Shoemaker-Levy 9/ 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 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 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 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 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 (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 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. For instance, are they really so and modes of oscillation. A particularly the infrared and submillimetre wave­ "dusty", as present observations seem interesting problem is whether Jupiter bands. to indicate, or do they contain large really possesses a core of metallic hy­ During its November 1993 meeting, amounts of gas? If so, what kind of drogen, as postulated by some scien­ the OPC granted extensive observing molecules are present? This will help to tists. time for observations with the ESO tele­ refine the predictions of the impact Infrared images will also be made by scopes of this event. effects. Klaus Jockers from the Max-Planck-In­ Some of the observations at ESO are Imaging and spectral observations of stitut fur Aeronomie with the ESO in­ aimed at the accurate determination of the comet for the same general pur­ frared IRAC camera at the MPIIESO 2.2­ the positions of the individual nuclei in poses will also be obtained in early July m telescope. Since they will be obtained order to improve the determination of with the EMMI instrument at the 3.5-m at shorter wavelengths than those at their orbits. By continuing this work until by an inter­ TIMMI, they will show higher layers of the very last day before the impacts, it national team headed by Rita Schulz, the atmosphere and the possible will hopefully be possible to achieve a formerly at the University of Maryland changes (streaming motions, new whirls final timing uncertainty of a few minutes and now at the Max-Planck-Institut fur and eddies?) which may result from the for these events. This will be of import­ Aeronomie. impacts. These programmes will there­ ance for all other observations, both No less than 46 observing hours have fore complement each other. from the ground, and especially for been allocated at the Swedish-ESO A total of no less than 13 half-nights those carried out from the spacecraft. Submillimetre Telescope (SEST) to an have been allocated at the 3.5-m New A team from the Munich Observatory international group headed by Daniel Technology Telescope. They will be (Germany) under the leadership of Heinz Gautier, Observatoire de Paris-Meudon. shared between two groups which will Barwig will perform rapid brightness During the impacts, the cometary both use the IRSPEC instrument to ob­ measurements of Jupiter's moons at the molecules will be mixed with those in tain detailed infrared spectra of the im­ predicted times of the impacts with the Jovian atmosphere, some of which pact sites. One team is headed by Rita their special high-speed photometer may come from very deep layers. To­ Schulz, the other by Therese Encrenaz attached to the ESO 1-metre telescope. gether they will be carried upwards in from Observatoire de Paris. Among The flashes from the impacts will be the plume, described above. This may many others, they hope to observe reflected from the surfaces of those provide a rare opportunity, not only to some of the molecules which may be Jupiter moons which are in view of the register the submillimetre emissions present in the deeper layers of the Jo­ impact sites. If this happens when a from those molecules which are already vian atmospheres, e.g., water, moon is in full sunlight, the relative in­ known to be present in the comet and ammonium and phosphine (PH 3). crease of intensity will probably only be on Jupiter, but also to detect new and Altogether, there are 12 individual of the order of 1 %. However, if one of unknown molecules otherwise not ac­ programmes at all of the major tele­ the moons is located in the shadow of cessible for direct observations, either scopes, including the 3.6-m, the NTI, Jupiter and is at the same time visible from the interior of the cometary nuclei the SEST, the 2.2-m MPI/ESO, the from the Earth, then the relative bright­ or from deep down in Jupiter's enorm­ 1.4-m CAT and the Danish 1.54-m tele­ ening may be very conspicuous. ous atmosphere. scope. Whether this will be the case will of Infrared observations will playa very course depend on the exact moments of important role during the ESO cam­ the impacts. paign. A new ESO-developed instru­ The Observations Are Difficult Also at the time of the impacts, a ment, TIMMI (Thermal Infrared Multi­ group of French astronomers, headed Mode Instrument) will be mounted at the The observers at ESO will profit from the by Bruno Sicardy of the Observatoire de ESO 3.6-m telescope and will provide simultaneous observations with many Paris, will mount a special CCD camera detailed infrared images of the impact different telescopes and observing at the Danish telescope, which will be areas when they become visible at the techniques at one site. In particular, they used for different types of observations. limb. Two teams will be active here; one will have contact with observers at the They also hope to be able to detect is led by Timothy A. Livengood from South African Astronomical Observatory some of the expected light flashes from NASA Goddard Space Flight Center (SAAO), where observations will start a the Jupiter moons. In addition, this pro­ (U.S.A.) and includes several ESO staff few hours before each evening. They gramme will monitor changes in the astronomers. Thanks to the excellent will then be able to better prepare them- 60 selves for unexpected developments, All in all, the observers face a difficult • Background material in the form of should such be observed at SMO. task and must be extremely alert, espe­ text and images, as well as related It is clear that these observations will cially around the predicted moments of video clippings (broadcast quality) be difficult, in particular because of the impact. This will demand very high con­ will be available at request from relatively short time that Jupiter and the centration and necessitate "training July 5. comet will be well above the horizon at runs" before the real observations be­ • From July 11, ESO will issue daily La Silla, at most a few hours each even­ gin. Some of these have already taken bulletins with the latest predictions ing. When Jupiter is very low in the sky, place - not surprisingly, various techni­ and other news, related to the prepa­ the viewing conditions are less favour­ cal problems were uncovered and are rations of observations at La Silla and able, since the light must traverse a now in the process of being resolved. elsewhere in the world. longer distance through the turbulent • Press Conferences will be arranged and absorbing terrestrial atmosphere. at the ESO Headquarters in Garching ESO's Special Services to the However, since Jupiter will be south of and at the ESO Office in Santiago de Media the celestial equator, observing condi­ on Saturday, July 16, just be­ tions will be even worse from obser­ In view of the unique nature of this event fore the first impacts. Following an in­ vatories located in the northern hemi­ and the associated astronomical obser­ depth briefing, some of the media sphere. vations, ESO has decided to provide representatives will pass the night at To record the best possible data (im­ special services to the media. In particu­ the ESO Headquarters from where ages, spectra, light curves, etc.), the lar, it is the intention to ensure that the they can follow the first observations telescopes must follow the motion of media will be able to follow the develop­ at La Silla at distance. Jupiter very accurately. Due to its orbital ments at La Silla closely and in near-real • There will be a Press Conference at motion in the solar system, Jupiter time, and at the same time will be kept the ESO HQ each following day at moves rather rapidly in the sky, and the informed about the observational results 11 :00 (CEST), summarizing the previ­ telescope motion must be precisely off­ at other observatories all over the world. ous night's results. Selected images set to continuously track the planet This service will be available from the obtained at ESO the night before will without "smearing" the images. This is ESO Headquarters in Garching near be available on these occasions. Spe­ not a simple task, also since the planet's Munich, Germany, and special arrange­ cial arrangements are also being rate of motion changes with time and ments are also being made for the made for the Chilean media. new corrections must be made several media in Chile. It will have the following Based on ESO Press Releases times each hour. elements: 02/94 and 10/94

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