Download From Inner Earth to Outer Space

From Inner Earth to Outer Space
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.
Joel Lee Groves
John Simonetti
Stefan Vajda
Wolfgang Ziegler
Princeton Junction, New Jersey, USA
Jacob I. Trombka
Goddard Space Flight Center
Greenbelt, 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.
1. Andersen RN, Jarrard R, Pezard P, Williams C and
Dove 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’s
Secrets,” 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 to
the acceleration due to gravity on the Earth’s surface at
sea level.
On a cold day in February 2001, a spacecraft
landed on 433 Eros, an asteroid between the
orbits of Mars and Jupiter. The spacecraft had
completed its five-year journey to investigate
fundamental questions about the nature and
origin of near-Earth objects for the first time.
The technical demands of the Near Earth
Asteroid Rendezvous NEAR-Shoemaker (NEAR)
mission were immense. A multidisciplinary team
of US National Aeronautics and Space
Administration (NASA) scientists and engineers
drew from many scientific and industrial
resources, including the predominantly innerEarth-focused oil and gas industry.
Applying technologies developed for oil and
gas exploration to scientific endeavors is not a
new practice. Oilfield technologies have often
been applied in the interest of science. For
example, deep-drilling projects conducted on
land and in most major oceans of the world have
contributed to our understanding of Earth’s past
as well as its future.
Engineers and scientists with the internationally funded Ocean Drilling Program began
subsea drilling operations in 1961 to explore the
hard outer layer of the Earth’s crust, or
lithosphere. Scientists used tools and techniques
developed for oil and gas exploration to document
continental drift and to generate a substantial
quantity of data relating to plate tectonics.1
> Distant spiral galaxy. The Hubble Space Telescope captured this image of light that left the spiral
galaxy NGC1300 more than 69 million years ago. Barred spirals differ from normal spiral galaxies in
that the arms of the galaxy do not spiral all the way into the center, but are connected to the two
ends 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’s
arms. The nucleus shows its own distinct spiral structure that is about 3,300 light-years across. The
image was constructed from exposures taken in September 2004 by the Advanced Camera for Surveys.
(Image courtesy of NASA.)
44
Oilfield Review
In 2004, engineers drilling in the Arctic Ocean
at the crest of the Lomonosov ridge provided
preliminary evidence that the Arctic was ice-free
and warm about 56 million years ago.2 Scientists
analyzed cores recovered from the drilling project
to help determine when, why and how the Arctic
temperature changed. They also gained insight
into current global-warming trends.3
Understanding the fundamental processes
that occur deep within the Earth’s crust has
contributed to our knowledge of many innerearth events, including volcanic activity, plate
tectonics, weather fluctuations, and chemical
and thermodynamic processes that lead to
mineral deposition.
Hydrocarbons are most often found in
forbidding environments. Tools and sensors are
stressed to their limits as boreholes are drilled
deeper into the Earth’s crust where high temperature and pressure and excessive vibrations are
common, and stress and shock forces reach
thousands of times the acceleration of gravity
(gn).4 Tools and instruments must also survive
extreme thermal cycles, from the cold surface of
the Arctic to temperatures higher than 204°C
[400°F] in the downhole environment. Drilling,
logging and measurement instruments have
evolved to meet these challenges. Today, oil and
gas E&P tools and instruments are designed and
thoroughly tested for extended exposure to these
harsh environments.
Similarly, the forces encountered while
launching and accelerating a vehicle into space
can be traumatic to equipment components. For
example, the shock of pyrotechnic-stage separation can reach over 4,000 gn, stressing both the
vehicle and its payload. Once in space, depending on orientation relative to the Sun, temperature extremes range from more than 100°C
[212°F] to below -200°C [-328°F]. Because of the
need to operate in harsh environments, the tools
and instrument packages designed for deep-well
drilling are inherently applicable to other
challenging environments, such as outer space.
Whether exploring inner space for scientific
purposes, searching for oil and gas or probing the
vastness of outer space, the desire to explore has
driven the history of modern civilizations. This
drive led, at least in part, to the conquest of the
moon in the 1960s, marking the beginning of a
new generation in space exploration and travel.
More recently, spacecraft, such as the Hubble
Space Telescope (HST), aided by technologies
developed for oil and gas exploration, have
peered from Earth orbit ever more sharply and
deeply into the universe beyond our solar system
(previous page).
Spring 2006
45
> Servicing the Hubble Space Telescope (HST). The Space Shuttle Discovery,
mission STS-82, lifts the HST from its service bay after the second Hubble
service mission. With a launch weight of 11,340 kg [25,000 lbm], the Hubble’s
main 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 telescope
itself 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. (Image
courtesy of NASA.)
As we move from exploration of inner space
to that of outer space, the tools and techniques
developed for exploration deep beneath the
Earth’s surface are helping to uncover the
mysteries of our solar system and the far reaches
of space. In this article, we discuss a few of the
recent contributions made to space exploration
by the scientists and engineers of the petroleum
industry. Although the mission of the NEAR
spacecraft has ended, oilfield technology aboard
the HST and the Cassini-Huygens Saturn probe
continues to expand our knowledge and chart
our way forward in the quest for knowledge.
Keeping the Hubble on Target
Throughout history, our understanding of the
universe has been limited by what we could see.
The invention of the telescope enhanced our
vision and allowed observations by Copernicus,
Kepler and Galileo in the 16th and 17th centuries
46
to show that the Earth was not the center of the
universe.5 By the 18th century, the development
of the telescope helped scientists investigate the
cosmos. Increasingly bigger and better telescopes
have routinely discovered and documented
planets, stars and nebulae that are invisible to
the naked eye.
As recently as the beginning of the 20th
century, most astronomers still believed that the
universe consisted of a single galaxy, the Milky
Way—a collection of stars, dust and gas in the
vastness of space. However, the universe as we
knew it changed in 1924 when American
astronomer Edwin Hubble used the 2.54-m
[100-in.] Hooker Telescope on Mount Wilson,
near Los Angeles, to observe billions of other
galaxies beyond the Milky Way.6
For astronomers like Edwin Hubble, there
has always been a major obstacle to a clear view
of the universe—the Earth’s atmosphere. Gases
and airborne particulates in the atmosphere blur
visible light, cause starlight to scintillate, or
twinkle, and hinder or totally absorb infrared,
ultraviolet, gamma ray and X-ray wavelengths .
To minimize atmospheric distortion,
scientists built observatories on mountaintops
and away from the areas of highly radiated light,
or sky glow, found near large cities. This effort
met with varying levels of success. Today,
adaptive optics and other image-processing
techniques have minimized, but not totally
eliminated, atmospheric effects.7
In 1946, Princeton astrophysicist Lyman
Spitzer documented the potential benefits of a
telescope in space, well above Earth’s
atmosphere. Then, following the launch of the
Soviet satellite Sputnik in 1957, NASA placed two
orbital astronomical observatories (OAO) into
Earth orbit. The OAOs made a number of
ultraviolet observations and established the
basic principles for the design, manufacture and
launch of future space observatories.8
Scientific, governmental and industrial
groups continued the move toward extraterrestrial exploration by planning the next step
beyond the OAO program. Spitzer gathered the
support of other astronomers for a large orbital
telescope, later called the Hubble Space
Telescope, and in 1969, the National Academy of
Sciences approved the project.9
NASA’s Goddard Space Flight Center in
Greenbelt, Maryland, USA, was responsible for
scientific instrument design and ground control
for the space observatory. In 1983, the Space
Telescope Science Institute (STScI) was
established at The Johns Hopkins University in
Baltimore, Maryland. The staff of STScI managed
the telescope’s observation time and data. NASA
chose the Marshall Space Flight Center in
Huntsville, Alabama, USA, as the lead NASA field
center for the design, development and
construction of the space telescope. Perkin-Elmer
Corporation, now Hughes Danbury Optical
Systems, developed the optical telescope assembly
and the fine-guidance sensor (FGS) system.
On April 24, 1990, after numerous project
delays, the space shuttle Discovery lifted off from
Earth carrying the HST in its cargo bay. The
following day, the school-bus-size space telescope
was deployed in low-Earth orbit (above left).
Free of atmospheric distortion, the giant
telescope mirror began its mission of gathering
photons from as far away as the edge of the
known universe.
Oilfield Review
Critical to the performance of the HST is
staying on target for extended periods of time.
