The image below is an up to date map of the solar system displaying the orbits of the terrestrial planets and the estimated position of thousands of known asteroids. This diagram is missing comets, space probes and, of course, the undiscovered asteroids. Even conservative estimates would suggest that for every asteroid on a dangerous Earth-Approaching orbit there are hundreds more which have yet to be discovered. There are over 300 known objects on Earth-crossing orbits, the majority of which are potentially capable of causing death and destruction on a scale unheard of in human history.
The terrestrial planets (Mercury, Venus, Earth and Mars) are shown on the diagram by Cyan or White squares, and their orbits are represented by the blue ellipses around the Sun (the yellow dot at the centre). The Earth is highlighted because of its special importance to us. Small green points mark the location of asteroids which do not approach close to the Earth right now. This does not exclude the possibility that they will do so in the future but generally we can consider the Earth to be safe from these for the near future. Yellow objects (with the exception of the one in the middle which we astronomers call the Sun ;-) are Earth approaching asteroids which are called Amors after the first one discovered. Amors have orbits which come close to the Earth but they don't cross the Earth's orbit. However, their orbits are close enough to the Earth that they could potentially be perturbed by the influence of the planets and begin to cross the Earth's orbit in a short time. There are over 300 known objects on such orbits.
Finally the red boxes mark the location of the Apollo and Aten asteroids. These cross the Earth's orbit and are the most directly identifiable astronomical threat for the near future. Included in this selection is the infamous asteroid, 1997XF11, which made a major impact on the world's headlines in March 1997 when observations indicated that it had a good chance of colliding with the Earth in 2028. Thankfully, new observations were made and the newly calculated orbit predicts a close approach of about 600,000 kilometres. Other asteroids which have orbits which may hit the Earth are 1999 AN 10 and 1998 OX 4. Further observation is required to determine their orbits in sufficient detail to predict an impact or a near miss.
It is estimated that there are perhaps 100,000 to 1,000,000 undiscovered asteroids on similar Earth crossing orbits.
Have a Nice Day.
I've also made up a short movie which shows 400 days in the life of the NEO population.
Alternatively, if you're adventurous you might want to take a look at this C programme for X which can use the current asteroid file to display a realtime animation. It's currently in development but you can play around with it if you want.
This map is a 2 dimensional projection, and it can look very busy in the vicinity of Earth. To get a better idea of the distribution of objects near the Earth I have a map showing the relative positions of all asteroids currently within 0.3 AU.
While you're here you might want to take a look at Armagheddon ;-).
Last Revised: 2010 July 26th