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J. D. Giorgini, L. A. M. Benner (JPL/CalTech), M. C. Nolan (Arecibo Observatory), S. J. Ostro (JPL/CalTech)
Asteroid 2004 MN4 is expected to pass 4.6 (+/- 1.6) Earth-radii above the surface of the Earth on 2029-Apr-13. Such close approaches by objects as large as 2004 MN4 (D \gtrsim0.3 km) are thought to occur at \gtrsim1000-year intervals on average. 2004 MN4 is expected to reach 3rd magnitude and thus be visible to the unaided eye. With a disk 2-4 arcseconds across, it may be resolved by ground-based telescopes. Arecibo (2380-MHz) delay-Doppler radar astrometry, obtained in late January 2005, significantly corrected 2004 MN4's orbit by revealing a 1.4 arcsecond bias in pre-discovery optical measurements. Doppler-shifted echoes were acquired 4.8\sigma (176.4 mm/s) away from the predicted frequency on Jan 27. Range on Jan 29 was found to be 747 km (2.8\sigma) closer to Earth than the pre-radar orbit predicted. Incorporation of these delay-Doppler measurements into a new weighted least-squares orbit solution moved the 2029-Apr-13 encounter prediction 5\sigma closer to the Earth, illustrating the problematic nature of prediction and statistical analysis with single-apparition optical data-sets. Without delay-Doppler data, the bias was not apparent, even when optical measurements spanned a full orbit period. The current combined data-set does not permit reliable trajectory propagation to encounters beyond 2029; Monte Carlo analysis shows that, by 2036, the 3\sigma confidence region wraps >300 degrees of heliocentric longitude around the Sun, with some sections of this statistical region experiencing low-probability encounters with the Earth in the 2030's, gravitationally scattering some possible trajectories inward to the orbit of Venus, or outward toward Mars. Future measurements from radar opportunities in August 2005 and May 2006 (SNR \approx5-10) have the potential to eliminate statistical encounters in the 2030's. Delay-Doppler astrometry from 2013 (SNR \approx30) should permit deterministic encounter prediction through 2070, shrinking the along-track uncertainty in 2036 by two orders of magnitude,from \gtrsim8(10)8 km to \lesssim7(10)6 km.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.