© 1985 SCIENTIFIC AMERICAN, INC

The Structure of Comet Tails

The plasma tall forms and disconnects from the comet in response

to the solar wind and its magnetic field. Observations of comets

Giacobini-Zinner and Halley may help to clarify such phenomena

by John C. Brandt and Malcolm B. Niedner,Jr.

The years 1985-86 will one day be regarded as a golden age for cometary astronomy. Indeed, if

we had been allowed to choose two years in which to be active as cometary scientists, these would have been our clear choices. Two important comets, Giacobini-Zinner and Halley;have ap­proached within range of observat'ion as they orbit around the sun. Giacobi­ni-Zinner has already yielded a boun­ty of information as a result of being the first comet to be visited by a space­craft, and astronomers have deployed an unprecedented array of resources to examine Comet Halley. Data are be­ing gathered by observatories on the earth's surface, by spacecraft orbit­ing the earth, by vehicles in space and in orbit around other planets and by six spacecraft that will fly near or into the comet's atmosphere.

It is fortunate that Giacobini-Zinner and Halley-among the few known periodic comets that are sufficiently bright and are also nearly complete in their range of cometary features-have come under close scrutiny at virtually the same time. Astronomers will com­pare findings derived from similar observational techniques for the two rather different comets. The massive efforts organized to study Giacobini­Zinner and Halley promise to provide direct evidence for theories about the origin, composition and dynamics of comets and their tails; we also expect them to raise many new questions.

The National Aeronautics and Space Administration's International Cometary Explorer (ICE) sped through the tail of Giacobini-Zinner on Sep-

tember 11, 1985, and relayed a mass of data that are still being evaluat­ed. Probes from the European Space Agency, Japan and the U.S.S.R. will venture to Halley in March, trans­mitting valuable measurements bear­ing on the structure, composition and physical conditions of cometary atmospheres, as well as providing the first images of a cometary nucleus. The missions will be supported by net­works of ground-based observations; concurrent observations will be made by the crew of NASA'S Astro 1 space shuttle mission.

The current missions extend a rich history of comet observation that

has stretched across the centuries. The word "comet" comes from the ter­minology of Greek astronomers, who first named these solar visitors "aster kometes," or long-haired star. It is now known that comets are composed of three main pa�ts: the atmosphere, the tail and the nucleus. The visual part of the atmosphere is often called the coma or head; it is an essentially spher­ical cloud of gas and dust. The atmos­phere may range from less than 1,000 kilometers in diameter to several mil­lion kilometers, depending on the spe­cies of gas. One or more tails, which are directed away from the sun, extend behind the atmosphere. There are two principal types of tails: dust tails and plasma tails.

Dust tails typically consist of sol­id, micrometer-size particles that have been pushed away from the coma by the pressure force resulting from sun­light striking the dust grains. They are

COMET WEST, photographed on March 9, 1976, was characterized by two types of com­et tail. The dust tail is the wide, diffuse tail to the left, consisting of three broad bands with faint emissions between them. It is composed of solid particles that have been pushed away from the comet's atmosphere by the pressure force resulting from sunlight striking the dust grains. The plasma tail is the narrower one to the right, which is threaded with intricate streamers. It is composed of molecules that have been trapped by interplanetary magnetic field lines (carried by particles of the solar wind) that have been wrapped around the comet.

curved and fuzzy, usually displaying little or no internal structure. Plas­ma tails are quite different. They are made of molecules that have been ion­ized in the atmosphere by solar radia­tion, trapped on interplanetary mag­netic fields generated by the sun, and wrapped around the comet so that the ions form a long, hairpin structure, which often exhibits threadlike for­mations, knots and large-scale distur­bances. The gases in the coma and the tail fluoresce, that is, they absorb sun­light and reradiate it.

