Constraining the Shape Distribution of Near Earth Objects from Partial Lightcurves

Abstract

In the absence of dense photometry for a large population of Near Earth Objects (NEOs), the best method of obtaining a shape distribution comes from sparse photometry and partial lightcurves. We have used 867 partial lightcurves obtained by Spitzer to determine a shape distribution for sub-kilometre NEOs. From this data we find a best fit average elongation $\frac{b}{a}=0.72 \pm 0.08$. We compare this result with a shape distribution obtained from 1869 NEOs in the same size range observed by Pan-STARRS 1 and find the Spitzer-obtained elongation to be in excellent agreement with this PS1 value of $\frac{b}{a}=0.70 \pm 0.10$. These values are also in agreement with literature values for $1<D<10$ km objects in the main asteroid belt, however, there is a size discrepancy between the two datasets. Using a smaller sample of NEOs in the size range $1<D<5$ km from PS1 data, we obtain an average axis ratio $b/a = 0.70 \pm 0.12$. This is more elongated than the shape distribution for main belt objects in the same size regime, although the current uncertainties are sizeable and this should be verified using a larger data set. As future large surveys come online it will be possible to observe smaller main belt asteroids to allow for better comparisons of different sub-kilometre populations.