Eros, Southern Hemisphere NEAR Shoemaker Altitude 200 km

1. Modeling Fluidization on Asteroids as Cause of Ponded CratersJ. D. Haseltine1,2, M. A. Franzen1, D.W. G. Sears,11 Arkansas Center for Space and Planetary Sciences, University of Arkansas, Fayetteville, Arkansas. 2 Abilene Christian University, Abilene, Texas. Eros Geology • NEAR Mission • Near-Earth Asteroid Rendezvous • Shoemaker spacecraft took photos of asteroid Eros. • Ponded Craters • Ponded craters on surface of Eros appear to be impact craters that have formed a flat layer of regolith on the crater floor. • These craters are found primarily in the equatorial region of the asteroid. • The average grain size of the regolith pond is substantially smaller than the average for the crater and surrounding area. • Possible Causes • Seismic shaking: The craters erode at a faster rate due to seismic movement. • Electrostatic levitation: The grain separation is caused by high electrostatic charge in the equatorial region. • Fluidization: The presence of volatiles beneath the asteroid surface causes fluidization, which results in grain separation and ponds. This research centers around the possibility of fluidization as the cause of ponded craters. Eros, Southern HemisphereNEAR Shoemaker Altitude 200 km Fig. 2 Crater through fluidization process, water as volatile Fluidization Model • Experimental Set-up • 18 cm regolith was placed in a .5-m diameter bucket within the Andromeda Chamber. • The volatile (20g ice or 10mL water) was placed beneath the surface of the regolith. • The chamber was pumped down to low pressure (Fig. 1). • Pressure and temperature were measured over time. • Current Results • Water as a volatile has produced similar results to those previously found (Fig. 2). • Redistribution and stratification of particles based on size occurs (Fig. 3). • Future experiments will focus on the effects of fluidization on possible asteroid conditions, including pre-existing craters and large boulders. • Size analysis is planned to determine the actual level of grain sorting occurring during fluidization. photos Fig. 1 Pressure Comparison for Initial Trials Fig. 3 Grain size sorting after fluidization Acknowledgements M. S. Robinson, et al. The Geology of Eros. Meteoritics & Planetary Science, 37 (2002). M. S. Kareev, et al. Origins of 433 Eros ponds: Laboratory Investigations of Regolith Fluidization by Volatile Degassing. Kunii, Daizo and Levenspiel, Octave. Fluidization Engineering. Butterworths: Boston, 1991.

2. Simulation of Near-Earth Asteroid Sample Collection at Non-normal Angles and Transverse VelocitiesJ. D. Haseltine1,2, L. A. Roe1,3 1 Arkansas Center for Space and Planetary Sciences, University of Arkansas, Fayetteville, Arkansas. 2 Abilene Christian University, Abilene, Texas. 3 Mechanical Engineering Department, University of Arkansas, Fayetteville, Arkansas. • Experimental Set-up • A rover was built with forward motion and lateral motion in two directions. • An actuator was attached to the rover to allow the connected TGIP sampler head to lower and raise. • Four types of regolith was created, matching the samples used in past experiments. • Large and small gravel (1-2 cm, <1 cm). • Three sand mixture: 210-300 μm; 600-850 μm; >850 μm. • Sand (300-425 μm), gravel, and iron filings (53-74 μm). • Gravel and sand stratified. • Current experiments include regolith of generic sand mixture was spread across a tarp at ~1-2 cm depth for sampling. • Introduction • HERA is a proposed mission to collect 3-100g samples from a near-Earth asteroid. • HERA is a joint project between the Arkansas Center for Space and Planetary Sciences and Jet Propulsion Laboratory. Fig. 1 Mass collected in initial runs Initial Runs Particle size 300-625 μm Rover and Sample Set-up Run 3 Run 4 Run 1 • Project History • Several sampler heads were considered before the current design (Touch-and-Go Impregnable Pad, or TGIP) was found to be most successful at sample pickup. • Past experiments have been done on the TGIP sampler to ensure that it will function under all possible conditions, including extreme temperatures and radiation exposure. • The TGIP sampler is filled with ~100 g of polydimethyl siloxane (PDMS) to act as an adhesive for sample collection. • Current experiments are being run to determine the reentry stability of the TGIP sampler and the return capsule and to discover the most efficient process for sample cleaning. Run 5 Run 2 Run 1 Run 2 Run 3 • Results • Experiments have currently been done with a sample formed of 300-625 μm sand. • No transverse velocities have been tested. • Contact time has been found to have more impact on sample size than angle of contact (Fig. 1). • The current sampler head does not rotate at sufficient angles to account for all possible contact angles. Acknowledgements Dougan, T. (2002) C.P Presentation: Building an Asteroid Sampler: Preliminary Design and Testing. Franzen, M. et al. (2005) Sample Collection from Small Airless Bodies: Examination of Temperature Constraints for the TGIP Sample Collector for the Hera Near-Earth Asteroid Sample Return Mission. Venechuk, E. M. et al. (2005) Radiation Resistance of a Silicone Polymer Grease Based Regolith Collector for the HERA Near-Earth Asteroid Sample Return Mission. Buffington, J. A. et al. (2005) Simulation of Extraterrestrial Sample Acquisition. Run 4 Run 5 • Plans for Future Research • The experiment will be continued to include data from all four sampling types at various angles and velocities.