Electromagnetic waves emitted from distant
objects are often faint or weak, so the HST must
stay perfectly positioned while the photons are
being collected in sufficient quantities to form
an image. To accomplish this, engineers used
the Schlumberger oilfield photomultiplier-tube
technology to design the FGS system.10
An FGS is essentially a targeting camera
capable of making celestial measurements,
locking onto guide stars and providing data for
maneuvering the telescope.11 Two FGSs are used
to point the telescope at an astronomical target
and hold that target in the telescope’s field of
view; the third FGS can then be used for
astrometry measurements.12
The FGS system can maintain pointing
accuracy to 0.007 arcseconds, allowing the
telescope’s pointing-control system (PCS) to
keep the Hubble telescope on target during
camera exposure times of 10 hours or more.13 The
PCS combines a number of different sensor
subsystems to achieve this milliarcsecond
pointing accuracy. This level of accuracy and
precision is comparable to training a laser beam
on a target the size of a thumbnail from a
distance of 442 km [275 miles].
Within the housing of each FGS instrument
are two orthogonal white-light, shearing
interferometers, their associated optical and
mechanical elements and four Schlumberger
S-20 photomultiplier tubes (PMTs) (above right).14
These PMTs are based on the same rugged
construction as those used in well-logging
instruments. The photocathode was manufactured using the same technology as tubes used
in oilfield service applications. For use on the
HST, the PMTs were designed to be sensitive over
a spectral range of 400 to 700 nanometers (nm),
with an efficiency of approximately 18% at the
blue end of the electromagnetic spectrum and
diminishing linearly to about 2% at the red end.
Each FGS interferometer consists of a
polarizing beam splitter followed by two Koesters
prisms. To measure the direction of the light
emitted by a guide star, the pairs of Koesters
prisms are oriented perpendicular to one
another. The angle of the wavefront in the X and
Y planes gives the precise angular orientation of
the guide star relative to the HST’s optical path.
These data, once fed into the PCS, are used to
control the telescope orientation relative to a
guide star.
Spring 2006
Aspheric collimating
mirror
Photomultiplier tube
with pinhole lens
assembly (4)
Star selector
mirrors
Correction group
Doublet lens (4)
Koesters prism
Deviation prism
Optical bench
Beam-splitter prism
Filters (5 in wheel)
Pickoff mirror
PMT B
PMT A
Field stop
Field lens
Field stop
Field lens
Positive
doublet
Positive
doublet
D
Dielectric
beam
splitter
Koesters
prism
A
B
C
Incident wavefront
Alpha
> Guiding Hubble. Light from the HST Optical Telescope Assembly (OTA) is intercepted by a pickoff
mirror in front of the HST focal plane and directed into the fine-guidance system (FGS) (left). The light
rays are collimated, or made parallel, and then compressed by an aspheric collimating mirror and
guided 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 incident
wavefront with respect to the Koesters prism (right). As the wavefront rotates about Point B, the
relative phase of the transmitted and reflected beams change as a function of angle alpha. When the
wavefront’s propagation vector is parallel to the plane of the dielectric surface, the relative intensities
of the two emergent beams detected by the photomultiplier tubes will be equal. When alpha is not
zero, the intensities of the left and right output beams will be unequal and the PMTs will record
different photon counts, thus providing the telescope guidance control system with data allowing for
pointing correction. [Images courtesy of NASA and The Johns Hopkins University Applied Physics
Laboratory (JHUAPL).]
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 effects
of rapidly changing optical distortion typically resulting
from changes in atmospheric conditions. Adaptive optics
works by measuring the distortion and rapidly
compensating for it using either deformable mirrors or
material 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 and
Schnorr D: “Saturation Monitoring With the RST
Reservoir 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 (accessed
March 14, 2006).
A guide star is one of many bright stars used for
telescope positioning and triangulation.
12. Astrometry is a branch of astronomy that deals with the
positions of stars and other celestial bodies, their
distances and movements.
13. A second of arc, or arcsecond, is a unit of angular
measurement that comprises one-sixtieth of an
arcminute, 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 1
unit diameter at a distance of 360x60x60/(2π) ≈ 206,265
units, such as (approximately) 1 cm at 2.1 km.
14. Interferometers were first used by Michaelson, who won
the Nobel Prize in 1907 for his work using an optical
interferometer to accurately measure the speed of light.
47
In addition to guiding the HST, the accuracy
of FGS sensors makes them useful for highprecision astrometric measurements. These
measurements help scientists determine the
precise positions and motions of stars. The FGS
sensors can provide star positions about 10 times
more precisely than measurements made with
ground-based telescopes. Scientists use astrometric measurements to help define wobble in
the motion of stars that might indicate the
presence of a planetary companion (below left).
The motions of stars can also determine whether
a star pair represents a true binary star system,
or simply an optical binary.15
Aided by elements of oilfield technology, the
Hubble Space Telescope continues its work
today. Scientists are using instruments like the
HST to search the far reaches of the universe and
uncover secrets of the past while reaching into
our future.
around the developing Sun. Within the disk, tiny
dust grains coagulated into larger and larger
bodies called planetesimals, many of which
eventually accreted into planets over a period of
100 million years. However, beyond the orbit of
Mars, gravitational interference from Jupiter
prevented protoplanetary bodies from growing to
diameters larger than about 1,000 km
[620 miles].16
Most asteroids are concentrated in an orbital
belt between Mars and Jupiter (below). These
space rocks orbit the Sun as planets do, but they
have no atmosphere and very little gravity. The
asteroids in the belt comprise a significant
Asteroids—Up Close and Personal
A little closer to home, technologies developed
for oilfield use are helping scientists explore
asteroids in our solar system. These large pieces
of rock are primordial objects left over from the
formation of the solar system. Some scientists
have suggested that asteroids are the remains of
a protoplanet that was destroyed in a massive
collision. However, the prevailing view is that
asteroids are leftover rocky matter that never
successfully coalesced into planets.
Scientists theorize that the planets of the
solar system formed from a nebula of gas and
dust that coalesced into a disk of dust grains
A
+
Mars
B
Mercury
+
Main
asteroid
belt
Earth
Venus
C
+
Trojan
asteroids
Trojan
asteroids
D
Jupiter
+
0
1.5
2.7
5.2
Astronomical units
> True binary stars. Each of the two stars in a
true binary system orbits around the center of
mass of the system. Kepler’s laws of planetary
motion govern how each star orbits the center
of mass. At aphelion (A), each of the two stars
are the farthest apart in their respective orbits.
At perihelion (C), the stars are the closest.
48
> Main asteroid belt. The asteroid belt is a region of the solar system falling roughly between the
planets Mars and Jupiter where the greatest concentration of asteroid orbits can be found. The main
belt region contains approximately 93.4% of all numbered minor planets. Trojan asteroids occupy two
regions centered 60° ahead of and behind Jupiter. Several hundred Trojans are known out of a total
population that includes an estimated 2,300 objects bigger than 15 km [9 miles] across and many
more of smaller size; most do not move in the plane of the planet’s orbit but rather in orbits inclined
by up to 40°.
Oilfield Review
The footprint of
asteroid Eros
Manhattan
> Impacting the Earth. An asteroid impacting the Earth some 49,000 year ago scarred the Earth
leaving this 1.2-Km [0.7-mile] crater. This view from the Space Shuttle shows the dramatic expression
of the crater in the arid landscape of Arizona, USA. (Image courtesy of the Earth Sciences and Image
Analysis Laboratory, NASA Johnson Space Center, STS040_STS040-614-58.)
amount of material—putting all of the asteroids
together would form a body about 1,500 km [930
miles] in diameter, roughly half the size of
Earth’s moon.17
Not all asteroids are far away in the asteroid
belt. Some, called near-Earth asteroids (NEAs),
have orbits that bring them close to Earth.
Astronomers believe NEAs to be fragments
ejected from the main asteroid belt by asteroidasteroid collisions or by gravitational perturbations from Jupiter. Some NEAs could also be the
nuclei of dead, short-period comets.
Since many asteroids have historically struck
Earth and its moon, understanding their
composition and origin may be a key to our past
as well as our future. Scientists believe that the
chemical building blocks of life and much of
Earth’s water may have arrived on asteroids or
comets that bombarded the planet in the early
stages of its development (above left). One
widely accepted theory suggests that an asteroid
measuring 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 approximately
800 NEAs found to date represent only a small
percentage of their total population. The largest
presently known is 1036 Ganymede, with an
approximate diameter of 41 km [25.5 miles].
NEAs with diameters greater than 1 km
[0.6 miles] are known as potentially hazardous
asteroids, suggesting that should they strike
Earth, they could threaten life as we know it.
Spring 2006
Of the more than 700 known potentially
hazardous asteroids, one of the largest is
Toutatis, an asteroid that is nearly 1.6 km
[1 mile] long and orbits around the Sun within
one-half degree of Earth’s orbital plane. In
December 1992, Toutatis passed within
0.024 astronomical units (AU), or 9.4 lunar
distances from Earth.18 Then, on September 29,
2004, Toutatis’s orbital path brought it within
0.01 AU of Earth—the closest approach of any
large asteroid in the 20th century.
