44 Oilfield Review

From Inner Earth to Outer Space

Joel Lee GrovesJohn SimonettiStefan VajdaWolfgang ZieglerPrinceton Junction, New Jersey, USA

Jacob I. TrombkaGoddard Space Flight CenterGreenbelt, Maryland, USA

For help in preparation of this article, thanks to Edward Durner, Steve Meddaugh, Jim Roderick and Joel Wiedemann, Princeton Junction, New Jersey. EcoScope is a mark of Schlumberger.Teflon is a mark of E.I. du Pont de Nemours and Company.

In the 1930s, Conrad and Marcel Schlumberger began development of tools and sensors

to explore Earth’s inner space. Some 75 years later, similar detectors are helping

scientists investigate the fundamental nature and origin of objects in outer space.

On a cold day in February 2001, a spacecraftlanded on 433 Eros, an asteroid between theorbits of Mars and Jupiter. The spacecraft hadcompleted its five-year journey to investigatefundamental questions about the nature andorigin of near-Earth objects for the first time.

The technical demands of the Near EarthAsteroid Rendezvous NEAR-Shoemaker (NEAR)mission were immense. A multidisciplinary teamof US National Aeronautics and SpaceAdministration (NASA) scientists and engineersdrew from many scientific and industrialresources, including the predominantly inner-Earth-focused oil and gas industry.

Applying technologies developed for oil andgas exploration to scientific endeavors is not anew practice. Oilfield technologies have oftenbeen applied in the interest of science. Forexample, deep-drilling projects conducted onland and in most major oceans of the world havecontributed to our understanding of Earth’s pastas well as its future.

Engineers and scientists with the interna-tionally funded Ocean Drilling Program begansubsea drilling operations in 1961 to explore thehard outer layer of the Earth’s crust, orlithosphere. Scientists used tools and techniquesdeveloped for oil and gas exploration to documentcontinental drift and to generate a substantialquantity of data relating to plate tectonics.1

> Distant spiral galaxy. The Hubble Space Telescope captured this image of light that left the spiralgalaxy NGC1300 more than 69 million years ago. Barred spirals differ from normal spiral galaxies inthat the arms of the galaxy do not spiral all the way into the center, but are connected to the twoends of a straight bar of stars containing the nucleus at its center. At Hubble’s resolution, fine details,some of which have never before been seen, show disk, bulge and nucleus throughout the galaxy’sarms. The nucleus shows its own distinct spiral structure that is about 3,300 light-years across. Theimage was constructed from exposures taken in September 2004 by the Advanced Camera for Surveys.(Image courtesy of NASA.)

1. Andersen RN, Jarrard R, Pezard P, Williams C andDove R: “Logging for Science,” The Technical Review 36,no. 4 (October 1988): 4–11.

2. Kerr RA: “Signs of a Warm, Ice-Free Arctic,” Science 305,no. 5691 (September 17, 2004): 1693.

3. For more on deep-ocean drilling: Brewer T, Endo T,Kamata M, Fox PJ, Goldberg D, Myers G, Kawamura Y,Kuramoto S, Kittredge S, Mrozewski S and Rack FR:“Scientific Deep-Ocean Drilling: Revealing the Earth’sSecrets,” Oilfield Review 16, no. 4 (Winter 2004/2005):24–37.

4. Acceleration is often expressed in units of g-force (gn),which is defined as 9.80665 m/s2, approximately equal tothe acceleration due to gravity on the Earth’s surface atsea level.

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In 2004, engineers drilling in the Arctic Oceanat the crest of the Lomonosov ridge providedpreliminary evidence that the Arctic was ice-freeand warm about 56 million years ago.2 Scientistsanalyzed cores recovered from the drilling projectto help determine when, why and how the Arctictemperature changed. They also gained insightinto current global-warming trends.3

Understanding the fundamental processesthat occur deep within the Earth’s crust hascontributed to our knowledge of many inner-earth events, including volcanic activity, platetectonics, weather fluctuations, and chemicaland thermodynamic processes that lead tomineral deposition.

Hydrocarbons are most often found inforbidding environments. Tools and sensors arestressed to their limits as boreholes are drilleddeeper into the Earth’s crust where high temper-ature and pressure and excessive vibrations arecommon, and stress and shock forces reachthousands of times the acceleration of gravity(gn).4 Tools and instruments must also surviveextreme thermal cycles, from the cold surface ofthe Arctic to temperatures higher than 204°C[400°F] in the downhole environment. Drilling,logging and measurement instruments haveevolved to meet these challenges. Today, oil andgas E&P tools and instruments are designed andthoroughly tested for extended exposure to theseharsh environments.

Similarly, the forces encountered whilelaunching and accelerating a vehicle into spacecan be traumatic to equipment components. Forexample, the shock of pyrotechnic-stage separa-tion can reach over 4,000 gn, stressing both thevehicle and its payload. Once in space, depend-ing on orientation relative to the Sun, tempera-ture extremes range from more than 100°C[212°F] to below -200°C [-328°F]. Because of theneed to operate in harsh environments, the toolsand instrument packages designed for deep-welldrilling are inherently applicable to otherchallenging environments, such as outer space.

Whether exploring inner space for scientificpurposes, searching for oil and gas or probing thevastness of outer space, the desire to explore hasdriven the history of modern civilizations. Thisdrive led, at least in part, to the conquest of themoon in the 1960s, marking the beginning of anew generation in space exploration and travel.More recently, spacecraft, such as the HubbleSpace Telescope (HST), aided by technologiesdeveloped for oil and gas exploration, havepeered from Earth orbit ever more sharply anddeeply into the universe beyond our solar system(previous page).

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As we move from exploration of inner spaceto that of outer space, the tools and techniquesdeveloped for exploration deep beneath theEarth’s surface are helping to uncover themysteries of our solar system and the far reachesof space. In this article, we discuss a few of therecent contributions made to space explorationby the scientists and engineers of the petroleumindustry. Although the mission of the NEARspacecraft has ended, oilfield technology aboardthe HST and the Cassini-Huygens Saturn probecontinues to expand our knowledge and chartour way forward in the quest for knowledge.

Keeping the Hubble on TargetThroughout history, our understanding of theuniverse has been limited by what we could see.The invention of the telescope enhanced ourvision and allowed observations by Copernicus,Kepler and Galileo in the 16th and 17th centuries

to show that the Earth was not the center of theuniverse.5 By the 18th century, the developmentof the telescope helped scientists investigate thecosmos. Increasingly bigger and better telescopeshave routinely discovered and documentedplanets, stars and nebulae that are invisible tothe naked eye.

As recently as the beginning of the 20thcentury, most astronomers still believed that theuniverse consisted of a single galaxy, the MilkyWay—a collection of stars, dust and gas in thevastness of space. However, the universe as weknew it changed in 1924 when Americanastronomer Edwin Hubble used the 2.54-m [100-in.] Hooker Telescope on Mount Wilson,near Los Angeles, to observe billions of othergalaxies beyond the Milky Way.6

For astronomers like Edwin Hubble, therehas always been a major obstacle to a clear viewof the universe—the Earth’s atmosphere. Gasesand airborne particulates in the atmosphere blur

visible light, cause starlight to scintillate, ortwinkle, and hinder or totally absorb infrared,ultraviolet, gamma ray and X-ray wavelengths .

To minimize atmospheric distortion,scientists built observatories on mountaintopsand away from the areas of highly radiated light,or sky glow, found near large cities. This effortmet with varying levels of success. Today,adaptive optics and other image-processingtechniques have minimized, but not totallyeliminated, atmospheric effects.7

In 1946, Princeton astrophysicist LymanSpitzer documented the potential benefits of atelescope in space, well above Earth’satmosphere. Then, following the launch of theSoviet satellite Sputnik in 1957, NASA placed twoorbital astronomical observatories (OAO) intoEarth orbit. The OAOs made a number ofultraviolet observations and established thebasic principles for the design, manufacture andlaunch of future space observatories.8

Scientific, governmental and industrialgroups continued the move toward extrater-restrial exploration by planning the next stepbeyond the OAO program. Spitzer gathered thesupport of other astronomers for a large orbitaltelescope, later called the Hubble SpaceTelescope, and in 1969, the National Academy ofSciences approved the project.9

NASA’s Goddard Space Flight Center inGreenbelt, Maryland, USA, was responsible forscientific instrument design and ground controlfor the space observatory. In 1983, the SpaceTelescope Science Institute (STScI) wasestablished at The Johns Hopkins University inBaltimore, Maryland. The staff of STScI managedthe telescope’s observation time and data. NASAchose the Marshall Space Flight Center inHuntsville, Alabama, USA, as the lead NASA fieldcenter for the design, development andconstruction of the space telescope. Perkin-ElmerCorporation, now Hughes Danbury OpticalSystems, developed the optical telescope assemblyand the fine-guidance sensor (FGS) system.

On April 24, 1990, after numerous projectdelays, the space shuttle Discovery lifted off fromEarth carrying the HST in its cargo bay. Thefollowing day, the school-bus-size space telescopewas deployed in low-Earth orbit (above left). Free of atmospheric distortion, the gianttelescope mirror began its mission of gatheringphotons from as far away as the edge of the known universe.

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> Servicing the Hubble Space Telescope (HST). The Space Shuttle Discovery,mission STS-82, lifts the HST from its service bay after the second Hubbleservice mission. With a launch weight of 11,340 kg [25,000 lbm], the Hubble’smain structure is 13 m [42.6 ft] long and 4.27 meters [14 ft] wide. Its twin solar arrays span 13.7 meters [45 feet] when deployed. The telescopeitself is a reflecting configuration termed a Cassegrain, comprising a 2.4-m[94.5-in.] primary mirror, and a 30-cm [12.2-in.] secondary mirror. (Imagecourtesy of NASA.)

