comet halley the view from pioneer venus

8/8/2019 Comet Halley the View From Pioneer Venus

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Of Space, Not of EarthEdmond Halley, scientist friend of Sir Isaac

Newton, placed comets in their rightful place ascelestial rather than terrestrial objects. Usingobservations of the bright comets of 1531,1607,and 1682, he applied Newtons method ofcomputing orbits of celestial bodies to calculatethe paths of these comets. From the similarity oftheir elliptical orbits he concluded that thesecomets were one and the same comet which visitsthe inner Solar System every 75-76 years. Earlierbeliefs that comets were atmospheric phenomenaakin to meteors and aurora were finally totallydispelled when the comet again returned in 1758and made a close approach to Earth as Halleyhad predicted.

Later, other astronomers searched ancientrecords and traced observations of the samecomet back into the Middle Ages and ultimatelyto comets recorded in China and Babylon as longago as 240 B.C. Sometimes i ts passage throughthe inner Solar System coincides with a favorableposition of Earth from an observational pointof view. At such times the comet becomes asplendid object in the sky. At other apparitions,such as the current one in 1986, the comet notonly f a i l s to pass near to Earth but also passesthrough perihelion, i ts closest approach to theSun, on the far side of the Sun from Earth.The most active and spectacular period of thecomets passage, generated by solar radiation,i s then hidden from Earth by the interveningbrilliance of the Sun itself.

Comets are important to study because theyare thought to preserve a record of the primordialnebula out of which the Solar System and itsplanets formed. Because comets spend most oftheir time in the cold outer reaches of the SolarSystem they are believed to preserve the recordbetter than the planets do, since planets aresubject to other modifying processes. Informationon cometary ingredients can provide importantclues about the early chemical and physicalhistory of the Solar System.

Several spacecraft are headed for HalleysComet on its current return, but they have al lbeen targeted to intercept the comet when it i sclose to the plane of Earths orbit, many weeksafter perihelion.

Only one spacecraft is situated to observeHalley at its perihelion when it wil l be mostactive; Pioneer Venus orbiting the planet Venus.This spacecraft will be on the far side of the Sunat the time of Comet Halleys passage.Another First for Pioneer

The Pioneer program has achieved manypioneering feats in the Solar System. The Pioneer10 spacecraft was first to show that a spacecraftcould pass safely through the asteroid belt.It became the first spacecraft to explore the giantplanet Jupiter and i t s planet-sized moons.Pioneer11was first to explore the ringed planetSaturn. Pioneer 10 also became the first spacecraftto escape from the Solar System into the inter-stellar void carrying a message to possibleextraterrestrials. Venus Pioneer probes penetratedthe enormously dense atmosphere. The PioneerVenus Orbiter charted the complex surfacefeatures, and observed for many years theparticles and fields around the planet andtheir interaction with the solar wind.

Th e spacecraft that will observe Comet Halleyi s Pioneer Venus Orbiter. This highly reliablespacecraft i s s t i l l gathering data about Venus andi t s interaction with the Sun to a high order ofaccuracy. It has already observed two other cometsusing an instrument that collects ultravioletradiation from them: Comet Encke in April 1984and Comet Giacobini-Zinner in September 1985.Giacobini-Zinner was observed just after the cometpassed perihelion and when the comet was about100 million miles (161 mill ion km) from thespacecraft and just beyond Earths orbit. Measure-ments were made simultaneously with the passageof the ICE spacecraft through the comets tail.

The Pioneer Venus Orbiter discovered thatEncke was losing water at a faster rate thanexpected. The observation suggests that water iceand dust are distributed unevenly in cometarynuclei. Also the observations showed thatGiacobini-Zinner i s much more active than Encke,but less active than i s expected of Halley.

Pioneer Venus observations also detectedLyman alpha ultraviolet emissions from thehydrogen coma as far as three million miles(5 million km) on either side of th e nucleusof G acobin -Zi n ner.

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ORIGINAL PAGePerihelion Passage COcdR PHOTOGRAPHComet Halley moves around the Sun opposite

to the Earths motion and on an elongatedelliptical orbit which i s inclined 18 degrees to thatof the Earth. The comets greatest distance fromthe Sun (aphelion) in 1948 was 3.28 bill ion miles(5.28 bill ion km), which i s beyond th e orbit ofNeptune. The comets closest approach to theSun (perihelion) on February 9,1986, wi ll be 54.6million miles (87.9 million km), which is insidethe orbit of Venus.

The comet consists of a nucleus which isbelieved to be an irregularly shaped, roughlyspherical body with a diameter of about five miles(eight km). From observations of the sunlightreflected from the nucleus, astronomers concludethat the nucleus of Comet Halley s rotating aboutonce in 2 to 3 days. This nucleus i s thought toconsist d ices- redorriinantly water ice-mixed with rocky dust particles, a mixture thathas been likened to a dirty snowball. Thisdescription was confirmed by the observationsof G acob n Zi n ner.

A cloud of gas and dust i s released from the cornets nucleus as itapproaches the sun.

