Spitzer Space Telescope - Directors Discretionary Time Proposal #223


The Size, Shape, and Albedo of Deep Impact Target 9P/Tempel 1

Principal Investigator: Michael A'Hearn
           Institution: University of Maryland

Co-Investigators: 
C. Lisse, ohns Hopkins University Applied Physics Lab                 
Y. Fernandez, IfA, University of Hawaii                               
M. Belton, Belton Space Exploration Initiatives                       
O. Groussin, University of Maryland                                   
K. Meech, IfA, University of Hawaii                                   
J. van Cleve, Ball Aerospace                                          

Science Category: comets 
 Observing Modes: IrsStare 
  Hours Approved: 6.0 

Abstract:
The Deep Impact mission, the eighth mission in NASA's Discovery
Program, will launch on 30  Dec 2004 and will impact the nucleus of
comet 9P/Tempel 1 on 4 July 2005.  Delivering an impactor to a
cometary nucleus and observing the results of the impact is a
challenging task.  Mission success depends critically on the ability
of the DI spacecraft to navigate to the comet.  Despite robust
targeting algorithms, large uncertainties in size, shape, albedo
distribution, and rotational state significantly degrade the
probability that the impactor will land in a sunlit portion of the
surface that is observable from the flyby.  The uncertainties in the
size, axial ratio, and rotational state further jeopardize the ability
of the flyby spacecraft to point its High Resolution Instrument at the
actual impact site with sufficient precision to ensure it is in the
field of view.  Current estimates of the comet's size using Keck LWS
observations are uncertain by 50% [1] and they have provided no
information on the distribution of albedo.  Although uncertainty in
the phasing of the optical lightcurve represents a large part of that
error, there is also a significant uncertainty due to the combination
of low SNR in the data from Keck and the limited spectral range over
which data could be obtained.  In order to improve our estimate of the
size and shape of the nucleus of Tempel 1, we must obtain data with
much higher SNR and with one-hour time resolution over a significant
portion of the rotational light curve when the comet is inactive.  We
must determine whether or not there are large variations in albedo
across the surface in order to ensure that the rotational light curve
from optical data can be used to predict the convex hull of the actual
shape.  The observations of comet Borrelly from the Deep Space 1 have
been interpreted as showing large variations in albedo but other
investigators have questioned the conclusion, suggesting that the
variations are primarily due to slope changes, thus leaving the
question of albedo variations undecided.  Tempel 1 is available in
only one SIRTF viewing window before it is expected to be close enough
to the sun to become active, namely the window from 27 February
through 29 April 2004, before the first GO observing period, at ~3.7
AU from the Sun.  The expected peak thermal flux will be ~ 10  mJy at
20  µm, easily detectable by SIRTF in a short integration time.