According to a generally accepted model proposed by Fred L. Whipple in 1950, the source of all cometary ma­terial is the nucleus, which is inside the atmosphere but is never observed through telescopes because it is too small. Whipple compared the nucleus to a dirty snowball; it consists of ices of water and other molecules. Dust grains, and possibly rocky material, are interspersed more or less uniform­ly throughout the icy matrix. The nu­cleus is often considered to pe some­what spherical, measuring several kil­ometers in diameter. How can such a minor body give rise to plasma tails that are sometimes 50 million kilome­ters long?

The answer lies in the dynamics of the material the nucleus releases into interplanetary space. As a comet ap­proaches the sun the nucleus absorbs sunlight and heats up until it reaches the temperature at which the ices sub­limate, or go directly from the solid phase into the gaseous one: The escap­ing gases leave the nucleus at an initial speed of several tenths of a kilometer per second; as they move outward they undergo many chemical reactions. In addition fleeing gas molecules collide with newly liberated dust grains and propel them outward. Many of the gas molecules absorb ultraviolet photons of sunlight and gain kinetic energy as they break into smaller molecules-a process called photodissociation. This

49 © 1985 SCIENTIFIC AMERICAN, INC

COMET GIACOBINI-ZINNER is seen in an electronic image made by Uwe Fink with a charge-coupled device at the Lunar and Planetary Laboratory in Tucson. The date was July 26, 1985. An extensive atmosphere surrounds the comet, somewhat elongated in a di­rection away from the sun. No plasma tail is seen, probably because the exposure was short.

conversion of sunlight into kinetic en­ergy and the pressure of denser gas forcing these molecules outward cause them to accelerate to speeds that aver­age about one kilometer per second. The gas and dust mixture expands to become the cometary atmosphere, which continues to expand and to be replenished as material constantly es­capes from the nucleus.

After the gases leave the nucleus they I\. undergo a complicated series of transformations that prod uce new mol­ecules; a recent computer simulation of these activities included more than 1,200 processes and reactions. The es­caping gases can also react with par­ticles that surround the comet as they speed away from the sun. In one type of reaction, called charge exchange, a proton captures an electron from a cometary gas molecule or atom to yield a neutral hydrogen atom and a positively charged ion. The escaping gases are also modified by reactions between neutral molecules and ions, in which they exchange charge. The var­ious reactions altering the original nuclear gases yield a concentration of ions in the coma that will become the gaseous constituents of the com­et's plasma tail.

Understanding of the formation and structure of the plasma tail was greatly advanced by the introduction of pho-

50

tography in the late 1800's. The first high-quality photographs of comets were probably the ones obtained by Sir David Gill in 1882, but the technique was not practiced regularly until Com­et Swift appeared a decade later. Ed­ward Emerson Barnard pioneered in the intensive exploitation of photogra­phy, working with such simple equip­ment as a portrait-lens camera. He was the first person to observe that plas­ma tails are highly intricate, threaded with ray like structures and streamers often accompanied by knots, conden­sations and helixes. In the photographs he obtained several times in the course of each clear night, he noted that parts of the plasma tail moved at high speeds. Indeed, photographs that had been made on successive nights usual­ly bore no resemblance to one anoth­er because of great changes in the tail. Although Barnard did not know the detailed composition of the tails he observed, he was able to distinguish them from the slower-changing tails now known to be dust tails.

Barnard's studies led to the major revelation of these early years of com­etary photography: the understand­ing that a plasma tail disconnects and floats away into space, to be replaced by a new tail. In a 1905 paper, "The Anomalous Tails of Comets," Barnard described this mystifying cyclic phe­nomenon and advocated that frequent

photographs be made throughout the night. He argued: "The day-to-day his­tory of a comet has too great an inter­val, and the changes are not necessari­ly at all connected. It is the hour-to­hour history that must be studied to understand the changes taking place in the comet. In the case of a very bright comet, exposures at intervals of half an hour should be made as long and as continuously as the conditions will permit. By this means it will be pos­sible to determine the exact value of the motion of the particles in the tails of various comets." Barnard conjec­tured from these studies that discon­nections are caused by interactions with "currents in interplanetary space across which the tail may sweep."

