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Periodic Comet Shoemaker-Levy 9 Collides with Jupiter

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Periodic Comet Shoemaker-Levy 9 Collides with Jupiter ED 374 016 SE 055'069' TITLE Periodic Comet Shoemaker-Levy 9 Collides with Jupiter. Background Material for Science Teachers. INSTITUTION Jet Propulsion Lab., Pasadena, Calif. PUB DATE 94 NOTE 28p.; Several figures based on images taken from spacecraft have marginal legibility. PUB TYPE Guides Classroom Use Teaching Guides (For Teacher) (052) EDRS PRICE MF01/PCO2 Plus Postage. DESCRIPTORS *Astronomy; Elementary Secondary Education; *Science Education; Space Sciences; Teaching Guides IDENTIFIERS *Comets; *Jupiter (Planet) ABSTR,77 In July of 1994, fragments of Comet Shoemaker-Levy collided-with Jupiter. This document has been provided to better inform students of the work that will be done by scientists and others involved in the study of this event. This document offers some background material on Jupiter, comets, what has and possibly will happen, and how scientists propose to take advantage of the impact events. The following sections are included: (1) What is A Comet?; (2) The Motion of Comets; .(3) The Fragmentation of Comets; (4) The Discovery and Early Study of Shoemaker-Levy 9;(5) The Planet Jupiter;(6) The Final Orbit of Shoemaker-Levy 9;(7) The Collisions; (8) How Can These Impacts and Their Consequences Be Studied?; and (9) What Do Scientists Expect to Learn from All of This? (ZWH) *********************************************************************A* Reproductions supplied by EDRS are the best that can be made from the original docuMent. * *********************************************************************** to O tt -tu Periodic CometShoemaker-Levy 9 U.S. DEPARTMENT OF EDUCATION 01 lice et Educational Research andtaiprovemeni Collides with Jupiter EDUCATIONAL RESOURCES INFORMATION CENTER IERICI 1..Zsdocument has been reproduced as pined Iron, the person or organization onginatrrig it 0 Minor changes have been madeto improve reproduction quality Points of view or opinions stated in this doCu. rent do not necessarily represent offidial Background Material for Science OERI position or policy Teachers 2 BEST COPY AVAILABLE or a period of about six days centered on July 19, Stupendous as these collisions will be, they will occur on the F1994, fragments of Comet Shoemaker-Levy 9 are far side of a body half a billion miles from Earth. There will expected to collide with Jupiter, the solar system's largest be no display yisible to the general public, not even a display_ planet. No such event has ever before been available for as obvious as a faint terrestrial meteor. Amateur astrono- study. The energy released by the larger fragments during mers may note a few seconds of brightening of the inner impact will be more than 10,000 times the energy released satellites of Jupiter during the impacts, and they might by a 100-megaton hydrogen bomb! Unfortunately for observe minor changes in the lovian cloud structure during observers, the collisions will occur on the. night side of the days following the impacts. The real value of this most Jupiter, which also will be the back side as seen from Earth. unusual event will come from scientific studies of the The collisions can still be studied. in many ways, nevertheless, comet's composition, of the imp& ihenomena-themselves, by spacecraft more advantageously located, by light of the. and of the response of a planetary at..losphere and mag- collisions reflected from Jupiter's satellites, and by the effects netosphere to such a series of "insults." of the impacts upon the Jovian atmosphere. (The impact This booklet offers some background material on Jupiter, sites will rotate into view from Earth about 20 minutes after comets, what has and possibly will happen, and how each collision.) scientists propose to take advantage of the impact events. R Cover Comet Shoemaker-Levy 9 is expected to collide with Jupiter in July 1994. From this historic event, scientist hope to learn more about comet, Jupiter, and the physics of high ve- locity planetary impacts. s a Comet? Comets are small, fragile, irregularly Comets, course,ust obey the same universal laws of shaped bodies composed of a mixture of non-volatile grains motion a do all o, er bodies. Where the orbits ofplanets and frozen gases. They usually follow highly elongated paths around t e Sun are nearly circular, however, theorbits of around the Sun. Most become visible,even in telescopes, comets re quite elongated. Nearly 100 known cometshave only when.they get near enough to the Sun for the Sun's ra- period (the time it takes them to makeone complete trip diation to start subliming the volatile gases, which in turn arou d the Sun) five to seven Earth years in length. Their far- blow away small bits of the solid material. These materials thest point from the Sun (their aphelion) isnear