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Critical to the performance of the HST isstaying on target for extended periods of time.Electromagnetic waves emitted from distantobjects are often faint or weak, so the HST muststay perfectly positioned while the photons arebeing collected in sufficient quantities to form an image. To accomplish this, engineers used the Schlumberger oilfield photomultiplier-tubetechnology to design the FGS system.10

An FGS is essentially a targeting cameracapable of making celestial measurements,locking onto guide stars and providing data formaneuvering the telescope.11 Two FGSs are usedto point the telescope at an astronomical targetand hold that target in the telescope’s field ofview; the third FGS can then be used forastrometry measurements.12

The FGS system can maintain pointingaccuracy to 0.007 arcseconds, allowing thetelescope’s pointing-control system (PCS) tokeep the Hubble telescope on target duringcamera exposure times of 10 hours or more.13 ThePCS combines a number of different sensorsubsystems to achieve this milliarcsecondpointing accuracy. This level of accuracy andprecision is comparable to training a laser beamon a target the size of a thumbnail from adistance of 442 km [275 miles].

Within the housing of each FGS instrumentare two orthogonal white-light, shearinginterferometers, their associated optical andmechanical elements and four Schlumberger S-20 photomultiplier tubes (PMTs) (above right).14

These PMTs are based on the same ruggedconstruction as those used in well-logginginstruments. The photocathode was manu-factured using the same technology as tubes usedin oilfield service applications. For use on theHST, the PMTs were designed to be sensitive overa spectral range of 400 to 700 nanometers (nm),with an efficiency of approximately 18% at theblue end of the electromagnetic spectrum anddiminishing linearly to about 2% at the red end.

Each FGS interferometer consists of apolarizing beam splitter followed by two Koestersprisms. To measure the direction of the lightemitted by a guide star, the pairs of Koestersprisms are oriented perpendicular to oneanother. The angle of the wavefront in the X andY planes gives the precise angular orientation ofthe guide star relative to the HST’s optical path.These data, once fed into the PCS, are used tocontrol the telescope orientation relative to aguide star.

5. NASA—Hubble’s Conception: http://hubble.nasa.gov/overview/conception-part1.php (accessed April 18, 2006).

6. NASA, reference 5.7. Adaptive optics is a technology used to improve the

performance of optical systems by reducing the effectsof rapidly changing optical distortion typically resultingfrom changes in atmospheric conditions. Adaptive opticsworks by measuring the distortion and rapidlycompensating for it using either deformable mirrors ormaterial with variable refractive properties.

8. Smith RW: The Space Telescope–A Study of NASA,Science, Technology and Politics. New York City:Cambridge University Press, 1989.

9. Smith, reference 8.10. For more on photomultiplier tubes: Adolph B, Stoller C,

Brady J, Flaum C, Melcher C, Roscoe B, Vittachi A andSchnorr D: “Saturation Monitoring With the RSTReservoir Saturation Tool,” Oilfield Review 6, no. 1(January 1994): 29–39.

11. Space Telescope Science Institute–FGS History:http://www.stsci.edu/hst/fgs/design/history (accessedMarch 14, 2006).A guide star is one of many bright stars used fortelescope positioning and triangulation.

12. Astrometry is a branch of astronomy that deals with thepositions of stars and other celestial bodies, theirdistances and movements.

13. A second of arc, or arcsecond, is a unit of angularmeasurement that comprises one-sixtieth of anarcminute, or 1⁄3,600 of a degree of arc or 1⁄1,296,000 ≈ 7.7x10-7

of a circle. It is the angular diameter of an object of 1unit diameter at a distance of 360x60x60/(2π) ≈ 206,265units, such as (approximately) 1 cm at 2.1 km.

14. Interferometers were first used by Michaelson, who wonthe Nobel Prize in 1907 for his work using an opticalinterferometer to accurately measure the speed of light.

> Guiding Hubble. Light from the HST Optical Telescope Assembly (OTA) is intercepted by a pickoffmirror in front of the HST focal plane and directed into the fine-guidance system (FGS) (left). The lightrays are collimated, or made parallel, and then compressed by an aspheric collimating mirror andguided to the optical elements of the star selector assembly. Small rotations of the star selector A and B assemblies alter the direction of the target’s collimated beam, and hence the tilt of the incidentwavefront with respect to the Koesters prism (right). As the wavefront rotates about Point B, therelative phase of the transmitted and reflected beams change as a function of angle alpha. When thewavefront’s propagation vector is parallel to the plane of the dielectric surface, the relative intensitiesof the two emergent beams detected by the photomultiplier tubes will be equal. When alpha is notzero, the intensities of the left and right output beams will be unequal and the PMTs will recorddifferent photon counts, thus providing the telescope guidance control system with data allowing forpointing correction. [Images courtesy of NASA and The Johns Hopkins University Applied PhysicsLaboratory (JHUAPL).]

Star selectormirrors

Correction group

Deviation prism

Pickoff mirrorFilters (5 in wheel)

Beam-splitter prism

Koesters prism

Doublet lens (4)

Photomultiplier tubewith pinhole lensassembly (4)

Aspheric collimatingmirror

PMT B

Field stop Field stopField lensField lens

Positivedoublet

KoestersprismDielectric

beamsplitter

Incident wavefront

Alpha

A C

D

B

Positivedoublet

PMT A

Optical bench

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In addition to guiding the HST, the accuracyof FGS sensors makes them useful for high-precision astrometric measurements. Thesemeasurements help scientists determine theprecise positions and motions of stars. The FGSsensors can provide star positions about 10 timesmore precisely than measurements made withground-based telescopes. Scientists use astro-metric measurements to help define wobble inthe motion of stars that might indicate thepresence of a planetary companion (below left).The motions of stars can also determine whethera star pair represents a true binary star system,or simply an optical binary.15

Aided by elements of oilfield technology, theHubble Space Telescope continues its worktoday. Scientists are using instruments like theHST to search the far reaches of the universe anduncover secrets of the past while reaching intoour future.

Asteroids—Up Close and PersonalA little closer to home, technologies developedfor oilfield use are helping scientists exploreasteroids in our solar system. These large piecesof rock are primordial objects left over from theformation of the solar system. Some scientistshave suggested that asteroids are the remains ofa protoplanet that was destroyed in a massivecollision. However, the prevailing view is thatasteroids are leftover rocky matter that neversuccessfully coalesced into planets.

Scientists theorize that the planets of thesolar system formed from a nebula of gas anddust that coalesced into a disk of dust grains

around the developing Sun. Within the disk, tinydust grains coagulated into larger and largerbodies called planetesimals, many of whicheventually accreted into planets over a period of100 million years. However, beyond the orbit ofMars, gravitational interference from Jupiterprevented protoplanetary bodies from growing todiameters larger than about 1,000 km[620 miles].16

Most asteroids are concentrated in an orbitalbelt between Mars and Jupiter (below). Thesespace rocks orbit the Sun as planets do, but theyhave no atmosphere and very little gravity. Theasteroids in the belt comprise a significant

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> True binary stars. Each of the two stars in atrue binary system orbits around the center ofmass of the system. Kepler’s laws of planetarymotion govern how each star orbits the center of mass. At aphelion (A), each of the two starsare the farthest apart in their respective orbits.At perihelion (C), the stars are the closest.

+

+

+

+

A

B

C

D

> Main asteroid belt. The asteroid belt is a region of the solar system falling roughly between theplanets Mars and Jupiter where the greatest concentration of asteroid orbits can be found. The mainbelt region contains approximately 93.4% of all numbered minor planets. Trojan asteroids occupy tworegions centered 60° ahead of and behind Jupiter. Several hundred Trojans are known out of a totalpopulation that includes an estimated 2,300 objects bigger than 15 km [9 miles] across and manymore of smaller size; most do not move in the plane of the planet’s orbit but rather in orbits inclinedby up to 40°.

Mars

Mercury

VenusEarth

Jupiter

Trojanasteroids

Trojanasteroids

Astronomical units

0 2.7 5.21.5

Mainasteroidbelt

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amount of material—putting all of the asteroidstogether would form a body about 1,500 km [930miles] in diameter, roughly half the size ofEarth’s moon.17

Not all asteroids are far away in the asteroidbelt. Some, called near-Earth asteroids (NEAs),have orbits that bring them close to Earth.Astronomers believe NEAs to be fragmentsejected from the main asteroid belt by asteroid-asteroid collisions or by gravitational perturba-tions from Jupiter. Some NEAs could also be thenuclei of dead, short-period comets.

Since many asteroids have historically struckEarth and its moon, understanding theircomposition and origin may be a key to our pastas well as our future. Scientists believe that thechemical building blocks of life and much ofEarth’s water may have arrived on asteroids orcomets that bombarded the planet in the earlystages of its development (above left). Onewidely accepted theory suggests that an asteroidmeasuring at least 10 km [6 miles] across,impacted the Earth some 65 million years ago,causing mass extinctions among many life forms,including the dinosaurs.

Astronomers suspect that the approximately800 NEAs found to date represent only a smallpercentage of their total population. The largestpresently known is 1036 Ganymede, with anapproximate diameter of 41 km [25.5 miles].NEAs with diameters greater than 1 km[0.6 miles] are known as potentially hazardousasteroids, suggesting that should they strikeEarth, they could threaten life as we know it.

