Terrestrial planets are believed to be formed via giant impacts of Mars-sized protoplanets .
Planets formed via giant impacts have highly eccentric orbits .
A swarm of planetesimals around the planets may lead to eccentricity damping for the planets via the equipartition of random energies ( dynamical friction ) .
However , dynamical friction increases eccentricities of planetesimals , resulting in high velocity collisions between planetesimals .
The collisional cascade grinds planetesimals to dust until dust grains are blown out due to radiation pressure .
Therefore , the total mass of planetesimals decreases due to collisional fragmentation , which weakens dynamical friction .
We investigate the orbital evolution of protoplanets in a planetesimal disk , taking into account collisional fragmentation of planetesimals .
For 100 km-sized or smaller planetesimals , dynamical friction is insignificant for eccentricity damping of planets because of collisional fragmentation .
On the other hand , giant impacts eject collisional fragments .
Although the total mass of giant impact ejecta is 0.1-0.3 Earth masses , the largest impact ejecta are \sim 1 , 000 km in size .
We also investigate the orbital evolution of single planets with initial eccentricities 0.1 in a swarm of such giant impact ejecta .
Although the total mass of giant impact ejecta decreases by a factor of 3 in 30 Myrs , eccentricities of planets are damped down to the Earth level ( \sim 0.01 ) due to interaction with giant impact ejecta .
Therefore , giant impact ejecta play an important role for determination of terrestrial planet orbits .