We are continuing to use the InfraRed Imaging Spectrometer (IRIS) on the 3.9 m Anglo-Australian Telescope (AAT) at Siding Spring Observatory (near Coonabarabran, Australia) to monitor the impacts of Comet Shoemaker-Levy 9 with Jupiter. On 22 July, observations were initiated at 04:28 UT. The the sun was still high in the sky (2:28 PM local time) but the sky was clear, and the seeing was initially about 1 arcsec. K- and H- grism image cubes of the Jovian disk were recorded between 4:30 and 5:30 UT. Images extracted from the K-grism cubes initially revealed the impact sites of fragments K and L as bright clouds in the morning and afternoon hemispheres at about 45 S latitude. We could not easily distinguish the impact site of P2 from that of the K impact.
K-grism drift scanning was used to monitor the R-fragment fireball with an integration time of 1.5 seconds per slit position. In this mode, we obtain spatially-resolved (0.6 arcsec/pixel), full-disk spectral image cubes (R ~ 300) of Jupiter once every 2 minutes.
The detection of the fragment R impact fireball was a special challenge because it occurred while the sun was up, and because it was predicted to occur just as the the impact sites of fragments D and G were approaching the morning limb. In addition, the seeing worsened to 2-3 arcsec just before the predicted impact time, further complicating the detection of the predicted impact fireball.
A distinct, bright, point-like source first appeared on the morning limb at 05:34 UT. This feature was tentatively identified as the impact fireball from fragment R. At 05:42, as the seeing improved, this feature brightened dramatically, saturating the detector at 05:45:33, and producing distinct diffraction spikes, like the fireballs associated with the impacts of fragments G and K observed earlier this week. Just before saturation, the feature was at least 200 times brighter than the South Polar Hood at 2.34 microns. The telescope was stopped down to 2.3 m to bring the detector back on scale at 05:47 UT. The brightness of the feature was still about 200 times brighter than the South Polar Hood at 05:48 UT. By 05:50, its brightness had decreased by at least a factor of 25. The mirror was therefore re-opened to its full 3.9 m aperture. At approximately 06:00 UT, a bright feature came over the limb. We could not determine if this was the impact site of fragment R, or that of G, D, or Q2.
We are continuing to monitor these impact sites.
AAT 3.9 m Observing Team - David Crisp (JPL)
Vikki Meadows (JPL)
Stuart Lumsden (AAO)
Steve Lee (AAO)