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P. A. Taylor, J.-L. Margot, P. D. Nicholson, D. B. Campbell (Cornell University), R. F. Jurgens, S. J. Ostro, L. A. M. Benner, J. D. Giorgini (Jet Propulsion Laboratory), M. C. Nolan, E. S. Howell, A. A. Hine (Arecibo Observatory), R. S. Hudson (Washington State University)
We have used radar observations from two sessions at Goldstone in 2001 and two sessions at Arecibo in 2004 to estimate physical and reflection properties of near-Earth asteroid 2000 PH5, the first object in a Sun-Earth horseshoe orbit for which we can obtain a detailed physical characterization. Its orbital parameters suggest 2000 PH5 may have originated within the Earth-Moon system [Margot and Nicholson 2003]. The observed radar albedo and polarization ratio of this object are typical of asteroid and lunar surfaces [Magri et al. 1999, Hagfors 1970]. We find that the only size-spin orientation combinations that satisfy our limb-to-limb bandwidth measurements range from a 90 meter average radius object with spin pole 32 degrees from the final observing date's line-of-sight to a 150 meter average radius object with spin pole 18 degrees from the final observing date's line-of-sight. On the previous observing dates, the spin pole was even closer to the line-of-sight. Over 200 high resolution delay-Doppler images from Arecibo are available, covering approximately 10 rotations of 2000 PH5 with 100+ pixels on the target per frame. The 7.5 meter resolution images show that the rotation period is 12.2 +/- 0.1 minutes and that 2000 PH5 has a peculiar shape. The range extent of the echo power varies roughly between 60 and 90 meters, which is the lower limit of the radius determined from bandwidth measurements. If the effective radius of 2000 PH5 is 90 meters, we can constrain the spin pole to within a few degrees. This spin and shape information is critical to link lightcurve data from successive apparitions and to model YORP torques [Margot et al. 2003].
Hagfors, Radio Science, 5, 1970.
Magri et al., Icarus, 140, 1999.
Margot and Nicholson, AAS/Division of Dynamical Astronomy, 34, 2003.
Margot et al., AAS/DPS, 35, 2003.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.