JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998) 95
NEAR OVERVIEW
T
NEAR Overview
Andrew F. Cheng, Robert W. Farquhar, and Andrew G. Santo
he Near Earth Asteroid Rendezvous (NEAR) mission inaugurates NASA’s
Discovery Program. It will be the first to orbit an asteroid and will make the first
comprehensive scientific measurements of an asteroid’s surface composition, geology,
physical properties, and internal structure. NEAR was launched successfully on 17
February 1996 aboard a Delta II-7925. It made the first reconnaissance of a C-type
asteroid during its flyby of the main-belt asteroid 253 Mathilde in June 1997 and will
orbit the unusually large near-Earth asteroid 433 Eros for about a year at a minimum
orbit radius of about 35 km from the center of the asteroid. NEAR will obtain new
information on the nature and evolution of asteroids, improve our understanding of
planetary formation processes in the early solar system, and clarify the relationship
between asteroids and meteorites. The NEAR Mission Operations Center and Science
Data Center are both located at APL. The latter will maintain the entire NEAR data
set on-line and will make data from all instruments accessible over the Internet to
every member of the NEAR science team.
(Keywords: Asteroids, Discovery Program, NEAR mission.)
INTRODUCTION
Of the more than 7000 asteroids that have been
named, most are found in the main asteroid belt between
the orbits of Mars and Jupiter, but those that
come within 1.3 AU of the Sun are known as near-
Earth asteroids. The orbits of these dynamically young
bodies have evolved on 100-million-year timescales
because of collisions and gravitational interactions
with planets. The present-day orbits of such asteroids
do not necessarily indicate where they formed. Some
are already in Earth-crossing orbits, and those that are
not are highly likely to evolve into one. About 250
near-Earth asteroids are known, and they appear to
typify a broad sample of the main-belt population.
Our current knowledge of the nature of asteroids
comes from three sources: Earth-based remote sensing,
data from the Galileo spacecraft flybys of the two
main-belt asteroids 951 Gaspra and 243 Ida, and laboratory
analyses of meteorites. Most meteorites are
believed to be collisional fragments of asteroids, but
they may represent a biased and incomplete sampling
of the materials actually found in near-Earth asteroids.
Firm links between meteorite types and asteroid types
have been difficult to establish.1 The uncommon
eucrite (a basaltic achondrite) meteorite has been
linked by visible and near-infrared reflectance measurements
to the relatively rare V-type asteroid.2,3
A. F. CHENG, R. W. FARQUHAR, AND A. G. SANTO
96 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998)
However, controversy remains over whether and how
the most common meteorite types (the ordinary chondrites)
may be linked to the most common asteroid
types (the S-type or stony asteroids) in the inner part
of the asteroid belt.4,5 (Gaspra, Ida, and the NEAR
target 433 Eros are all S-type asteroids.)
The S-type asteroids are a diverse class of objects
known to contain the silicate minerals olivine and
pyroxene plus an admixture of iron/nickel metal. Some
appear to be fragments of bodies that underwent substantial
melting and differentiation. Others may consist
of primitive materials like ordinary chondrites that
never underwent melting and that may preserve characteristics
of the solid material from which the inner
planets accreted.
The Galileo flybys provided the very first highresolution
images of asteroids, revealing complex surfaces
covered by craters, fractures, grooves, and subtle
color variations.6,7 Galileo also discovered a satellite at
Ida, which is a member of the Koronis family (Eros is
not an asteroid family member). The near-infrared
spectrum of Gaspra indicates a high olivine abundance
such that it is inferred to be a fragment of a differentiated
body. Conversely, Ida and Eros display infrared
spectra that may be consistent with a silicate mineralogy
like that in ordinary chondrites.8,9 The Galileo
instrument complement did not include any capability
to measure elemental composition, and debate continues
about whether ordinary chondrites are related to
S-type asteroids.
The NEAR mission will spend about a year in orbit
around Eros. It will make the first comprehensive,
spatially resolved measurements of the geology, mineralogy,
and elemental composition of an S-type asteroid.
