Goldstone and Arecibo Radar Imaging of Near-Earth Asteroid 2014 HQ124

Wednesday, 17 December 2014
Lance Benner1, Marina Brozovic1, Jon D Giorgini1, Michael Busch2, Michael C Nolan3, Patrick A Taylor3, Ellen S Howell3, Alessondra Springmann3, Joseph S Jao1, Clement G Lee1, Martin A Slade4, Amanda K. Mainzer1, Joseph T. Pollock5, Dan Reichart6, Joshua B. Haislip6, Nathan R. Frank6 and Aaron P. LaCluyze6, (1)NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States, (2)NRAO, Socorro, NM, United States, (3)Arecibo Observatory, Arecibo, PR, United States, (4)NASA Jet Propulsion Laboratory, Pasadena, CA, United States, (5)Appalachian State University, Boone, NC, United States, (6)University of North Carolina at Chapel Hill, Physics and Astronomy, Chapel Hill, NC, United States
2014 HQ124 was discovered by the NEOWISE spacecraft on 2014 April 23, about six weeks before the asteroid made a very close Earth flyby within 0.0084 au (3.2 lunar distances) on June 8. Prior to the encounter, A. K. Mainzer used NEOWISE data and a thermal model to estimate a diameter of 330 +- 90 m. The NEOWISE data and photometry obtained by J. T. Pollock yielded a rotation period of ~20 h and a large lightcurve amplitude of 0.8 mag. The close approach, diameter, and slow rotation period indicated that this object would provide an outstanding opportunity for radar imaging, physical characterization, and orbit refinement. We observed 2014 HQ124 on June 8 and 10 with radar at Goldstone, Arecibo, and elements of the Very Long Baseline Array (VLBA). We conducted monostatic radar imaging observations at X-band (8560 MHz, 3.5 cm) with the 70 m DSS-14 antenna and at S-band (2380 MHz, 13 cm) with Arecibo; bistatic X-band observations using DSS-14 to transmit and Arecibo and the 34 m DSS-13 antenna to receive; and S-band radar speckle observations to constrain the spin state using Arecibo as a transmitter and the Pie Town, Los Alamos, Ft. Davis, and Kitt Peak VLBA facilities as receivers. Radar astrometry improved the orbit significantly and increased the interval of reliable orbit estimation by a factor of two to ~900 years. The images achieve a resolution of 3.75 m x 0.00625 Hz and provide some of the most detailed radar views ever obtained for any near-Earth object. 2014 HQ124 is elongated, angular, and bifurcated with a long axis of at least 400 m. The asteroid has at least one large concavity, possible ridges, and small-scale surface features such as numerous radar-bright spots that are candidates for boulders. The larger lobe has a narrow, sinuous, ~100-m-long radar-dark feature that may be a scarp or perhaps a fault. These observations were the first test of new data taking equipment at Arecibo that can acquire dual-polarization images at 3.75 m resolution, which is twice as fine as the highest range resolution that Arecibo can achieve with monostatic observations. Receiving echoes at Arecibo using transmissions from Goldstone also boosts the signal-to-noise ratio by about a factor of five relative to monostatic Goldstone observations and is ideal for resolving very close and/or small near-Earth objects with slow spin states.