STEREO has provided some spectacular observations of comets.
This STEREO HI movie above shows Comet McNaught, seen in January, 2007. The structure in the tail of the comet can be clearly seen. The bright comet head is to the right the field of view and saturates the images. These images are probably the most detailed images ever taken of a comet while it is so close (0.17 Astronomical Units, which is even closer than Mercury) to the Sun.
Also visible in this movie is Venus (the bright object left of centre at the bottom) and Mercury (appears from the right later in the sequence). Their brightness creates saturation streaks.
In the movie above, STEREO has captured a collision between a coronal mass ejection (CME), and the comet ENCKE. The collision was so fierce that it ripped off the comet tail (this happens just after half way through the movie). Comets are icy leftovers from the formation of the solar system, billions of years ago. They usually hang out in the cold, distant regions of the solar system, but occasionally a strong gravitational tug pulls them into the inner solar system. Once there, the Sun's heat and radiation vaporizes gas and dust from the comet, forming its tail. Comets typically have two tails, one made of dust and a fainter one is made of electrically conducting gas, called plasma.