Hayabusa-2 Mission Target Asteroid 162173 Ryugu (1999 JU3): Searching for the Object's Spin-Axis Orientation (1611.05625v1)

Abstract: The JAXA Hayabusa-2 mission was approved in 2010 and launched on December 3, 2014. The spacecraft will arrive at the near-Earth asteroid 162173 Ryugu in 2018 where it will perform a survey, land and obtain surface material, then depart in Dec 2019 and return to Earth in Dec 2020. We observed Ryugu with the Herschel Space Observatory in Apr 2012 at far-IR thermal wavelengths, supported by several ground-based observations to obtain optical lightcurves. We reanalysed previously published Subaru-COMICS and AKARI-IRC observations and merged them with a Spitzer-IRS data set. In addition, we used a large set of Spitzer-IRAC observations obtained in the period Jan to May, 2013. The data set includes two complete rotational lightcurves and a series of ten "point-and-shoot" observations. The almost spherical shape of the target together with the insufficient lightcurve quality forced us to combine radiometric and lightcurve inversion techniques in different ways to find the object's key physical and thermal parameters. We find that the solution which best matches our data sets leads to this C class asteroid having a retrograde rotation with a spin-axis orientation of (lambda = 310-340 deg; beta = -40+/-15 deg) in ecliptic coordinates, an effective diameter (of an equal-volume sphere) of 850 to 880 m, a geometric albedo of 0.044 to 0.050 and a thermal inertia in the range 150 to 300 Jm-2s-0.5K-1. Based on estimated thermal conductivities of the top-layer surface in the range 0.1 to 0.6 WK-1m-1, we calculated that the grain sizes are approximately equal to between 1 and 10 mm. The finely constrained values for this asteroid serve as a `design reference model', which is currently used for various planning, operational and modelling purposes by the Hayabusa2 team.

Analysis of Asteroid 162173 Ryugu for Hayabusa-2 Mission

The paper investigates the spin-axis orientation and other vital physical characteristics of the near-Earth asteroid (162173) Ryugu, the target for the JAXA Hayabusa-2 mission. This mission, approved in 2010, aims to provide crucial insights by surveying Ryugu, collecting samples, and returning them to Earth. The research combines extensive data from notable space observatories such as Herschel, Spitzer, and AKARI, alongside ground-based telescopic observations, to ascertain the asteroid's spin-axis orientation, shape, size, albedo, thermal inertia, and regolith grain size.

Key Observations and Techniques

1. Spin-Axis Orientation: Through a combination of radiometric analysis and lightcurve inversion techniques, the authors determine that Ryugu exhibits a retrograde rotation. The most probable spin-axis orientation lies within ecliptic coordinates ($\lambda$ = 310$^{\circ}$ to 340$^{\circ}$; $\beta$ = -40$^{\circ}$ $\pm$ $\sim$15$^{\circ}$).
2. Shape and Size Estimation: The asteroid's nearly spherical configuration posed challenges in obtaining precise lightcurve data. Effective diameter estimates range from 850 to 880 meters, aligning closely with previously published models. These results were achieved by synthesizing visual and infrared data and applying the thermophysical model (TPM).
3. Thermal Inertia: Ryugu's thermal inertia is estimated to span 150 to 300 J m$^{-2}$ s$^{-0.5}$ K$^{-1}$, indicating a relatively conductive surface compared to other asteroids. The thermal properties, determined using phase angles and wavelength observations, suggested a smooth surface.
4. Albedo Measurements: The geometric albedo was found to be between 0.044 and 0.050, lower than prior estimates, refining our understanding of Ryugu's reflectivity.
5. Regolith Grain Size: Using thermal inertia data, the paper estimates the grain sizes in Ryugu's top-layer surface to be approximately 1 to 10 mm. This small grain size may offer clues regarding the aggregation and surface processes on C-class asteroids.

Implications and Future Directions

The precise determination of Ryugu's spin-axis and thermal properties has significant implications for the upcoming phases of the Hayabusa-2 mission, including navigation, landing site selection, and sample retrieval strategies. Moreover, the methodologies adopted in this paper could be adapted for similar analyses of other asteroid mission targets. The research paves the way for more refined thermophysical models which take into account asteroid heterogeneity and shape complexities.

Looking forward, continued missions and observational programs will likely expand our understanding of the relationships between asteroid thermal properties and their spectral characteristics, ultimately contributing to a more comprehensive understanding of asteroid surface physics.

In summary, this multifaceted research on Ryugu integrates complex observational data with advanced modeling techniques, setting a precedent for future asteroid analysis endeavors. The insights from this paper will not only enhance the Hayabusa-2 mission's success but also inform broader asteroid characterization in planetary science.