This project will systematically investigate the validity of the small-body thermal models used to determine size and albedo information. The commonest models generally assume a spherical geometry and small phase angles. Continued advances in infrared capabilities, including the Spitzer Space Telescope and the Stratospheric Observatory For Infrared Astronomy (SOFIA), are increasing the observational data at a rapid pace, which makes it imperative to assess quantitatively the effect of those assumptions. This project will investigate possible biases through combined analysis of infrared and radar observations of a number of asteroids. Realistic shape models derived from the radar observations will be used to calculate a more accurate thermal emission model, and comparing shape-based models to the most common thermal models provides a quantitative analysis of the biases those models introduce into the size and albedo estimates. In particular, this work will examine whether more realistic shapes will reduce the biases, and will derive a more representative thermal phase function for small, irregular bodies.
The impact of this research is far reaching, including helping with proper mitigation of the hazard of Earth impact by asteroids and comets. Numerous educational opportunities for students at all levels and for teachers will be actively pursued, including involvement in mentoring and internship vehicles already in place at The Johns Hopkins University's Applied Physics Laboratory. The Arecibo Observatory regularly engages students, and conducts both student and teacher workshops. There will also be opportunities to involve students at small universities and colleges that otherwise might not get to participate in a large research project.
Our view of the early solar nebula and planetary accretion depends on understanding the most primitive solar system bodies, yet as we sample ever smaller sizes of near-Earth asteroids (NEAs), we see an increasing variation in the range of physical properties. The regolith on an asteroid surface controls its thermal properties and often its radar reflectance as well, and at smaller sizes the irregular shape plays an increasingly important role. The size and albedo distribution are still poorly understood for the smallest objects. Whereas spacecraft missions will reveal details of a few NEAs, only ground-based observations will provide an overall understanding of the population of small bodies. Each radar experiment is like a "flyby mission", showing a diverse range of shapes, surface features, and rotation states among NEAs. Infrared observations of these objects are equally varied, illustrating a range of spectral types and thermal characteristics. Aside from those few objects directly measured with radar, most information about the sizes of asteroids comes from modeling the thermal emission and deriving the size from the visible reflected light and thermal flux. With few exceptions, these models assume only a spherical shape along with a simple standard phase curve. Smaller asteroids are expected to have increasingly non-spherical shapes and thus self-shadowing and changes in surface regolith will have an increasingly large effect. The goal of this investigation was to use both measured radar reflectance and thermal properties to better understand the regolith of different types and shapes of NEAs and to use that knowledge to quantify the systematic biases in existing thermal models that are based on simple assumptions of regolith properties and shape. The last decade has seen remarkable improvements in our capability to determine accurate shape models for small bodies using radar techniques. These realistic shape models can be used to calculate a more accurate model thermal emission for comparison to the IR observations. Radar-derived shape models were used to investigate the thermal emission of these asteroids, using the IR observations to constrain the thermal models. In particular, the possibility that more realistic shapes allow a more complete investigation of the surface properties was examined, and a better understanding of how thermal models apply to small, irregular asteroids was derived. The observations, conducted at Arecibo Observatory and the NASA Infrared Telescope Facility, showed that multiple observations of the same asteroid do not fit the commonly applied simple thermal models. Detailed studies of a few NEAs showed that these small irregular objects have a wide range of spin rates and albedos, which affects their thermal properties. Diameters derived using simple thermal models do not always match radar-derived values. A shape-based thermal model, SHERMAN, explains some of the discrepancies. But shape alone does not explain all of the thermal observations, particularly at high solar phase angles, illustrating that even shape-based models are lacking in some of the physics needed for a self-consistent picture of asteroid properties. As part of this project, educational opportunities for both students and teachers were pursued. Through the NSF Research Experience for Undergraduates (REU) program at Arecibo, three students worked on radar data and shape modeling. The results of these student projects were presented at scientific meetings. Two successful teacher workshops were also conducted, bringing local teachers from all over Puerto Rico to Arecibo for a weekend to help enhance science in the classroom. The Arecibo Observatory currently has the only scientific outreach program in Puerto Rico and reaches a large portion of the Hispanic community there. Continued teacher education programs help to increase the visibility of science careers for Puerto Rican students.
