Albedo and Size Determination of Potentially Hazardous Asteroids and the Case of (99942) Apophis
Delbo, Marco¹; Alberto, Cellino^2
1UNS-OCA-CNRS; ²INAF-Osservatorio Astronomico di Torino

Determination of sizes and albedos is a fundamental task for the physical characterization of of near-Earth asteroids (NEAs). Knowledge of the size distribution of the NEA population is crucial for an accurate assessment of the impact risk that these bodies pose to our planet. NEA sizes are also required for the calculation of the size-dependent force arising from the thermal emission of these bodies. This force causes the so-called Yarkovsky effect, a slow drifting of the orbits of NEAs that is not negligible when accurate orbital evolution is required, as in the case of Earth impact probability predictions.

However, sizes for most NEOs are based on an assumed albedo, which is known to vary by factor of 4. This results in a factor of 2 uncertainty in the size. Since the impact energy is proportional to the cube of the size, the impact energy is therefore uncertain by a factor of 8.

We will describe the most recent progress and results in the albedo and size determination of NEAs and potentially hazardous asteroids by means of observations in the thermal infrared and optical polarimetry.

Thermal infrared observations have provided the large majority of asteroids sizes and for some objects also information about the surface thermal inertia. The latter determines the temperature distribution over the surface of an asteroid and therefore governs the magnitude the Yarkovsky effect.

Polarimetry allows the albedo of an asteroid to be derived from an empirical relation between this parameter and the rate of change of the degree of linear polarization of the light reflected by the asteroid surface as function of the solar phase angle. The largest source of uncertainty comes from the error in the determination of some coefficients appearing in the albedo-polarization relationship. Based on recent VLT observations of (99942) Apophis at V = 19.5 PHA at solar elongations between 64° and 83°, from which we obtained an albedo with an uncertainty of ~6%, we show that useful polarimetric observations can be obtained on V = 22 mag NEOs in 2-hrs (wall-clock time).