But how do you know which comets and asteroids can be viewed? The answers are found in the links below.
Bright Comets Currently Visible: Skyhound's monthly list of Comets Visible This Month.
Bright Asteroids Currently Visible: The Minor Planet Center at Harvard maintains a list of Bright Asteroid Oppositions for each Year. Bright asteroids are named, and for each one is given its date of opposition from the Sun, and its visible magnitude at that time. The visibility of each asteroid will be best around the time of its opposition, and worst about six months before or after. All of these asteroids should be easily visible in telescopes of at least 5" diameter; start with the brightest ones (Vesta, Ceres, etc.). To identify it, use Skycharts, and observe on two or more different nights to verify that the object has changed its position.
Near-Earth Asteroids: When the Near-Earth Object box is checked, minute-by-minute positions are downloaded for fast-moving asteroids. However, only 24 hours' worth of positions are obtained. Therefore, you can set RTGUI to some non-real time (for example, the beginning of the night on which you plan to observe the object) to obtain positions starting at that time. The 24 hour period begins at whatever time is set in RTGUI. If the object will be visible on two or more nights, you can create separate files for each night you plan to observe.
NASA's compendium of Recent and forthcoming near-earth asteroid approaches. (Names from this list should be entered without parentheses.)
Another list: Tom Fine's asteroid flyby page.
So far, none of the objects listed here has hit the Earth. Keep your fingers crossed.....