## 5. Collisional properties of individual Hilda asteroidsThe collision probability of the Hilda asteroids are dominated by HM collisions that only occurs when they are relatively close to their perihelia. Depending on the individual eccentricities of the Hilda asteroids they will reach unequally deep into the main-belt. The deeper into the main-belt the Hilda asteroids can reach they will increase their probability of encountering a main-belt asteroid for two reasons. The Hilda orbit will cross the orbits of a larger fraction of main-belt objects, and the Hilda asteroid will be in the `main-belt space' during a larger fraction of its orbit. Schubart (1982) showed that the eccentricity of individual Hilda objects oscillates around a fixed mean value for at least years. An extension of Schubart's study has been made (unpublished) with a numerical integration taking into account perturbations from Venus to Neptune. The results showed that the Hilda orbits are stable for at least years. Also Franklin et al. (1993) concluded that the orbits of Hilda asteroids seem to be stable during the history of the Solar System. The stability of the mean eccentricity gives the expectation that there will be a strong correlation between mean eccentricity and collision probability for Hilda objects. This is also indicated by the wide range of among the Hilda asteroids seen in Fig. 5. This is verified in Fig. 6, which shows a very strong correlation
between the mean eccentricity during time
Fig. 6 (lower panel) gives the mean relative velocity versus
for the Hilda asteroids. The mean velocities
range from 3.3 to 6.0 , and a tentative
correlation between and
is indicated in the data (
To illustrate the scatter of collision properties of the Hilda population, the distribution of collision velocities of four Hilda asteroids are given in Fig. 7. The areas under the histograms are proportional to the collision probability (and ) of the objects. In the velocity distribution of 190 Ismene, 3% of the encounters have a very low velocity, 0.3 . All these encounters are with the Hilda asteroid 2246 Bowell. This shows that due to the similar shapes of Hilda orbits the relative velocity between two objects can be very low at an encounter, or at a series of encounters. This gives a low velocity tail in the velocity distributions for some of the Hilda asteroids, which is also seen in the velocity distribution of HH collisions in Fig. 1.
The great difference in collisional probability among the Hilda
asteroids are illustrated by 1256 Normannia and 2483 Guinevere, which
has the lowest and highest , respectively. The
spread in collision velocities among the Hilda population are
illustrated by 1256 Normannia ( =
3.39 ) and the high inclination object
1877 Marsden ( © European Southern Observatory (ESO) 1998 Online publication: July 27, 1998 |