Long-term evolution of observable comets. The time is given in millions of years before the time of observation. The open circles correspond to the initial position, the filled circles correspond to the original orbit at the perihelion preceding the observation, and the stars correspond to the original orbit when the comet is observable. The evolution is given by one point every 250 Myr. For each panel, the upper diagrams show the evolution versus time of the semimajor axis (dashed line) and perihelion distance (full line); the two lower diagrams are, from left to right, the motion of the comet in the (Ω_G, cos i_E) plane and in the (l_G, sin b_G) plane. The black region in the (Ω_G, cos i_E) plane is a forbidden region. In the (l_G, sin b_G) plane, the full black curve and the dashed black curve are, respectively, the trace of the ecliptic plane and the EE plane. The boldface letter in the upper-right corner of each panel refers to the long-term behavior described in the main text.