Spitzer Space Telescope - Directors Discretionary Time Proposal #292

Thermophysical Mapping of 25143 Itokawa

Principal Investigator: Mark Sykes 
Institution: Planetary Science Institute

Co-Investigators:
Robert Gaskell, Planetary Science Institute
Matthew Chamberlain, Planetary Science Institute
Paul Abell, Planetary Science Institute
William Reach, Spitzer Science Center
Faith Vilas, MMT Observatory
Susan Lederer, California State University San Bernadino
Deborah Domingue, Applied Physics Laboratory


Science Category: Asteroids
Observing Modes: IRS Staring  
Hours Approved: 3.9

Abstract: We have a unique opportunity to map at subhemispheric resolution the 
thermophysical properties of the small Earth-crossing asteroid 25143 Itokawa 
this April/May 2007 by combining high-resolution shape and topography models, 
recently derived from imagery of the asteroid obtained by the Japanese Hyabusa 
mission, with rotationally well- sampled thermal spectra obtained with Spitzer 
IRS. Prior groundbased observations in the N and Q bands (limited in rotational
coverage,  wavelength coverage, with substantially lower signal-to-noise) provide 
only a single global value of thermal inertia, compared to the possible 40 
surface resolution elements this program may obtain. Itokawa has a block-strewn 
surface combined with smooth areas with no definitive large craters and an 
apparent deficiency of small craters  - the first clear example of a 
'rubble-pile', that may be characteristic of most small NEOs. Itokawa makes 
its closest and best approach to the Spitzer spacecraft (0.09 AU) in April at 
an observational phase angle providing an excellent view of the  terminator 
across which surface temperature changes are maximum. Significant changes in 
shape and spectral peak of Itokawa's SED as the asteroid rotates are simulated. 
The high signal-to-noise of the proposed Spitzer IRS observations will 
well-resolve these spectral differences. Though Itokawa is not spatially 
resolved by Spitzer, a priori knowledge of its detailed shape and topography 
from the Hyabusa mission allows us to divide its surface into subunits with  
independent thermal properties, and constrain them by grid search, finding 
those values or range of values that reproduce the numerous spectra obtained, 
where different combinations of surface units contribute to each spectrum as 
they move from evening to morning to afternoon and in and out of view (sometimes 
blocked by  nearby units). Maximizing rotational sampling maximizes the 
longitudinal resolution of our thermophysical maps and the number of resolution 
elements covering its surface.