NEAR will orbit Eros at low altitude, about 1 body
radius above the surface. This long-duration, lowaltitude
orbit will allow NEAR instruments to measure
the asteroid’s abundant key elements with a maximum
spatial resolution of about 2 km, more than an order
of magnitude greater than Galileo. The NEAR data,
especially when combined with those from the Galileo
flybys, will greatly advance our understanding of Stype
asteroids and their possible relationships to other
small bodies of the solar system. NEAR will also map
the entire surface of the asteroid in the near infrared
and make a thorough search for satellites of Eros.
SCIENCE OBJECTIVES
The overall objectives of the NEAR mission are to
rendezvous with a near-Earth asteroid, achieve orbit
around such an asteroid, and conduct the first systematic
scientific exploration of a near-Earth asteroid.
NEAR will study the nature and evolution of S-type
asteroids, improve our understanding of processes and
conditions relevant to the formation of planets in the
early solar system, and clarify the relationship between
asteroids and meteorites.
Specific science questions to be addressed by NEAR
are as follows.
• What are the morphological and textural characteristics
of the asteroid surface, and how do they compare
with those on larger bodies?
• What is the elemental and mineralogical composition
of the asteroid? Is there evidence of compositional
or structural heterogeneity?
• Is the asteroid a solid fragment of a larger parent body
or a rubble pile?
• Is the asteroid’s precursor body(ies) primitive or
differentiated?
• Is there evidence of past or present cometary activity?
• Is the asteroid related to a meteorite type or types?
• Does an intrinsic magnetic field exist? What is it like?
• Does the asteroid have any satellites, and how might
they compare with Eros?
SPACECRAFT DESIGN
NEAR is a solar-powered, three-axis–stabilized
spacecraft10 with a launch mass, including propellant,
of 805 kg and a dry mass of 468 kg. The spacecraft is
simple and highly redundant (Fig. 1). It uses X-band
telemetry to the NASA deep space network; data rates
at Eros will be selectable in the range of 2.9 to 8.8 kbps
using a 34-m high-efficiency antenna. With a 70-m
antenna, the data rates from Eros will range from 17.6
to 26.5 kbps. The command and telemetry systems are
fully redundant. Two solid-state recorders are accommodated
with a combined memory capacity of 1.8
Gbit.
Spacecraft attitude is determined using a star camera,
a fully redundant inertial measurement unit, and
Figure 1. Structural view of the NEAR flight configuration after
deployment.
Solar panels
Propulsion
subsystem
X-ray solar
monitors
(not shown)
Forward
deck
Aft deck
Side panels
Spacecraft
adapter
JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998) 97
NEAR OVERVIEW
redundant digital Sun sensors. The propulsion subsystem
is dual mode (hydrazine is used as fuel for both
the monopropellant and bipropellant systems) and
includes one 450-N bipropellant thruster for large
maneuvers, four 21-N thrusters, and seven 3.5-N
thrusters for fine velocity control and momentum
dumping. Attitude can be controlled by a redundant
set of four reaction wheels or by the thruster complement
to within 1.7 mrad. NEAR’s line-of-sight pointing
stability is within 50 mrad over 1 s, and postprocessing
attitude knowledge is within 50 mrad.
Forward and aft aluminum honeycomb decks are
connected with eight aluminum honeycomb side panels.
Mounted on the outside of the forward deck are
a fixed, 1.5-m-dia. X-band high-gain antenna (HGA),
four fixed solar panels, and the X-ray solar monitor
system. When the solar panels are fully illuminated,
the Sun is in the center of the solar monitor field of
view (FOV). No booms are accommodated on the
spacecraft. The electronics are mounted on the inside
of the forward and aft decks.
NEAR contains six scientific instruments, which
are detailed in the next section.