Tittle progress was made beyond Bar­L nard's prescient work until 1951. In that year Ludwig Biermann began to discern the nature of the currents by analyzing the motions and accelera­tions of structures observed in come­tary plasma tails .. He calculated that the pressure of the sun's light was not nearly sufficient to account for the strength of the force pushing the tail away from the sun. Biermann argued that some form of "corpuscular radi­ation" continuously emitted from the sun must collide with cometary ions to form the tail and produce the large ob­served accelerations.

The next step was to determine the nature of these hypothesized solar par­ticles and explain how they account for the plasma tail's intricate morphol­ogy. It was known that hydrogen and helium are ionized in the corona (the tenuous outer part of the sun's atmos­phere) and that the resulting protons and electrons flow away from the sun as a result of the high solar tempera­tures. E. N. Parker of the University of Chicago determined that these atoms must accelerate as they leave the sun's gravitation and travel through inter­planetary space. He coined the term solar wind to account for the radia­tion's dynamic motion as it sweeps across the solar system. In 1957, a year before Parker's pioneering paper was published, Hannes Aifven of the Roy­al Institute of Technology in Stock­holm speculated that this corpuscular radiation probably carries the sun's magnetic field into space. His paper showed that the solar wind's magnetic field couples the solar-wind plasma with the cometary ions to form plasma tails. Aifven's "magnetic flux tube" model is widely accepted today. Our work and the observations of Giacobi­ni-Zinner and Halley continue to add detail to his theory.

At the heart of the theory is the vio­lent collision between the solar wind

© 1985 SCIENTIFIC AMERICAN, INC

and the comet's atmospheric gases. The solar wind and its magnetic field both flow toward the comet at typical speeds of 400 kilometers per second. In contrast to the solar-wind plasma traveling away from the sun, the new­ly created cometary ions are traveling toward the sun at perhaps one kilome­ter per second. Some basic knowledge of electricity and magnetism helps to predict what will happen when the gas­es collide. It is significant that charged particles cannot freely cross a magnet­ic field, but instead perform helical orbits along its lines of force. When ions from the outer regions of the at­mosphere (500,000 kilometers or more from the nucleus) are deposited into the solar wind, they are "captured" on the solar wind's magnetic lines of force and consequently travel back toward the comet in the same direction as the solar wind.

Because the solar wind has had mass added to it in the form of cometary ions, it must slow dowrr to conserve momentum. This deceleration process continues as progressively more ions are captured during the solar wind's flight toward the inner atmosphere. Eventually a point is reached where so

many ions have been captured, and the flow has been so decelerated, that the outward pressures from the ions and other gases closer to the nucleus are balanced by the inward pressures ex­erted by the solar wind carrying the captured ions. At this point the flow of the solar wind stops; it is said to stagnate. The magnetic fields it carries, which have been continuously com­pressed, form a magnetic barrier that is also at rest. This takes place far in­side the atmosphere; in the case of a bright comet such as Halley it may oc­cur between 1,000 and 10,000 kilome­ters from the nucleus.

Since the comet is an obstacle to the solar wind, a "bow shock" is

formed between perhaps 50,000 and 100,000 kilometers from the nucleus. It is similar to the bow wave made by a ship moving through the water. Far off to the sides of the comet smaller num­bers of ions are captured, and conse­quently the solar wind is not signifi­cantly impeded. The magnetic fields in these areas, which are connected to fields in the barrier, wrap behind the comet, forming two lobes of opposite polarity. This magnetic tail can be ob-

PLANE OF THE ORBIT ECLIPTIC OF EARTH

served because it channels fluorescing cometary ions such as those of carbon monoxide and water vapor.