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ORIGINAL PAGECOLOR PHOTOGRAPH

As the comet approaches th e Sun, solarradiation heats the surface of the nucleus andchanges th e ices into gas, thereby releasing acloud of gas and dust which spreads outwardaround th e nucleus. This cloud i s referred to asthe coma. Sunlight reflected from the gas anddust of the coma produces the visible comet seenfrom Earth in the night sky. The nucleus is toosmall to be seen, but th e coma can extend aroundthe small nucleus for tens of thousands of miles.The coma reaches i ts greatest size when a cometis about twice Earths distance from th e Sun.The visible gas cloud i s thought to consist mainlyof carbon-bearing molecules with traces of othergases. The water vapor i s not visible because ofit s transparency, but i s inferred from ultravioletobservations.

Material that forms the coma emerges fromthe nucleus irregularly, and as the material i spushed from th e nucleus into the coma it acts asan array of weak rocket jets to affect the rotationrate and orbital motion of the comet. Jets fromComet Halley, confirmed in detail by new image-processing techniques applied to 1910 photo-graphs, reveal dust particles spiraling outwardinto the coma. Jets from the nucleus, in additionto gravitational effects of th e planets, change thecomets orbit. In the 30 recorded appearances ofHalleys Comet it has sometimes taken just over74 years to return to perihelion, while a t othersit has taken nearly 80 years.

Surrounding he visible coma, and travelingwith it, i s an invisible hydrogen cloud extendingfor millions of miles around the nucleus of thecomet. The presence of hydrogen, oxygen, andcarbon atoms was speculated from theory, butdirect observation required ultraviolet-sensitiveinstruments. The halo was confirmed by space-craft observing ultraviolet light from comets.

Action of solar radiation on th e coma producestwo tails, a dust tai l and a plasma or ion tail, bothof which grow as th e comet approaches th e Sun.These tails point away from the Sun. Plasma tailsare wispy in appearance; dust tails are smooth.The dust particles in the tail are accelerated bysolar radiation pressure and move relativelyslowly away from the comet. As a result the dusttail tends to curve away from the comet becausethe comets speed changes along i t s elliptical


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path. The dust ta i l can appear as a fan-shapednebulosity when it is viewed broadside becausedust particles of differing masses are acceleratedto different velocities. The ion ta i l moves verymuch faster and i s relatively straight. I t resultsfrom solar radiation ionizing neutral molecules,and th e resulting electrically charged ions beingaccelerated by the magnetic field of the solarwind a t i t s enormous velocity of hundreds ofmiles per second. Th e ion ta i l stretchesmillions of miles from the comet in a directionaway from the Sun. The dust tai l i s much shorter.Tails usually begin to develop when a comet i sabout one and a half times Earths distance fromthe Sun (1.5 astronomical units or 140 millionmiles, 225 million km). Before i t s ta i l forms,a comet is a fairly inconspicuous, nebulous blobof light.

ORIGINAL PAGECOLOR PHOTOGRAPH

The View from Pioneer VenusOn February 9,1986, Comet Halley will hurtle

at over one hundred thousand miles per hourwithin 54.6 mi ll ion miles (87.9 million km) of theSun before starting i t s outward journey again.At the perihelion passage solar radiation wil l havemaximum impact upon the nucleus. The blizzardof charged particles and associated magneticfields of the solar wind will tear at the hydrogenhalo and the coma. At this time the comet willhave passed by Venus; i ts closest approach toVenus being February 4,1986, five days beforeperihelion. At the time of closest approach toVenus, the comet wil l be above the plane of

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Venus orbit and some 25 mil lion miles(40 million km) from the planet, betweenthe planet and the Sun.

Preparing for the unique opportunity toobserve Comet Halley at close range during i t sperihelion passage, the Pioneer MissionsOperations Team wil l send commands to thespacecraft to change the tilt of i t s spin axis.Pioneer Venus Orbiter spins on this axis f ive timesa minute as it travels around Venus. Thrusters onth e spacecraft wil l fire at a predetermined time ineach spin so as to tilt the spacecraft to th e desired


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Lick Observatory photograp h

position for i ts ultraviolet instrument to scan thecomet instead of Venus. A similar maneuver wassuccessfully made in 1984 to inspect Comet Enckeand in 1985 to inspect Comet Giacobini-Zinner.The thrusters will fire about 100 half-secondpulses to move the spin axis.

Because the ultraviolet spectrometer carried byPioneer was designed with a small field of view tomap variations in Venus atmosphere, a generalpicture of the comet can be gathered as a seriesof strips, one each spin, with the comet movingslightly between each spin to expose a sequenceof strips along i ts length. The ability to scan a thigh resolution across the comets coma, tail, andhydrogen halo, allows the distribution of particlesand gases to be determined. The ultraviolet datarecorded by this instrument wil l be studied byscientists a t the University of Colorado, Boulder.There, a team of researchers, headed by Dr. A. IanF . Stewart, the principal investigator for theultraviolet spectrometer instrument on thePioneer Venus Orbiter, will use the new data tolearn more about the makeup, evolution, andactivities of the comet.

