Astron. Astrophys. 356, 747-756 (2000)

## 5. Conclusions

This paper shows that ejecta from cratering events can be placed into temporary orbits about asteroids or comets. The perturbations due to the solar radiation pressure can alter the initial orbital elements after the impact and transform re-impact orbits into a temporary capture orbits. By using an approximate analytical theory (RPA) we are able to explicitly compute the lifetime of particles as a function of their size, ejection site, ejection velocity, and ejection angles and . We derive also the ranges of ejection velocity leading to capture instead of re-impact or hyperbolic escape.

We have performed numerical comparisons showing that the theory is qualitatively correct, making it useful for statistical studies at the least. Through this comparison we can also understand more precisely the physics behind temporary capture of particles about comets and asteroids. We used as a test bench the DI experiment at comet Tempel 1 and we found that dust grains from submillimeter to centimeters in size can be injected into temporary capture orbits around the comet after the experiment. We defined ranges in ejection velocity and ejection angles for trapping. These ranges are qualitatively correct and are intended as useful indications to the more sophisticated numerical models that will be developed once the shape, gravity of the comet and outgassing activity of a particular comet are known.

There are a number of effects that are not accounted for in the RPA approximation, like irregularity of the gravity field, solar tide, and cometary outgassing. During our numerical study we could delineate how the predictions of RPA are affected by solar tide. However, we can only estimate the changes in the orbital evolution induced by the other two perturbing forces. Gravity field perturbations affect only a small percentage of particles whose periapses are very close to the nucleus during the temporary capture. If the pericenter passage occurs within 5 nucleus radii, the irregular field of the body can either eject the particle in a hyperbolic orbit or cause the particle to be more closely bound to the nucleus. Comet outgassing, if collimated into jets, represents a significant perturbation only if the particle crosses the jet. An extended outgassing field may alter the eccentricity of the orbit and again cause escape or even trapping.

© European Southern Observatory (ESO) 2000

Online publication: April 10, 2000