Although astronomers have known about
asteroids for nearly 200 years, until recently, their
basic properties, their relationship to meteorites
found on Earth and their origins remained a
mystery. NASA and the scientific community,
driven by both the desire to understand asteroids
and the threat to Earth presented by NEAs more
than 1 km in diameter, set in motion the plans for
the NEAR project.
A Mission of Many Firsts
In 1990, NASA introduced a new program of
planetary missions called the Discovery program.
By 1991, the first mission was chosen—a
rendezvous with near-Earth asteroid 433 Eros.
The Johns Hopkins University Applied Physics
Laboratory (JHUAPL) was chosen to manage the
project, and in 1995, the NEAR spacecraft was
shipped to the Kennedy Space Center in Florida.19
> A large asteroid. The outline of Eros (red) is
superimposed on the island of Manhattan, New York
City, showing the relative size of the asteroid.
Discovered in 1898, the NEA Eros is one of
the largest and best-observed asteroids.20 With
dimensions 33 by 13 by 13 km [21 by 8 by
8 miles], Eros is about the size of Manhattan,
New York, USA (above). It accounts for nearly
half of the volume of all near-Earth asteroids.
15. The term binary star refers to a double-star system, or a
union of two stars into one system based on the laws of
attraction. Any two closely spaced stars might appear
from Earth to be a double-star pair when, in fact, they
are a foreground and background star pair widely
separated in space. These systems are typically referred
to 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 distance
from the Earth to the Sun, or approximately 149,000,000 km
[92,500,000 miles].
19. The NEAR spacecraft was renamed NEAR–Shoemaker
to honor planetary geologist Eugene Shoemaker
(1928–1997).
20. Farquhar RW: “NEAR Shoemaker at Eros: Mission
Director’s Introduction,” Johns Hopkins APL Technical
Digest 23, no. 1 (2002): 3–5.
49
> Approaching Eros. This image of the southern hemisphere of Eros offers a long-distance look at the
asteroid’s cratered terrain. (Image courtesy of NASA/JHUAPL.)
Mathilde
Gaspra
Ida
> Asteroids close up. Shown are views of the three asteroids that had been imaged at close range
by spacecraft prior to NEAR’s arrival at Eros. The image of Mathilde (left) was taken by the NEAR
spacecraft on June 27, 1997. Images of the asteroids Gaspra (middle) and Ida (right) were taken by
the Galileo spacecraft in 1991 and 1993, respectively. All three objects are presented at the same
scale. The visible part of Mathilde is 59 km wide by 47 km long [37 by 29 miles]. (Images courtesy of
NASA/JHUAPL.)
The large S-type potato-shaped asteroid is one of
the most elongated asteroids. It orbits around
the Sun, rotating on its axis once every 5.27
hours, with a perihelion of 1.13 AU and an
aphelion of 1.78 AU (top).21
NEAR departed Earth for asteroid Eros on
February 17, 1996, riding on top of a Delta-II
launch vehicle. One year later, on February 18,
1997, NEAR reached its most distant point from
the Sun, 2.18 AU, setting a new distance record
50
for a spacecraft with instrumentation powered by
solar cells.
By the end of its five-year mission, NEAR
would produce an impressive list of spacecraft
firsts: the first spacecraft with instrumentation
solely powered by solar cells to operate beyond
the orbit of Mars, the first to encounter a C-type
asteroid, the first to encounter a near-Earth
asteroid, the first to orbit a small body, and the
first spacecraft to land on a small body.
NEAR—The Scientific Mission
Prior to the NEAR mission, our knowledge of
asteroids came primarily from three sources:
Earth-based remote sensing, data from the
Galileo mission flybys of the two main-belt
S-type asteroids 951 Gaspra and 243 Ida, and
laboratory analyses of meteorites recovered after
impact with the Earth.
Although astronomers theorize that most
meteors result from the collision of asteroids,
they may not be completely representative of all
materials that comprise NEAs.22 Clear links
between meteorite types and asteroid types
proved difficult to establish.23
Some S-type asteroids appear to be fragments
of bodies that underwent substantial melting and
differentiation, while others consist of what
appears to be nonmelted primitive materials like
chondrites.24 Scientists believe that nonmelted Stype asteroids may have preserved the
characteristics of the solid material from which
the inner planets accreted.
The Galileo mission flybys provided the first
high-resolution images of asteroids in the early
1990s. Images revealed complex surfaces covered
by craters, fractures, grooves and subtle
color variations (left).25 However, Galileo’s
instrumentation was not capable of measuring
elemental composition, so prior to the NEAR
mission, scientists continued to be unsure of the
relationship between ordinary chondrites and Stype asteroids.
Mission engineers believed that the NEAR
data, when combined with those from the Galileo
flybys, would help scientists understand the
relationship between S-type asteroids and other
small bodies of the solar system. The NEAR
mission’s primary objectives were to rendezvous
with, achieve orbit around and conduct the first
scientific exploration of a near-Earth asteroid.
The NEAR Spacecraft
Engineers designed NEAR’s systems to be solarpowered, simple and highly redundant.26 Onboard
NEAR were five instruments designed to make
detailed scientific observations of the gross
physical properties, surface composition and
morphology of Eros. These five were the multispectral imager (MSI), near-infrared spectrometer (NIS), magnetometer (MAG), NEAR laser
rangefinder (NLR) and the combined X-ray,
gamma ray spectrometer (XGRS) (next page).
The MSI imaged the surface morphology of
Eros with spatial resolutions down to 5 m
[16.4 ft], while scientists used the NIS to
measure mineral abundances at a spatial
Oilfield Review
resolution on the order of 300 m [984 ft]. The
MAG was used to define and map intrinsic
magnetic fields on Eros.
Scientists used the NLR to enhance the
surface morphology profiles derived from NEAR’s
imaging camera. The NLR is a laser altimeter
that measures the distance from the spacecraft
to the asteroid surface by sending out a short
burst of laser light and then recording the time
required for the signal to return from the
asteroid. The ranging data were used to
construct a global shape model and a global
topographic map of Eros with a spatial resolution
of about 5 m.
The XGRS was the primary tool used for
surface and near-surface elemental analysis of
Eros. Scientists combined data from the XGRS,
MSI and the NIS to produce global maps of Eros’s
surface composition.
Development of the complex XGRS system
began about three years prior to launch. The
instrument was designed to detect and analyze
X-ray and gamma ray emissions from the asteroid
surface from orbital altitudes of 35 to 100 km
[22 to 62 miles]. Although spectroscopy of
remote surfaces is possible during spacecraft
flyby operations, measurements made while
orbiting allow longer observation times and
produce higher quality spectral data.
X-rays emitted from the Sun shining on Eros
produce X-ray fluorescence from the elements
contained in the top 1 mm [0.04 in.] of the
asteroid’s surface. In the absence of any
significant atmosphere that might otherwise
absorb X-ray emissions, elements fluoresce at
energy levels that are characteristic of specific
elements. Scientists used the X-ray fluorescence
energy detected in the 1- to 10-keV level to infer
surface elemental composition.
The XRS subunit consists of three identical
gas-filled proportional counters that provide a
large active surface area and therefore the
sensitivity required for remote sensing. Similar
detectors have been used on lunar orbital
missions and most recently on Apollo missions.
The X-ray gas tubes are not particularly
sensitive to temperature change, since the
multiplication effect depends more on the
number of gas molecules than the gas pressure.
However, the gain in the gas tubes is sensitive to
voltage variations.
Gamma ray spectrometry provides a
complementary measurement of near-surface
elemental composition. The gamma ray
spectrometer (GRS) detects discrete-line gamma
ray emissions in the 0.1- to 10-MeV energy range.
Spring 2006
Solar panel
X-ray solar
monitors
Propulsion system
Forward deck
Side panels
Gamma ray
spectrometer
Aft deck
Multispectral imager
Near-infrared
spectrometer
X-ray
spectrometer
NEAR laser rangefinder
> NEAR spacecraft systems. NEAR’s basic design and primary systems are shown. (Image courtesy
of NASA/JHUAPL.)
At these energy levels, oxygen [O], silicon [Si],
iron [Fe] and hydrogen [H] become excited, or
radioactively activated, from the continual influx
of cosmic rays. The GRS also detects naturally
radioactive elements such as potassium [K],
thorium [Th] and uranium [U]. The measurements have been used for years in oil and gas
well logging to determine the physical and
elemental composition of reservoir rock.
Unlike the low-energy X-rays, gamma rays are
not as easily absorbed and therefore can escape
from regions beneath the surface, allowing the
GRS to reveal elemental composition to depths
as much as 10 cm [4 in.] below the surface.