Of the more than 700 known potentiallyhazardous asteroids, one of the largest isToutatis, an asteroid that is nearly 1.6 km[1 mile] long and orbits around the Sun withinone-half degree of Earth’s orbital plane. InDecember 1992, Toutatis passed within0.024 astronomical units (AU), or 9.4 lunardistances from Earth.18 Then, on September 29,2004, Toutatis’s orbital path brought it within0.01 AU of Earth—the closest approach of anylarge asteroid in the 20th century.

Although astronomers have known aboutasteroids for nearly 200 years, until recently, theirbasic properties, their relationship to meteoritesfound on Earth and their origins remained amystery. NASA and the scientific community,driven by both the desire to understand asteroidsand the threat to Earth presented by NEAs morethan 1 km in diameter, set in motion the plans forthe NEAR project.

A Mission of Many FirstsIn 1990, NASA introduced a new program ofplanetary missions called the Discovery program.By 1991, the first mission was chosen—arendezvous with near-Earth asteroid 433 Eros.The Johns Hopkins University Applied PhysicsLaboratory (JHUAPL) was chosen to manage theproject, and in 1995, the NEAR spacecraft wasshipped to the Kennedy Space Center in Florida.19

Discovered in 1898, the NEA Eros is one ofthe largest and best-observed asteroids.20 Withdimensions 33 by 13 by 13 km [21 by 8 by8 miles], Eros is about the size of Manhattan,New York, USA (above). It accounts for nearlyhalf of the volume of all near-Earth asteroids.

15. The term binary star refers to a double-star system, or aunion of two stars into one system based on the laws ofattraction. Any two closely spaced stars might appearfrom Earth to be a double-star pair when, in fact, theyare a foreground and background star pair widelyseparated in space. These systems are typically referredto as optical binaries.

16. NASA–Eros or Bust: http://science.nasa.gov/headlines/y2000/ast08feb_1.htm (accessed April 14, 2006).

17. NASA, reference 16.18. NASA/ Jet Propulsion Laboratory–Asteroid 4179 Toutatis:

http://echo.jpl.nasa.gov/asteroids/4179_Toutatis/toutatis.html (accessed April 14, 2006).An astronomical unit (AU) is equivalent to the distancefrom the Earth to the Sun, or approximately 149,000,000 km[92,500,000 miles].

19. The NEAR spacecraft was renamed NEAR–Shoemakerto honor planetary geologist Eugene Shoemaker(1928–1997).

20. Farquhar RW: “NEAR Shoemaker at Eros: MissionDirector’s Introduction,” Johns Hopkins APL TechnicalDigest 23, no. 1 (2002): 3–5.

> Impacting the Earth. An asteroid impacting the Earth some 49,000 year ago scarred the Earthleaving this 1.2-Km [0.7-mile] crater. This view from the Space Shuttle shows the dramatic expressionof the crater in the arid landscape of Arizona, USA. (Image courtesy of the Earth Sciences and ImageAnalysis Laboratory, NASA Johnson Space Center, STS040_STS040-614-58.)

> A large asteroid. The outline of Eros (red) issuperimposed on the island of Manhattan, New YorkCity, showing the relative size of the asteroid.

Manhattan

The footprint ofasteroid Eros

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The large S-type potato-shaped asteroid is one ofthe most elongated asteroids. It orbits aroundthe Sun, rotating on its axis once every 5.27hours, with a perihelion of 1.13 AU and anaphelion of 1.78 AU (top).21

NEAR departed Earth for asteroid Eros onFebruary 17, 1996, riding on top of a Delta-IIlaunch vehicle. One year later, on February 18,1997, NEAR reached its most distant point fromthe Sun, 2.18 AU, setting a new distance record

for a spacecraft with instrumentation powered bysolar cells.

By the end of its five-year mission, NEARwould produce an impressive list of spacecraftfirsts: the first spacecraft with instrumentationsolely powered by solar cells to operate beyondthe orbit of Mars, the first to encounter a C-typeasteroid, the first to encounter a near-Earthasteroid, the first to orbit a small body, and thefirst spacecraft to land on a small body.

NEAR—The Scientific MissionPrior to the NEAR mission, our knowledge ofasteroids came primarily from three sources:Earth-based remote sensing, data from theGalileo mission flybys of the two main-belt S-type asteroids 951 Gaspra and 243 Ida, andlaboratory analyses of meteorites recovered afterimpact with the Earth.

Although astronomers theorize that mostmeteors result from the collision of asteroids,they may not be completely representative of allmaterials that comprise NEAs.22 Clear linksbetween meteorite types and asteroid typesproved difficult to establish.23

Some S-type asteroids appear to be fragmentsof bodies that underwent substantial melting anddifferentiation, while others consist of whatappears to be nonmelted primitive materials likechondrites.24 Scientists believe that nonmelted S-type asteroids may have preserved thecharacteristics of the solid material from whichthe inner planets accreted.

The Galileo mission flybys provided the firsthigh-resolution images of asteroids in the early1990s. Images revealed complex surfaces coveredby craters, fractures, grooves and subtle color variations (left).25 However, Galileo’sinstrumentation was not capable of measuringelemental composition, so prior to the NEARmission, scientists continued to be unsure of therelationship between ordinary chondrites and S-type asteroids.

Mission engineers believed that the NEARdata, when combined with those from the Galileoflybys, would help scientists understand therelationship between S-type asteroids and othersmall bodies of the solar system. The NEARmission’s primary objectives were to rendezvouswith, achieve orbit around and conduct the firstscientific exploration of a near-Earth asteroid.

The NEAR SpacecraftEngineers designed NEAR’s systems to be solar-powered, simple and highly redundant.26 OnboardNEAR were five instruments designed to makedetailed scientific observations of the grossphysical properties, surface composition andmorphology of Eros. These five were the multi-spectral imager (MSI), near-infrared spectrom-eter (NIS), magnetometer (MAG), NEAR laserrangefinder (NLR) and the combined X-ray,gamma ray spectrometer (XGRS) (next page).

The MSI imaged the surface morphology ofEros with spatial resolutions down to 5 m[16.4 ft], while scientists used the NIS tomeasure mineral abundances at a spatial

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> Approaching Eros. This image of the southern hemisphere of Eros offers a long-distance look at theasteroid’s cratered terrain. (Image courtesy of NASA/JHUAPL.)

> Asteroids close up. Shown are views of the three asteroids that had been imaged at close rangeby spacecraft prior to NEAR’s arrival at Eros. The image of Mathilde (left) was taken by the NEARspacecraft on June 27, 1997. Images of the asteroids Gaspra (middle) and Ida (right) were taken bythe Galileo spacecraft in 1991 and 1993, respectively. All three objects are presented at the samescale. The visible part of Mathilde is 59 km wide by 47 km long [37 by 29 miles]. (Images courtesy ofNASA/JHUAPL.)

Mathilde Gaspra Ida

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resolution on the order of 300 m [984 ft]. TheMAG was used to define and map intrinsicmagnetic fields on Eros.

Scientists used the NLR to enhance thesurface morphology profiles derived from NEAR’simaging camera. The NLR is a laser altimeterthat measures the distance from the spacecraftto the asteroid surface by sending out a shortburst of laser light and then recording the timerequired for the signal to return from theasteroid. The ranging data were used toconstruct a global shape model and a globaltopographic map of Eros with a spatial resolutionof about 5 m.

The XGRS was the primary tool used forsurface and near-surface elemental analysis ofEros. Scientists combined data from the XGRS,MSI and the NIS to produce global maps of Eros’ssurface composition.

Development of the complex XGRS systembegan about three years prior to launch. Theinstrument was designed to detect and analyzeX-ray and gamma ray emissions from the asteroidsurface from orbital altitudes of 35 to 100 km [22 to 62 miles]. Although spectroscopy ofremote surfaces is possible during spacecraftflyby operations, measurements made whileorbiting allow longer observation times andproduce higher quality spectral data.

X-rays emitted from the Sun shining on Erosproduce X-ray fluorescence from the elementscontained in the top 1 mm [0.04 in.] of theasteroid’s surface. In the absence of anysignificant atmosphere that might otherwiseabsorb X-ray emissions, elements fluoresce atenergy levels that are characteristic of specificelements. Scientists used the X-ray fluorescenceenergy detected in the 1- to 10-keV level to infersurface elemental composition.

The XRS subunit consists of three identicalgas-filled proportional counters that provide alarge active surface area and therefore thesensitivity required for remote sensing. Similardetectors have been used on lunar orbitalmissions and most recently on Apollo missions.

The X-ray gas tubes are not particularlysensitive to temperature change, since themultiplication effect depends more on thenumber of gas molecules than the gas pressure.However, the gain in the gas tubes is sensitive tovoltage variations.

Gamma ray spectrometry provides acomplementary measurement of near-surfaceelemental composition. The gamma rayspectrometer (GRS) detects discrete-line gammaray emissions in the 0.1- to 10-MeV energy range.

At these energy levels, oxygen [O], silicon [Si],iron [Fe] and hydrogen [H] become excited, orradioactively activated, from the continual influxof cosmic rays. The GRS also detects naturallyradioactive elements such as potassium [K],thorium [Th] and uranium [U]. The measure-ments have been used for years in oil and gaswell logging to determine the physical andelemental composition of reservoir rock.

