Radar Images of near-Earth Asteroid (53319) 1999 JM8

Radar Observations of Asteroid 1999 JM8
L. A. M. Benner, S. J. Ostro, M. C. Nolan, J. L. Margot, J. D. Giorgini, R. S. Hudson
R. F. Jurgens, M. A. Slade, E. S. Howell, D. B. Campbell, and D. K. Yeomans.
Meteoritics and Planetary Science 37, 779-792 (2002).
Abstract:
We report results of delay-Doppler observations of 1999 JM8 with the Goldstone 8560 MHz
(3.5 cm) and Arecibo 2380 MHz (13 cm) radars over 18 days in July-August 1999.  The images
place thousands of pixels on the asteroid and achieve range resolutions as fine as 15 m/pixel.
The images reveal an asymmetric, irregularly-shaped object with a typical overall dimension
within 20% of 7 km.  If we assume that 1999 JM8's effective diameter is 7 km, then the 
absolute magnitude, 15.15, and the average Goldstone radar cross secction, 2.49 km^2, 
correspond to optical and radar albedos of 0.02 and 0.06, establishing that 1999 JM8 is a 
dark object at optical and radar wavelengths.  The asteroid is in a non-principal axis spin
state that, although not yet well determined, has a dominant periodicity of about 7 days.  
However, images obtained between July 31 and August 9 show apparent regular rotation of
features from day to day, suggesting that the rotation state is not far from principal
axis rotation.  1999 JM8 has regions of pronounced topographic relief, prominent facets
several kilometers in extent, numerous crater-like features between about 100 m and 1.5 km in
diameter, and features whose structural nature is peculiar.  Arecibo images provide the
strongest evidence to date for a circular polarization ratio feature on any asteroid.
Combined optical and radar observations from April 1990 to December 2000 permit computation
of planetary close approach times to within +- 10 days over the interval from 293 to at 
least 2907, one of the longest spans for any potentially hazardous asteroid.  Integration
of the orbit into the past and future shows close approaches to Earth, Mars, Ceres, and
Vesta, but the probability of the object impacting Earth is zero for at least the next
nine centuries.

Figure 1: Radar images from July 20-August 1

Figure 2: Radar images from August 2-9

Caption: Sequences of delay-Doppler images obtaind at Goldstone and Arecibo.
In each image range increases toward the bottom and Doppler frequency 
increases toward the left, so rotation is clockwise.  The height in each frame is 6.0 km 
(40 microseconds).  The images are shown with a Doppler extent of 3.7 Hz when adjusted 
to a transmitter frequency of 8560 MHz in order to facilitate direct comparison between 
Goldstone and Arecibo images.  The images have logarithmic contrast stretches in order 
to take advantage of the dynamic range. The collage shows one image per day, where each 
frame is the sum of all the highest resolution images on a given day.  "A" and "G" 
indicate images obtained at Arecibo and Goldstone, respectively.  
      


A press release is also available:
  

1999 JM8 Press Release