ADDENDUM -------- A quick note to alert everyone to an important change in the SL9 impact predictions. As previously noted, the new impact times are a day or so earlier than previously thought, with the first nucleus hitting Jupiter around July 16.8. Even more interesting, the impact point has changed as well as the impact times. IAU Circular 5909, issued on December 17th, reports than calculations by Yeomans and Chodas, based on the new astrometry, give the impacts occuring ONLY 5-10 DEGREES BEHIND JUPITER'S MORNING LIMB, at 47 - 49 degrees S latitude. This is great news for studying the after-effects of the impacts: the impact sites will be visible only 12 - 24 minutes after impact rather than the couple of hours previously supposed. And anything ejected higher than 500-1000 km above the cloud tops during the impacts will be directly visible from Earth! Galileo will get a direct view of the impacts rather than the grazing limb view previously expected. The only bad part about this new prediction is that reflections off the satellites will not be visible for orbital longitudes in the 300-350 degree range, before Jupiter occultation. But this will be amply compensated by the chance to see the flashes reflected in any coma still attached to each nucleus by the time it has traversed the magnetosphere, or the chance for direct observations of impact debris. Note, by the way, that the nuclei will hit behind the dark limb of Jupiter: the phase angle is 10.6 degrees so the impact sites will have to rotate an additional 10 degrees after coming into view from Earth before they come into sunlight. John Spencer 1993/12/18. -------------- posted 20 Dec 1993, acr =========================================================================== NEW IMPACT TIMES AND SATELLITE REFLECTOR AVAILABILITY IAUC 5906, issued on December 14th, used new astrometry from last week's recovery of the comet by the Spacewatch team to give much-improved impact time predictions for selected nuclei, probably good to 0.1 days and thus suitable for planning observations from a particular site. Note that the impact times are about a day earlier than previous estimates! Here's a table based on IAUC 5906, giving impact times for all nuclei, the orbital longitudes of the satellites at each impact, and the likely suitability of the satellites as reflectors of the impact flashes. In the table, a "+" next to the satellite orbital longitude means that the impact will be visible from the satellite (I'm assuming the impacts are 20 degrees past local midnight on Jupiter). Longitudes near the ends of the visibility range will be less useful as the phase angle of the reflected light will be high. A "*" means the satellite will be visible in eclipse, allowing higher sensitivity observations, and an "o" means it will be occulted by Jupiter. Timing uncertainties are still too large to predict eclipses and occultations at impact times with certainty, though. I've included Callisto (J4) for completeness but it's likely to be too far out to be useful- the same may be true for Ganymede (J3). ------------------------------------------------------------------------------ Nucleus UT date of Designation impact (July) Satellite Orbital Longitudes ----------------- ------------- (degrees past superior conj.) Jewitt Sekanina Fitted IAUC Brightness --------------------------------- DPS abs. Preprint Time 5906 Index J5 J1 J2 J3 J4 ------------------------------------------------------------------------------ 21 A 16.81 1 194 340+ 103+ 75+ 35+ 20 B 17.08 1 25* 34+ 130 89+ 41+ 19 C 17.27 1 168 74+ 150 99+ 45+ 18 D 17.46 1 304 112 169 108 49+ 17 E 17.61 17.6 2 49+ 142 183 115 52+ 16 F 17.98 2 320+ 218 221 134 60+ 15 G 18.29 18.3 2 184 281 253 150 67+ 14 H 18.79 18.8 2 183 22+ 303+ 175 78+ 13 J 19.08 1 37+ 83+ 333+ 190 84+ 12 K 19.41 19.4 2 272 149 6* 206 91+ 11 L 19.91 19.9 2 271 250 57+ 231 102+ 10 M 20.24 1 152 318+ 90+ 248 109+ 9 N 20.40 1 265 350+ 106 256 112 8 P 20.61 2 58+ 33+ 128 266 117 7 Q 20.82 20.8 3 207 75+ 149 277 121 6 R 21.21 21.3 2 135 156 189 297+ 130 5 S 21.63 21.6 2 78+ 241 231 318+ 139 4 T 21.73 1 150 261 241 323+ 141 3 U 21.91 1 281 298 260 332+ 145 2 V 22.19 2 117 353o 287 345+ 151 1 W 22.33 22.3 2 219 22+ 302+ 353+ 154 Uncertainties: 0.1 0.1 72 20 10 5 2 ------------------------------------------------------------------------------ The "brightness index" subjectively rates comet fragment brightnesses, 3 being brightest. Relative timings of impacts are taken from the Sekanina, Chodas, and Yeomans preprint, with absolute times adjusted for best fit to the IAUC 5906 times. Only Io is suitably placed for the impact of the brightest fragment, (7 or Q), and even Io's location isn't great unless the impact is at the early end of the uncertainty range. There are several possibilities for eclipse observations but none are certain yet. The table below gives the orbital longitudes (in degrees) of satellites when in Jupiter eclipse and occultation (used to annotate the above table). Values should be good to about one degree. ------------------------------------- Satellite Occulted Eclipsed ------------------------------------- J5: Amalthea 337 - 023 023 - 034 J1: Io 351 - 009 009 - 020 J2: Europa 355 - 005 006 - 016 J3: Ganymede 358 - 002 009 - 013 J4: Callisto No occultations or eclipses ------------------------------------ John Spencer 1993/12/15 ============================= posted 17 Dec 1993, acr.