Unlike the low-energy X-rays, gamma rays arenot as easily absorbed and therefore can escapefrom regions beneath the surface, allowing theGRS to reveal elemental composition to depthsas much as 10 cm [4 in.] below the surface. By comparing elemental analysis from the XRSand GRS, scientists inferred the depth andextent of the dust layer, or regolith, covering thesurface of Eros.27

21. Asteroids are classified based on reflectance spectrumand light-reflection characteristics, or albedo, which areindicators of surface composition. S-Type (silicaceous)asteroids are more prevalent in the inner part of themain asteroid belt, while C-Type (carbonaceous)asteroids are found in the middle and outer parts of thebelt. Together, these two types account for about 90% ofthe asteroid population.Perihelion and aphelion are the orbital points nearestand farthest from the center of attraction—in this case,the Sun.

22. A meteorite is a solid portion of a meteoroid thatsurvives its fall to Earth. Meteorites are classified asstony meteorites, iron meteorites and stony ironmeteorites, and further categorized according to theirmineralogical content. They range in size frommicroscopic to many meters across. Of the several tensof tons of cosmic material entering Earth’s atmosphereeach day, only about one ton reaches the ground.

23. Cheng AF, Farquhar RW and Santo AG: “NEAROverview,” Johns Hopkins APL Technical Digest 19, no. 2(1998): 95–106.

24. Chondrites are a type of stony meteorite made mostly ofiron- and magnesium-bearing silicate minerals.Chondrites are the most common type of meteorite,accounting for about 86% that fall to Earth. Theyoriginate from asteroids that never melted, or underwentdifferentiation. As such, they have the same elementalcomposition as the original solar nebula. Chondritesderive their name from the fact that they containchondrules—small round droplets of olivine andpyroxene that apparently condensed and crystallized inthe solar nebula and then accreted with other materialsto form a matrix within the asteroid.

25. Cheng et al, reference 23.26. Cheng et al, reference 23.27. Regolith is a layer of loose material, including soil,

subsoil and broken rock, that covers bedrock. On Earth’smoon and many other bodies in the solar system, itconsists mostly of debris produced by meteorite impactsand blankets most of the surface.

> NEAR spacecraft systems. NEAR’s basic design and primary systems are shown. (Image courtesyof NASA/JHUAPL.)

Gamma rayspectrometer

X-rayspectrometer

X-ray solarmonitors

NEAR laser rangefinderNear-infraredspectrometer

Multispectral imager

Aft deck

Forward deckSide panels

Propulsion system

Solar panel

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The GRS central detector assembly is based ona ruggedized thallium-activated [Tl] sodium iodide[NaI] scintillator unit used in oilwell loggingoperations, designed and built by Schlumberger(below). NaI-based scintillators are widely used indownhole logging-tool applications to makemeasurements of density, natural radioactivity and elemental spectra. As an example, theEcoScope multifunction logging-while-drilling tooluses a NaI detector to make while-drillingspectroscopy measurements.28 Other logging toolsuse different materials.

Interactions of gamma rays with solidmaterials depend on the energy of the gamma raysand on the density and the atomic number of thematerials being investigated. These interactionscan be classified by the level of energy absorbedby the substrate material.

At lower energy levels, the photoelectriceffect, or Compton scattering, is prevalent. Inthis case, only a fraction of the gamma ray energy is deposited, and the rest leaves thematerial as low-energy photons. At higher

gamma ray energy levels, above 3 MeV, pairproduction becomes dominant.29

Identification of elemental compositions isperformed primarily by measuring the charac-teristic photoelectric energy of individualnuclear varieties when excited by an externalradiation source, such as solar wind or othercosmic rays. At higher energy levels the pair-production mechanism generates well-definedspectra. As such, the most accurate GRSmeasurements were made during periods of highsolar-flare activity when gamma ray energy levelswere at their highest.

To improve the elemental identificationcapability of the GRS, an active detector cup shieldwas designed especially for NEAR. It wasfabricated from a single bismuth germanate[BGO] crystal. The dense BGO cup acted as anactive scintillator while providing direct andpassive shielding from the local gamma rayenvironment and reducing unwanted back-ground signals.

The new design replaced the more expensiveand less reliable long booms used in other missionsto reduce unwanted signals from the activation ofthe spacecraft body itself by cosmic radiation. TheGRS also provided sensitivity to the direction fromwhich the gamma rays were coming.

Detour to a C-Type AsteroidIn early December 1993, NEAR mission managersat The Johns Hopkins University Applied PhysicsLaboratory reviewed a list of asteroids that mightbe in close proximity to NEAR’s flight path (nextpage, top). Asteroid 253 Mathilde was found to bewithin 0.015 AU, or about 2.25 million km[1.4 million miles], of NEAR’s planned orbitalpath. Engineers calculated that with slightchanges in NEAR’s planned trajectory, it couldencounter 253 Mathilde with only a 57 m/s[187 ft/s] change in velocity, well within thespacecraft’s velocity margin.30

Although the dark asteroid was discovered in1985, little was known about Mathilde. Newastronomical observations from ground-basedtelescopes showed it to be a C-type asteroid withan unusual rotation period of 15 days, almost anorder of magnitude slower than most otherknown asteroid rotation periods.

NEAR encountered Mathilde on the way toEros after five trajectory-correction maneuversabout 2 AU from the Sun.31 At this distance,available power from the spacecraft’s solar-powered system was down nearly 75%. With thislimited power, astronomers could use only theMSI to explore the surface of the asteroid, andradio-tracking data, before and after approach,to help determine the mass of the asteroid.

During the flyby, Mathilde exerted a slightgravitational pull on the NEAR spacecraft.Because of Mathilde’s mass, the gravitationaleffects on NEAR’s path were detectable in thespacecraft’s radio-tracking data.

Data from radio-tracking mass estimatesalong with volume approximations helpedscientists calculate the asteroid’s approximatedensity of 1.3 ± 0.3 g/cm3 [81.16 ± 18.73 lbm/ft3].Because of the asteroid’s spectra, Mathilde wasbelieved to be similar in composition tocarbonaceous-chondrite meteorites. However,Mathilde’s density was half of that expected,implying either a high internal porosity orsignificant void space within the asteroid.

Scientists imaged Mathilde over a 25-minuteperiod during the spacecraft’s approach at adistance of 1,200 km [746 miles] and a speed of9.93 km/s [22,213 mi/h]. A total of 534 images

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> XGRS imaging systems. The combined X-ray, gamma ray spectrometer system (XGRS) is shownmounted on the NEAR spacecraft (top left). Shown on the right side of the XRGS instrument is thegamma ray spectrometer. The assembly is mounted to the aft deck of the NEAR spacecraft (top right).The sensor assembly (bottom left) contains the NaI(Tl) detector that is positioned within the bismuthgermanate (BGO) cup shield to reduce unwanted background signals by almost three orders ofmagnitude. The Schlumberger photomultiplier tubes (PMTs) at each end convert the light output ofthe scintillation detectors into electrical signals. (Image and diagram courtesy of NASA/JHUAPL.)

Teflon spacers

Support

Aft deck

Thermalspacers

Gamma raydetector

Connector

Clamp

Spring

SpringTeflon wedge

SmallPMT

Opticalcoupling

NaI (Tl)crystal

BGOshield

LargePMT

Opticalcoupling

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were obtained during this interval at resolutionsranging from 200 to 500 m [656 to 1,640 ft] (above).

Images obtained during the flyby of Mathildeshow an asteroid with a heavily cratered surface.At least four giant craters have diameters thatare comparable to the asteroid’s mean radius of26.5 km [16.5 miles]. The magnitude of theimpacts required to create craters of this size issignificant. Scientists suspect that Mathilde didnot break apart during any of these impacts

because of the asteroid’s high porosity.Laboratory data suggest that cratering in highlyporous targets is governed more by compactionof the target material than by fragmentation andexcavation.32 Cratering processes governed bystructural properties such as porosity producecraters with steep walls, crisp rims and with littleejecta, similar to those imaged on Mathilde.

The images also show Mathilde is remarkablyuniform. The NEAR observations revealed noevidence of any regional albedo, or spectral

variations, implying a homogeneous composition.Further, the measured albedo was consistentwith ground-based telescopic observations.

Although significant data were gained by theMathilde flyby, numerous questions about C-typeasteroids remain unanswered. Mathilde’s densitywas inconsistent with common carbonaceous-chondrite meteorites found on Earth, and theasteroid’s surface appears homogeneous. So, thequestion remains: what connection, if any, existsbetween dark asteroids and meteors found in thesolar system?

Detecting Gamma Ray BurstsGamma ray bursts (GRBs) have remained one ofthe great mysteries of astrophysics since theirdiscovery more than 30 years ago. NASA’s HubbleSpace Telescope made the first observation of anobject associated with a GRB that was detected bythe Italian BeppoSAX satellite in February 1997.33

Scientists believe that GRBs result frommassive explosions in the distant universe thatrelease waves of high-energy photons. GRBsseem to occur daily and emanate from randomparts of the sky. GRBs represent the mostpowerful events known in the universe, emittingin one second as much energy as the Sun willemit in its lifetime. Spectroscopic analyses offaint, but long-lasting GRB optical afterglowshave, in a number of cases, indicated Dopplershifts in the red spectrum that indicate acosmological origin of GRBs.34 Time is critical infollow-up observation efforts, since GRBafterglows fade quickly, in the radio as well asoptical spectrum, making it difficult forastronomers to locate the emission source.