The magnetic field embedded in the solar wind also interacts periodically with the magnetic field of the comet's tail to cause the kind of detachment Barnard observed. Our study of Com­et Kohoutek in 1973-74 shows how the process works. The comet was pho­tographed extensively at the Joint Ob­servatory for Cometary Research near Socorro, N. M. Its images revealed a wealth of structural detail in the plas­ma tail. The photographs made on one night indicated that the tail extended outward to a point some distance from the head, stopped and then seemed to start again. We studied Barnard's writ­ings on what are now called discon­nection events carefully as we sought to determine whether the Kohoutek photographs were indeed demonstrat­ing a property of comets or whether we were simply misinterpreting faint images on the film. His photographs provided the support we needed to view disconnection events as general properties of comets.

We next returned to data from Ko­houtek, seeking to understand how

SUN

ORBIT OF COMET HALLEY

ORBIT OF COMET GIACOBINI­ZINNER

GIACOBINI-ZINNER AND HALLEY are periodic comets, which visit the inner solar system in the course of their regular journeys around the sun. Halley's comet is observable from the earth ap-

proximately once every 76 years and Giacobini-Zinner approach­es the earth every 6.5 years. The colored dots on the three orbits in­dicate the position of the two comets and the earth as of January 15.

51 © 1985 SCIENTIFIC AMERICAN, INC

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MAGNETIC FIELD LINES from the incident solar wind are com­pressed in the comet's atmosphere, whose ions become captured by the magnetic field lines as the solar wind streams into the cometary atmosphere. The compression is caused by the increasing mass of the solar wind as a result of picking up ions; conservation of mo­mentum dictates that the flow must slow down. At some point deep inside the atmosphere, where the concentration of captured ions has

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become very large, the pressure of outwardly flowing ions matches the pressure of the inwardly flowing solar wind. This balance makes it impossible for the solar-wind plasma, and the lines of force it carries, to penetrate any farther into the atmosphere; the result is a magnetic-free region in the comet. Off to the sides the magnetic field lines WC2p around and behind the comet. These lines drag ions in the atmosphere along with the solar wind, away from the sun.

AREA OF MAGNETIC FIELD REVERSAL

IONS IN COMET TAIL are channeled along the magnetic field lines of the solar wind as it wraps around the comet. A sheet of electric current (vertical arrow) and a magnetic field of Idw strength separate lobes of opposite magnetic field polarity in the tail. When

52

the supersonic solar wind has captured a large enough mass of com­etary ions, it is abruptly slowed as it passes through a "bow shock," which is analogous to the shock wave caused by a supersonic air­craft or the bow wave caused by a boat as it moves through water.

© 1985 SCIENTIFIC AMERICAN, INC

disconnections can take place even though magnetic field lines have no be­ginning and no end. The Eighth Inter­planetary Monitoring Platform satel­lite measured the solar-wind plasma and the magnetic field near the earth. By translating these data to the posi­tion of Kohoutek we found the likely cause of disconnection events. Kohou­tek lost its tail when the comet passed through a magnetic sector boundary: a border between sectors of opposite po­larity in the magnetic field. We con­cluded that a tail disconnection proba­bly occurs every time a comet passes from one magnetic sector to another. There are plenty of opportunities for comets to lose their tails; during the sun's 25-day rotation period it forms four magnetic sectors of alternate po­larities that expand constantly as they turn with the sun. When the sectors are detected in the plane of the earth's or­bit, they appear in the shape of four swirling spirals.

According to our model, a comet I\. drops its tail when it CroSSBS a sec­tor boundary because the new sector contains a magnetic field that opposes the field from which the tail developed. From a plasma-physics point of view, the opposition of the two fields creates a situation that could hardly be more unstable. The result is a process known variously as magnetic reconnection, magnetic merging or magnetic annihi­lation. Although the theoretical details of the phenomenon are poorly under­stood in spite of decades of study, it is generally accepted that the topology of the magnetic field in the cometary atmosphere changes in a fundamental way. When the old magnetic field lines of the comet are approached by the magnetic field lines of the new sector, the old field lines are cut and recon­nected into the pattern of the field lines of the new sector. When the field lines of the comet are thus cut, the material they contain remains trapped in the old field lines while the comet contin­ues to move into the new magnetic field. The tail appears to detach when the last part of the material embedded in the old field moves away from the comet. When the disconnection proc­ess is complete, the comet immediately starts to grow a new plasma tail: one whose polarity corresponds to that of the new magnetic sector.