By comparing elemental analysis from the XRS
and GRS, scientists inferred the depth and
extent of the dust layer, or regolith, covering the
surface of Eros.27
21. Asteroids are classified based on reflectance spectrum
and light-reflection characteristics, or albedo, which are
indicators of surface composition. S-Type (silicaceous)
asteroids are more prevalent in the inner part of the
main asteroid belt, while C-Type (carbonaceous)
asteroids are found in the middle and outer parts of the
belt. Together, these two types account for about 90% of
the asteroid population.
Perihelion and aphelion are the orbital points nearest
and farthest from the center of attraction—in this case,
the Sun.
22. A meteorite is a solid portion of a meteoroid that
survives its fall to Earth. Meteorites are classified as
stony meteorites, iron meteorites and stony iron
meteorites, and further categorized according to their
mineralogical content. They range in size from
microscopic to many meters across. Of the several tens
of tons of cosmic material entering Earth’s atmosphere
each day, only about one ton reaches the ground.
23. Cheng AF, Farquhar RW and Santo AG: “NEAR
Overview,” Johns Hopkins APL Technical Digest 19, no. 2
(1998): 95–106.
24. Chondrites are a type of stony meteorite made mostly of
iron- and magnesium-bearing silicate minerals.
Chondrites are the most common type of meteorite,
accounting for about 86% that fall to Earth. They
originate from asteroids that never melted, or underwent
differentiation. As such, they have the same elemental
composition as the original solar nebula. Chondrites
derive their name from the fact that they contain
chondrules—small round droplets of olivine and
pyroxene that apparently condensed and crystallized in
the solar nebula and then accreted with other materials
to 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’s
moon and many other bodies in the solar system, it
consists mostly of debris produced by meteorite impacts
and blankets most of the surface.
51
gamma ray energy levels, above 3 MeV, pair
production becomes dominant.29
Identification of elemental compositions is
performed primarily by measuring the characteristic photoelectric energy of individual
nuclear varieties when excited by an external
radiation source, such as solar wind or other
cosmic rays. At higher energy levels the pairproduction mechanism generates well-defined
spectra. As such, the most accurate GRS
measurements were made during periods of high
solar-flare activity when gamma ray energy levels
were at their highest.
To improve the elemental identification
capability of the GRS, an active detector cup shield
was designed especially for NEAR. It was
fabricated from a single bismuth germanate
[BGO] crystal. The dense BGO cup acted as an
active scintillator while providing direct and
passive shielding from the local gamma ray
environment and reducing unwanted background signals.
The GRS central detector assembly is based on
a ruggedized thallium-activated [Tl] sodium iodide
[NaI] scintillator unit used in oilwell logging
operations, designed and built by Schlumberger
(below). NaI-based scintillators are widely used in
downhole logging-tool applications to make
measurements of density, natural radioactivity
and elemental spectra. As an example, the
EcoScope multifunction logging-while-drilling tool
uses a NaI detector to make while-drilling
spectroscopy measurements.28 Other logging tools
use different materials.
Interactions of gamma rays with solid
materials depend on the energy of the gamma rays
and on the density and the atomic number of the
materials being investigated. These interactions
can be classified by the level of energy absorbed
by the substrate material.
At lower energy levels, the photoelectric
effect, or Compton scattering, is prevalent. In
this case, only a fraction of the gamma ray
energy is deposited, and the rest leaves the
material as low-energy photons. At higher
Aft deck
Support
Thermal
spacers
Teflon spacers
Clamp
Optical
coupling
Gamma ray
detector
NaI (Tl)
crystal
Connector
Small
PMT
BGO
shield
Optical
coupling
Large
PMT
Spring
Teflon wedge
Spring
> XGRS imaging systems. The combined X-ray, gamma ray spectrometer system (XGRS) is shown
mounted on the NEAR spacecraft (top left). Shown on the right side of the XRGS instrument is the
gamma 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 bismuth
germanate (BGO) cup shield to reduce unwanted background signals by almost three orders of
magnitude. The Schlumberger photomultiplier tubes (PMTs) at each end convert the light output of
the scintillation detectors into electrical signals. (Image and diagram courtesy of NASA/JHUAPL.)
52
The new design replaced the more expensive
and less reliable long booms used in other missions
to reduce unwanted signals from the activation of
the spacecraft body itself by cosmic radiation. The
GRS also provided sensitivity to the direction from
which the gamma rays were coming.
Detour to a C-Type Asteroid
In early December 1993, NEAR mission managers
at The Johns Hopkins University Applied Physics
Laboratory reviewed a list of asteroids that might
be in close proximity to NEAR’s flight path (next
page, top). Asteroid 253 Mathilde was found to be
within 0.015 AU, or about 2.25 million km
[1.4 million miles], of NEAR’s planned orbital
path. Engineers calculated that with slight
changes in NEAR’s planned trajectory, it could
encounter 253 Mathilde with only a 57 m/s
[187 ft/s] change in velocity, well within the
spacecraft’s velocity margin.30
Although the dark asteroid was discovered in
1985, little was known about Mathilde. New
astronomical observations from ground-based
telescopes showed it to be a C-type asteroid with
an unusual rotation period of 15 days, almost an
order of magnitude slower than most other
known asteroid rotation periods.
NEAR encountered Mathilde on the way to
Eros after five trajectory-correction maneuvers
about 2 AU from the Sun.31 At this distance,
available power from the spacecraft’s solarpowered system was down nearly 75%. With this
limited power, astronomers could use only the
MSI to explore the surface of the asteroid, and
radio-tracking data, before and after approach,
to help determine the mass of the asteroid.
During the flyby, Mathilde exerted a slight
gravitational pull on the NEAR spacecraft.
Because of Mathilde’s mass, the gravitational
effects on NEAR’s path were detectable in the
spacecraft’s radio-tracking data.
Data from radio-tracking mass estimates
along with volume approximations helped
scientists calculate the asteroid’s approximate
density of 1.3 ± 0.3 g/cm3 [81.16 ± 18.73 lbm/ft3].
Because of the asteroid’s spectra, Mathilde was
believed to be similar in composition to
carbonaceous-chondrite meteorites. However,
Mathilde’s density was half of that expected,
implying either a high internal porosity or
significant void space within the asteroid.
Scientists imaged Mathilde over a 25-minute
period during the spacecraft’s approach at a
distance of 1,200 km [746 miles] and a speed of
9.93 km/s [22,213 mi/h]. A total of 534 images
Oilfield Review
Earth swingby
01/22/98
1,186-km altitude
variations, implying a homogeneous composition.
Further, the measured albedo was consistent
with ground-based telescopic observations.
Although significant data were gained by the
Mathilde flyby, numerous questions about C-type
asteroids remain unanswered. Mathilde’s density
was inconsistent with common carbonaceouschondrite meteorites found on Earth, and the
asteroid’s surface appears homogeneous. So, the
question remains: what connection, if any, exists
between dark asteroids and meteors found in the
solar system?
Eros
Earth orbit
orbit
Launch 02/17/96
C3 = 25.9 km2/s2
Sun
Eros arrival
01/09–02/06/99
Deep-space
maneuver
03/07/97
∆V = 215 m/s
> Destination Eros. The NEAR spacecraft was successfully launched in
February 1996, taking advantage of the unique alignment of Earth and Eros
that occurs only once every seven years. A Delta-II rocket placed NEAR into
a two-year Earth gravity-assist trajectory. The gravity-assist maneuver decreased
the aphelion distance while increasing the inclination from 0 to about 10°.
20 km
> A quick look at asteroid Mathilde. This view of 253 Mathilde, taken from a
distance of about 1,200 km, was acquired shortly after the NEAR spacecraft’s
closest approach to the asteroid. Showing on Mathilde are numerous impact
craters, 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 from
these craters traveled only short distances before falling back to the surface;
straight sections of some crater rims indicate the influence of large faults or
fractures 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 the
NEAR flyby. (Image courtesy of NASA/JHUAPL.)
were obtained during this interval at resolutions
ranging from 200 to 500 m [656 to 1,640 ft] (above).
Images obtained during the flyby of Mathilde
show an asteroid with a heavily cratered surface.
At least four giant craters have diameters that
are comparable to the asteroid’s mean radius of
26.5 km [16.5 miles]. The magnitude of the
impacts required to create craters of this size is
significant. Scientists suspect that Mathilde did
not break apart during any of these impacts
Spring 2006
because of the asteroid’s high porosity.
Laboratory data suggest that cratering in highly
porous targets is governed more by compaction
of the target material than by fragmentation and
excavation.32 Cratering processes governed by
structural properties such as porosity produce
craters with steep walls, crisp rims and with little
ejecta, similar to those imaged on Mathilde.