28. For more on while-drilling spectroscopy measurements:Adolph B, Stoller C, Archer M, Codazzi D, el-Halawani T,Perciot P, Weller G, Evans M, Grant J, Griffiths R,Hartman D, Sirkin G, Ichikawa M, Scott G, Tribe I andWhite D: “No More Waiting: Formation Evaluation WhileDrilling,” Oilfield Review 17, no. 3 (Autumn 2005): 4–21.

29. Pair production is the chief method by which energyfrom gamma rays is observed in condensed matter.Provided there is enough energy available to create thepair, a high-energy photon interacts with an atomicnucleus and an elementary particle and its antiparticleare created.

30. Dunham DW, McAdams JV and Farquhar RW: “NEARMission Design,” Johns Hopkins APL Technical Digest 23,no. 1 (2002): 18–33.

31. Cheng et al, reference 23.32. Domingue DL and Cheng AF: “Near Earth Asteroid

Rendezvous: The Science of Discovery,” Johns HopkinsAPL Technical Digest 23, no. 1 (January-March 2002):6–17.

33. The Johns Hopkins University Applied PhysicsLaboratory–Near Spacecraft Gets Unexpected View ofMysterious Gamma-Ray Burst: http://www.jhuapl.edu/newscenter/pressreleases/1998/gamma.htm (accessedApril 5, 2006).

34. NASA–Automatic NEAR-XGRS Data Processing Systemfor Rapid and Precise GRB Localizations with theInterplanetary Network: http://gcn.gsfc.nasa.gov/gcn/near.html (accessed April 5, 2006).

> Destination Eros. The NEAR spacecraft was successfully launched inFebruary 1996, taking advantage of the unique alignment of Earth and Erosthat occurs only once every seven years. A Delta-II rocket placed NEAR intoa two-year Earth gravity-assist trajectory. The gravity-assist maneuver decreasedthe aphelion distance while increasing the inclination from 0 to about 10°.

Sun

Earthorbit

Earth swingby01/22/981,186-km altitude

Eros arrival01/09–02/06/99

Deep-spacemaneuver03/07/97∆V = 215 m/s

Erosorbit

Launch 02/17/96C3 = 25.9 km2/s2

> A quick look at asteroid Mathilde. This view of 253 Mathilde, taken from adistance of about 1,200 km, was acquired shortly after the NEAR spacecraft’sclosest approach to the asteroid. Showing on Mathilde are numerous impactcraters, ranging from more than 30 km [18 miles] to less than 0.5 km [0.3 miles]in diameter. Raised crater rims suggest that some of the material ejected fromthese craters traveled only short distances before falling back to the surface;straight sections of some crater rims indicate the influence of large faults orfractures on crater formation. Mathilde has at least five craters larger than 20 km [12 miles] in diameter on the roughly 60% of the body viewed during theNEAR flyby. (Image courtesy of NASA/JHUAPL.)

20 km

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Since 1993, astronomers have used speciallyinstrumented spacecraft to help identify thesource of GRBs. These include the Ulyssesspacecraft and several spacecraft near the Earth:the BeppoSAX, Wind observatory, the ComptonGamma-Ray Observatory (CGRO) and the RossiX-Ray Timing Explorer. Unfortunately, these near-Earth spacecraft are too close to each other toallow a definitive triangulation of burst locations.

The loss of the Pioneer Venus orbiter andMars Observer in the early 1990s meant thatastronomers lacked a third detector source foraccurate triangulation of deep-space GRBs. Theaddition of the NEAR spacecraft to theinterplanetary network greatly increased theprobability of associating a GRB with a particularsource using optical and radio telescopes.

The GRS onboard NEAR was not originallyintended to begin its work until the spacecraftreached Eros. However, while en route to Eros,simple software changes to the XGRS systemallowed scientists to use the spectrometer forGRB detection. By adding the NEAR spacecraftto the GRB interplanetary network (IPN) andtaking advantage of significant improvements intelemetry rate and computational capability,NEAR helped reduce GRB detection andtriangulation times from months to seconds.

As an example, gamma ray detectors on theNEAR and Ulysses spacecraft first recordedgamma ray burst GRB000301C on March 1,2000.35 Initially, the sky coordinates of the burstwere not well-defined, but with data from theNEAR and Ulysses spacecraft, an area of the skyabout 4.2 arcminutes wide and 180 degrees inlength was identified as the potential source. Asecond position from the Rossi X-Ray TimingExplorer reduced the error to 4.2 degrees longand 8.7 arcminutes wide. Triangulation of thethree data points further narrowed the gammaray emission zone to within a 50 arcminutesquare, thus allowing a much quicker search ofthe sky by the HST and ground-based telescopes.

Over a 15-month period from December 1999to February 2001, the IPN, including NEAR,detected over 100 GRBs.36 Of these, 34 werelocalized rapidly and precisely enough to allowoptical and radio telescope follow-up observa-tions. The suspected GRB emission locations weredetermined with accuracies of the order ofseveral arcminutes. One of the most interestingresults was the detection of a GRB originating inthe southern constellation Carina. Opticalobservations of an extreme red-shift indicatedthat the source of the GRB was about 12.5 billionlight-years from Earth, making it the mostdistant GRB yet detected.

Unlocking the Secrets of ErosThe NEAR spacecraft entered Eros orbit onFebruary 14, 2000, beginning its one-year missionto explore Eros. Orbital characteristics rangedfrom elliptical to circular and took NEAR within35 km [22 miles] of the surface of Eros. Then,almost six years to the day after launch,engineers at JHUAPL brought NEAR’s mission toits culmination with a successful controlleddescent to the surface of Eros.

Although the primary mission of NEAR was toinvestigate the mineralogy, composition,magnetic fields, geology and origin of Eros, NEARobtained much more detailed information duringits orbital encounter with Eros.

Images, laser altimetry and radio-sciencemeasurements provided strong evidence thatEros is a consolidated, yet fractured asteroidwith a regolith cover varying dramatically indepth from near zero to as much as 100 m[328 ft] in some areas.37 Scientists believe thatthe presence of joined and well-defined craters isindicative of cohesive strength within theasteroid. Surface images show the geometricrelationship of grooves and cuts in the surface,suggesting that the rock is competent and not aloosely bound agglomeration of smaller rocks.

The gravity field on Eros appeared to beconsistent with that expected from a uniform-density object of the same shape. The measureddensity of Eros indicates that it has a bulkporosity of 21 to 33%, implying that even thoughthe asteroid’s mass is uniformly distributed, it issignificantly porous and potentially fractured,but to a lesser extent than Mathilde.

Imaging at resolutions of a few centimetersper pixel revealed a complex and active regoliththat has been significantly modified andredistributed by gravity-driven slope processes.High-albedo features noted in images takenaround crater walls that slope in excess of 25°were often 1.5 times brighter than theirsurroundings, indicating recent changes insurface characteristics due to regolith sloughing(above right).38

Silicate mineralogy analysis performed by theNIS was consistent with ordinary chondritemeteorites. Spatially resolved measurements of theasteroid’s surface provided no evidence for mineralcompositional variation. Scientists believe that thespectral uniformity of Eros may have resulted froma uniformly high degree of space weatheringcaused by micrometeorite bombardment.

The NEAR spectrographs, XRS, GRS and NISmeasured the elemental and mineral composi-tion of Eros. Data acquired by the XRS duringorbiting showed calcium, aluminum, magnesium,iron and silicon abundances consistent withordinary chondrite and certain primitiveachondrite meteorites. However, the level ofsulfur typical of chrondritic meteorites wasabsent or depleted on Eros.

Although the surface of Eros appears to beelementally homogeneous, the XRS instrumentcan measure only surface composition, so it isunknown if the sulfur depletion is a surface effector consistent through the core of the asteroid. Ifthe sulfur depletion is consistent across the bulkof the asteroid, this would imply an associationwith primitive achondrite meteorites.

The orbital GRS measurements had lowersignal levels than predicted, so the elementalratios with the highest precision were measuredafter landing. GRS data showed the Mg/Si andSi/O ratios and the abundance of K to beconsistent with chondritic meteorite values, butfound Fe/Si and Fe/O levels to be lower thanwhat would be expected with chrondriticmeteorites. Since these measurements were

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> Close-approach Eros crater wall. Material on theinner wall of the crater in the center of the imageis brighter than the surrounding regolith and isthought to be subsurface material exposed whenoverlying, darker regolith slid off. The field ofview is 1.2 km [0.7 miles] across, taken from 38km [24 mi] above Eros. (Images courtesy ofNASA/JHUAPL.)

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made after landing and the GRS instrument canprobe tens of centimeters below the surface,these measurements reflect a volume of about1 m3 [35.3 ft3] around the detector. From theGRS data alone, scientists could not determinewhether the Fe depletion is a global composi-tional property of Eros or a localized property ofthe landing zone.

Although the XGRS system observed Erosduring a one-year orbital period, the useful timefor data collection was considerably shorter.Engineers were limited by the angular require-ments of the solar panels relative to the sun,telemetry time and periods when the surface ofEros was properly lit by the Sun. In the end,scientists found that the best quality composi-tional data were acquired during low-altitudeorbits and after landing on Eros (right). OnceNEAR was on the surface, the gamma rayspectrometer obtained in-situ measurements ofthe regolith for a period of about 14 days.39

The surface composition of Eros suggests thatthe asteroid is similar in bulk composition to arange of meteorites that have experiencedminimal thermal alteration since their formationat the birth of the solar system. Scientists believethat Eros is primitive in its chemical compositionand has not experienced differentiation into acore, mantle and crust. Differences between XRSand GRS data in Fe/Si ratio and an apparentdeficiency of sulfur at the surface of Eros couldreflect either alteration processes in the regolithduring the last millions to billions of years orpartial melting in the first 10 million years ofsolar system history.

