I am leading a spitzer Space Telescope survey of Jupiter-family comets, to make the connection between comets and meteor streams and determine what comets are made of. Historically we look at the material comets shed as they approach the Sun, and guess that it is representative of the nucleus. Of course, it would be much better to go to the nucleus and sample it, but the only significant effort to do that is the European Rosetta mission, which is on the way to comet 67P/Churyumov-Gerasimenko, and the NASA Stardust mission, which recently passed through the coma of 81P/Wild 2 and is soon to drop a capsule containing comet and interstellar dust nack down to Earth. NASA tried to turn the tables in 2004 by smashing a projectile into comet 9P/Tempel 1 as part of the Deep Impact" mission. The only way to determine what other comets are made of (and even to make significant progress on those two comets) is to study the particles and gas they produce.
From the Giotto mission that passed through the coma of 1P/Halley in 1986 as well as 21P/Grigg-Skjellerup, from the Stardust encounter with 81P/Wild 2, and from modeling images taken from the ground, we know that the mass of particles produced by comets is dominated by the largest particles. Large particles were definitely encountered by these spacecraft: they damaged the instruments (ripping the cover off the Giotto camera) and they changed the spacecraft trajectory upon impact. We also all know and love these large particles as meteors, when the enter the Earth's atmosphere. Meteor streams are produced when the Earth passes through the orbit of a comet and particles from 100 microns to 1 mm in size zip into the atmosphere, generating an ionized streak.
Our infrared survey uses the Spitzer Space Telescope to image the distribution of large particles from 30 comets. We can separate the large particles form the small ones becuase the small ones are affected by radiation pressure, blowing back into the well-known "dust tail". The large particles feel only a very small effect, and have orbits very similar to those of the parent comet. They gradually fill the orbit as tiny perturbations, both due to radiation pressure and the pulls of the planets, change the particles orbits relative to that of the nucleus.
The image above is comet Shoemaker-Levy 3, imaged at 24 microns. Here is the image release on the SSC website: Press image release on Spitzer images of comet trails
And here is material that I wrote to describe our work with the Infrared Space Observatory: Comet Encke (1997 observations and analysis)