GEOPHYSICAL RESEARCH LETTERS, VOL. 22, NO. 12, PAGES 1565-1568,JUNE 15, 1995 Direct observations of the Comet Shoemaker-Levy 9 fragment G impact by Galileo UVS C. W. Hord, W. R. Pryor, A. I. F. Stewart,K. E. Simmons, J. J. Gebben,C. A. Barth, W. E. McClintock, andL. W. Esposito Labo•:atory forAtmospheric and Space Physics, University of Colorado, Boulder, Colorado W. K. Tobiska, R. A. West, S. J. Edberg, J. M. Ajello, K L. Naviaux JetPropulsion Laboratory, Pasadena, California Abstract. The Galileo Ultraviolet Spectrometer (UVS) team has detected the Shoemaker-Levy 9 fragment G impacton Jupiter in data recentlyplayed back from the spacecraft tape recorder. A 20% brightening of the disc-integrated signal of Jupiter was detected at 292 nm duringa swathacross Jupiter that lasted1.6 secandwascentered at 1994-July 18 (day 199)/07:33:31 UT (all times in this paper arecorrected to be the timeof theevent asseen from Earth). The emissionbrightness, when combinedwith simultaneous Photopolarimeter Radiometer (PPR) measurements at 945 nm, is consistent with thermal radiation at a temperature of 7800 (+500, -600) K emiued overan area of 40 (+60, -25) km 2. No excess signal was seenduring swaths 5 1/3 secbefore and after the detection swath. Introduction nm thesensitivity of thistube drops sharply.An F-channel fixed wavelength of 292 nm (1.4 nm band pass), near the peak response to Jupiter (Fig. 1), was chosento maximize the probability of detecting thermal emission from the impact. This wavelength is at the energyemission peak for blackbody thermalradiationat a temperature of 10 n K. There are no distinctive absorption or emission features in thereflected solar spectrum at 292 nm. During the fragment G observation on July 18, 1994, Galileo was 240 million km from Jupiter,whose angular diameterwas 0.034 deg. The planet undeffilled the UVS instrument slit (0.10 ø x 0.33ø). The angular rate of the scan platform sweeps was such thatJupiter traversed the 0.1o width of the UVS slit in about 1.6 s. The observing geometry is shown in Fig. 2. Brightness measurements werereported every7.6 ms, each representing 6 ms of integration. The Galileo spacecraft (Johnson et al., 1992) witnessed the unique Comet Shoemaker-Levy 9 (SL9) impactevents on Jupiter. The sun-Jupiter-spacecraft phase angle of 51 degprovided direct viewing of the fragment G impact event, which occurred just before dawnbeyond the limb of Jupiter as seen from Earth. The G impact site was at planetocentric latitude 43.66+1.0 ø and system121I longitude 26.8+2.0ø (Hammel et al., 1995). Three Galileo scan platform instruments: the Near-Infrared Mapping Spectrometer (NIMS), the PPR, and the UVS performed simultaneous, boresighted measurements duringthe periodwhen theimpact of fragment G wasexpected. The scan platform swept the instrumental fields of view across Jupiter from the dark to brightlimb and then back in a pattern that repeated every 10 2/3 sec. This paper will briefly discussand interpret the UVS detection of the fragment G impact event. Experimental Design The UVS observing strategy for the fragmentG observation was chosen to search for thermalblackbody emission from the impact event. The Galileo UVS instrument usesa Cassegrain telescope, an Ebert-Fastie spectrometer, 3 photomultiplier tubes anda scanning grating drive to record UV spectra from 115430 nm (Hord et al., 1992). The UVS F-channel has a photomultiplier tubewith a cesium telluride photocathode (160-320 nm) and is suitable for observing planets illuminated by sunlight. Above 300 Copyright 1995by the American Geophysical Union. Papernumber95GL01414 0094-8534/95/95GL-01414503.00 Observations Galileo UVS data at 292 nm from 73 full or partial 1.6 s traverses across Jupiter were returned to Earth. Of this set, 33 were sequential swaths from the period 1994-199/07:33:12to 07:36:06. A total of 53 swaths were used (including thedetection swath) in this analysis. The wholedataset spanned the period from 07:31:44to 07:41:51. The average count ratefrom 52 such traverses (withno evidence of excess signal) was 6.6 counts per6 ms at 292 nm, corresponding to an energyflux at Galileo of 2.1x1044 watt cm '2 nm 4. Onlyone traverse across Jupiter at 1994-199/07:33:31, detailed below,produced UVS count levels significantly different from theaverage. The average count levels for each traverse weredistributed according to Poisson statistics, as expected, except for the one bright swath. Jupiter measurements 5 1/3 s earlier and5 1/3 s later thanthe bright swath were not significantly different fromtheother background Jupiter measurements, indicating that theUV flash was brief. The impactflash was seen duringa right-to-left traverse of Jupiter acrossthe UVS field of view. On such traverses, the terminator crossed intotheslit first,followed 0.45 s later by the bright limb. The average of 15 such traverses, notincluding the impact traverse, isshown in Figure 3 as a heavy line. Also plotted is the impact traverse; both curves have been smoothed over 13 samples or 0.1 s. All thetraverses were hand-aligned before the average was taken, to counteract the effects of small errors in pointing and/or timing. The RMS correction was0.09 mrad or 80 ms. Theimpact site entered the slit0.42s before the bright limb, making theimpact swath wider than thebackground swaths. In Figure 3 the impact traverse was aligned with the average at the right-hand edge, afterthe impact sitehad left the field of view. The difference between these two curves is shown as a histogram representing the average excess counts seenin successive 0.1 s 1565