The shape and spin state of (275677) 2000 RS11 from ground-based radar and optical observations

Abstract

Near-Earth asteroid (275677) 2000 RS11 was observed over 5 days in March 2014 with both the Arecibo (2380 MHz, 12.6 cm) and Goldstone (8560 MHz, 3.5 cm) planetary radar systems. The continuous-wave spectra and delay-Doppler images collected revealed a sub-km-sized object with a strongly bifurcated shape. We used these radar observations, in combination with 7 optical lightcurves collected in 2014 and one lightcurve from 2023, to create a comprehensive shape and spin-state model for RS11. We find a rotation period of P = (4.445+-0.001) hours around a pole of lambda = (225+-80) and beta = (-80+-9) relative to the plane of the ecliptic. The shape of RS11 is unusual in that it does not resemble many of the other near-Earth asteroids modelled with ground-based radar. Whilst RS11 consists of a largely spherical, smaller lobe attached to an elongated, larger lobe via a narrow neck, the smaller lobe is not aligned with the long axis of the larger lobe, but is closer to the larger lobe's shortest principal axis. In combination with a large concavity observed on the outer face of the larger lobe, this may point to an unusual formation or event in the object's past. We estimate that RS11 has an geometric albedo of (0.16+-0.06) and a radar albedo between 0.08 and 0.16. Analysis of its gravitational environment reveals that for standard S-type asteroid densities, we would not expect rotational instability and it is possible for RS11 to be a low tensile strength rubble-pile asteroid.

Figures (11)

• Figure 1: The summed CW spectra collected on each night between the 12th and 17th of March, 2014. The solid line shows the opposite-sense circularly polarised signal received, whilst the dotted line is the same-sense circularly polarised signal. Observations on the 12th and 13th of March were taken at Goldstone, and those from the 14th-17th of March were collected with Arecibo. The resolution of each CW is described in Table \ref{['tab:RadarInfo']}.
• Figure 2: A collage of the first delay-Doppler for each night between 2014-03-14 and 2014-03-17 from Arecibo. These images show the Doppler shift on the horizontal axis, increasing from left to right, and the time delay on the vertical axis, increasing from top to bottom. Markings on the images mark the location of (A) the concave neck structure between lobes, and (B) evidence for a crater or concavity on the larger lobe. The resolution for each day of observations is described in Table \ref{['tab:RadarInfo']}. Here, each image is scaled and cropped so as to be $3$ Hz by $2~\mu s$ in size.
• Figure 3: Pole scan generated using a spherical harmonic model to approximate the shape of RS$_{11}$ using SHAPE. The scan used all available radar and lightcurve data, with the initial conditions set to a spherical-harmonic description of a sphere with a radius estimated from the radar images. For each data point, all shape parameters and the period were allowed to vary, but the rotational pole orientation was kept constant. Results are shown in ecliptic coordinates, with darker areas indicating better fits. White areas have a reduced $\chi^2$ above 10% of the best value.
• Figure 4: The six principal axis views of the final shape model of RS$_{11}$ generated with both optical and radar data in SHAPE. The model has pole solution $\lambda = 225^{\circ}$ and $\beta = -80^{\circ}$. The average length between the $1500$ vertices is $26.9$ metres. Red shading is applied to facets not viewed by the delay-Doppler radar imaging, while yellow shading is applied to facets viewed only at scattering angles greater than $60^\circ$ in the delay-Doppler images.
• Figure 5: The gravitational potential (\ref{['fig:GravPotential']}) and gravitational slopes (\ref{['fig:GravSlopes']}) of RS$_{11}$ , assuming a uniform density of $\rm 2.66~g.cm^{-3}$. Gravitational slopes are given relative to the normal of each facet, such that an angle of $0^{\circ}$ points into the surface, and any angle greater than $90^{\circ}$ would cause regolith to separate from the surface. The six plots show the model in both directions along the X, Y, and Z axes.