These spectral measurements providedscientists with a new set of questions. While thespectral observations are consistent with anordinary chondritic meteorite composition, themeasurements did not establish an undisputed linkbetween Eros and a specific meteorite type. Thequestion remains whether Eros is unrelated to anyknown meteorite type, or is actually a chondritetype at depth, below the surface layers that mayhave been altered by weathering processes.

35. NASA–Amateurs Catch a Gamma-Ray Burst:http://science.nasa.gov/headlines/y2000/ast14mar_2m.htm (accessed April 5, 2006).

36. Trombka JI et al: NASA Goddard Space Flight Center:http://www.dtm.ciw.edu/lrn/preprints/4631trombka.pdf(accessed April 5, 2006).

37. Domingue and Cheng, reference 32.38. Domingue and Cheng, reference 32.39. Trombka et al, reference 36.

> Landing on Eros. The location of NEAR Shoemaker’s planned landing site (top right) is shown in thisimage (yellow circle) mosaic taken on December 3, 2000, from an orbital altitude of 200 km [124miles]. NEAR’s imaging systems were recording (bottom 4 images) as the spacecraft performed acontrolled landing on the surface of Eros. At a range of 1,150 m, NEAR captured an image that spans54 m [177 ft] of the asteroid’s surface (1). The large rock at the lower left corner of the imagemeasures 7.4 m [24 ft] across. NEAR then recorded images at ranges of 700 m (2), 250 meters (3),followed by the last image before landing (4) at a range of 120 m [394 ft]. The field of view in this finalimage measures 6 m [20 ft] across. The large rock at the top of the image measures 4 m [12 ft]across. The streaky lines at the bottom indicate loss of signal as the spacecraft touched down on theasteroid during image transmission. Once on the surface, the GRS system generated gamma rayspectrum data for a period of seven days (graph, top left). These scientific data were the first evercollected on the surface of an asteroid. The gamma ray instrument has two sensors (red and bluelines) that detected clear signatures of key elements in the composition of Eros. These data, whichsurpass in quality all the data accumulated by this instrument from orbit, helped the NEAR scienceteam relate the composition of Eros to that of meteorites that have fallen to Earth. (Images courtesyof NASA/JHUAPL.)

1

2

3

4

Final Eros images: range 1,150 m (3,773 ft)

Final Eros images: range 700 m (2,300 ft)

Final Eros images: range 250 m (820 ft)

Last, closest image of Eros

106

105

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Scientists were surprised that Eros appearsto have little or no magnetic field. Mostmeteorites, including chondrites, tend to bemore magnetized than Eros. Perhaps its lowlevels of iron and the fact that it never has beenheated to melting play a role in thisdifferentiation. The spectral homogeneity of Eroscombined with gravity-field measurements,structural characteristics and indications ofstructural coherence suggests that Eros is acollisional fragment of a larger parent body.

The NEAR mission, a mission of many firsts inNASA’s Discovery Program, substantiallyincreased our knowledge of primitive bodies inour solar system. Although the data returned byNEAR have revealed many secrets of asteroids,many questions remain unanswered, and morewill be learned from future missions.

Exploring Gas GiantsThe goal of the Cassini mission is to exploreSaturn, its many known moons and those yet tobe discovered. Managed by NASA’s Jet PropulsionLaboratory (JPL) in Pasadena, California, USA,Cassini is a joint endeavor of NASA, theEuropean Space Agency (ESA) and the Italian

space agency, Agenzia Spaziale Italiana (ASI). Itis one of the most ambitious efforts in planetaryspace exploration.40

Because of the low level of sunlight reachingSaturn, solar arrays are not feasible as a powersource. Engineers employed a set of radioisotope-thermoelectric generators similar to those used

56 Oilfield Review

40. NASA/Jet Propulsion Laboratory–Cassini Mission toSaturn: http://www.jpl.nasa.gov/news/fact_sheets/cassini.pdf (accessed April 13, 2006).

> Preparing Cassini for flight. Technicians reposition and level the Cassini orbiter in the Payload Hazardous Servicing Facility at the Kennedy Space Centerin July 1997, after stacking the craft’s upper equipment section on the propulsion module (left). The orbiter’s primary systems are shown (right). (Imagescourtesy of NASA/JPL.)

4-m high-gain antennaLow-gain antenna (1 of 2)

11-m magnetometerboom

Radio/plasma wavesubsystem antenna(1 of 3)

Remote sensinginstruments

445 N engine (1 of 2)

Radar bay

Huygens Titanprobe

Radioisotopethermoelectricgenerator (1 of 3)

> Imaging Saturn’s rings. The Ultraviolet Imaging Spectrograph (UVIS) is a setof telescopes used to measure ultraviolet light from the Saturn system’satmospheres, rings and surfaces. The UVIS has two spectrographic channelsor instruments: the extreme ultraviolet channel and the far ultraviolet (FUV)channel. Each instrument is housed in aluminum cases, and each contains areflecting telescope, a concave grating spectrometer and an imaging, pulse-counting detector. The UVIS also includes a high-speed photometer (HSP)channel, a hydrogen-deuterium absorption cell (HDAC) channel and anelectronic and control subassembly. (Image courtesy of NASA/Laboratory for Atmospheric and Space Physics.)

HDAC

FUV spectrograph

HSP

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on the previous Galileo and Ulysses missions.With these systems, heat from the natural decayof plutonium-238 is used to generate electricityto operate Cassini’s systems.

The Cassini spacecraft is equipped with18 instruments, 12 on the orbiter and another sixon the Huygens probe, which is designed toseparate from the main spacecraft andparachute through the atmosphere of Titan,Saturn’s largest moon. The 12 instruments on theorbiter are currently conducting in-depth studiesof Saturn, its moons, rings and magneticenvironment (previous page, bottom).

Key to Cassini’s science mission is theUltraviolet Imaging Spectrograph (UVIS), aninstrument based on Schlumberger sensors andpackaging, and designed to operate in harshenvironments like those found in oil and gaslogging operations (previous page, right). TheUVIS is now helping scientists determineatmospheric chemistry, the nature of clouds andring systems, and the atmospheric energybalance on Saturn and its moon Titan.

The UVIS comprises a set of telescopes thatmeasure ultraviolet light from the Saturnsystem’s atmospheres, rings and surfaces. Theinstrument has two spectrographs: the farultraviolet channel (FUV), 110 to 190 nm, and theextreme ultraviolet channel (EUV), 56 to 118 nm.

The FUV and EUV channels in the UVISspectrometer require different detectors tooptimize sensitivity to the wavelength rangerequired by the Cassini project. In cooperationwith the Laboratory for Atmospheric and SpacePhysics (LASP) at the University of Colorado,Schlumberger designed the detector response tomeet these requirements.

The FUV detector was assembled using acesium iodide photocathode with a magnesiumfluoride window. This detector was vacuum-sealed and included an integrated pump thatmaintained an ultrahigh vacuum during thespacecraft assembly and launch. Once in space,the detector was equalized to the vacuum ofspace for the voyage to Saturn.

The EUV detector utilizes a potassiumbromide photocathode and has no window sincetransmission of all known substances is very poorin this short wavelength range. Fortunately,potassium bromide is a very robust photocathodeand can be exposed to dry air for the short timerequired for testing and assembly. Once in thevacuum of space, the detector cover was opened,allowing light to enter the instrument.

Both detectors utilize specially selectedmicrochannel plates (MCP). MCP technology hasa long history in spaceflight imaging instruments.

Quality-control procedures during manufacturingallowed only MCPs with very low-defect densitiesto be used for final assembly. Once an MCP wasavailable, LASP and Schlumberger scientistsworked together during the final assemblyprocess. The units were then transported toNASA laboratories for final testing.

Two FUV and two EUV detectors that met thestringent quality requirements for space travel toSaturn were assembled at the SchlumbergerPrinceton Technology Center (PTC) in NewJersey. One pair of detectors was designated asflight units while the second set was kept inreserve as a backup.

The UVIS also includes a high-speedphotometer (HSP) channel, a hydrogen-deuterium absorption cell (HDAC) channel andelectronic and control subassemblies. Scientistsare using the HSP to make stellar occultationmeasurements of the structure and density ofmaterial in Saturn’s rings.

Cassini was launched on October 15, 1997,from Cape Kennedy, Florida, aboard a TitanIVB/Centaur rocket, the most powerful launchvehicle in the US fleet (above). After Cassini wasplaced in orbit around Earth, the upper stagefired to send Cassini on an interplanetarytrajectory that would eventually deliver thespacecraft to Saturn.

> Launching Cassini. A Titan IVB/Centaur launch vehicle propelled the Cassini spacecraft and its attachedHuygens probe into space from Cape Kennedy Air Station’s Launch Complex 40, Florida. Visible in thisview are the 20-m [66-ft] long, 5-m [17-ft] wide payload atop the vehicle holding the Cassini spacecraft.Cassini’s planned interplanetary flight path (chart inset) began with launch from Earth on October 15,1997, followed by gravity assist flybys of Venus, Earth and Jupiter. The gravity-assist flybys of thedifferent planets are designed to increase the spacecraft’s velocity relative to the Sun so it can reachSaturn. With the gravity-assist trajectory, it took more than 61⁄2 years for the Cassini spacecraft toarrive at Saturn. (Images courtesy of NASA.)