The images also show Mathilde is remarkably
uniform. The NEAR observations revealed no
evidence of any regional albedo, or spectral
Detecting Gamma Ray Bursts
Gamma ray bursts (GRBs) have remained one of
the great mysteries of astrophysics since their
discovery more than 30 years ago. NASA’s Hubble
Space Telescope made the first observation of an
object associated with a GRB that was detected by
the Italian BeppoSAX satellite in February 1997.33
Scientists believe that GRBs result from
massive explosions in the distant universe that
release waves of high-energy photons. GRBs
seem to occur daily and emanate from random
parts of the sky. GRBs represent the most
powerful events known in the universe, emitting
in one second as much energy as the Sun will
emit in its lifetime. Spectroscopic analyses of
faint, but long-lasting GRB optical afterglows
have, in a number of cases, indicated Doppler
shifts in the red spectrum that indicate a
cosmological origin of GRBs.34 Time is critical in
follow-up observation efforts, since GRB
afterglows fade quickly, in the radio as well as
optical spectrum, making it difficult for
astronomers 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 and
White D: “No More Waiting: Formation Evaluation While
Drilling,” Oilfield Review 17, no. 3 (Autumn 2005): 4–21.
29. Pair production is the chief method by which energy
from gamma rays is observed in condensed matter.
Provided there is enough energy available to create the
pair, a high-energy photon interacts with an atomic
nucleus and an elementary particle and its antiparticle
are created.
30. Dunham DW, McAdams JV and Farquhar RW: “NEAR
Mission 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 Hopkins
APL Technical Digest 23, no. 1 (January-March 2002):
6–17.
33. The Johns Hopkins University Applied Physics
Laboratory–Near Spacecraft Gets Unexpected View of
Mysterious Gamma-Ray Burst: http://www.jhuapl.edu/
newscenter/pressreleases/1998/gamma.htm (accessed
April 5, 2006).
34. NASA–Automatic NEAR-XGRS Data Processing System
for Rapid and Precise GRB Localizations with the
Interplanetary Network: http://gcn.gsfc.nasa.gov/
gcn/near.html (accessed April 5, 2006).
53
Since 1993, astronomers have used specially
instrumented spacecraft to help identify the
source of GRBs. These include the Ulysses
spacecraft and several spacecraft near the Earth:
the BeppoSAX, Wind observatory, the Compton
Gamma-Ray Observatory (CGRO) and the Rossi
X-Ray Timing Explorer. Unfortunately, these nearEarth spacecraft are too close to each other to
allow a definitive triangulation of burst locations.
The loss of the Pioneer Venus orbiter and
Mars Observer in the early 1990s meant that
astronomers lacked a third detector source for
accurate triangulation of deep-space GRBs. The
addition of the NEAR spacecraft to the
interplanetary network greatly increased the
probability of associating a GRB with a particular
source using optical and radio telescopes.
The GRS onboard NEAR was not originally
intended to begin its work until the spacecraft
reached Eros. However, while en route to Eros,
simple software changes to the XGRS system
allowed scientists to use the spectrometer for
GRB detection. By adding the NEAR spacecraft
to the GRB interplanetary network (IPN) and
taking advantage of significant improvements in
telemetry rate and computational capability,
NEAR helped reduce GRB detection and
triangulation times from months to seconds.
As an example, gamma ray detectors on the
NEAR and Ulysses spacecraft first recorded
gamma ray burst GRB000301C on March 1,
2000.35 Initially, the sky coordinates of the burst
were not well-defined, but with data from the
NEAR and Ulysses spacecraft, an area of the sky
about 4.2 arcminutes wide and 180 degrees in
length was identified as the potential source. A
second position from the Rossi X-Ray Timing
Explorer reduced the error to 4.2 degrees long
and 8.7 arcminutes wide. Triangulation of the
three data points further narrowed the gamma
ray emission zone to within a 50 arcminute
square, thus allowing a much quicker search of
the sky by the HST and ground-based telescopes.
Over a 15-month period from December 1999
to February 2001, the IPN, including NEAR,
detected over 100 GRBs.36 Of these, 34 were
localized rapidly and precisely enough to allow
optical and radio telescope follow-up observations. The suspected GRB emission locations were
determined with accuracies of the order of
several arcminutes. One of the most interesting
results was the detection of a GRB originating in
the southern constellation Carina. Optical
observations of an extreme red-shift indicated
that the source of the GRB was about 12.5 billion
light-years from Earth, making it the most
distant GRB yet detected.
54
Unlocking the Secrets of Eros
The NEAR spacecraft entered Eros orbit on
February 14, 2000, beginning its one-year mission
to explore Eros. Orbital characteristics ranged
from elliptical to circular and took NEAR within
35 km [22 miles] of the surface of Eros. Then,
almost six years to the day after launch,
engineers at JHUAPL brought NEAR’s mission to
its culmination with a successful controlled
descent to the surface of Eros.
Although the primary mission of NEAR was to
investigate the mineralogy, composition,
magnetic fields, geology and origin of Eros, NEAR
obtained much more detailed information during
its orbital encounter with Eros.
Images, laser altimetry and radio-science
measurements provided strong evidence that
Eros is a consolidated, yet fractured asteroid
with a regolith cover varying dramatically in
depth from near zero to as much as 100 m
[328 ft] in some areas.37 Scientists believe that
the presence of joined and well-defined craters is
indicative of cohesive strength within the
asteroid. Surface images show the geometric
relationship of grooves and cuts in the surface,
suggesting that the rock is competent and not a
loosely bound agglomeration of smaller rocks.
The gravity field on Eros appeared to be
consistent with that expected from a uniformdensity object of the same shape. The measured
density of Eros indicates that it has a bulk
porosity of 21 to 33%, implying that even though
the asteroid’s mass is uniformly distributed, it is
significantly porous and potentially fractured,
but to a lesser extent than Mathilde.
Imaging at resolutions of a few centimeters
per pixel revealed a complex and active regolith
that has been significantly modified and
redistributed by gravity-driven slope processes.
High-albedo features noted in images taken
around crater walls that slope in excess of 25°
were often 1.5 times brighter than their
surroundings, indicating recent changes in
surface characteristics due to regolith sloughing
(above right).38
Silicate mineralogy analysis performed by the
NIS was consistent with ordinary chondrite
meteorites. Spatially resolved measurements of the
asteroid’s surface provided no evidence for mineral
compositional variation. Scientists believe that the
spectral uniformity of Eros may have resulted from
a uniformly high degree of space weathering
caused by micrometeorite bombardment.
> Close-approach Eros crater wall. Material on the
inner wall of the crater in the center of the image
is brighter than the surrounding regolith and is
thought to be subsurface material exposed when
overlying, darker regolith slid off. The field of
view is 1.2 km [0.7 miles] across, taken from 38
km [24 mi] above Eros. (Images courtesy of
NASA/JHUAPL.)
The NEAR spectrographs, XRS, GRS and NIS
measured the elemental and mineral composition of Eros. Data acquired by the XRS during
orbiting showed calcium, aluminum, magnesium,
iron and silicon abundances consistent with
ordinary chondrite and certain primitive
achondrite meteorites. However, the level of
sulfur typical of chrondritic meteorites was
absent or depleted on Eros.
Although the surface of Eros appears to be
elementally homogeneous, the XRS instrument
can measure only surface composition, so it is
unknown if the sulfur depletion is a surface effect
or consistent through the core of the asteroid. If
the sulfur depletion is consistent across the bulk
of the asteroid, this would imply an association
with primitive achondrite meteorites.
The orbital GRS measurements had lower
signal levels than predicted, so the elemental
ratios with the highest precision were measured
after landing. GRS data showed the Mg/Si and
Si/O ratios and the abundance of K to be
consistent with chondritic meteorite values, but
found Fe/Si and Fe/O levels to be lower than
what would be expected with chrondritic
meteorites. Since these measurements were
Oilfield Review
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.
Spring 2006
NEAR Surface GRS Spectra
Iron
Potassium
106
Silicon
Relative intensity
made after landing and the GRS instrument can
probe tens of centimeters below the surface,
these measurements reflect a volume of about
1 m3 [35.3 ft3] around the detector. From the
GRS data alone, scientists could not determine
whether the Fe depletion is a global compositional property of Eros or a localized property of
the landing zone.
Although the XGRS system observed Eros
during a one-year orbital period, the useful time
for data collection was considerably shorter.
Engineers were limited by the angular requirements of the solar panels relative to the sun,
telemetry time and periods when the surface of
Eros was properly lit by the Sun. In the end,
scientists found that the best quality compositional data were acquired during low-altitude
orbits and after landing on Eros (right). Once
NEAR was on the surface, the gamma ray
spectrometer obtained in-situ measurements of
the regolith for a period of about 14 days.39
The surface composition of Eros suggests that
the asteroid is similar in bulk composition to a
range of meteorites that have experienced
minimal thermal alteration since their formation
at the birth of the solar system. Scientists believe
that Eros is primitive in its chemical composition
and has not experienced differentiation into a
core, mantle and crust. Differences between XRS
and GRS data in Fe/Si ratio and an apparent
deficiency of sulfur at the surface of Eros could
reflect either alteration processes in the regolith
during the last millions to billions of years or
partial melting in the first 10 million years of
solar system history.
