chosen C-type target would be lower, around 1 g \( \mathrm { c m } ^ { - 3 } \)(Britt et al., 2002). The composition, mass and internal properties (rubble pile versus monolith) will all play a role here.

# 4. Conclusions

We have collected the available data on very small asteroids (VSAs) with the highest quality light curves. Unsurprisingly, a VSA will most likely be found to have a period under the “spin barrier” of 2.2 hrs; the average period from the \( D \le \) 60 m sample analyzed here is 0.67 hrs or 40 minutes. The lower limit for the period of the current sample reaches down to 25 seconds (2010 \( \mathrm { J L } _ { 8 8 } \), a 13 m diameter S-type) or even less (2014 RC, a 12-22 m Sq-type, with a period of 16 seconds) though shorter periods are possible.

With respect to structure, our results imply that a VSA will probably be a monolithic structure in which a singular boulder is held together by its own tensile strength, as opposed to a “rubble pile” in which many boulders are gravitationally bound together, although arguments from Holsapple (2003) and new evidence from Mommert et al. (2014) show that this may not necessarily be true.

We used the information on the light curves provided by various surveys to estimate the axial ratio. The VSAs in our samples have an average minimum \( a / b \) ratio of about 1.4, and the VSA with the greatest axial ratio was found to be 2007 \( \mathrm { T S _ { 2 4 } } \) at 2.8. Alternate analyses of some asteroid light curves have given slightly different values, but all VSAs observed to date are consistent with axial ratios less than three. The mission outlined by the KISS report discussed a capture bag capable of accommodating a 10x15 meter asteroid with a 2:1 axis ratio. Most (\( > \) 90%) of our \( D \le \) 60 m restricted sample have an \( a / b \) ratio less than 2, but a few exceed this value. We do note that our method of determining axial ratios from Kwiatkowski et al. (2010a) provides the minimum axial ratio consistent with the light curve amplitude, and so the values reported here are lower limits.

The composition of most potential targets is likely to be rich in silicates (S-type taxonomic class). The KISS study suggested that C-type asteroids would make more interesting targets be-cause of their more diverse composition, which include water, carbon compounds, rock and metal. However, such asteroids are not common within the currently characterized small near-Earth as-teroid population, though four C-group (including sub-types B and F) and three X-types appear in our sample. Though the real NEA population is not necessarily this low in C types, the list of potential ARM targets is likely to be poorer in carbonaceous bodies than might otherwise be expected.

# 5. Acknowledgements

The authors thank Duncan Steel and David Asher for thoughtful comments which much im-proved this paper. This work was supported in part by the Natural Sciences and Engineering Council of Canada and the Canadian Space Agency (CSA) through the ASTRO CSA Cluster.

# References

Britt, D. T., Yeomans, D., Housen, K., and Consolmagno, G. (2002). Asteroid Density, Porosity, and Structure. Asteroids III, pages 485–500.

Brophy, J., Culick, F., Friedman, L., and 31 others (2012). Asteroid retrieval feasibility study. Technical report, Keck Institute for Space Studies, Pasadena, California.