Physical characterization of the potentially hazardous contact-binary asteroid (153201) 2000 WO107

Introduction

The near-Earth asteroid (153201) 2000 WO107 was selected as an M-asteroid candidate based on its known albedo value p_v = 0.129 ± 0.058 [1] and the taxonomic X-type determined from spectral observations [2]. The ambiguous spectral X-type suggests one of the three E-, P-, or M-types, but in this case, given the known albedo, the asteroid is most consistent with an M-type. It has a significantly elongated orbit with e = 0.781, which crosses the orbits of all major inner planets, enters the orbit of Mercury, and approaches the Sun at a distance of q = 0.20 AU. The asteroid has a small size of about 0.5 km and is available for observation only during close approaches to the Earth.

We initiated photometric observations of 2000 WO107 in the 2020 opposition aimed to obtain rotation parameters and estimate its shape and bulk density.

Observations

We carried out photometric observations of (153201) 2000 WO107 in November-December 2020, when the asteroid passed at a minimum distance of 0.029 AU from the Earth. Our observations were made in a coordinated manner at five observatories: the 1-m telescope of the Crimean Astrophysical Observatory, the 70-cm telescope of Abastumani Observatory, the 61-cm telescope of Skalnaté Pleso Observatory, the 36-cm telescope of Kitab Observatory, and the 0.25-m BART telescope of the Roque de los Muchachos Observatory. The observations covered the time interval from Nov. 28 to Dec. 9, 2020, and were done using CCD cameras in R-filter and, moreover, in Kitab without a filter. In addition, the asteroid was also observed in BVI filters on some nights. The measured lightcurves showed a very high amplitude (up to 1.24 mag at the phase angles of 65-70 deg) and made it possible to determine the rotation period of 5.03 hrs (Fig.1). Such a high amplitude characterizes the body of the asteroid as very elongated and possibly binary. Note the unusual flat minima are seen in the light curve. They are a characteristic manifestation of incomplete illumination of an asteroid at large phase angles for synchronous, as well as very elongated bodies of a nonconvex shape. It means that the asteroid is either a binary or a contact binary.

Our assumptions are confirmed by radar observations carried out using the 70-m antenna at the Goldstone Observatory in the same opposition with our observations, which showed that the asteroid is a contact binary with a larger elongated primary component and a smaller secondary close to the sphere [3]. Moreover, the smaller body is attached to the end of the longest axis of the larger body.

 

Figure 1. The composite light curve obtained from observational data on the night of November 28–29 during the passage of an asteroid at a minimum distance from the Earth.

 

Modelling

We simulated the scattering of light and radio waves from a bi-lobal asteroid in the assumption of two contacting ellipsoids. The axes of the two ellipsoids, the distance between their centers, the orientation of the rotation axis, the initial rotation phase and the period were used as fitting parameters in a Markov chain Monte Carlo (MCMC) algorithm. The resulted shape model fits well both the optical lightcurves and delay–Doppler images of 2000 WO107 obtained by Goldstone on Nov. 28, 29 and 30, 2020. We estimate the density of 1.5 g/cm³ from the requirement that the resulting shape of the asteroid is mechanically stable.

An independent estimate of the asteroid density can be obtained from its Yarkovsky effect. Greenberg et al. (2020) analyzed measured positions for 2000 WO107 and found the Yarkovsky drift rate to be −24.37 ± 12.7 AU/My [4]. We simulated the Yarkovsky effect of a bi-lobal asteroid using the shape and size of the asteroid obtained from our modelling. Thus, we calculated the theoretical value of the Yarkovsky force acting on this asteroid. By comparing the theoretical Yarkovsky force with the observed Yarkovsky drift rate, we estimated the density of 2000 WO107. Within the error margin, the obtained value agrees with the density determined from the shape modeling.

Conclusions

Combination of photometric and radar data allows us to determine the rotation period and the position of the asteroid's pole, to predict its shape and size, and to measure the asteroid's density.

The resulted density of the near-Earth asteroid (153201) 2000 WO107 is rather low, which implies the asteroid has a predominantly non-metallic composition and most probably belongs to primitive asteroids.

Acknowledgment

Ukrainian team was supported by the National Research Foundation of Ukraine (grant No. 2020.02/0371 “Metallic asteroids: search for parent bodies of iron meteorites, sources of extraterrestrial resources”). The authors are thankful to A. Novichonok and A. Zhornichenko for their help with observations.

 

References

[1] Mainzer, A., Grav, T., Bauer, J. et al. 2011. Astron. J. 743, Art. 156

[2] Binzel, R. P., Rivkin, A. S., Stuart, J. S., et al. 2004. Icarus 170, 259–294

[3] Benner, L. A. M. Goldstone Radar Observations Planning: (7753) 1988 XB, 2017 WJ16, and 2000 WO107. NASA JPL database “Asteroid Radar Research”. https://echo.jpl.nasa.gov/asteroids/1988XB/1988xb.2020.goldstone.planning.html

[4] Greenberg, A.H., Margot, J.-L., Verma, A.K., Taylor, P.A., Hodge, S.E. 2020. Astron. J. 159, 92