These spectral measurements provided
scientists with a new set of questions. While the
spectral observations are consistent with an
ordinary chondritic meteorite composition, the
measurements did not establish an undisputed link
between Eros and a specific meteorite type. The
question remains whether Eros is unrelated to any
known meteorite type, or is actually a chondrite
type at depth, below the surface layers that may
have been altered by weathering processes.
Oxygen
105
Iron
Silicon
Oxygen
4
10
Iron
Outer detector
10
Inner detector
3
0
2,000
4,000
6,000
8,000
10,000
Energy, keV
1
Final Eros images: range 1,150 m (3,773 ft)
3
Final Eros images: range 250 m (820 ft)
2
Final Eros images: range 700 m (2,300 ft)
4
Last, closest image of Eros
> Landing on Eros. The location of NEAR Shoemaker’s planned landing site (top right) is shown in this
image (yellow circle) mosaic taken on December 3, 2000, from an orbital altitude of 200 km [124
miles]. NEAR’s imaging systems were recording (bottom 4 images) as the spacecraft performed a
controlled landing on the surface of Eros. At a range of 1,150 m, NEAR captured an image that spans
54 m [177 ft] of the asteroid’s surface (1). The large rock at the lower left corner of the image
measures 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 final
image 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 the
asteroid during image transmission. Once on the surface, the GRS system generated gamma ray
spectrum data for a period of seven days (graph, top left). These scientific data were the first ever
collected on the surface of an asteroid. The gamma ray instrument has two sensors (red and blue
lines) that detected clear signatures of key elements in the composition of Eros. These data, which
surpass in quality all the data accumulated by this instrument from orbit, helped the NEAR science
team relate the composition of Eros to that of meteorites that have fallen to Earth. (Images courtesy
of NASA/JHUAPL.)
55
Scientists were surprised that Eros appears
to have little or no magnetic field. Most
meteorites, including chondrites, tend to be
more magnetized than Eros. Perhaps its low
levels of iron and the fact that it never has been
heated to melting play a role in this
differentiation. The spectral homogeneity of Eros
combined with gravity-field measurements,
structural characteristics and indications of
structural coherence suggests that Eros is a
collisional fragment of a larger parent body.
The NEAR mission, a mission of many firsts in
NASA’s Discovery Program, substantially
increased our knowledge of primitive bodies in
our solar system. Although the data returned by
NEAR have revealed many secrets of asteroids,
many questions remain unanswered, and more
will be learned from future missions.
Exploring Gas Giants
The goal of the Cassini mission is to explore
Saturn, its many known moons and those yet to
be discovered. Managed by NASA’s Jet Propulsion
Laboratory (JPL) in Pasadena, California, USA,
Cassini is a joint endeavor of NASA, the
European Space Agency (ESA) and the Italian
space agency, Agenzia Spaziale Italiana (ASI). It
is one of the most ambitious efforts in planetary
space exploration.40
Because of the low level of sunlight reaching
Saturn, solar arrays are not feasible as a power
source. Engineers employed a set of radioisotopethermoelectric generators similar to those used
HDAC
HSP
40. NASA/Jet Propulsion Laboratory–Cassini Mission to
Saturn: http://www.jpl.nasa.gov/news/fact_sheets/
cassini.pdf (accessed April 13, 2006).
aph
rogr
pect
s
FUV
> Imaging Saturn’s rings. The Ultraviolet Imaging Spectrograph (UVIS) is a set
of telescopes used to measure ultraviolet light from the Saturn system’s
atmospheres, rings and surfaces. The UVIS has two spectrographic channels
or instruments: the extreme ultraviolet channel and the far ultraviolet (FUV)
channel. Each instrument is housed in aluminum cases, and each contains a
reflecting telescope, a concave grating spectrometer and an imaging, pulsecounting detector. The UVIS also includes a high-speed photometer (HSP)
channel, a hydrogen-deuterium absorption cell (HDAC) channel and an
electronic and control subassembly. (Image courtesy of NASA/Laboratory
for Atmospheric and Space Physics.)
4-m high-gain antenna
Low-gain antenna (1 of 2)
11-m magnetometer
boom
Radar bay
Radio/plasma wave
subsystem antenna
(1 of 3)
Huygens Titan
probe
Remote sensing
instruments
Radioisotope
thermoelectric
generator (1 of 3)
445 N engine (1 of 2)
> Preparing Cassini for flight. Technicians reposition and level the Cassini orbiter in the Payload Hazardous Servicing Facility at the Kennedy Space Center
in July 1997, after stacking the craft’s upper equipment section on the propulsion module (left). The orbiter’s primary systems are shown (right). (Images
courtesy of NASA/JPL.)
56
Oilfield Review
on the previous Galileo and Ulysses missions.
With these systems, heat from the natural decay
of plutonium-238 is used to generate electricity
to operate Cassini’s systems.
The Cassini spacecraft is equipped with
18 instruments, 12 on the orbiter and another six
on the Huygens probe, which is designed to
separate from the main spacecraft and
parachute through the atmosphere of Titan,
Saturn’s largest moon. The 12 instruments on the
orbiter are currently conducting in-depth studies
of Saturn, its moons, rings and magnetic
environment (previous page, bottom).
Key to Cassini’s science mission is the
Ultraviolet Imaging Spectrograph (UVIS), an
instrument based on Schlumberger sensors and
packaging, and designed to operate in harsh
environments like those found in oil and gas
logging operations (previous page, right). The
UVIS is now helping scientists determine
atmospheric chemistry, the nature of clouds and
ring systems, and the atmospheric energy
balance on Saturn and its moon Titan.
The UVIS comprises a set of telescopes that
measure ultraviolet light from the Saturn
system’s atmospheres, rings and surfaces. The
instrument has two spectrographs: the far
ultraviolet channel (FUV), 110 to 190 nm, and the
extreme ultraviolet channel (EUV), 56 to 118 nm.
The FUV and EUV channels in the UVIS
spectrometer require different detectors to
optimize sensitivity to the wavelength range
required by the Cassini project. In cooperation
with the Laboratory for Atmospheric and Space
Physics (LASP) at the University of Colorado,
Schlumberger designed the detector response to
meet these requirements.
The FUV detector was assembled using a
cesium iodide photocathode with a magnesium
fluoride window. This detector was vacuumsealed and included an integrated pump that
maintained an ultrahigh vacuum during the
spacecraft assembly and launch. Once in space,
the detector was equalized to the vacuum of
space for the voyage to Saturn.
The EUV detector utilizes a potassium
bromide photocathode and has no window since
transmission of all known substances is very poor
in this short wavelength range. Fortunately,
potassium bromide is a very robust photocathode
and can be exposed to dry air for the short time
required for testing and assembly. Once in the
vacuum of space, the detector cover was opened,
allowing light to enter the instrument.
Both detectors utilize specially selected
microchannel plates (MCP). MCP technology has
a long history in spaceflight imaging instruments.
Spring 2006
Venus swingby 04/26/98
Venus swingby 06/24/99
Saturn Arrival
07/01/04
Orbit of
Jupiter
Orbit of Earth
Deep-space maneuver
12/03/98
Orbit of Venus
Launch 10/15/97
Orbit of
Saturn
Jupiter swingby
12/30/00
Earth swingby 08/18/99
> Launching Cassini. A Titan IVB/Centaur launch vehicle propelled the Cassini spacecraft and its attached
Huygens probe into space from Cape Kennedy Air Station’s Launch Complex 40, Florida. Visible in this
view 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 the
different planets are designed to increase the spacecraft’s velocity relative to the Sun so it can reach
Saturn. With the gravity-assist trajectory, it took more than 61⁄2 years for the Cassini spacecraft to
arrive at Saturn. (Images courtesy of NASA.)
Quality-control procedures during manufacturing
allowed only MCPs with very low-defect densities
to be used for final assembly. Once an MCP was
available, LASP and Schlumberger scientists
worked together during the final assembly
process. The units were then transported to
NASA laboratories for final testing.
Two FUV and two EUV detectors that met the
stringent quality requirements for space travel to
Saturn were assembled at the Schlumberger
Princeton Technology Center (PTC) in New
Jersey. One pair of detectors was designated as
flight units while the second set was kept in
reserve as a backup.
The UVIS also includes a high-speed
photometer (HSP) channel, a hydrogendeuterium absorption cell (HDAC) channel and
electronic and control subassemblies. Scientists
are using the HSP to make stellar occultation
measurements of the structure and density of
material in Saturn’s rings.
