Trail left by Chelyabinsk fireball on February 15th, 2013.The Chelyabinsk asteroid is estimated to have had an original diameter of about 17 metres and was undetected prior to its atmospheric entry. Tracking and calculating the orbits of these potentially hazardous bodies is currently a high priority for astronomers and planetary scientists. image: M. Ahmetvaleev/NASA JPL
Earth-crossing asteroids are those that traverse the Earth’s orbit and as a result have the potential to impact our planet. The orbital characteristics of the largest of these bodies has now been determined and as a consequence their potential threat level is known with reasonable certainty. Smaller asteroids, such as the one that produced the 2013 Chelyabinsk fireball, have yet to be fully evaluated and hence still pose a significant and as yet undetermined level of threat.
Asteroid 3200 Phaethon is an interesting example of an Earth-crossing asteroid in that it is also associated with a well-known annual meteor shower, the Geminids, which takes place each year in early December. Image: D.S. Lauretta/C. Hergenrother