Our model calling for disconnec­tion events every time a comet cross­es a sector boundary, or roughly ev­ery week, is supported by high correla­tions between observed disconnection events and crossings of magnetic sec­tor boundaries. We have also found agreement between observed morpho­logical changes during a disconnec-

\ I I MAGNETIC-SECTOR

BOUNDARy ........

SPIRAL SHAPE of the solar wind's magnetic field (as seen from above the plane of the earth's orbit) results from the sun's rotation as it emits plasma carrying the field. Four "field sectors" are usually observed with each rotation. Successive sectors are of opposite polarity.

tion event and those expected from the model. Other mechanisms have been proposed to explain disconnec­tion events, but the explanation based on the cutting and reconnection of magnetic field lines at a sector bound­ary seems to be the most viable.

There are basic weaknesses in this model of disconnection events that should be solved by the extensive current observations of Halley. The first weakness is that our macroscopic model of the morphology of discon­nection events and the evolution of plasma tails has been plagued by frag­mentary data. The picture has been pieced together by utilizing images of different comets and analyzing data from various observatories that rely on a variety of instruments, emulsions and exposure times. We expect that the Large-Scale Phenomena Network of the International Halley Watch will help by providing a comprehensive record of Halley's journey. The net­work consists of approximately 100 facilities around the world, each one equipped with wide-field photograph­ic instruments that should record a major fraction of the comet's plasma and dust tails. These observatories

are tracking Halley from its passage across the northern hemisphere of the celestial sphere on its inbound journey through its return to the outer solar system across the southern hemisphere this spring.

In order to achieve extended periods of frequent coverage, good weather and an even distribution of observa­tion in both the earth's hemispheres is required. In the Southern Hemisphere, which is largely covered by oceans, four island-network sites have been es­tablished, two in the Pacific and one each in the Atlantic and Indian oceans. Portable telescopes will be set up at these sites to fill in what would oth­erwise be sparse coverage as the com­et moves across the Southern Hemi­sphere in March and April.

Halley's dust tail and plasma tail should both be observable. The tails will point roughly in the same direction on the comet's journey toward the sun; on its outbound leg the tails should be separated by a wide angle. There is ev­ery reason to expect that the impres­sive tail lengths observed during the 1910 apparition will be repeated, al­though the tails will not appear as long this time owing to foreshortening and

53 © 1985 SCIENTIFIC AMERICAN, INC

the greater distance from the earth. Analysis of photographs made in 1910 suggests that Halley's tail achieved lengths of several tenths of an astro­nomical unit. (A tenth of an astronom­ical unit is eq ual to 15 million kilome­ters.) As a result of highly favorable geometry and projection factors, the angular tail lengths in 1910 were as great as 50 degrees. In 1985-86 these apparent lengths should be in the range of from 10 to 15 degrees.

The coverage by the Large-Scale Phenomena Network will be sup­

plemented by photography of Halley by the wide-field cameras on board the Astro 1 space shuttle mission. Dur-

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ing one week in March the cameras will obtain images every six hours. They will also record the comet every hour and a half for one-day intervals when Halley is approached by two spacecraft, the U.S.S.R.'s Vega-2 and the European Space Agency's Giotto.

The second weakness in our model­insufficient data on the microscopic conditions of cometary plasma and the tail's magnetic fields-is being ad­dressed by six spacecraft. One, ICE, has already had a successful encounter with Giacobini-Zinner. The other five will encounter Halley in March of this year, and ICE will be an upstream mon­itor. Prior to these missions there have been no in situ measurements of any

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cometary parameters; we have had no accurate values for the magnetic field, temperature, density, composition or bulk velocities in any cometary plas­ma. Data on these parameters are nec­essary to determine which processes are important and how rapidly chang­es in the comet can be expected.

