We use large cosmological N-body simulations to study the subhalo population in galaxy group sized halos. In particular, we look for fossil group candidates with typical masses ~10-25% of Virgo cluster but with an order of magnitude less substructure
. We examine recent claims that the earliest systems to form are deficient enough in substructure to explain the luminosity function found in fossil groups. Although our simulations show a correlation between the halo formation time and the number of subhalos, the maximum suppression of subhalos is a factor of 2-2.5, whereas a factor of 6 is required to match fossil groups and galaxies. While the number of subhalos depends weakly on the formation time, the slope of the halo substructure velocity function does not. The satellite population within Cold Dark Matter (CDM) halos is self-similar at scales between galaxies and galaxy clusters regardless of mass, whereas current observations show a break in self-similarity at a mass scale corresponding to group of galaxies.
Asteroids can be eclipsed by other bodies in the Solar System, but no direct observation of an asteroid eclipse has been reported to date. We describe a statistical method to predict an eclipse for an asteroid based on the analysis of the orbital ele
ments covariance matrix. By propagating a set of Virtual Asteroids to an epoch correspondent to a close approach with a Solar System planet or natural satellite, it is possible to estimate the probability of a partial or total eclipse. The direct observation of an eclipse can provide data useful to improve the asteroid orbit, especially for dim asteroids typically observed only for a few days. We propose two different methods: the first, based on the inclusion of the apparent magnitude residuals into the orbits least squares minimization process, capable of improving the asteroids nominal orbit and the related covariance matrix; the second, based on weighting different Virtual Asteroids in relation to their apparent magnitude during the eclipse, useful for recovery purposes. As an application, we have numerically investigated the possibility of a Near Earth Asteroid eclipsed by the Moon or the Earth in the 1990-2050 period. A total of 74 distinct eclipses have been found, involving 59 asteroids. In particular, the asteroid (99942) Apophis has a probability of about 74% to enter the Moons penumbra cone and a probability of about 6% to enter the umbra cone on April 14, 2029, less than six hours after a very close approach to Earth.
