Clark R. Chapman
6 April 1994
Nearly all observations of the SL-9 impacts will be made from the Earth, or from Earth-orbiting observatories. Several spacecraft will also be observing Jupiter in mid-July. However, the Galileo spacecraft, headed for Jupiter orbit in late 1995, has the best seat in the house. It will be the only observatory capable of actually "seeing" (i.e. resolving) the impacts directly: the impact sites will be on the side of Jupiter facing the spacecraft, and Galileo's CCD camera can resolve phenomena on Jupiter as well as can be done from most ground-based observatories (Jupiter will be about 60 pixels across). Other Galileo instruments will be observing, as well.
The sequences are currently being designed and there is little room for further modification. Moreover, the work is being done with a shoestring budget on a "best effort" basis. The information below is tentative and may be inaccurate in some respects. It is being provided, at the request of the managers of the bulletin board, in the interest of developing needed coordination.
The premier data will be taken with Galileo's camera. About 6 of the 19 impacts (tentatively D, E, K, N, V, and W) will be imaged in one of two basic ways. For half of the opportunities, the camera will snap pictures in a time-lapse mode using a new on-chip mosaicking capability. Images will be recorded every 2 1/3 second (tentatively for fragment N) through the period that includes the bolide flash and any subsequent fireball and lasting until the impact site rotates past Jupiter's morning terminator (typically 6 to 10 minutes later). Pictures will be taken every 8 2/3 seconds for two other events (probably V and D), which would permit successive images to be taken through a repeated cycle of 4 color filters and will reduce the 33% dead-time that characterizes the 2 1/3 second mode; this 8 2/3 second mode is more conservative of resources but would permit a brief bolide to slip through undetected, so it is best for the subsequent fireball phase. The images will be recorded on Galileo's tape recorder as arrays of 7 by 7 or 8 by 8 images per frame.
A second scanning mode will be most useful for recording the time history of the brief bolide flashes as the comet fragments plunge for a couple seconds through Jupiter's upper atmosphere. Galileo's scan platform will be moved so that the image of Jupiter drifts across the CCD detector with the shutter open (through a narrow filter). The scans will sacrifice one spatial dimension -- so the pictures won't be "pretty" -- but the mode should permit the rise and fall of a bolide flash to be measured every two-tenths of a second, or so. Diagonal scans across half of the CCD are tentatively planned for events E and W and should be sensitive to very faint phenomena (e.g. bolides from fragments as small as 100 m, meteor storms, aurorae, etc.) as well as to bright phenomena. A more efficient horizontal scanning approach is tentatively planned for fragment K, but Jupiter will be superimposed on the flashes, so faint phenomena will not be detected.
For five of the 19 impacts (tentatively C, F, G, P, and R), the camera will not be used. Instead, the other scan platform instruments -- including the Near Infrared Mapping Spectrometer, the Photopolarimeter Radiometer, and the Ultraviolet Spectrometer -- will try to record the bolide and fireball phases of the impacts. Together, they cover a much wider range of wavelengths than the camera, and often with better time resolution. However, these instruments lack the camera's spatial resolution and will record the impacts simply as enhancements to Jupiter's total radiation at those wavelengths.
The remaining impacts (A, B[?], H, L, Q, S, T, U) will not be observed by either Galileo's camera or most of Galileo's scan platform instruments. For those events, the spacecraft will not be in sight of Deep Space Network antennas on Earth; scan platform operations are not permitted when engineers on Earth are not in contact with the spacecraft. However, some monitoring of Jupiter's long radio-wavelength radiation by the Plasma Wave Subsystem can be done during those events without operating the scan platform; a low-rate PPR mode may alternate with PWS.
Most of the SL-9 impact data will be recorded on Galileo's tape recorder for later play-back to Earth. It is expected that most of the desired observations can be fit onto the tape recorder. The trick will be to identify the important data, because only a few percent of it can eventually be transmitted back to Earth over Galileo's small, low-gain antenna given the limited allocations by the Deep Space Network. (Galileo's large, high-gain antenna failed to open several years ago.) In order to find the valuable data, Galileo engineers will rely on exact timings of phenomena from Earth-based telescopic observers. They also hope to use the measurements of the impact of an early fragment by the PPR to calibrate the timings; if the absolute timing of that impact can be measured, then the relative times of the remaining impacts will be predictable to within a few minutes (or so we think).
It remains to be determined just how quickly Galileo will be able to respond to late changes in the predicted (or observed) impact times. The basic commands will be radioed out to the spacecraft a week or so in advance of the mid-July commencement of impacts. They will tell the spacecraft to observe (e.g. snap the camera shutter repeatedly) for an hour on either side of the predicted impact times. Later information from telescopic astrometry is expected to narrow the predicted impact times to plus-or-minus 10 minutes during the final days, and it is hoped that late commands can be sent up to Galileo to record data on the tape recorder for plus-or-minus 20 minutes centered on such late predictions (it would overtax the capacity of the tape recorder to record data for an hour or two at each impact). Galileo may have the opportunity to change the record times for the last several impacts based on observed times for the early impacts. Finally, it is hoped that the actual times of the events will become known to a couple of minutes, so that the appropriate parts of the recorded data can eventually be returned to Earth.
The Galileo experiment will be GREATLY ENHANCED by the most rapid communication of (a) updated, more accurate pre-impact predictions of impact times from astrometry, (b) preliminary indications of when major impacts may have occurred, and (c) refined estimates of impact times from synthesis of all groundbased and spacebased data.