This spectrometer, one of many scientificinstruments carried by the Venus Orbiter, detectslight in the ultraviolet region of the spectrum.Most atoms emit ultraviolet light when exposedto sunlight. By measuring the wavelength andintensity of the emitted ultraviolet light,researchers can determine which elements arein the comet and in what relative quantities.Thus, data from this experiment will inform us of

the comets gas composition, i t s water vaporiza-tion rate, and the ratio of gas to dust in the comaand in the nucleus.Comets are thought to be frozen remnants of

the material that formed the Sun and the planets,so information on cometary ingredients canprovide important clues about the early chemicaland physical history of the Solar System.

Objectives of the ultraviolet experiment are todetermine the composition of the gaseous comaand to measure the total gas production rate ofthe comet continuously during t s passage throughperihelion and, in particular, the rate a t whichwater sublimates from the nucleus. This can beestablished by measuring the ultraviolet emissionfrom hydrogen, oxygen, and the hydroxyl radicalin the coma and thereby determine the relativeamounts of these gases. Nitrogen, carbon, sulfur,carbon monoxide, and carbon dioxide will alsobe searched for.

Such observations through the perihelionpassage are very important to ascertaining theeffects of solar radiation and the solar wind uponthe comet. Rapid and large changes of the anglea t the comet between the Sun and the spacecraftduring about two months of observations willallow scientists to investigate the way light isscattered by the dust at different phase anglesto help estimate the density and distributionof sizes of dust particles.

ORIGINAL P AG COLOR PHOTOGRAPH


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Pioneer Orbiter will observe Comet Halley indetail for about f ive weeks in February and March1986. During that time the comet will travel athigh speed away from Venus. It will be about25 mil lion miles (40.2 million km) from thespacecraft a t the time of perihelion passage onFebruary 9, and some 125 million miles (200million km) away when the observations endabout March 6. The Orbiter will also makelong-range, less-detailed observations of thecomet in December 1985. Through the month ofJanuary1986, comet observations will be curtailedby solar interference with communications asVenus passes on the fa r side of the Sunfrom Earth.

ExpectedResultsUltraviolet spectra at much closer range than

possible from Earth will be obtained during theperihelion passage and the post-perihelionperiod. These wil l be correlated with observationsby Earth-orbiting astronomical observatorieswhere possible. Materials present in the comaand ta i l will be identified, and the rate thesematerials are evaporated from the nucleus andwithin th e coma will be established. Changes inevaporation rate and gaseous components willhelp us understand the uneven composition ofthe nucleus. Substances not previously identifiedin comets may be discovered, thereby providinga more detailed description of the compositionof the nucleus, and hence of primordial material

In addition, the spacecraft will continuemonitoring the solar wind with particle and fieldssensors to provide important data on the state ofthe wind affecting the comet f o r correlation withobservations by other spacecraft and ground-based observers. Pioneer Orbiter has a specialrelationship to the study of comets interactionswith the solar radiation and plasma wind, because

of the Solar System. From these data, scientistsexpect to obtain a better understanding of thechemical processes taking place in the inner comaand how the coma reacts with the solar wind.

Coupled with ground-based observations andobservations from other spacecraft on inboundand outbound legs of the comets path, PioneerVenus Orbiter wil l help provide anotherimportant piece of the jigsaw puzzle of how solarsystems are formed and how planets develop tothe point a t which they can support biologicalsystems with the abil i ty to question their ownorigin and purpose.

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Halley's CometOrbital inclinationOrbital periodLongitude of

ascending node

162 degrees18 degrees retrograde75 years

58 degreesArgument of perihelion 112 degreesSemi-major axis 17.9185 A.U.Perihelion passage 9 February 1986

distance 0.5871 A.U.54.6 million miles87.9 million km

Aphelion passage 1948distance 35.25 A.U.3.28 billion miles

5.28 bill ion kmEarth close approaches 27 November 1985

0.62 A.U.57.6 million miles92.7 mil lion km11April 19860.42 A.U.39.0 mil lion miles62.8 mil lion km4 February 19860.27 A.U.25.1 million miles40.4 mil lion km

Venus close approach

Pioneer Venus OrbiterLaunched 20 May, 1978Arrived at Venus 4 December, 1978Weight 824 Ib

374 kgThe Pioneer Orbiter i s controlled and managedfrom NASA Ames Research Center, Moffett Field,California.Project Manager, Richard0.FimmelScience Chief, Roger A. CraigSpace Operations Office Chief, JohnW. DyerFlight Director, Robert W. JacksonAssistant Flight Director, David W. LozierTrajectory Analyst, James A. PhillipsPower Systems and Hardware, Martha A. SmithGround Data System Manager, Manfred N. WirthUltraviolet SpectrometerPrincipal InvestigatorOptics type

aperturef/num berfocal lengthf/num berdiffraction grating

Monochromator type

Spectral resolutionGrating steps

(Note: one A.U. is92,900,000 miles (1495OO,OOO km), Wavelength rangesthe mean distance of Earth from the Sun.)

A. Ian F. StewartCassegrain telescope5 cm5Ebert-Fastie12.5 cm53600 grooves/mm13 A4.4 A1100-1900 A1800-3400 A

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comet halley the view from pioneer venus

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