URGENT PROPOSAL FOR RADAR OBSERVATIONS OF ASTEROID 1999 JM8 S. J. Ostro, L. A. M. Benner, R. S. Hudson, M. C. Nolan, J. D. Giorgini, J.-L. Margot, D. B. Campbell, D. K. Yeomans This very large (H = 15; diameter = 3 to 6 km), recently discovered Earth-crosser approaches to within 0.06 AU just before it enters Arecibo's declination window, on Aug. 1. The predicted signal-to-noise ratios (SNRs; Table 1) should be adequate for decameter-resolution delay-Doppler imaging and shape reconstruction, and may well be higher than values achieved in any asteroid radar experiment since Toutatis. ------------------------------------------------------------ Table 1. Ephemeris and SNR Estimates (Assumptions: diameter = 3 km, period = 4 h, antenna gain =7.3 K/Jy, Tsys = 30 K) R.A. DEC Dist. SNR/ Max AST Date (hh mm)(deg) (AU) Date SNR/Run 1999-Aug-01 05 14 +34 0.059 4000 760 1999-Aug-02 05 00 +29 0.061 6900 980 1999-Aug-03 04 48 +23 0.064 6600 850 1999-Aug-04 04 39 +19 0.067 5600 710 1999-Aug-05 04 31 +14 0.071 4500 580 1999-Aug-06 04 24 +10 0.076 3400 470 1999-Aug-07 04 18 +07 0.081 2500 380 1999-Aug-08 04 13 +04 0.086 1800 300 ------------------------------------------------------------ To take advantage of this unusual opportunity, we propose observations on five days during the period from Aug. 1 to Aug. 8 (Table 2). Clearly, the higher-SNR dates are desirable. (Determination of the rotation period optically or at Goldstone would let us define an optimum rotation-phase-coverage strategy at Arecibo, and we will notify NAIC if this information becomes available.) ----------------------------------------- Table 2. Requested AST Observing Periods. Begin Rise-Set Stop Aug 1 06:45 08:17-10:00 10:00 Aug 2 06:00 07:35-10:00 10:00 Aug 3 06:00 07:09-09:59 10:00 Aug 4 05:45 06:54-09:44 09:45 Aug 5 05:30 06:48-09:29 09:30 Aug 6 05:30 06:42-09:14 09:15 Aug 7 05:30 06:36-08:50 09:00 Aug 8 05:30 06:39-08:30 08:30 ----------------------------------------- The current one-sigma pointing uncertainty on the first proposed date, about 1.5 arcmin, should be reduced dramatically by planned optical astrometry and perhaps by Goldstone radar astrometry. However, it is likely we will need to use Arecibo delay-Doppler astrometry to refine the ephemerides. Therefore it is desirable that closed-loop tests that calibrate and validate time-delay corrections be performed in advance. 1999 JM8 is the fifth largest asteroid classified as potentially hazardous by the Minor Planet Center. The object's 1999 close approach is its closest for at least two centuries.