Comet Encke observed by ISO: a closer look at a frozen mudball in action
Below is a schematic view of the dust trail, comparing it to the other well-known parts of a comet--the nucleus, coma, and tail. The trail makes a straight line across the image. It contains substantially more mass than the tail and coma, even though it is not as bright, because the trail is composed of large particles that have relatively less surface area. Just as optical observations are biased toward the emission from gas, the infrared surface brightness observations are biased toward the smaller dust particles that have more surface area per unit mass. The sensitive observations possible from space revealed the dust trail, which continues well out of the picture shown here, stretching over at least 60 degrees of the comet's orbit!
We took a picture of comet 2P/Encke using the Infrared Space Observatory (ISO) this July (14th). The image was made by pointing the telescope in a regular grid of positions and taking a picture at each position using the mid-infrared camera (ISOCAM). The comet was moving pretty fast--it moved about half-way across the area that we mapped, during our observation! So to put all the iamges together, I shifted each individual image by exactly opposite the motion of the comet. The picture above is actually a black and white, smoother version of the ISO image--it is not an artist's rendition! Look at the real picture by clicking the link here. In the real picture, the stars appear as multiple "dots" because the telescope moved with respect to them, but the comet appears in a single frame. The goal of these observations was to observe the large particles that are ejected by the comet. The "tail" of a comet is well-known from the appearance of the comet in visible light: it is due to very small dust particles and also emission from ices and vapors emitted by the comet. The "trail" of a comet is a different phenomenon: large particles (rocks and boulders) that fall off of a comet have orbits not too different from the comet itself. They are blown off of the comet relatively gently and then they slowly drift away. (The small particles move more quickly away from the comet because they are dragged by the fast-moving gas and because they feel radiation pressure from sunlight.) These large particles for what we call a "trail" that fills the orbit of the parent comet.
The observations shown below are an attempt to see at higher resolution how the trail connects to the parent comet. We already know that the trail contains much more material than the tail, and that the "rocks" that comprise the trail are numerous enough that the comet cannot survive too long before losing a good fraction of its total mass. More details will be forthcoming. This project is a collaboration between myself (William Reach) and Mark Sykes of U. Arizona, John Davies of the Joint Astronomy Center in Hawaii, and David Lien of Vanguard Research in Scotts Valley California.
