46th Lunar and Planetary Science Conference (2015)
IMPROVED SHAPE MODEL OF 1627 IVAR. Jenna L. Crowell1, Ellen S. Howell2, Christopher Magri3, Yanga
R. Fernandez1, Sean E. Marshall4, Brian Warner5 and Ronald J. Vervack, Jr.6, 1University of Central Florida, Orlando, FL, USA. 2Arecibo Observatory, Arecibo, Puerto Rico. 3University of Maine at Farmington, Farmington, ME,
USA 4Cornell University, Ithaca, NY, USA 5Center for Solar System Studies - Palmer Divide Station 6The Johns
Hopkins University Applied Physics Laboratory, Laurel, MD, USA
Introduction: 1627 Ivar is an Amor class near
Earth asteroid with a taxonomic type of Sqw, [1] and a
rotation period of 4.7979 ± 0.0001 hours [2]. Ivar was
the first asteroid to be imaged by radar in 1985 at a
resolution of 4 Hz [3]. These radar images show Ivar to
be elongated with estimated dimensions of 11.5 x 6 km
[3]. Its large size and close approach to Earth in 2013
(minimum distance 0.32 AU) provided an opportunity
to observe the asteroid over many different viewing
angles for an extended period of time. Here we present
an improved shape model for Ivar using these data.
Observations: During Ivar’s apparition, we were
able to obtain CCD light curves, radar delay-Doppler
data, and near-IR spectra. Radar delay-Doppler data
consists of CW runs and imaging with 300 m resolution and were obtained using the Arecibo Observatory’s 2380 MHz radar. Light curve data were gathered
using the 0.35m telescope at the Palmer Divide Station.
The NIR spectra encompass reflected and thermal
wavelengths (0.8 – 4.1 µm) and were acquired using
the SpeX instrument at the NASA IRTF.
Background: We have used the software SHAPE
[4] to incorporate these recent radar and light curve
datasets in order to determine the best shape model for
Ivar that updates the results presented by Kaasalainen
et al. [5], which were based solely on light curves.
Figure 1 shows both the original light curve model by
Figure 1: The model on the right illustrates the nonconvex
model of Ivar by Kaasalainen et al. while the one on the left
demonstrates the features that begin to take shape in the fitting
process run by SHAPE.
Kaasalainen et al. and an enhanced model produced by
shape that provides a closer match to the 2013 data.
Our approach is similar to that of Magri et al. [6] for
1580 Betulia. The software uses penalty functions and
parameters that must be tailored specifically to the target asteroid in order to
steer asteroid shapes
away from those that are
overly complex and implausible. Figure 2 illustrates the results of not
using proper penalty
functions. SHAPE produces synthetic light
curves and radar images
Figure 2: This model represents the “best fit” probased on the resulting
duced by SHAPE without
shape model that can
appropriate penalty functhen be compared to the
tions to provide reasonable
original data. The results
of our work clearly
demonstrate the merit of combining the additional delay-Doppler data.
Future work: We will include using the resulting
shape model of Ivar with our thermal-modeling code
SHERMAN [7,8,9]. Input parameters for SHERMAN
include the asteroid’s IR emissivity, optical scattering
law and thermal inertia in order to complete thermal
computations based on the shape model. The software
then creates synthetic near-IR spectra that can be compared to our own spectra, which cover a wide range of
Ivar’s rotational longitudes and viewing geometries.
The spectra show changes with rotation and phase angle. SHERMAN lets us determine which reflective,
thermal, and surface properties for Ivar best reproduce
our spectra. For example, we will be able to investigate
the effects of phase reddening on Ivar and compare
these with the results of testing phase reddening effects
on crushed samples of ordinary chondrites in a laboratory [10]. With these comparisons and the results pro-
46th Lunar and Planetary Science Conference (2015)
duced by SHERMAN, we will learn more about the
detailed regolith and surface properties of Ivar and how
those properties compare to those of other S-complex
References: DeMeo et al. 2009, Icarus 202, 160180. [2] Hahn, G. et al. 1989, Icarus 78, 363-381. [3]
Ostro, S. et al. 1990, Astron. J., 99, 2012-2018 [4]
Magri, C. et al. 2011, Icarus 214, 210-227. [5] Kaasalainen, M. et al. 2004, Icarus 167, 178-196. [6] Magri C. et al. 2007, Icarus 186, 152-177. [7] Howell, E.
et al. 2012, AAS/DPS 44. [8] Marshall, S. et al. 2013,
AAS/DPS 45. [9] Crowell, J. et al. 2014, AAS/DPS 46.
[10] Sanchez, J. et al. 2012, Icarus 220, 36-50. We
thank NSF (AST-1109855) and the CLASS SSERVI
for their support of this work.