1. Multispectral Imager (MSI)
2. Near-Infrared Spectrograph (NIS)
3. X-Ray Spectrometer (XRS)
4. Gamma-Ray Spectrometer (GRS)
5. NEAR Laser Rangefinder (NLR)
6. Magnetometer (MAG)
The MAG is mounted on top of the HGA feed,
where it is exposed to the minimum level of spacecraftgenerated
magnetic fields. The remaining instruments
(MSI, NIS, XRS, GRS, and NLR) are all mounted on
the outside of the aft deck. They are on fixed mounts
and are co-aligned to view a common boresight direction.
The NIS has a scan mirror that allows it to look
more than 90° away from the common boresight.
Key properties of the mission design permit the use
of this fixed spacecraft geometry. Throughout the
course of the rendezvous with Eros, the angle between
the Sun and the Earth, as seen from the spacecraft, will
remain less than about 30°. In addition, the mission
aphelion will be reached during cruise. Hence, if the
solar panels are sufficiently large to sustain NEAR at
aphelion, there will be sufficient power margin at Eros
for the spacecraft to pull its solar panels 30° off full
illumination to point the HGA at Earth. Moreover,
the rendezvous orbit plane will be maintained so that
the orbit normal points approximately at the Sun. In
this case, as NEAR orbits Eros, it will be able to roll
around the HGA axis so as to keep the instruments
pointed at the asteroid. (The instruments are usually
pointed away from the asteroid when the HGA is used
to downlink to Earth.) This mode of operation motivated
the requirement for onboard data storage. With
onboard image compression, NEAR can store more
than 1000 images and downlink them within 10 h at
its maximum data rate of 26.5 kbps.
The spacecraft is designed using a distributed architecture
(Fig. 2), partitioned so that subsystems do not
share common hardware or software. One major benefit
of this approach is that careful design of interfaces
allows development, test, and integration of subsystems
in parallel. In addition, this architecture has
a natural advantage of built-in contingencies and
design margins. Truly parallel subsystem development
requires independence at the subsystem interface,
through careful partitioning of functional requirements
and ample design margins at subsystem interfaces.
On NEAR, subsystems are interfaced through a
MIL-STD-1553 data bus, chosen because it is compatible
with many off-the-shelf industry components.
The data bus has additional attractive features: fewer
interconnecting cables; built-in redundancy and
cross-strapping; simplification of interface definition;
a fault-tolerant, transformer-coupled interface; a common
data architecture for sharing information among
subsystems; and a flexible software-defined interface
instead of a rigid hardware-defined interface.
Measured in constant dollars, NEAR is the lowestcost
U.S. planetary mission ever. The spacecraft’s
27-month development schedule was unusually rapid.
The distributed architecture and the selection of the
1553 data bus were key to developing NEAR on time
and under budget. Previous planetary missions have
not used a distributed architecture because they have
been optimized for performance, i.e., to return maximum
science within available technology. The distributed
architecture approach comes with a mass
penalty, and therefore a performance penalty: some
hardware that can be combined at the system level is
duplicated at the subsystem level. The distributed
architecture approach for NEAR features interface
margin and testability, optimizing the spacecraft for
low cost and rapid schedule. Nevertheless, the performance
penalty is minuscule, and the mass penalty for
using the distributed architecture approach is only
about 10 kg.
INSTRUMENT TASKS
Details on the many science objectives of the
NEAR instruments can be found elsewhere.11–16 A
brief summary of instrument characteristics is given in
this section. (Full descriptions of each science investigation
and instrument will appear in a special upcoming
issue of Space Science Reviews.) Detailed instrument
descriptions and results of ground and
in-flight calibrations appear in the companion articles
of this issue of the Technical Digest.
A. F. CHENG, R. W. FARQUHAR, AND A. G. SANTO
98 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998)
Multispectral Imager
The main goals of the MSI are to determine the
shape of Eros and to map the mineralogy and morphology
of features on its surface at high spatial resolution.
MSI is a 537 ´ 244 pixel charge-coupled device camera
with five-element radiation-hardened refractive optics.