The first of the missions, ICE, passed through the plasma tail of Giacobini­Zinner some 8,000 kilometers from the nucleus on September 11, 1985. It provided significant evidence for the shape of the captured magnetic field, the two-lobed tail and the flow of elec­tric current in the tail-features that follow from Alfven's 1957 model. ICE measured a striking amount of turbu-

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PLASMA TAIL DISCONNECTS when the comet crosses a sector boundary, moving from a sector where the magnetic field has the same polarity as the tail to a sector where the field has opposite polarity. When the new field enters the coma and is pressed into the oppositely directed old fields threading the tail, the old field lines are cut by a process known as magnetic reconnection. The old field

lines retain ions they have captured, but they are no longer bound to the comet. When all the old magnetic fields have been ejected from the atmosphere, the newly created ions in the atmosphere have no magnetic connection to the plasma tail. The tail appears to dis­connect from the coma. The atmosphere immediately supplies ions for a new tail having the polarity of the new magnetic field sector.

54 © 1985 SCIENTIFIC AMERICAN, INC

lence as it encountered the bow-shock region. The mission also prod uced two major surprises: not all the expect­ed signs of bow shock were observed and some unexpected high-energy ions were detected by two experiments. The observations of the bow shock have led some workers to speculate that comet plasma may under certain con­ditions gently slow the solar wind in­stead of decelerating and shocking it more rapidly, as was predicted in the classical model. Detailed study of the ICE mission's data should help to ex­plain these findings and others.

The launchings of the Halley-bound spacecraft are timed to minimize

the launch energy necessary to reach the comet. Aiming for the comet when it is passing through the earth's orbit­al path takes advantage of the mo­mentum of the earth's orbital mo­tion, which transfers momentum to the spacecraft and thus effectively boosts their energy during the launching. The most daring of these missions will be carried out by Giotto, which will en­counter a large concentration of com­etary dust as it flies within 500 kilo­meters of the nucleus. The leading sur­face of Giotto is protected by a double shield. The first shield should fragment and slow the incoming particles; the second shield is intended to prevent the particles from penetrating the main body of the spacecraft.

In order to approach closely, Giotto will need data on the position of the nucleus. The U.S.S.R. will provide this information through an agreement called Pathfinder. Cameras on the So­viet probes Vega-l and Vega-2 will re­cord the position of the nucleus, which will be relayed under the auspices of an Inter-Agency Consultative Group composed of the four flight agencies and the International Halley Watch. Vega-l will fly within 10,000 kilome­ters of the nucleus; Vega-2 may be tar­geted closer to it. The Vega and Giotto missions should supply the first photo­graphs of the nucleus of a comet. The Japanese probes, Sakigake and Suisei, will provide additional information about Halley while staying far enough away from the comet to avoid the haz­ard of cometary debris. Sakigake will monitor the solar wind streaming onto the comet from a position about one million kilometers or more upstream of the comet. Suisei will penetrate the comet's atmosphere at a distance of 200,000 kilometers and obtain data on plasma density and velocity. It will also obtain ultraviolet images of the comet's atmosphere.

The massive efforts directed toward Halley and Giacobini-Zinner should dramatically advance understanding

DISCONNECTION of Comet Halley's tail was photographed during its last visit, in 1910. Disconnections were first reported by Edward Emerson Barnard, who correctly hypothe­sized in 1899 that they were caused by some interaction with the interplanetary medium. He advocated photographing comets frequently to trace such changes. Halley's discon­nection event was pieced together from different photographs made in 1910 on June 6 at the Yerkes Observatory (top) and at Honolulu (middle) and on June 7 at Beirut (bottom).

