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L. A. M. Benner (JPL/Caltech), M. C. Nolan (NAIC/Arecibo Observatory), J. L. Margot (Caltech), S. J. Ostro, J. D. Giorgini (JPL/Caltech)
Arecibo delay-Doppler radar images of 1998 ST27 obtained during Oct. 7-11, 2001, reveal two objects with separations in range and Doppler frequency that change as a function of time. The primary has a visible range extent of ~400 m and the delay-Doppler dispersion of a spheroid, suggesting that its diameter is about 800 m. The visible range extent of the secondary is ~60 m; if we assume that it is a sphere, then its diameter about 120 m. The primary/secondary diameter ratio is close to 7 and is the largest known among the seven binary near-Earth asteroids (NEAs) detected by radar to date. 1998 ST27's absolute magnitude of 19.5 combined with the primary's apparent diameter suggest a visual albedo of only 0.05. The primary and secondary sizes and bandwidths of ~9 Hz and ~0.6 Hz constrain their rotation periods to be less than 3 h and 6 h, respectively. Preliminary orbit estimations suggest that the orbital period is several days, that the semimajor axis could be as large as ~7 km, and that the eccentricity is at least 0.3. Among binary NEAs detected by radar that have well-constrained orbits, the orbital period of 1998 ST27 is the longest and most eccentric. The rotation period of the secondary is more than an order of magnitude more rapid than its orbital period -- the first such case among binary NEAs. The rapid spin of the secondary and the non-zero eccentricity of its orbit hint that this may be a geologically youthful system. Echoes from 1998 ST27's secondary are weak on three of the four days we observed the system, implying that other radar-detected NEAs could have unrecognized companions that were too weak to detect.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.