And the answer is...

Shuttle

While the Sun gravitationally pulls the asteroids inward, the gravitational pull of Jupiter is pulling the asteroids outward. Since the belt is closer to Jupiter than it is to the Sun, Jupiter exerts more of an influence on the asteroids. As a result, the asteroids are held in orbit away from the inner planets.

Shuttle

The StarChild site is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), within the Astrophysics Science Division (ASD) at NASA/ GSFC.

StarChild Authors: The StarChild Team
StarChild Graphics & Music: Acknowledgments
StarChild Project Leader: Dr. Laura A. Whitlock
Curator: J.D. Myers
Responsible NASA Official: Amber Straughn