NASA's Asteroid Redirect Research Articles

Resistance forces during boulder extraction from an asteroid

Planning for NASA's Asteroid Redirect Mission (ARM) requires estimating the forces that appear during extraction of a boulder from the surface of an asteroid with unknown surface regolith properties. These forces are estimated for a vertical constant force or acceleration pull and a rolling, constant force, torque (peel) on a 4-m diameter spherical boulder using both analytic and discrete element method (DEM) models considering the effects of microgravity and regolith cohesion using Johnson–Kendall–Roberts (JKR) model. Estimates of the bulk asteroid regolith cohesion strength derived from lunar and asteroid regolith studies ranged from 25Pa to 250Pa. JKR cohesive forces at particle contacts depend on particle surface energy and effective curvature radius (particle size). DEM particle size dependent cohesion parameters are linked to estimated regolith cohesion strength by simulating shear and tension tests over a range of DEM particle surface energies resulting in the formulation of the dependence of particle surface energy as a function of cohesion strength and particle size. Maximum extraction forces occur for a vertical pull through the boulder center of mass with constant acceleration. Extraction force decreases for a constant force pull to 0.62pcS where S is the boulder surface area embedded in the regolith and pc is the cohesion strength of the regolith. Boulder extraction by peeling produces the smallest forces by up to more than a factor of 2, as the failure across the boulder surface increases progressively rather than being fully engaged as occurs during a vertical pull extraction. Variations between DEM and analytic results differed from 9% to 17% over the range of regolith cohesion values and peel extraction leverage.

On the rotation rates and axis ratios of the smallest known near-Earth asteroids—The archetypes of the Asteroid Redirect Mission targets

NASA's Asteroid Redirect Mission (ARM) has been proposed with the aim to capture a small asteroid a few meters in size and redirect it into an orbit around the Moon. There it can be investigated at leisure by astronauts aboard an Orion or other spacecraft. The target for the mission has not yet been selected, and there are very few potential targets currently known. Though sufficiently small near-Earth asteroids (NEAs) are thought to be numerous, they are also difficult to detect and characterize with current observational facilities. Here we collect the most up-to-date information on near-Earth asteroids in this size range to outline the state of understanding of the properties of these small NEAs. Observational biases certainly mean that our sample is not an ideal representation of the true population of small NEAs. However our sample is representative of the eventual target list for the ARM mission, which will be compiled under very similar observational constraints unless dramatic changes are made to the way near-Earth asteroids are searched for and studied.We collect here information on 88 near-Earth asteroids with diameters less than 60m and with high quality light curves. We find that the typical rotation period is 40min. Relatively few axis ratios are available for such small asteroids, so we also considered the 92 smallest NEAs with known axis ratios. This sample includes asteroids with diameters up to 300m. The mean and median axis ratios were 1.43 and 1.29, respectively.Rotation rates much faster than the spin barrier are seen, reaching below 30s, and implying that most of these bodies are monoliths. Non-principal axis rotation is uncommon. Axial ratios often reach values as high as two, though no undisputed results reach above three. We find little correlation of axis ratio with size. The most common spectral type in the sample of small NEAs is S-type (>90%), with only a handful of C and X types known.