http://en.wikipedia.org/wiki/List_of_asteroids_in_astrology
Asteroids in astrology are used by some astrologers in a similar way to the planets. Asteroids are planetoids that orbit the Sun between Mars and Jupiter, and are not to be confused with centaurs or trans-Neptunian objects. The asteroids are relatively new to astrology, having only been discovered in the 19th century. However, some of them (especially the largest of them), are believed by some astrologers to influence human affairs. Still though, they are often ignored within mainstream systems of astrology, especially in more traditional astrology systems like Vedic astrology or Hellenistic astrology. Their use has become significant to a few Western astrologers yet still only a minority of astrologers use the asteroids in chart interpretation.
I don't know much about them. But, then I don't know a whole lot about planetary astrology either. Alex Miller is a well respected astrologer. I have no reason not to pay attention to his analysis and explanation. Portions of this article from Daykeeper is also posted on Mountain Astrologer, a favorite of mine.
From what I've read so far, the asteroids (at least the first four discovered) are associated with the Feminine. That's going to give what we think of as "dark" energy, as darkness is a feminine aspect. Judging by this article, they take events deeper. Even Chiron has been considered an asteroid.
http://www.astrology3d.com/educenter/asteroids.html
Over the past two decades the use of the first four discovered asteroids has become popular in astrology. The asteroids are a series of minor planets mostly between the orbits of Mars and Jupiter. Some think the asteroids are the remnants of a planet which exploded. Others believe they are a planet which never coalesced. not many years ago a more recently discovered asteroid named Chiron )located between the orbits of Saturn and Uranus) was also added to the astrological toolbox.
What follows is a brief description of these five asteroids along with some suggested meanings. As with the planets,the influences of the asteroids are strongly linked to the mythological legend for which they are named. It should be stressed that a final determination the asteroids' meanings await astrologers' use, experience and interpretation. In general, the first four asteroids add a feminine influence to the masculine-dominated hierarchy of the standard ten planets. In fact, these four asteroids can be correlated to the four archetypes of woman as given in Edmund Whitman's book The Symbolic Quest. Another book, Asteroid Goddess by Demetra George, also comprehensively examines the feminine principle of the first four asteroids.