It covers the spectral range from 0.4 to 1.1 mm,
and it has an eight-position filter wheel containing
filters chosen to optimize the sensitivity of MSI to
minerals expected or known to occur on Eros. The
camera has an FOV of 2.90 ´ 2.25° and a pixel resolution
of 95 ´ 161 mrad. It has a maximum framing
rate of 1 per second with images digitized to 12 bits
and a dedicated digital processing unit with an image
buffer in addition to both lossless and lossy onboard
image compression.
Near-Infrared Spectrograph
The NIS will measure the spectrum of sunlight reflected
from Eros in the near-infrared range from 0.8
to 2.6 mm in 64 channels to determine the distribution
and abundance of surface minerals like olivine and
pyroxene. This grating spectrometer disperses the light
from the slit FOV (0.38 ´ 0.76° in its narrow position
and 0.76 ´ 0.76° in the wide position) across a pair of
passively cooled one-dimensional array detectors, one a
germanium array covering the lower wavelengths 0.8 to
1.5 mm and the other an indium/gallium-arsenide array
covering 1.3 to 2.6 mm. The slit can be closed for dark
current measurement. The NIS has a scan mirror that
enables it to view the common boresight direction
(within the MSI FOV) or directions more than 90°
away. Spectral images can be built up by a combination
of scan mirror and spacecraft motions. In addition, the
NIS has a gold calibration target that can be positioned
to scatter sunlight into the instrument and provide a
quantitative, in-flight spectral calibration.
X-Ray Spectrometer
The XRS is an X-ray resonance fluorescence spectrometer
that detects the characteristic X-ray line
emissions excited by solar X-rays from major elements
in the asteroid’s surface. It covers X-rays in the energy
range from 1 to 10 keV using three gas proportional
counters. The balanced, differential filter technique is
used to separate the closely spaced Mg, Al, and Si lines
lying below 2 keV. The gas proportional counters directly
resolve higher-energy line emissions from Ca
Power
amplifier
GRS/XRS
data processor
unit
Propulsion
subsystem
Attitude
interface
unit
Reaction
wheels
NLR
NIS/MAG
data processor
unit
MSI
data processor
unit
RF switch
assembly
Command/
telemetry
processor
X-band
transponder Diplexer
Solid-state
recorders
Power
switch
Power
amplifier
Command/
telemetry
processor
X-band
transponder Diplexer Star
camera
Flight
computers
High-gain
antenna
Mediumgain
antenna
Low-gain
antennas
GRS/XRS
Sun
sensors
NIS
MAG
MSI
Command and
data handling
1553 data bus
Guidance and
control 1553
data bus
Shunts
Power
system
electronics
L
28 ± 6 V DC bus
9-A•h
NiCd
battery
Inertial
measurement
unit
Figure 2. Block diagram illustrating the NEAR spacecraft’s distributed architecture.
JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998) 99
NEAR OVERVIEW
and Fe. A mechanical collimator gives the XRS a 5°
FOV, with which it will map the chemical composition
of the asteroid at spatial resolutions as great as 2 km
in the low orbits. It also includes a separate solar
monitor system to measure continuously the incident
spectrum of solar X-rays, using both a gas proportional
counter and a high-spectral-resolution silicon X-ray
detector. The XRS performs in-flight calibration using
a calibration rod with Fe-55 sources that can be rotated
into or out of the detector FOV.
Gamma-Ray Spectrometer
The GRS detects characteristic gamma rays in the
0.3- to 10-MeV range that are emitted from specific
elements in the surface. Some of these emissions are
excited by cosmic rays and some arise from natural
radioactivity in the asteroid. The GRS uses a bodymounted
passively cooled NaI scintillator detector
with a bismuth germanate anticoincidence shield that
defines a 45° FOV. Abundances of several important
elements such as K, Si, and Fe will be measured in four
quadrants of the asteroid.
