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In this paper we present the observational campaign carried out at ESO NTT and VLT in April and May 2006 to investigate the nature and the structure of the Near Earth Object (144898) 2004 VD17. In spite of a great quantity of dynamical information, according to which it will have a close approach with the Earth in the next century, the physical properties of this asteroid are largely unknown. We performed visible and near--infrared photometry and spectroscopy, as well as polarimetric observations. Polarimetric and spectroscopic data allowed us to classify 2004 VD17 as an E-type asteroid. A good agreement was also found with the spectrum of the aubrite meteorite Mayo Belwa. On the basis of the polarimetric albedo (p_v=0.45) and of photometric data, we estimated a diameter of about 320 m and a rotational period of about 2 hours. The analysis of the results obtained by our complete survey have shown that (144898) 2004 VD17 is a peculiar NEO, since it is close to the breakup limits for fast rotator asteroids, as defined by Pravec and Harris (2000). These results suggest that a more robust structure must be expected, as a fractured monolith or a rubble pile in a "strength regime" (Holsapple 2002)., Comment: 32 pages, 7 figure, paper accepted for publication in Icarus

We consider a small sample of known near Earth objects (NEOs), both asteroids and comets, with low minimum orbital intersection distance (MOID). Through a simple numerical procedure we generate slightly different orbits from this sample in such a way that these bodies will collide with the Earth at a specific epoch. Then we study the required change in orbital velocity (along track Δv) in order to deflect these NEOs at different epochs before the impact event. The orbital evolution of these NEOs is performed through a full N-body numerical integrator. A comparison with analytical estimates is also performed in selected cases. Interesting features in the Δv/time before impact plots are found; as a prominent result, we find that close approaches to the Earth before the epoch of the impact can make the overall deflection easier.

A simple probabilistic model for estimating the population of near-Earth asteroids (NEAs) by comparing the rate of redetections of previously known asteroids of a given size with new detections during a test interval of time is described. We apply this method to the whole sample of NEA observations (discovery observations and observations from apparitions of previously known asteroids) made by the Lincoln near-Earth Asteroids Research Project within the period Jan. 1, 1999–Dec. 31, 2000 and published by the Minor Planet Center. The numbers of discoveries and redetections are sufficient to estimate the total population in the absolute magnitude range 13.5 H ≤20.0. By this method we obtain an estimate of the number of NEAs of H ≤18.0 (diameter ≥ 1 km) of 855±101. We note, however, that the expected biases of this method all work in the direction of underestimating the true population; thus our estimate is consistent with other recent estimates, for example, W. F. Bottke et al. (2000, Science 288 , 2190–2194), who obtain 910±110.