Jupiter'S or- expand into an enormous escaping atmosphere called the bit, with the closest point (perihelion) being muchnearer to coma, which becomes far bigger than a planet, and they are Earth. A few comets like Halley have their aphelionsbeyond forced back into long tails of dust andgas by radiation and Neptune (which is six times as far from the Sunas Jupiter). charged particles flowing from the Sun. Comets are cold Other comets come from much farther outyet, and it may bodies, and we see them only because the gases in their co- take them thousands or even hundreds of thousands of mae and tails fluoresce in sunlight (somewhat akin to a fluo- years to make one comp,ete orbit around the Sun. In all rescent light) and because of sunlight reflected from the cases, if a comet approaches near to Jupiter, it is strongly at- solids. Comets are regular members of the solar system fam- tracted by the gravitational pull of that giantamong planets, ily, gravitationally bound to the Sun.They are generally be- and its orbit is perturbed (changed), sometimesradically. lieved to be made of material, originally inthe outer part of This is part of what happened to Shoemaker-Levy9. (See the solar system, that didn't get. incorporated into the plan- Sections 2 and 4 for more details.) ets leftover debris, if you will. It is thevery fact that they are thought to be composed of such unchanged "primitive" The nucleus of a comet, which is its solid, persistingpart, has material that makes them extremely interesting to scientists been called an icy conglomerate, a dirty snowball,and other who wish to learn about conditions duringthe earliest pe- colorful but even less accurate descriptions.Certainly a riod of the solar system. comet nucleus contains silicates akin to some ordinary Earth rocks in composition, probably mostly invery small grains Comets are very small in size relative to planets. Their aver- and pieces. Perhaps the grains are "glued" togetherinto age diameters usually range from 750 m or less to about larger pieces by the frozen gases. A nucleusappears to in- 20 km. Recently, evidence has been foundfor much larger clude complex carbon compounds and perhapssome free distant comets, perhaps having diameters of300 km or carbon, which make it very black in color. Most notably,at more, but these sizes are still small compared to planets. least when young, it contains many frozengases, the most Planets are usually more or less spherical in shape, usually common being ordinary water. In the low pressure condi- bulging slightly at the. equator. Comets are irregular in tions of space, water sublimes, that is, itgoes directly from shape, with their longest dimension often twice the shortest. solid to gas just like dry ice does on Earth. Water probably (See Appendix A, Table 3.) The best evidence suggests that makes up 75-80% of the volatile material inmost comets. comets are very fragile. Their tensile strength (the stress they Other common ices are carbon monoxide (CO), carbondiox- can take without being pulledapart) appears to be only about ide (CO2), methane (CH4), ammonia (NH3), and formalde- 1,000 dynes/cm2 (about 2 Ibift.2). You could take a big hyde(H2C0). Volatiles and solids appear to be fairly well piece of cometary material and simply pull it in two with your mixed throughout the nucleus of anew comet approaching bare hands, something likea poorly compacted snowball. the Sun for the first time. As a cometages from many trips 4 What Is a Comet? 1 clo5e to the Sun, there is evidence that it loses most of its the glare of the coma. A great deal was learned when the ices, or at least those ices anywhere near the nucleus sur- European Space Agency, the Soviet Union, and the Japanese face, and becomes just a very fragile old "rock" in appear- sent spacecraft to fly by Comet Halley in 1986. For the first ance, indistinguishable at a distance from an asteroid. time, actual images of an active nucleuswere obtained (see Figure 1) and the composition Of the dust andgases flowing A comet nucleus is small, so its gravitational pull is very from it was directly measured. Early in thenext century the weak. You could run and jump completely off of it (if you Europeans plan to send a spacecraft called Rosetta toren- could get traction). The escape velocity is only about 1 m/s dezvous with a comet and watch it closely fora long period (compared to 11 km/s on Earth). As a result, the escaping of time. Even this sophisticated mission is not likelyto tell gases and the small solid particles (dust) that they drag with scientists a great deal about the interior structure of comets, them never fall back to the nucleus surface. Radiation pres- however. Therefore, the opportunity to reconstruct the sure, the pressure of sunlight, forces the dust particles back events that.occurred when Shoemaker-Levy 9 split and to into a dust tail in the direction opposite to the Sun.
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