Spitzer observations of spacecraft target 162173 (1999 JU3)
Abstract
Context: NearEarth asteroid 162173 (1999 JU3) is the primary target of the Japanese Aerospace Exploration Agency (JAXA) Hayabusa2 sample return mission, and is also on the list of potential targets for the European Space Agency (ESA) Marco Polo sample return mission. Earthbased studies of this object are fundamental to these missions.
Aims: Our aim is to provide new constraints on the surface properties of this asteroid.
Methods: We present a midinfrared spectrum (538 μm) obtained with NASA's Spitzer Space Telescope in May 2008 and results from the application of thermal models.
Results: These observations place new constraints on the surface properties of this asteroid. To fit our spectrum we used the nearEarth asteroid thermal model (NEATM) and the more complex thermophysical model (TPM). However, the position of the spinpole, which is uncertain, is a crucial input parameter for constraining the thermal inertia with the TPM; hence, we consider two pole orientations. First is the extreme case of an equatorial retrograde geometry from which we derive a rigorous lower limit to the thermal inertia of 150 Jm^{2} s^{0.5} K^{1}. Second, when we adopt the pole orientation of Abe et al. (2008a, 37^{th} COSPAR Scientific Assembly) our bestfit thermal model yields a value for the thermal inertia of 700 ± 200 Jm^{2} s^{0.5} K^{1} and even higher values are allowed by the uncertainty in the spectral shape due to the absolute flux calibration. Our best estimates of the diameter (0.90 ± 0.14 km) and geometric albedo (0.07 ± 0.01) of asteroid 162173 are consistent with values based on previous midinfrared observations.
Conclusions: We establish a rigorous lower limit to the thermal inertia, which is unlikely but possible, and would be consistent with a fine regolith similar to wthat is found for asteroid 433 Eros. However, the thermal inertia is expected to be higher, possibly similar to or greater than that on asteroid 25143 Itokawa. An Accurately determining the spinpole of asteroid 162173 will narrow the range of possible values for its thermal inertia.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 August 2009
 DOI:
 10.1051/00046361/200912374
 arXiv:
 arXiv:0908.0796
 Bibcode:
 2009A&A...503L..17C
 Keywords:

 minor planets;
 asteroids;
 infrared: solar system;
 space vehicles;
 Astrophysics  Earth and Planetary Astrophysics
 EPrint:
 4 pages, 2 figures