Cassini was launched on October 15, 1997,
from Cape Kennedy, Florida, aboard a Titan
IVB/Centaur rocket, the most powerful launch
vehicle in the US fleet (above). After Cassini was
placed in orbit around Earth, the upper stage
fired to send Cassini on an interplanetary
trajectory that would eventually deliver the
spacecraft to Saturn.
57
Cassini flew twice past Venus, then once past
Earth and Jupiter. The spacecraft’s speed
relative to the Sun increased as it approached
and swung around each planet, giving Cassini the
cumulative boost it needed to reach Saturn with
minimal fuel consumption. After reaching
Saturn, Cassini fired its main engine for about 96
minutes, reducing the spacecraft’s speed and
allowing it to be captured in an orbit around
Saturn. On January 5, 2005, Cassini released the
European-built Huygens probe toward Titan.
> Titan image. In this infrared view of Titan, features on the leading hemisphere
are shown, including the bright, crescent-shaped Hotei arcus (right of center),
often referred to as “the smile” by researchers. The view is centered on the
bright region called Xanadu. The image was taken with the Cassini spacecraft
narrow-angle camera using a spectral filter sensitive to wavelengths of
infrared light centered at 938 nm. The image was acquired at a distance of
approximately 1.3 million km [800,000 miles] from Titan. (Image courtesy of
NASA/JPL/Space Science Institute.)
Entry speed: 6.2 km/s
1,000
Peak deceleration: 10gn to 25gn
500
300
Main chute deploys
Altitude, km
192
Instrument
inletDecelerator
port
jettisons
opens
170
Drogue chute deploys
Probe
performs
surface
science
0
0
2.5
Time, hours after entry
> Descent to Titan. The Huygens probe analyzed Titan’s atmosphere and
recorded a significant amount of data and images on its journey to the
surface of Titan. (Image courtesy of NASA/JPL.)
58
Journey to a Distant Moon
With a diameter larger than the planet Mercury,
Titan is one of the most interesting moons in the
solar system. The surface of this moon lies
hidden beneath an opaque atmosphere more
than 50% denser than that of Earth (left).
Titan’s atmosphere is filled with a brownishorange haze composed of complex organic
molecules falling like rain from the sky to the
surface. Most scientists agree that conditions on
Titan are too cold for life to have evolved—
although there are theories concerning the
possibility of life forms in covered lakes of
liquid hydrocarbons warmed by the planet’s
internal heat.
The Huygens probe entered Titan’s atmosphere on January 14, 2005, deployed its
parachutes and began its scientific observations
during a descent through the moon’s dense
atmosphere lasting close to 21⁄2 hours (below
left).41 Instruments onboard the probe detected a
surface temperature of 94K at the landing site.
Images taken by the probe while descending
showed surface channels that appeared to
indicate rain or fluid flow, possibly in the form of
liquid methane. Ridges as tall as 100 m were
observed near the landing area (next page, top).
High quantities of methane were detected in the
lower atmosphere, with nitrogen predominating
in the upper atmosphere. Oxygen was not
detected probably because it is tied up as frozen
water. This would also prevent the formation of
carbon dioxide.
Laboratory tests recreated the impact
measurements derived from the onboard
penetrometer. These tests indicate that the
surface in the landing area may be composed of
fine particles with a thin crust. Accelerometer
measurements suggest the probe settled 10 to
15 cm [4 to 6 in.] into the surface. Heat from
instruments then evaporated liquid methane in
the soil and released it around the spacecraft as
methane gas. The Huygens probe continued
Oilfield Review
making measurements and transmitting data to
Cassini for 72 minutes after landing until power
limitations and deterioration of the spacecraft
due to extreme surface conditions on Titan
resulted in loss of signal.
Exploring the Ringed Planet
Aside from Titan, more moons of greater variety
orbit Saturn than any other planet. So far,
observations from Earth and by spacecraft have
found Saturnian satellites ranging from small
asteroid-size bodies to those as large as Titan.
Saturn is the second-largest planet in the
solar system. Like the other gaseous outer
planets—Jupiter, Uranus and Neptune—it has
an atmosphere made up mostly of hydrogen and
helium, and like them, it is ringed. Saturn’s
distinctive bright rings are made up of ice and
rock particles ranging in size from grains of sand
to small houses.
Although the face of Saturn appears calm, the
planet has a windswept atmosphere where an
equatorial jet stream blows at 1,800 km/h
[1,118 mi/h], and swirling storms churn beneath
the cloud tops. Early explorations by NASA’s
Pioneer 11 spacecraft in 1979, and the Voyager 1
and 2 spacecraft in 1980 and 1981, found Saturn
to have a huge and complex magnetic environment where trapped protons and electrons
interact with each other, the planet, the rings
and the surfaces of many of Saturn’s moons.
From Earth, Saturn’s rings appear as only a
few monolithic bands, while in reality, they
consist of thousands of rings and ringlets, with
particles sometimes arranged in complicated
orbits by the gravitational interaction of small
moons previously unseen from Earth (right).
Scientists are using data from the UVIS in
detailed computer models to simulate the
complex motion of these rings.
Second in size only to Jupiter, Saturn has
more than 750 times the volume of Earth.
Combined with the planet’s low density,
less than half that of water, its fast rotation
promotes a bulge of material near the equator.
Saturn is shaped like a flattened ball; its pole-topole diameter is only 108,728 km [67,560 miles],
compared to about 120,536 km [about
74,898 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).
Spring 2006
> Under Titan’s atmosphere. The perspective view of the surface of Titan near the Huygens probe
landing site (top) is color-coded, with blue the lowest altitude and red the highest. The total area
covered by the image is about 1 by 3 km [0.6 by 2 miles]. A pair of images (inset) was acquired from
the 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 the
medium-resolution imager. (Images courtesy of ESA/NASA/JPL/University of Arizona/USGS.)
> Exploring Saturn’s rings. Images taken during the Cassini spacecraft’s orbit around Saturn show
compositional variation in Saturn’s rings (top). The red in the image indicates sparser ringlets that
probably 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 was
taken with the Ultraviolet Imaging Spectrograph (UVIS) instrument, which is capable of resolving the
rings to show features up to 97 km [60 mi] across, roughly 100 times the resolution of ultraviolet data
obtained by the Voyager 2 spacecraft. The false-color view of Saturn’s A ring (bottom left) was also
taken 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 the
Keeler Gap. The insert (bottom right) is a computer simulation about 150 m [490 ft] across, illustrating
a clumpy region of icy particles in the A ring. (Images courtesy of NASA/JPL/University of Colorado.)
59
Unlike rocky inner planets such as Earth,
Saturn has no surface on which to land. A
spacecraft descending into its atmosphere would
simply find the surrounding gases becoming
denser, and the temperature progressively
hotter; eventually the craft would be crushed and
melted. Detailed analysis of Saturn’s gravitational field leads astronomers to believe that the
deepest interior of Saturn must consist of a
molten rock core about the same size as the
planet Earth, but much denser.
Spectroscopic studies by the Voyager
spacecraft found Saturn to be made up of about
94% hydrogen and 6% helium. Hydrogen and
helium are the primary constituents of all the
giant gas planets, the Sun and the stars. Gravity
at the top of Saturn’s clouds is similar to that
near the surface of Earth. The temperature near
the cloud tops is about -139°C [-218°F],
increasing toward the planet’s core due to
increased atmospheric pressure. At the core,
Saturn’s temperature is predicted to be about
10,000°C [18,000°F].
On June 21, 2005, the UVIS detected auroral
emissions from both Saturn’s northern and
southern poles (above right).42 These emissions
are believed to be similar to Earth’s Northern
Lights yet are invisible to the naked eye.
Ultraviolet images captured the entire oval of the
auroral emissions from hydrogen gas excited by
electron bombardment. Time-lapse images
indicate that aurora lights are dynamic,
responding rapidly to changes in the solar wind.
New Moons
There were only 18 known moons orbiting Saturn
when the Cassini spacecraft began its mission to
Saturn in 1997. During Cassini’s seven-year
journey, Earth-based telescopes uncovered 13
more moons. Soon after the spacecraft reached
Saturn, the Cassini team discovered two more
tiny moons, Methone and Pallene. The two new
moons are approximately 3 km [1.8 miles] and
4 km [2.5 miles] across.
Scientists suspected that more tiny Saturnian
moons might be found within the gaps in Saturn’s
rings. On May 1, 2005, using a sequence of timelapse images from Cassini’s cameras, astronomers confirmed the presence of a tiny moon
hidden in a gap in Saturn’s A ring.43 The images
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 an
Active, 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.
