Computer Science – Learning
Scientific paper
Jan 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994eso..pres....2.&link_type=abstract
ESO Press Release, 01/1994
Computer Science
Learning
Scientific paper
Astronomers all over the world are preparing themselves for observations of a most unique event: during a period of six days in July 1994, at least 21 fragments of comet Shoemaker-Levy 9 will collide with giant planet Jupiter. At the European Southern Observatory, an intensive observational campaign with most of the major telescopes at La Silla is being organized with the participation of a dozen international teams of astronomers.
This is the first time ever that it has been possible to predict such a collision. Although it is difficult to make accurate estimates, it is likely that there will be important, observable effects in the Jovian atmosphere.
WHAT IS KNOWN ABOUT THE COMET ?
Comet Shoemaker-Levy 9 is the ninth short-period comet discovered by Gene and Carolyn Shoemaker and David Levy. It was first seen on a photographic plate obtained on 18 March 1993 with the 18-inch Schmidt telescope at the Mount Palomar Observatory, California. It was close in the sky to Jupiter and orbital calculations soon showed that it moves in a very unusual orbit. While other comets revolve around the Sun, this one moves in an elongated orbit around Jupiter. It is obvious that it must have been ``captured'' rather recently by the gravitational field of the planet.
It was also found that Shoemaker-Levy 9 consists of several individual bodies which move like ``pearls on a string'' in a majestic procession. It was later determined that this is because the comet suffered a dramatic break-up due to the strong attraction of Jupiter at the time of an earlier close passage to this planet in July 1992.
High-resolution Hubble Space Telescope images have shown the existence of up to 21 individual fragments (termed ``nuclei''), whose diameters probably range between a few kilometres and a few hundred meters. There is also much cometary dust visible around the nuclei; it is probably a mixture of grains of different sizes, from sub-millimetre sand up to metre-sized boulders. No outgassing has so far been observed from Shoemaker-Levy 9, but this is not unusual for a comet at this large distance from the Sun, about 750 million kilometres.
The most recent observations obtained with telescopes at Hawaii in mid-January 1994 have shown changes in the relative brightness of the individual nuclei, and many of them have now developed individual comet ``tails''.
Where and when will the collision take place ?
Accurate determinations of the positions of the individual nuclei have permitted to calculate quite precise orbits and it can now be said with 99 percent certainty that all of them will indeed collide with Jupiter. The points of impact are in the Jovian southern hemisphere, at -43 +- 0.6 deg latitude. Unfortunately, these impacts will happen just behind Jupiter's limb, i.e., out of sight from the Earth. However, due to the rapid rotation of the planet, the impact sites will come into view only about 10 minutes later at the very limb where they will be seen ``from the side''. It is also fortunate that the American spacecraft Galileo, now approaching Jupiter, will then be ``only'' about 200 million km away and will have a good view of the impact sites.
On the basis of the recent observations, the impact times can now be predicted to about +-40 minutes. The first, rather small nucleus (``A'') will hit the upper layers of Jupiter's atmosphere on July 16, 1994 at about 18:45 Universal Time (UT); the apparently biggest nucleus (``Q'') on July 20, also at 18:45 UT, and the last one in the train (``W''), on July 22 at 07:00 UT.
WHAT IS LIKELY TO HAPPEN AT JUPITER ?
The comet nuclei will hit Jupiter at a high velocity, about 60 km/sec. The correspondingly large motion energy (the ``kinetic energy'') will all be deposited in the Jovian atmosphere. For a 1 km fragment, this is about equal to 10^28 erg, or no less than about 250,000 Megatons.
When one of the cometary nuclei enters the upper layers of the Jovian atmosphere, it will be heated by the friction, exactly as a meteoroid in the Earth's atmosphere, and its speed will decrease very rapidly. Depending of the size of the fragment, it may evaporate completely within a few seconds, while it is still above the dense cloud layer that forms the visible ``surface'' of Jupiter, or it may plunge right through these clouds (and therefore out of sight) into increasingly denser, lower layers, where it ultimately comes to a complete stop and disintegrates in a giant explosion.
All of the kinetic energy is released during this process. One part will heat the surrounding atmosphere to very high temperatures; this will result in a flash of light that lasts a few seconds. Within the next minutes, a plume of hot gas will begin to rise over the impact site. It may reach an altitude of several hundred kilometres above the cloud layers and will quickly spread out in all horizontal directions.
Another part of the energy will be transformed into shock waves that will propagate into the interior of Jupiter, much as seismic waves from an earthquake do inside the Earth. When these waves again reach the upper layers of the atmosphere, they may be seen as slight increases in the local temperature along expanding circles with the impact sites at their centres (like waves on a water surface). The shock waves may also start oscillations of the entire planet, like those of a ringing bell.
During the past months, atmospheric scientists have attempted to calculate the details of these impacts, but the uncertainties are still rather large. Moreover, the magnitudes of the overall effects are entirely dependent on the energies involved, i.e., on the still not well determined sizes (masses) of the cometary nuclei.
It is also expected that there will be some kind of interaction between the cometary dust and Jupiter's strong magnetic field. The fast-moving dust grains may become electrically charged. This will possibly have a significant influence on Jupiter's radio emission and therefore be directly observable with Earth-based radio telescopes, as well as from several spacecraft, including Ulysses, now en route towards its first pass below the Sun. There may also be changes in the plasma torus that girdles Jupiter near the orbit of the volcanic moon Io, and some cometary dust particles may collect in Jupiter's faint ring.
All in all, this spectacular event offers a unique opportunity to study Jupiter and its atmosphere. It may also provide a first ``look'' into its hitherto unobservable inner regions. Nobody knows for sure, how dramatic the effects of the impacts will actually be, but unless we are prepared to observe them, we may lose a great chance that is unlikely to come back in many years, if ever.
WHICH PREPARATIONS HAVE BEEN MADE AT ESO ?
In November of last year, a group of 25 cometary and planetary specialists from Europe and the U.S.A. met at ESO to discuss possible observations from the ESO La Silla observatory in connection with the cometary impacts at Jupiter. In a resulting report, they emphazised that ESO is in a particularly advantageous situation in this respect, because the excellent site of this observatory is located in the south and Jupiter will be 12 degrees south of the celestial equator at the time of the event and therefore well observable from here; the time available from observatories in the northern hemisphere will be much more restricted. Moreover, many different observing techniques are available at La Silla; this provides optimal conditions for effective coordination of the various programmes, in particular what concerns imaging and spectral observations in the infrared and submillimetre wavebands.
A joint request for a coordinated observing programme was submitted by the group to the ESO Observing Programmes Committee. During its meeting at the end of November 1993, this committee reacted very positively and a substantial number of observing nights at the major telescopes at La Silla was granted at the time of the impacts in July 1994. The total amount of observing time to this programme is just over 40 full nights, a quantity never allocated for any single astro
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