During this final orbit, after breakup in July 1992, Shoemaker- Levy 9 was followed carefully from discovery in March 1993 until the time the comet's angular distance from the Sun became too small to permit observations. The last useful astrometric (positional) observations reported before the fragments were lost in the glare of the Sun were made on July 11. The comet was recovered (found again after almost five months without observations) by Scotti and Tom Gehrels on December 9, with the comet rising above the horizon a bit more than three hours before the Sun in the morning sky. (The comet is so faint that it cannot be observed in twilight or too low on the horizon.) The quality of the predictions for the time of impact of the individual fragments on Jupiter will depend upon the number of high-quality astrometric observations of each comet fragment made between December 1993 and the time of impact. A week before the impacts the times should be known at least to 110 minutes (with 50% confidence), improving to perhaps 15 minutes a half day before impact.
At a fall planetary astronomy meeting (DPS) Jewitt, Luu, and Chen exhibited an image showing 21 distinct fragments in the Shoemaker- Levy 9 nucleus train. At discovery in March, this train was about 50 arc seconds or 162,000 km in length as projected on the sky. This angular distance had increased by about 40% (and the true linear distance by about 50%, since Jupiter was then farther from Earth) by the time the comet was lost in the glare of the Sun in July. The spreading is caused mainly by the fact that the piece closest to Jupiter at breakup was some 9 km closer than the farthest piece (the diameter of the comet) and therefore entered a faster orbit. The orbits are all so elongated that from Earth they appear to be nearly a straight line with the fragments strung out along it. The fragment nearest to Jupiter at breakup remains nearest to it and will be the first to impact. At this writing, Chodas and Yeomans predict that the train will reach an apparent length of some 1,286 arc seconds at the time the first of the fragments enters Jupiter's atmosphere. The true length of the train will be 4,900,000 km at that time, and it will require 5.5 days for all of the major fragments to impact.
The new data taken following solar conjunction (the closest apparent approach to the Sun as projected against the sky) more than doubled the length of time since discovery for which cometary positions were available. With this new data, it appears that the impacts will be centered on about July 19.5, a day and a half earlier than the first predictions. The approach to Jupiter that shattered the comet appears to have been even closer than first thought, about 96,000 km from the planet's center and only 25,000 km above the clouds. The revised orbit has also moved the impact points closer to the visible hemisphere, but unfortunately still on the back side as seen from Earth. The brightest fragment, of which there is some indication that it itself is fragmented, will impact on about July 20.78 and contains about 10% of the total mass of the comet. The other 20 observed fragments contain more than 80% of the mass. The remaining mass is contained in all of the dust and small pieces in the train, the trails, and the tails. Most of this mass also will hit Jupiter over a period of several months beginning about July 10, but it probably will cause few or no detectable effects.
Meanwhile, the dust trails of small debris will continue to spread, as will the major fragments. The east-northeast trail is expected to reach a maximum apparent length of some 70 arc minutes in late June of 1994 and then decrease again in apparent length with the tip turning around into a "V" shape. The west-southwest trail may reach a length of almost 100 arc minutes before impacts begin. Only the larger trail material will actually impact Jupiter. The earliest dust will begin to hit about July 10, and impacts will continue into October. The smaller dust will be moved into the tail by solar radiation pressure and will miss the planet completely.
If upon further study it is found that the pieces of Shoemaker- Levy 9 have continued to fragment, then predicting impact times will be much more difficult and the predictions less reliable. Such continued fragmentation of pieces already badly fractured is very possible, but fragmentation in the last day or two, when it is most likely to occur, will have no significant effect on the predicted times of impact. The pieces typically separate with a velocity of less than a meter per second. There are 86,400 seconds in a day, so even pieces separating at a full meter per second would be only 86.4 km apart after one day. Moving jointly at a velocity which reaches 60 km/s at impact, the pieces would hit within a few seconds of each other. The effect of further splitting upon the impact phenomena would be far greater and is discussed in the next section.
Figure 9 (currently not available) by David Seal shows the final segment of the comet fragment's trajectories. They will impact near a latitude of 44 deg. S and a longitude 70 deg. past the midnight meridian, still 10 deg. beyond the limb of Jupiter as seen from Earth, impacting the atmosphere at an angle of about 42 deg. from vertical.
Observing conditions from Earth will not be ideal at the time of the impacts, since there will be only about two deg. hours of good observing time for large telescopes at any given site after the sky gets good and dark and before Jupiter comes too close to the horizon to observe. At least it will be summer in the northern hemisphere, and there will be a better chance for good weather where many observatories are located. With 21 pieces hitting over a 5.5-day period, there will be an impact on average about every 6 hours, so any given site should have about one chance in three of observing at the actual time of an impact each night. Since the impacts will be on the back side of Jupiter, light from the impacts can only be observed by reflection from Jupiter's moons or perhaps from the rings or the dust comae of the comet fragments. Those attempting observations of the effects of the impacts on Jupiter can begin about 20 minutes after the impacts, when the impact area rotates into view from Earth.