Radar and Lightcurve Observations and a Physical Model of Potentially Hazardous Asteroid 1981 Midas

Abstract

We report observations of the Apollo-class potentially hazardous asteroid 1981 Midas, which passed 0.090 au from Earth (35 lunar distances) on 2018 March 21. During this close approach, Midas was observed by radar both from the Arecibo Observatory on March 21 through 25 (five nights) and from NASA’s Goldstone Deep Space Communications Complex on March 19 and 21. Optical lightcurves were obtained by other observers during four apparitions (1987, 1992, 2004, and 2018), which showed a rotation period of 5.22 hr. By combining the lightcurves and radar data, we have constructed a shape model for Midas. This model shows that Midas has two lobes separated by a neck, which, at its thinnest point, is about 60% of the width of the largest lobe. We also confirm the lightcurve-derived rotation period and show that Midas has a pole direction within 6° of ecliptic longitude and latitude (λ, β) = (39°, −60°) and dimensions of (3.41 ± 9%) × (1.90 ± 11%) × (1.27 ± 29%) km. Analysis of gravitational slopes on Midas indicates that nearly all of the surface has a slope less than the typical angle of repose for granular materials, so it does not require cohesion to maintain its shape. In addition, we measured a circular polarization ratio of 0.83 ± 0.04 at Arecibo’s 13 cm wavelength, which is the highest seen to date for any near-Earth asteroid with visible and near-infrared spectral type V.

Arecibo Planetary Radar Observations of Near-Earth Asteroids: 2017 December–2019 December

“…The number of NEAs that are V types, X complex, or L/Ld types is too low to provide statistical significance. The relative uncertainty for the radar albedo of Midas is estimated at ∼40% using a shape model (McGlasson et al 2022); therefore, the radar albedos of the other less-constrained cases are estimated to be at least 50%.…”

“…The Doppler-echopower spectra for these objects are displayed in Appendix A, Figure 6, excluding (3752) Camillo and 2013 CW32 that were used as examples in Figure 1. Moreover, radar analysis of (3200) Phaethon is available in Taylor et al (2019a); a full radar analysis and a shape model of (1981) Midas are available in McGlasson et al (2022), and a radar analysis of 2019 OK is available in Zambrano-Marin et al (2022). Some observations were led by other teams, so a full radar analysis is in preparation, and those NEAs were therefore omitted from this section, including (433) Eros (e.g., Hinkle et al 2019), (163899) 2003SD220 (e.g., Rivera-Valentín et al 2019EB (e.g., Brozovic et al 2018.…”

Numerical Evaluation of Planetary Radar Backscatter Models for Self-Affine Fractal Surfaces

“…For example, Mitchell et al [6] used only the Gaussian law, cosine law [7,8], and a "flat law" [9], for attempts to fit radar scattering by (1) Ceres, (2) Pallas, and (4) Vesta. The cosine law remains the most used scattering law option in the shape modeling of asteroids based on radar data (e.g., [10][11][12][13]); however, the physical interpretation of the fit parameters of the cosine law is not well constrained as will be discussed further below.…”