chance to gather data on comets, for example, by analyzing the spectra of meteors, we can tell what these small particles of comet are composed of. Since meteoroids share very similar orbits to their parent bodies, this also gives us clues as to the orbits of comets.