Shoemaker-Levy 9/Jupiter Collision to be Observed atESO The upcoming collision between July 20, at 20h UT, and the last one in magnetic field. The fast-moving dust comet Shoemaker-Levy 9 and giant the train ("W"), on July 22 at about Sh grains may become electrically charged. planet Jupiter has led to intensive pre­ UT. This will possibly have a significant influ­ parations by astronomers allover the ence on Jupiter's radio emission and world and it is obvious that this unique therefore be directly observable with Possible Effects event has also caught the imagination of Earth-based radio telescopes, as well as the public. According to the latest calcu­ The comet nuclei will hit Jupiter at a high from several spacecraft, including lations the impacts will commence in the velocity, ~ 60 km/sec. The correspond­ Ulysses, now en route towards its first evening of July 16 and end in the morn­ ingly large motion energy (the "kinetic pass below the Sun. There may also be ing of July 22, 1994. There will be no energy") will all be deposited in the changes in the plasma torus that girdles less than 12 different investigations at Jovian atmosphere. For a 1 km frag­ Jupiter near the orbit of the volcanic the ESO La Silla observatory during this ment, this is about equal to 1028 erg moon 10, and some cometary dust par­ period. (~250,000 Megatons). ticles may collect in Jupiter's faint ring. When one of the cometary nuclei en­ All in all, this spectacular event offers ters the upper layers of the Jovian atmo­ a unique opportunity to study Jupiter A Unique Event sphere, it will be heated by the friction, and its atmosphere. It may also provide This is the first time ever that it has exactly as a meteoroid in the Earth's a first "look" into its hitherto unobserv­ been possible to predict such a colli­ atmosphere, and its speed will decrease able inner regions. Nobody knows for sion. Although it is difficult to make very rapidly. Depending on the size of sure, how dramatic the effects of the accurate estimates, it is likely that there the fragment, it may evaporate com­ impacts will actually be, but unless we will be important, observable effects in pletely within a few seconds, while it is are prepared to observe them, we may the Jovian atmosphere. still above the dense cloud layer that lose a great chance that is unlikely to High-resolution Hubble Space Tele­ forms the visible "surface" of Jupiter, or come back in many years, if ever. scope images have shown that the it may plunge right through these clouds (and therefore out of sight) into increas­ comet has broken up into 21 or more Some Recent Developments individual fragments (termed "nuclei"), ingly denser, lower layers, where it ulti­ whose diameters probably range be­ mately comes to a complete stop and Both Jupiter and the cometary nuclei tween a few kilometres and a few disintegrates in a giant explosion. have been extensively observed during hundred meters. There is also much All of the kinetic energy is released the past months. However, while we cometary dust visible around the nuclei; during this process. One part will heat now possess more accurate information it is probably a mixture of grains of dif­ the surrounding atmosphere to very about the comet's motion and the times ferent sizes, from sub-millimetre sand high temperatures; this will result in a of impact, there is still great uncertainty up to metre-sized boulders. No outgas­ flash of light that lasts a few seconds. about the effects which may actually be sing has so far been observed from Within the next minutes, a plume of hot observed at the time of the impacts. Shoemaker-Levy 9, but this is not un­ gas will begin to rise over the impact This is first of all due to the fact that it usual for a comet at a heliocentric dis­ site. It may reach an altitude of several has not been possible to measure the tance of 5 A.U. hundred kilometres above the cloud lay­ sizes and masses of the individual com­ Accurate determinations of the posi­ ers and will quickly spread out in all etary nuclei and thereby to estimate the tions of the individual nuclei have per­ horizontal directions. amount of energy which will be liberated mitted to calculate quite precise orbits Another part of the energy will be at the collisions. and it is certain that all of them will transformed into shock waves that will Despite intensive spectroscopic ob­ indeed collide with Jupiter. The points of propagate into the interior of Jupiter, servations, no gas has yet been de­ impact are in the Jovian southern hemi­ much as seismic waves from