The DECam Ecliptic Exploration Project (DEEP): Examining the Size-Shape Dependence of MBAs and NEOs.

The small bodies of the solar system are powerful tracers of the dynamical, collisional, and compositional history of the solar system. One of the most interesting populations of these bodies are the Near-Earth Objects (NEOs), objects with orbits that pass very close to that of Earths. NEOs are a fascinating and highly-studied population because their proximity to Earth makes them more accessible for observation than similarly-sized main-belt asteroids (MBAs). Because of this observational bias, the majority of known NEOs have diameters smaller than those of known MBAs. McNeill et al. (2019) found that the sub-kilometer NEOs had, on average, higher lightcurve amplitudes than the larger MBAs and are therefore more elongated on average. There is an outstanding question about why small NEOs are more elongated than larger MBAs: size, or dynamics. Here, we examine a sample of ~60,000 main-belt asteroid lightcurves measured by the DECam Ecliptic Exploration Project (DEEP). We find that within the DEEP dataset, different size regimes of MBAs display different average lightcurve amplitudes and that average lightcurve amplitude decreases with size. We find that photometric error is not sufficient to produce this trend, and instead suggest that smaller MBAs are more elongated than larger MBAs. The similar lightcurve amplitude distribution for sub-km MBAs and sub-km NEOs supports the hypothesis that the shape distribution of NEOs is preserved as they enter near-Earth space from the Main Belt, and that dynamical and collisional effects in the near-Earth region do not control the overall shape distribution. This has implications for objects such as Bennu and Ryugu, which likely brought their “top-shaped” structures from the Main Belt. This work is supported by grants from both NASA and NSF.