55 © 1985 SCIENTIFIC AMERICAN, INC

VEHICLE

LAUNCH DATA

GIACOBINI-ZINNER

NASA

ICE

AUGUST 1978

(DECEMBER 1983)

HALLEY

U.S.S.R. EUROPEAN SPACE AGENCY

VEGA-1

DECEMBER 1984

VEGA-2 GIOTTO

DECEMBER 1984 JULY 1985

JAPAN

SAKIGAKE SUISEI

JANUARY 1985 AUGUST 1985

ENCOUNTER: DATE SEPTEMBER 11, 1985 MARCH 6, 1986 MARCH 9, 1986 MARCH 13, 1986 MARCH 8, 1986 MARCH

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COMET EXPERIMENTS 8 13

SPACECRAFT ENCOUNTERS with two important comets are listed in this table. The encounter with Giacobini-Zinner provid­ed valuable data on the structure of comet tails when the Nation­al Aeronautics and Space Administration's Illtematiollal Cometary Explorer (ICE) passed through the plasma tail last September 11.

13 10 3

(ICE was launched into orbit between the earth and the sun in Au­gust, 1978, and was retargeted in December, 1983, for the encoun­ter with Giacobini-Zinner.) The missions that will intercept Halley in March should provide a more detailed understanding of the prop­erties of comet tails by supplying information from the sunward side.

of cometary physics, but many q ues­tions, including those that emerge from the new data, will remain. The direct exploratory missions provide in­formation based on a series of snap­shots taken along single trajectory lines. Global data recording cometary

changes over time will be needed to deepen understanding of comet tails. NASA'S Comet Rendezvous and Aster­oid Flyby mission, planned for launch­ing in' the early 1990's, should pro­vide this important information. The vehicle is expected to approach Com-

et Wild II in 1995 and record valuable data as it travels with the comet along its orbital path for approximately two and a half years. If the mission is suc­cessful, it will mark the next logical step in attempts to explore and under­stand the nature of comets.

GREAT COMET OF 1843 dominated the northern skies. It was depicted in this lithograph as it was seen over Paris on March 19. The comet, perhaps the brightest one of the past two centuries, has

not lJeen seen since the 1843 appearance. The tail was about 300

56

million kilometers long, or longer than the distance from the sun to the orbit of Mars. The artist recorded a bright tail that is probably of the dust variety. Although there must have been a plasma tail, it was

probably embedded in the dust tail and was too faint to be observed.

© 1985 SCIENTIFIC AMERICAN, INC

Anti-lock braking pOlNer you can grade on a curve.

You' re driving 55 MPH on a rain-slick curve. Suddenly the unexpected: You stand on the brake pedal and steer to stay in your lane. You might expect Europe's most exotic cars to handle such a crisis effortlessly. Yet for all its awesome straight-line braking ability, Ferrari 308 GT Si failed to negotiate a 150-foot radius curve at maximum braking in USAC-certified testing. Lamborghini Countach failed. Lotus Esprit Turbo failed. Porsche 944 failed. Only the 1986 Corvette demonstrated the ability to steer and stop in these conditions at the same time. Only Corvette made the turn while coming to a controlled stop. When conditions turn foul, Corvette's new computerized Bosch ABS II anti-lock braking sy stem is designed to help improve a driver's ability to simultaneously brake and steer out of trouble. Why does the Corvette feature the world's most advanced braking technology? Because a world-class champion should give y ou the edge in an emergency. Corvette. A world-class champion.

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- - -- _ .. -- - -_ ... - - .. ----'!!!!!:::".!!!! !!!!!!!!!!!!!=:: -.r_ !!!!!!!!!!!. '!!!::::!!!!!' ___ !!!!!!!=:: � �

© 1985 SCIENTIFIC AMERICAN, INC

AIetificial Intelligence: Summary: GTE research in Artificial Intel­ligence has produced exciting results in several areas ofknowl­edge-based systems. In addition, research is under way to teach computers to learn by them­selves, much as humans do.

