Radar Observations and the Shape of Near-Earth Asteroid 2008 EV5

Abstract: We observed the near-Earth asteroid 2008 EV5 with the Arecibo and Goldstone planetary radars and the Very Long Baseline Array during December 2008. EV5 rotates retrograde and its overall shape is a 400 /pm 50 m oblate spheroid. The most prominent surface feature is a ridge parallel to the asteroid's equator that is broken by a concavity 150 m in diameter. Otherwise the asteroid's surface is notably smooth on decameter scales. EV5's radar and optical albedos are consistent with either rocky or stony-iron composition. The equatorial ridge is similar to structure seen on the rubble-pile near-Earth asteroid (66391) 1999 KW4 and is consistent with YORP spin-up reconfiguring the asteroid in the past. We interpret the concavity as an impact crater. Shaking during the impact and later regolith redistribution may have erased smaller features, explaining the general lack of decameter-scale surface structure.

Analysis of Radar Observations of Near-Earth Asteroid 2008 EV5

This paper presents a comprehensive study of the near-Earth asteroid (NEA) 2008 EV5, relying predominantly on radar observations conducted using the Arecibo and Goldstone planetary radars and supplemented by measurements from the Very Long Baseline Array. The asteroid was observed during its close approach to Earth in December 2008, over a distance conducive to detailed radar imaging. The characterization of 2008 EV5's physical and dynamical properties yields valuable insights into its potential suitability as a target for space missions and furthers our understanding of NEAs in general.

Key Findings and Methodology

1. Shape and Rotation: The study indicates that 2008 EV5 is an oblate spheroid with a diameter of approximately 400 ± 50 meters, showcasing relatively smooth surface features disrupted primarily by an equatorial ridge and a significant concavity. Delay-Doppler imaging confirms a retrograde rotation with a period of 3.725 ± 0.001 hours, reinforcing the lightcurve data obtained in optical wavelengths.
2. Surface Composition and Structure: Radar albedo measurements suggest a surface composition that could be rocky or stony-iron. Notably, the equatorial ridge bears resemblance to features seen on other rubble-pile asteroids such as 66391 1999 KW4, suggesting the likelihood of YORP (Yarkovsky-O'Keefe-Radzievskii-Paddack effect) spin-up influences on its past configuration. The large concavity is interpreted as an impact crater, with its morphology following typical crater parameters, including depth and circumference ratios.
3. Radar Observational Techniques: The paper notably employs both continuous-wave (CW) and delay-Doppler radar imaging. The CW signals at Arecibo provide superior signal-to-noise ratios and calibration, assisting significantly in determining EV5's radar cross-section and reflectivity. Furthermore, the radar speckle tracking between VLBA stations helped ascertain the asteroid’s retrograde spin, solidifying pole orientation estimation.
4. Absence of Satellites: Contrary to what might be expected of a body with a pronounced equatorial ridge, high-resolution radar images did not resolve any satellite companions within the Hill sphere of 2008 EV5. This poses questions about the asteroid's rotation history and the potential transient existence of satellites that might have altered its spin state before departing the system.

Implications and Future Prospects

The structural analysis of 2008 EV5 implies a rubble-pile composition common among NEAs, with potential implications for its internal cohesion and thermal properties. Future missions, be they robotic exploration or sample return endeavors, stand to benefit from the detailed morphometric and compositional data accrued through these radar observations. The asteroid's proximity and low delta-v further enhance its attractiveness as a future mission target.

Additionally, subsequent close Earth approaches, notably in 2023 and 2039, promise opportunities for refining orbit predictions through additional radar imaging. These future observations hold promise for further dissecting the YORP and Yarkovsky influences on the asteroid, ultimately enhancing our understanding of small body evolution in the solar system.

Overall, the detailed investigation of 2008 EV5 provides a valuable data point in the broader context of NEA studies, contributing towards the understanding of their dynamical behaviors, structural characteristics, and potential threat mitigation strategies.