Ejection Velocity Field Research Articles

The Puzzling Case of the Nysa–Polana Family

The Nysa–Polana dynamical family is one of the most complex groupings identified in the asteroid main belt. It has a generally twofold structure and is located in a region of the belt showing an unusual abundance of the fairly rare F-type taxonomic class. We present here new spectra of 22 family members, observed at ESO (European Southern Observatory) and at CASLEO (Complejo Astronomico El Leoncito, Argentina). We find evidence that the family is actually formed by two distinct groupings mutually overlapping in the space of orbital proper elements. The first group of objects, consisting of dark asteroids including several F-type members, can be named after its least-numbered member, 142 Polana. The second group consists of S-type asteroids and should be named after its most plausible least-numbered member, 878 Mildred. Some detailed physical analysis of the two groupings can be already attempted at this stage, including a reasonable reconstruction of the original ejection velocity field for the Mildred family. However, we face here a number of intriguing problems that deserve further analysis. For instance, it is not clear how to interpret the role of the two largest nominal members of the clan, 44 Nysa and 135 Hertha, as well as the presence of several large F-type asteroids apparently not related, but very close to the clan. Moreover, recent discoveries of common spectral features indicating hydrated minerals in the spectra of Nysa and Hertha add complexity to an already puzzling scenario.

Maria's Family: Physical Structure and Possible Implications for the Origin of Giant NEAs

An extensive analysis of the Maria family is presented. A reconstruction of the original ejection velocity field of the fragments suggests that a substantial number of relatively large fragments may have been injected into the neighboring 3/1 mean motion resonance with Jupiter. This also agrees with an analysis of the size distribution of the family, suggesting that about 10 objects in the size range 15–30 km have probably been lost. As a consequence, Maria's family can be seen as one of the most promising candidates for the source of the “giant” near-Earth asteroids 433 Eros and 1036 Ganymed. This is also supported by an extensive spectroscopic survey of the family members in the spectral range 5000–10000 Å. The spectra obtained are very similar and are consistent with the known spectra of Eros and Ganymed.

Reconstructing the Original Ejection Velocity Fields of Asteroid Families

The kinematical properties of the fragment ejection processes in the events that produced the presently known asteroid families have long been poorly understood due to the presence of a couple of unknown angles (the true anomaly and the argument of the perihelion of the parent body at the epoch of the family formation) in Gauss's equations. In the present paper a general procedure for obtaining a reliable estimate of the unknown angles starting from the known values of the proper semimajor axis, eccentricity, and inclination of the family members is described and extensively tested. The results of numerical simulations are highly encouraging when we consider some plausible structures of the ejection velocity fields in agreement with those observed in laboratory experiments. In these cases, we show that the unknown angles can be reliably determined, and the ejection velocities of the fragments can be fairly well reconstructed. An application of this technique to a sample of real asteroid families shows structures of the resulting ejection velocity fields to be fairly plausible according to evidence from laboratory experiments. An interesting by-product of this analysis is an evaluation in each case of the direction of impact of the original projectile in the reference frame of the orbiting parent body. The relevance of these results for the physical studies of the asteroids and their collisional evolution is discussed.

Field Fragmentation of Macroscopic Targets Simulating Asteroidal Catastrophic Collisions

We report the results from a series of experiments in which spherical macroscopic targets of artificial rock were disrupted using contact charges, and concentrate on the detailed analysis of four tests. The experiments were performed in the open, so we have been able to identify the shape and measure the sizes of fragments from the primary fragmentation event and to record the uninterrupted ballistic trajectories of the fragments. The targets were spheres of about 21 cm in diameter, two of them with stronger cores. From the recorded distribution of fragments on the ground, and using fast framing photography and our own image processing computer system, it has been possible to obtain the mass, shape, velocity, and angular velocity for several hundred fragments and from these to deduce the corresponding statistical distributions, as well as information concerning the geometry of the ejection velocity field and the correlations between different fragment properties. Most results compare satisfactorily with the predictions of the numerical model by Paolicchi et al. ( Icarus 77, 187-212 (1989); Celest. Mech. 57, 49-56 (1993)). The presence of a core inside the target is found to influence the dynamics of the fragmentation process and the resulting velocity field, albeit not the fragment size distribution. The experimental fragments share some observed statistical properties of the asteroid families, in particular the higher dispersions of ejection velocity and rotation rate for smaller bodies.

The accuracy of proper orbital elements and the properties of asteroid families: Comparison with the linear theory

The accuracy and reliability of the proper orbital elements used to define asteroid families are investigated by simulating numerically the dynamical evolution of families assumed to arise from the “explosion” of a parent object. The orbits of the simulated family asteroids have then been integrated in the frame of the elliptic restricted three-body problem Sun-Jupiter-asteroid, for times of the order of the circulation periods of perihelia and nodes. By filtering out short-periodic perturbations, we have monitored the behavior of the proper eccentricities and inclinations, computed according to the linear secular perturbation theory. Significant long-period variations have been found especially for families having nonnegligible eccentricities and/or inclinations (like the Eos family), and strong disturbances due to the proximity of mean motion commensurabilities with Jupiter have been evidenced (for instance, in the case of the Themis family). These phenomena can cause a significant “noise” on the proper eccentricities and inclinations, probably affecting in some cases the derived family memberships. They can also give rise to a spurious anisotropy in the fragment ejection velocity fields computed from the dispersion in proper elements observed in each family, and this could explain the puzzling anisotropies of this kind actually found in real families by D. Brouwer (1951, Astron. J. 56, 9–32) and by V. Zappalà, P. Farinella, Z. Knežević, and P. Paolicchi (1984), Icarus 59, 261–285).