60
> The southern lights of Saturn. Images of Saturn obtained by Cassini’s UVIS
show auroral emissions at its poles similar to Earth’s Northern Lights. The two
UV 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 show
similar emissions at Saturn’s north pole. In these false-color images, blue
represents aurora emissions from hydrogen gas excited by electron
bombardment, while red-orange represents reflected sunlight. These images
were taken 1 hour apart; during this time the brightest spot in the left aurora
image fades and a bright spot appears in the middle of the aurora in the right
image. (Images courtesy of NASA/JPL/University of Colorado.)
Moon
Perturbations
caused by moon
> Perturbations caused by a tiny moon. This image confirmed earlier suspicions that a small moon
was 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 main
rings. 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, when
they saw perturbations in the ring structure of the Keeler Gap’s outer edge. These images were
obtained with the Cassini spacecraft narrow-angle camera on May 1, 2005, at a distance of
approximately 1.1 million km [680,000 miles]. (Image courtesy of NASA/JPL/Space Science Institute.)
show the tiny object in the center of the Keeler
Gap and the wavy patterns in the gap edges that
are generated by the moon’s gravitational
influence (above).
The new object, Daphnis, is about 7 km
[4 miles] across and reflects about half the light
falling on it—a brightness that is typical of the
particles in the nearby rings. As Cassini
continues to explore Saturn and its moons,
scientists expect to uncover more of the secrets
of this vast planetary system.
Signs of an Atmosphere
Although the moon Enceladus is covered with ice
composed of water, like Saturn’s other moons, it
displays an abnormally smooth surface with very
few impact craters. With a diameter of only
500 km [310 mi], Enceladus would fit into
the state of Arizona. Yet despite its small size,
Enceladus exhibits one of the most interesting
surfaces of all the icy satellites. Enceladus
reflects about 90% of the incident sunlight as if
Oilfield Review
Enceladus
Hot plasma flow
Saturn
Vapor cloud
> Shifting magnetic fields. This artist’s conception shows the detection of a
dynamic atmosphere on Saturn’s icy moon Enceladus. The Cassini
magnetometer is designed to measure the magnitude and direction of the
magnetic fields of Saturn and its moons. During Cassini’s three close flybys
of Enceladus on Febuary 17, March 9, and July 14, 2005, the instrument
detected a bending of the magnetic field around Enceladus thought to be
caused by electric currents generated by the interaction of atmospheric
particles and the magnetosphere of Saturn. The graphic shows the magnetic
field observed by Cassini, as well as the predicted vapor cloud being vented
from the south pole of Enceladus. Cassini’s magnetometer observed bending
of the magnetic field consistent with its draping around a conducting object.
(Image courtesy of NASA/JPL.)
A
B
Lambda Scorpius
Starlight
Starlight
Bellatrix
July
February
> Indications of an atmosphere. On July 11, 2005, the Cassini ultraviolet imaging spectrograph
observed the star Bellatrix as it passed behind Enceladus, as seen from the spacecraft. The starlight
was observed to dim when it got close to Enceladus, indicating the presence of an atmosphere
isolated to the southern pole (A). The ultraviolet imaging spectrograph indicated that the atmosphere
was water vapor, based on absorption features in the spectrum of the star. The colors show the
undimmed star signal (blue) versus the dimmed star signal (red). As Bellatrix reemerged from behind
Enceladus, there was no observed dimming of the starlight. In another occultation (B) of the star
Lambda Scorpius, no sign of an atmosphere was detected, implying that the atmosphere is localized
toward the southern pole. (Image courtesy of NASA/JPL/Space Science Institute.)
Spring 2006
covered with fresh-fallen snow, placing it among
the most reflective objects in the solar system.
Although Enceladus was previously thought to be
a cold and dead rock mass, data from the Cassini
spacecraft indicate evidence of ice volcanism,
which might explain its smooth surface features.
In July 2005, Cassini’s instruments detected a
cloud of water vapor over the moon’s southern
pole and warm fractures where evaporating ice
probably supplies the vapor cloud.44 So far,
Enceladus is the smallest body found that
displays evidence of active volcanism. Scientists
theorize that warm spots in the moon’s icy and
cracked surface are probably the result of heat
from tidal energy like the volcanoes on Jupiter’s
moon Io. Its geologically young surface of waterbase ice, softened by heat from below, resembles
areas on Jupiter’s moons, Europa and Ganymede.
Cassini flew within 175 km [109 miles] of
Enceladus on July 14, 2005. Data collected
during that flyby confirm an extended and
dynamic atmosphere. This atmosphere was first
detected by Cassini’s magnetometer during a
distant flyby earlier in 2005 (above left).
Cassini’s magnetometer detected disturbances in the magnetic field caused by small
currents of ionized gas from the atmosphere
around this moon. These could be detected by
the instrument long before imaging instruments
could be applied to confirm this finding.
As Cassini approached this small body,
imaging instruments were able to make
measurements that showed gas composition,
further confirming the presence of an
atmosphere. The ion and natural mass
spectrometers and the UVIS showed that the
southern atmosphere contains water vapor
(left). The mass spectrometer found that water
vapor comprises about 65% of the atmosphere,
with molecular hydrogen at about 20%. The rest
is mostly carbon dioxide and some combination
of molecular nitrogen and carbon monoxide. The
variation of water-vapor density with altitude
suggests that the water vapor may come from a
localized source comparable to a geothermal hot
spot. The ultraviolet results strongly suggest a
local vapor cloud. The fact that the atmosphere
persists on this low-gravity world, instead of
instantly escaping into space, suggests that the
moon is geologically active enough to replenish
the water vapor at a slow, continuous rate.
High-resolution images show that the south
pole has an even younger and more fractured
appearance than the rest of Enceladus, complete
61
> Imaging Enceladus. This view (top left) is a mosaic of four high-resolution images taken by the
Cassini spacecraft narrow-angle camera during its close flyby of Saturn’s moon Enceladus. The view
is 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 to
10,833 miles]. The southern polar terrain of Enceladus (bottom left) appears strewn with great
boulders 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 approximately
208 km [129 miles]. The enhanced color view of Enceladus (right) is principally of the southern
hemisphere. 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 that
includes images taken at wavelengths from the ultraviolet to the infrared portion of the light spectrum.
(Images courtesy of NASA/JPL/Space Science Institute.)
Enceladus Temperature Map
Temperature, kelvin
85
80
75
70
65
Predicted temperatures
Observed temperatures
> A hot southern pole. This map represents the surface temperature of
Enceladus as seen by the composite infrared spectrometer. The observed
temperatures included an unexpected hot spot at the south pole. On average
the region is 15K warmer than expected; in some places hot spots greater
than 140K were observed. The hottest spots line up with the blue fracture
stripes visible in the previous image (above). (Images courtesy of NASA/JPL/
Goddard Space Flight Center.)
45. Sagan C: Cosmos. New York City: Carl Sagan
Productions and Random House (1980): 4.
62
with icy boulders the size of large houses and
long, bluish cracks or faults (left).
Another Cassini instrument, the composite
infrared spectrometer (CIRS), demonstrates
that the southern pole is warmer than
anticipated (below left). Temperatures near the
equator were found to reach a frigid 80K.
Scientists believe that the poles should be even
colder because of the low level of energy received
from the Sun. However, south polar average
temperatures reached 85K, much warmer than
expected. Small areas of the pole, concentrated
near the fractures, are even warmer: higher than
140K in some places.
Scientists find the temperatures difficult to
explain if sunlight is the only heat source. More
likely, a portion of the polar region, including
observable fractures, is warmed by heat escaping
from the interior. Evaporation of this “warm” ice
at several locations within the region could
explain the density of the water-vapor cloud
detected by Cassini’s instruments. How a 500-km
[310-mile] diameter moon can generate this
much internal heat and why it is concentrated at
the southern pole are still a mystery.
Similar to multiple well-logging instruments
working together deep beneath the Earth’s
surface, the discovery of an atmosphere on
Enceladus resulted from an array of different
sensors working in synergy to acquire data and
maximize scientific value.
The Challenge of Space
Advances in technology, particularly during the
last 100 years, have helped change the way we
view the Earth, our solar system and the universe
beyond. From the E&P industry’s early beginnings, engineers, geoscientists and many other
dedicated men and women have led the way in
exploration of our inner space environment.
Today, this same innovative spirit, and in many
cases, similar technologies, are taking us beyond
the confines of Earth’s environment into the vast
unknowns of outer space.
The examples presented in this article are
just a few of the contributions made by the
oilfield service industry to space exploration. In
the future, we can expect to see more terrestrial
technology applied in the quest for extraterrestrial understanding. The late astrophysicist
Carl Sagan wrote, “Imagination will often carry
us to worlds that never were. But without it, we
go nowhere.”45 It is this imagination and
creativity that have driven the E&P industry to
explore deep beneath the Earth’s surface and
that will inevitably launch the first drilling
expeditions to Mars and beyond.
—DW
Oilfield Review