an tected in any of the nuclei. We only see sphere, at ~ -450 latitude. Unfortu­ earthquake do inside the Earth. When dust around the nuclei which are com­ nately, these impacts will happen just these waves again reach the upper lay­ pletely hidden from our view within behind Jupiter's limb, i.e., out of sight ers of the atmosphere, they may be these clouds. The amount of the dust from the Earth. However, due to the seen as slight increases in the local tem­ has been steadily decreasing; this is be­ rapid rotation of the planet, the impact perature along expanding circles with cause the dust production from the indi­ sites will come into view only the impact sites at their centres (like vidual nuclei - which began when the ~ 10-20 minutes later at the very limb, waves in a water surface). The shock parent body broke up at the time of the where they will be seen "from the side". waves may also start oscillations of the near-collision with Jupiter in July 1992 ­ It is also fortunate that the American entire planet, like those of a ringing bell. is slowly diminishing with time. spacecraft Galileo, now approaching During the past year, atmospheric sci­ Some of the smaller nuclei have re­ Jupiter, will have a direct view of the entists have attempted to calculate the cently disappeared from view, probably impact sites. details of these impacts, but the uncer­ because they have ceased to produce On the basis of the recent astrometric tainties are still rather large. Moreover, dust. It is not clear, however, whether observations, including some with the the magnitudes of the overall effects are this also implies that they no longer exist Danish 1.5-m telescope at La Silla, the entirely dependent on the energies in­ at all, or whether they are just too small impact times can now (June 20) be pre­ volved, i.e., on the still not well deter­ to be seen with available telescopes. dicted to about ± 30 minutes (20). The mined sizes (masses) of the cometary first, rather small nucleus ("A") will hit nuclei. The Observations at ESO the upper layers of Jupiter's atmosphere It is also expected that there will be on July 16, 1994 at about 20h (UT); the some kind of interaction between the In November 1993, a group of 25 com­ apparently biggest nucleus ("0") on cometary dust and Jupiter's strong etary and planetary specialists from 59 Europe and the U.S.A. met at ESO to cloud structure around the impact sites. imaging capabilities of TIMMI in the far­ discuss possible observations from the It may also be possible to obtain low­ infrared spectral region, this group will ESO La Silla observatory in connection resolution spectra which will show the be able to look far down into the atmo­ with the cometary impacts at Jupiter. In temperature of the flashes, but in view sphere and to measure minute tempera­ a resulting report, they emphasized that of their very short duration, a few sec­ ture variations. This should make it pos­ ESO is in a particularly advantageous onds at most, this will not be easy. sible to register the effects of the shock­ situation in this respect, because the The same instrument will also be used waves that arise when the cometary excellent site of this observatory is lo­ by Nick Thomas of the Max-Planck-In­ energy is deposited in the atmosphere. cated in the south and Jupiter will be stitut fUr Aeronomie (Lindau, Germany) The second group at the 3.6-m tele­ 12° south of the celestial equator at the to image the Jupiter plasma torus in scope, led by Benoit Mosser from In­ time of the event and therefore well ob­ order to detect possible changes after stitut d'Astrophysique, Paris, will be servable from here; the time available the impacts. looking for short- and long-term oscilla­ from observatories in the northern hemi­ Spectral observations of the comet tions of the entire planet during the days sphere will be much more restricted. have been made with the ESO 1.5-m and nights following the impacts. It is Moreover, many different observing telescope in April by Heike Rauer, also agreed that such observations will not techniques are available at La Silla; this from the Max-Planck-Institut fur Aero­ be easy, but they offer the best hope we provides optimal conditions for effective nomie. They are expected to lead to a presently have of learning about the in­ coordination of the various pro­ better knOWledge about the physical ternal structure of Jupiter. It may be grammes, in particular what concerns and chemical state of the impacting deduced from the observed frequencies imaging and spectral observations in bodies.