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Comet Impact Observations from CTIO (1Sep94) (from NOAO HIGHLIGHTS!, NOAO Newsletter No. 39, 1 September 1994) The much-anticipated collision of Comet Shoemaker-Levy 9 with Jupiter disappointed nobody. The effects, which many astronomers had expected to be negligible, were spectacular and very well-observed, thanks to an unprecedented global observational campaign. The sequence of observable events for the larger impacts, gathered from many observatories, went something like this. As the nuclei approached Jupiter, the clouds of dust which surrounded them, and made them visible, were stretched out towards Jupiter by tidal forces, and the previous anti-sunward tails, produced by solar radiation pressure on the dust, disappeared. The actual impacts produced rather faint 30-second flashes at 0.95 fm, seen by the Galileo spacecraft; visible-wavelength flashes reflected off the satellites have not been confirmed at the time of writing. Within a couple of minutes, "fireballs" developed which were big enough to rise above Jupiter's limb and be seen directly from Earth: some of these were incredibly bright, brighter than Jupiter, between 2 and 10 fm. Spectra revealed emission lines due to CO and other species. These fireballs faded over tens of minutes, by which time a cloud at the impact site began to rotate into view. The clouds were very large, more than 10^4 km across, even when new, and were probably generated in situ by the effects of the fireballs as they spread laterally, predominantly to the south of the impact points. The largest impacts showed spectacular rings, presumably atmospheric shock waves of some sort, spreading out from the impact points in the first hour after impact. The oldest impact-generated clouds are a week old at the time of writing and show little sign of fading. They are dark at visible wavelengths and are very conspicuous. In the infrared, they are bright against the dark planet at wavelengths absorbed by methane, indicating a very high altitude, above most of the methane absorption: residual CH4 absorption can be used to estimate cloud altitudes. HST and ground-based images reveal a complex dark core and an extended crescent-shaped halo to the south of each impact site. Not all of the 20 impacts produced such dramatic effects. Several of the fragments seen in the pre-impact images produced no observable effects at all, while others of similar pre-impact brightness were spectacular. Intriguingly, most of the fragments which "fizzled" were displaced away from the linear alignment of the other fragments in pre-impact images. CTIO played an important part in this campaign, despite bad luck with both the weather and the faintness of the impacts that were visible from there. John Spencer, Darren DePoy, Jay Frogel, and Nick Schneider, using the OSIRIS camera at the 4-m, obtained some of the first images of the persistent cloud at the first impact site (fragment "A") in the hours after the impact, at 2.3 um, and continued to image the development of the clouds as they accumulated over the following week. OSIRIS also obtained 1-2.5 um spectra with a resolution of 500 of several of the impact sites: these show CH4 absorption above the clouds and will provide estimates of cloud altitude (see Figure 1 for an example). [Figure not included] Sang Kim, Christophe Dumas, Jay Elias, and Richard Elston used the IRS 1-5 um spectrometer at the 1.5-m telescope, and among other things detected CH4 v=3 emission from the "A" impact site, which will constrain physical conditions at this site in the hours after impact (see Figure 2). [Figure not included] IRS 5-um spectra of the "D" site showed very strong continuum emission and several molecular species, and many other spectra were obtained. CCD imaging by Brad Schaefer and Fred Ringwald at the 0.9-m telescope and Charles Ford at the Lowell 0.6-m telescope showed the development of the dark impact clouds in the visible, and the 0.4-m telescope provided spectacular visual views of the impact sites. Prior to the event, Pat Seitzer had been providing astrometry of the comet fragments using the Curtis Schmidt telescope. One of the last pictures taken of the comet is shown in Figure 3. This was taken the night of 10/11 July, when the comet was less than a degree from Jupiter; the image is the sum of several exposures which were shifted, added, and corrected for scattered light from the planet. [Figure not included] John Spencer (Lowell Obs.), Jay Elias (CTIO)
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