It's extremely tedious and difficult to teach a computer to respond to spe­cific problems in an intelligent way.

Despite this, GTE has created several workable systems, which are in the field now.

But training a computer to respond to analogous or unexpected situations -teaching it to learn-is a very differ­ent challenge. And this is one of our long-range programs in AI research.

The ultimate brain-picking. The Expert-Systems version of AI

is literally the result of programming the experiences of experts into a com­puter.

Which of these questions is easier for a computer to answer? The apparently simple greeting is loaded with semantic traps. On the other hand,

Once these human reasoning proc­esses have been codified, the com­

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puter has the information it needs to mimic the experts' responses to an immense variety of problems.

COMPASS (Cen­tral Office Maintenance Printout Analysis and Suggestion System) is an Expert System we devised for tele­communications. It is being phased into field use to monitor switch per-

the complex question relating to traffic redirection can be tackled by Expert Systems.

© 1985 SCIENTIFIC AMERICAN, INC

reality and promise. formance, diagnose problems and rec­ommend corrective actions in large communications networks.

Say hello to FRED. The proliferation of databases and

their integration in a large informa­tion system is increasing computer uses. Increasing user friendliness is becoming all the more necessary for computers to be used by less skilled operators.

GTE has developed FRED (Front End for Databases), which enables operators to frame information requests from mUltiple databases, in plain English. FRED untangles the request, breaks it into segments the computer understands-and provides the data, in plain English.

For its next evolution, we are teach­ing FRED to approach several data­bases at once (rather than one at a time), and put all relevant data into a single reply.

The nature of thought. Another of our AI research direc­

tions is basic, long-range research into ways of teaching computers to learn for themselves, through experience and/or inference.

This involves research into such an area as the way children learn, as well as deep studies into the nature of deci­sion-making itself.

Much remains to be discovered, of course-but the promise oftrue machine learning is perhaps the most exciting in the entire computer field.

The outcome of these projects­some near-term, some more in the future-will be to make the computer a far more useful and friendly tool for an immense variety of industrial and human problems.

The box lists some of the pertinent papers GTE personnel have pub­lished on various aspects of Artificial

Intelligence. For any of these, you are invited to write to GTE Marketing Services Center, Department AI, 70 Empire Drive, West Seneca, NY 14224. Or call 1-800-828-7280 (in New York State 1-800-462-1075).

Pertinent Papers "CO MPASS: An Expert System for

Telephone Switch Maintenance;' S. K.

Goyal, D. S. Prerau, A. V. Lemmon, A. S.

Gunderson and R. E. Reinke, Expert

Systems: The International Journal of

Knowledge Engineering, Vol. 2, No. 3,

August 1985. pp 112-126.

"Selection of an Appropriate D omain L..,..""'"" .... .;.;.;. ..... .....:IL_.;.;.;._ ....... _.-, ................ __ �_'fuwJ! for an Expert System;' D. 5. Prerau, AI Magazine, Vol. 4, No. 2, Summer

1985; pp 26-30.

" A Natural Language Interface for

Medical Information Retrieval;' G.

Jakobson, C. LaFond, E. Nyberg and V. Shaked. T hird A ASM I Joint National

Congress on Computer Applications in

Medicine, May 1984, San Francisco,

California. pp 405-409.

Computer Experience and Cognitive

Development, R. W Lawler. Ellis

Horwood Limited, Chichester, U.K.

(1985). (Summary of book.)

"T he Learning of World Models by

Connectionist Networks;' R. S. Sutton

and B. Pinette. Proceedings of the

Seventh Annual Conference on

Cognitive Science Society, 54 (August

1985).

"Training and Tracking in Robotics;'

O. G. Selfridge, R. S. Sutton, A. G. Barto.

Proceedings of the Ninth International

Joint Conference on Artificial

Intelligence, 670 (August 1985).

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© 1985 SCIENTIFIC AMERICAN, INC