Orbit of Jupiter

Saturn Arrival07/01/04

Jupiter swingby12/30/00

Orbit ofSaturnOrbit of Venus

Launch 10/15/97

Orbit of Earth

Deep-space maneuver12/03/98

Earth swingby 08/18/99

Venus swingby 06/24/99

Venus swingby 04/26/98

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Cassini flew twice past Venus, then once pastEarth and Jupiter. The spacecraft’s speedrelative to the Sun increased as it approachedand swung around each planet, giving Cassini thecumulative boost it needed to reach Saturn withminimal fuel consumption. After reachingSaturn, Cassini fired its main engine for about 96minutes, reducing the spacecraft’s speed andallowing it to be captured in an orbit aroundSaturn. On January 5, 2005, Cassini released theEuropean-built Huygens probe toward Titan.

Journey to a Distant MoonWith a diameter larger than the planet Mercury,Titan is one of the most interesting moons in thesolar system. The surface of this moon lieshidden beneath an opaque atmosphere morethan 50% denser than that of Earth (left).

Titan’s atmosphere is filled with a brownish-orange haze composed of complex organicmolecules falling like rain from the sky to thesurface. Most scientists agree that conditions onTitan are too cold for life to have evolved—although there are theories concerning thepossibility of life forms in covered lakes of liquid hydrocarbons warmed by the planet’sinternal heat.

The Huygens probe entered Titan’s atmos-phere on January 14, 2005, deployed itsparachutes and began its scientific observationsduring a descent through the moon’s denseatmosphere lasting close to 21⁄2 hours (belowleft).41 Instruments onboard the probe detected asurface temperature of 94K at the landing site.Images taken by the probe while descendingshowed surface channels that appeared toindicate rain or fluid flow, possibly in the form ofliquid methane. Ridges as tall as 100 m wereobserved near the landing area (next page, top).High quantities of methane were detected in thelower atmosphere, with nitrogen predominatingin the upper atmosphere. Oxygen was notdetected probably because it is tied up as frozenwater. This would also prevent the formation of carbon dioxide.

Laboratory tests recreated the impactmeasurements derived from the onboardpenetrometer. These tests indicate that thesurface in the landing area may be composed offine particles with a thin crust. Accelerometermeasurements suggest the probe settled 10 to15 cm [4 to 6 in.] into the surface. Heat frominstruments then evaporated liquid methane inthe soil and released it around the spacecraft asmethane gas. The Huygens probe continued

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> Titan image. In this infrared view of Titan, features on the leading hemisphereare shown, including the bright, crescent-shaped Hotei arcus (right of center),often referred to as “the smile” by researchers. The view is centered on thebright region called Xanadu. The image was taken with the Cassini spacecraftnarrow-angle camera using a spectral filter sensitive to wavelengths ofinfrared light centered at 938 nm. The image was acquired at a distance ofapproximately 1.3 million km [800,000 miles] from Titan. (Image courtesy ofNASA/JPL/Space Science Institute.)

> Descent to Titan. The Huygens probe analyzed Titan’s atmosphere andrecorded a significant amount of data and images on its journey to thesurface of Titan. (Image courtesy of NASA/JPL.)

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making measurements and transmitting data toCassini for 72 minutes after landing until powerlimitations and deterioration of the spacecraftdue to extreme surface conditions on Titanresulted in loss of signal.

Exploring the Ringed PlanetAside from Titan, more moons of greater varietyorbit Saturn than any other planet. So far,observations from Earth and by spacecraft havefound Saturnian satellites ranging from smallasteroid-size bodies to those as large as Titan.

Saturn is the second-largest planet in thesolar system. Like the other gaseous outerplanets—Jupiter, Uranus and Neptune—it hasan atmosphere made up mostly of hydrogen andhelium, and like them, it is ringed. Saturn’sdistinctive bright rings are made up of ice androck particles ranging in size from grains of sandto small houses.

Although the face of Saturn appears calm, theplanet has a windswept atmosphere where anequatorial jet stream blows at 1,800 km/h[1,118 mi/h], and swirling storms churn beneaththe cloud tops. Early explorations by NASA’sPioneer 11 spacecraft in 1979, and the Voyager 1and 2 spacecraft in 1980 and 1981, found Saturnto have a huge and complex magnetic environ-ment where trapped protons and electronsinteract with each other, the planet, the ringsand the surfaces of many of Saturn’s moons.

From Earth, Saturn’s rings appear as only afew monolithic bands, while in reality, theyconsist of thousands of rings and ringlets, withparticles sometimes arranged in complicatedorbits by the gravitational interaction of smallmoons previously unseen from Earth (right).Scientists are using data from the UVIS indetailed computer models to simulate thecomplex motion of these rings.

Second in size only to Jupiter, Saturn hasmore than 750 times the volume of Earth.Combined with the planet’s low density,less than half that of water, its fast rotationpromotes a bulge of material near the equator.Saturn is shaped like a flattened ball; its pole-to-pole diameter is only 108,728 km [67,560 miles],compared to about 120,536 km [about74,898 miles] for the equatorial diameter.

41. European Space Agency–Cassini-Huygens:http://huygens.esa.int/science-e/www/object/index.cfm?fobjectid=36396 (accessed April 13, 2006).

> Exploring Saturn’s rings. Images taken during the Cassini spacecraft’s orbit around Saturn showcompositional variation in Saturn’s rings (top). The red in the image indicates sparser ringlets thatprobably comprise “dirty,” and possibly smaller particles than those in the icier turquoise ringlets. The red band roughly three-fourths of the way outward is known as the Encke Gap. This image wastaken with the Ultraviolet Imaging Spectrograph (UVIS) instrument, which is capable of resolving therings to show features up to 97 km [60 mi] across, roughly 100 times the resolution of ultraviolet dataobtained by the Voyager 2 spacecraft. The false-color view of Saturn’s A ring (bottom left) was alsotaken by the UVIS. The ring is the bluest in the center, where the gravitational clumps are the largest.The thickest black band in the ring is the Encke Gap, and the thin black band farther to the right is theKeeler Gap. The insert (bottom right) is a computer simulation about 150 m [490 ft] across, illustratinga clumpy region of icy particles in the A ring. (Images courtesy of NASA/JPL/University of Colorado.)

> Under Titan’s atmosphere. The perspective view of the surface of Titan near the Huygens probelanding site (top) is color-coded, with blue the lowest altitude and red the highest. The total areacovered by the image is about 1 by 3 km [0.6 by 2 miles]. A pair of images (inset) was acquired fromthe Huygens descent imager/spectral radiometer. The left image was acquired from 14.8 km [9 miles]above the surface with the high-resolution imager and the right from 6.7 km [4 miles] altitude with themedium-resolution imager. (Images courtesy of ESA/NASA/JPL/University of Arizona/USGS.)

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Unlike rocky inner planets such as Earth,Saturn has no surface on which to land. Aspacecraft descending into its atmosphere wouldsimply find the surrounding gases becomingdenser, and the temperature progressivelyhotter; eventually the craft would be crushed andmelted. Detailed analysis of Saturn’s gravita-tional field leads astronomers to believe that thedeepest interior of Saturn must consist of amolten rock core about the same size as theplanet Earth, but much denser.

Spectroscopic studies by the Voyagerspacecraft found Saturn to be made up of about94% hydrogen and 6% helium. Hydrogen andhelium are the primary constituents of all thegiant gas planets, the Sun and the stars. Gravityat the top of Saturn’s clouds is similar to thatnear the surface of Earth. The temperature nearthe cloud tops is about -139°C [-218°F],increasing toward the planet’s core due toincreased atmospheric pressure. At the core,Saturn’s temperature is predicted to be about10,000°C [18,000°F].

On June 21, 2005, the UVIS detected auroralemissions from both Saturn’s northern andsouthern poles (above right).42 These emissionsare believed to be similar to Earth’s NorthernLights yet are invisible to the naked eye.Ultraviolet images captured the entire oval of theauroral emissions from hydrogen gas excited byelectron bombardment. Time-lapse imagesindicate that aurora lights are dynamic,responding rapidly to changes in the solar wind.

New MoonsThere were only 18 known moons orbiting Saturnwhen the Cassini spacecraft began its mission toSaturn in 1997. During Cassini’s seven-yearjourney, Earth-based telescopes uncovered 13more moons. Soon after the spacecraft reachedSaturn, the Cassini team discovered two moretiny moons, Methone and Pallene. The two newmoons are approximately 3 km [1.8 miles] and4 km [2.5 miles] across.

Scientists suspected that more tiny Saturnianmoons might be found within the gaps in Saturn’srings. On May 1, 2005, using a sequence of time-lapse images from Cassini’s cameras, astrono-mers confirmed the presence of a tiny moonhidden in a gap in Saturn’s A ring.43 The images

show the tiny object in the center of the KeelerGap and the wavy patterns in the gap edges thatare generated by the moon’s gravitationalinfluence (above).

The new object, Daphnis, is about 7 km[4 miles] across and reflects about half the lightfalling on it—a brightness that is typical of theparticles in the nearby rings. As Cassinicontinues to explore Saturn and its moons,scientists expect to uncover more of the secretsof this vast planetary system.

Signs of an AtmosphereAlthough the moon Enceladus is covered with icecomposed of water, like Saturn’s other moons, itdisplays an abnormally smooth surface with veryfew impact craters. With a diameter of only500 km [310 mi], Enceladus would fit into the state of Arizona. Yet despite its small size,Enceladus exhibits one of the most interestingsurfaces of all the icy satellites. Enceladusreflects about 90% of the incident sunlight as if

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42. Laboratory for Atmospheric and Space Physics–Cassini-UVIS Mission to Saturn and Titan:http://lasp.colorado.edu/cassini/whats_new/ (accessed April 13, 2006).

43. NASA/Jet Propulsion Laboratory–Cassini Finds anActive, Watery World at Saturn’s Enceladus: http://www.nasa.gov/mission_pages/cassini/media/cassini-072905.html (accessed April 13, 2006).

44. NASA/Jet Propulsion Laboratory, reference 43.

> Perturbations caused by a tiny moon. This image confirmed earlier suspicions that a small moonwas orbiting within the narrow Keeler Gap in Saturn’s A ring. The Keeler Gap is located about 250 km[155 miles] inside the outer edge of Saturn’s A ring, which is also the outer edge of the bright mainrings. The new moon, Daphnis, is about 7 km across and reflects about 50% of incident sunlight.Scientists predicted the moon’s presence and its orbital distance from Saturn after July 2004, whenthey saw perturbations in the ring structure of the Keeler Gap’s outer edge. These images wereobtained with the Cassini spacecraft narrow-angle camera on May 1, 2005, at a distance ofapproximately 1.1 million km [680,000 miles]. (Image courtesy of NASA/JPL/Space Science Institute.)

Moon

Perturbationscaused by moon

> The southern lights of Saturn. Images of Saturn obtained by Cassini’s UVISshow auroral emissions at its poles similar to Earth’s Northern Lights. The twoUV images are the first from the Cassini-Huygens mission to capture the entire“oval” of the auroral emissions at Saturn’s southern pole. They also showsimilar emissions at Saturn’s north pole. In these false-color images, bluerepresents aurora emissions from hydrogen gas excited by electronbombardment, while red-orange represents reflected sunlight. These imageswere taken 1 hour apart; during this time the brightest spot in the left auroraimage fades and a bright spot appears in the middle of the aurora in the rightimage. (Images courtesy of NASA/JPL/University of Colorado.)

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covered with fresh-fallen snow, placing it amongthe most reflective objects in the solar system.Although Enceladus was previously thought to bea cold and dead rock mass, data from the Cassinispacecraft indicate evidence of ice volcanism,which might explain its smooth surface features.

In July 2005, Cassini’s instruments detected acloud of water vapor over the moon’s southernpole and warm fractures where evaporating iceprobably supplies the vapor cloud.44 So far,Enceladus is the smallest body found thatdisplays evidence of active volcanism. Scientiststheorize that warm spots in the moon’s icy andcracked surface are probably the result of heatfrom tidal energy like the volcanoes on Jupiter’smoon Io. Its geologically young surface of water-base ice, softened by heat from below, resemblesareas on Jupiter’s moons, Europa and Ganymede.

Cassini flew within 175 km [109 miles] ofEnceladus on July 14, 2005. Data collectedduring that flyby confirm an extended anddynamic atmosphere. This atmosphere was firstdetected by Cassini’s magnetometer during adistant flyby earlier in 2005 (above left).

Cassini’s magnetometer detected distur-bances in the magnetic field caused by smallcurrents of ionized gas from the atmospherearound this moon. These could be detected bythe instrument long before imaging instrumentscould be applied to confirm this finding.

As Cassini approached this small body,imaging instruments were able to makemeasurements that showed gas composition,further confirming the presence of anatmosphere. The ion and natural massspectrometers and the UVIS showed that thesouthern atmosphere contains water vapor(left). The mass spectrometer found that watervapor comprises about 65% of the atmosphere,with molecular hydrogen at about 20%. The restis mostly carbon dioxide and some combinationof molecular nitrogen and carbon monoxide. Thevariation of water-vapor density with altitudesuggests that the water vapor may come from alocalized source comparable to a geothermal hotspot. The ultraviolet results strongly suggest alocal vapor cloud. The fact that the atmospherepersists on this low-gravity world, instead ofinstantly escaping into space, suggests that themoon is geologically active enough to replenishthe water vapor at a slow, continuous rate.

High-resolution images show that the southpole has an even younger and more fracturedappearance than the rest of Enceladus, complete

> Shifting magnetic fields. This artist’s conception shows the detection of adynamic atmosphere on Saturn’s icy moon Enceladus. The Cassinimagnetometer is designed to measure the magnitude and direction of themagnetic fields of Saturn and its moons. During Cassini’s three close flybysof Enceladus on Febuary 17, March 9, and July 14, 2005, the instrumentdetected a bending of the magnetic field around Enceladus thought to becaused by electric currents generated by the interaction of atmosphericparticles and the magnetosphere of Saturn. The graphic shows the magneticfield observed by Cassini, as well as the predicted vapor cloud being ventedfrom the south pole of Enceladus. Cassini’s magnetometer observed bendingof the magnetic field consistent with its draping around a conducting object.(Image courtesy of NASA/JPL.)

Hot plasma flow

Saturn

Enceladus

Vapor cloud

> Indications of an atmosphere. On July 11, 2005, the Cassini ultraviolet imaging spectrographobserved the star Bellatrix as it passed behind Enceladus, as seen from the spacecraft. The starlightwas observed to dim when it got close to Enceladus, indicating the presence of an atmosphereisolated to the southern pole (A). The ultraviolet imaging spectrograph indicated that the atmospherewas water vapor, based on absorption features in the spectrum of the star. The colors show theundimmed star signal (blue) versus the dimmed star signal (red). As Bellatrix reemerged from behindEnceladus, there was no observed dimming of the starlight. In another occultation (B) of the starLambda Scorpius, no sign of an atmosphere was detected, implying that the atmosphere is localizedtoward the southern pole. (Image courtesy of NASA/JPL/Space Science Institute.)

A

Starlight

Bellatrix

Lambda Scorpius

Starlight

July February

B

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with icy boulders the size of large houses andlong, bluish cracks or faults (left).

Another Cassini instrument, the compositeinfrared spectrometer (CIRS), demonstratesthat the southern pole is warmer thananticipated (below left). Temperatures near theequator were found to reach a frigid 80K.Scientists believe that the poles should be evencolder because of the low level of energy receivedfrom the Sun. However, south polar averagetemperatures reached 85K, much warmer thanexpected. Small areas of the pole, concentratednear the fractures, are even warmer: higher than140K in some places.

Scientists find the temperatures difficult toexplain if sunlight is the only heat source. Morelikely, a portion of the polar region, includingobservable fractures, is warmed by heat escapingfrom the interior. Evaporation of this “warm” iceat several locations within the region couldexplain the density of the water-vapor clouddetected by Cassini’s instruments. How a 500-km[310-mile] diameter moon can generate thismuch internal heat and why it is concentrated atthe southern pole are still a mystery.

Similar to multiple well-logging instrumentsworking together deep beneath the Earth’ssurface, the discovery of an atmosphere onEnceladus resulted from an array of differentsensors working in synergy to acquire data andmaximize scientific value.

The Challenge of SpaceAdvances in technology, particularly during thelast 100 years, have helped change the way weview the Earth, our solar system and the universebeyond. From the E&P industry’s early begin-nings, engineers, geoscientists and many otherdedicated men and women have led the way inexploration of our inner space environment.Today, this same innovative spirit, and in manycases, similar technologies, are taking us beyondthe confines of Earth’s environment into the vastunknowns of outer space.

The examples presented in this article arejust a few of the contributions made by theoilfield service industry to space exploration. Inthe future, we can expect to see more terrestrialtechnology applied in the quest for extrater-restrial understanding. The late astrophysicistCarl Sagan wrote, “Imagination will often carryus to worlds that never were. But without it, wego nowhere.”45 It is this imagination andcreativity that have driven the E&P industry toexplore deep beneath the Earth’s surface andthat will inevitably launch the first drillingexpeditions to Mars and beyond. —DW

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> A hot southern pole. This map represents the surface temperature ofEnceladus as seen by the composite infrared spectrometer. The observedtemperatures included an unexpected hot spot at the south pole. On averagethe region is 15K warmer than expected; in some places hot spots greaterthan 140K were observed. The hottest spots line up with the blue fracturestripes visible in the previous image (above). (Images courtesy of NASA/JPL/Goddard Space Flight Center.)

Enceladus Temperature Map

Predicted temperatures Observed temperatures

85

75

80

70

Tem

pera

ture

, kel

vin

65

> Imaging Enceladus. This view (top left) is a mosaic of four high-resolution images taken by theCassini spacecraft narrow-angle camera during its close flyby of Saturn’s moon Enceladus. The viewis about 300 km [186 miles] across and shows a myriad of faults, fractures, folds, troughs and craters.The images were taken in visible light at distances ranging from of 26,140 to 17,434 km [16,246 to10,833 miles]. The southern polar terrain of Enceladus (bottom left) appears strewn with greatboulders of ice in the wide-angle camera image; more details are shown in the high-resolution,narrow-angle camera image (inset). The two images were acquired at an altitude of approximately208 km [129 miles]. The enhanced color view of Enceladus (right) is principally of the southernhemisphere. The south polar terrain is marked by a striking set of ‘blue’ fractures and encircled by a conspicuous and continuous chain of folds and ridges. This mosaic is a false-color view thatincludes images taken at wavelengths from the ultraviolet to the infrared portion of the light spectrum.(Images courtesy of NASA/JPL/Space Science Institute.)

45. Sagan C: Cosmos. New York City: Carl SaganProductions and Random House (1980): 4.

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