NEAR Laser Rangefinder
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. It uses a chromium/neodymium/yttriumaluminum-
garnet (Cr-Nd-YAG) solid-state laser and a
compact reflecting telescope. It sends a small portion
of each emitted laser pulse through
an optical fiber of known length
and into the receiver, providing a
continuous in-flight calibration of
the timing circuit. The ranging
data will be used to construct a
global shape model and a global
topographic map of Eros with horizontal
resolution of about 300 m.
The NLR will also measure detailed
topographic profiles of surface
features on Eros with a best
spatial resolution of about 5 m. The
topographic profiles will enhance
and complement the study of surface
morphology from imaging.
Magnetometer
This fluxgate magnetometer
uses ring core sensors made of highly
magnetically permeable material.
MAG will search for and map
intrinsic magnetic fields of Eros.
The recent Galileo flybys of the S-type asteroids Gaspra
and Ida revealed that both of these bodies are magnetic,
although the evidence is ambiguous.17 Discovery of an
intrinsic magnetic field at Eros would be the first definitive
detection of magnetism at an asteroid and
would yield important insights about its thermal and
geological history.
Radio Science
In addition to the six major instruments, a coherent
X-band transponder will be used to conduct a radio
science investigation by measuring the Doppler shift
from the spacecraft’s radial velocity component relative
to the Earth. Accurate measurements of the
Doppler shift as the spacecraft orbits Eros will allow us
to map the asteroid’s gravity field. In conjunction with
MSI/NIS and NLR data, gravity determinations will be
combined with global shape and rotation state data to
constrain the internal density structure of Eros, possibly
revealing heterogeneity.
MISSION PROFILE
The NEAR spacecraft was successfully launched in
February 1996, taking advantage of the unique alignment
of Earth and Eros that occurs only once every
7 years.18 A Delta-II 7925 rocket placed NEAR into
a 2-year DV (trajectory correction maneuver)/Earth
gravity-assist trajectory (DVEGA, Fig. 3). This trajectory
represents a new application of the DVEGA
technique: Instead of using an Earth swingby maneuver
to increase the aphelion of the spacecraft
Figure 3. NEAR trajectory profile ( C3 = launch energy).
Sun
Mathilde flyby
(27 Jun 1997)
V = 9.9 km/s
(433) Eros
orbit
Earth
orbit
Deep space
maneuver
(3 Jul 1997)
D V = 0.28 km/s
Earth swingby
(23 Jan 1998)
(478-km altitude)
Launch
(17 Feb 1996)
C3 = 26 km2/s2
(253) Mathilde
orbit
Eros arrival
(10 Jan 1999)
D V = 0.95 km/s
A. F. CHENG, R. W. FARQUHAR, AND A. G. SANTO
100 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998)
trajectory, the maneuver actually decreases the aphelion
distance while increasing the inclination from 0
to about 10°. The circuitous 3-year flight path to Eros
is the result of a Discovery Program requirement to use
an inexpensive, but less capable, launch vehicle. With
a larger launch vehicle such as an Atlas or Titan, a 1-
year direct trajectory could have been used, but the
total mission cost would have increased by at least $50
million. NEAR’s total mission cost, including the
launch vehicle, is about $210 million.
The Mathilde encounter occurred 1 week before
the deep space maneuver on 3 July 1997. The Earth
swingby occurred on 23 January 1998. Rendezvous
operations at Eros are scheduled to begin on 20 December
1998. The initial close encounter with Eros
(distance »500 km) will occur on 10 January 1999.
Later, as NEAR is maneuvered closer to Eros, it will
conduct science investigations from orbits that come
as close as 15 km to the asteroid. In the last 2 months
of the orbital phase, the minimum altitude will be
lowered further, and on 6 February 2000 NEAR will
attempt a soft landing on Eros.
Mathilde Flyby
Asteroid 253 Mathilde was discovered on 12 November
1885 by Johann Palisa in Vienna, Austria. The
name was suggested by V. A. Lebeuf (1859–1929), a
staff member of the Paris Observatory, who first computed
an orbit for the new asteroid. The name is
thought to honor the wife of astronomer Moritz Loewy
(1833–1907), then the vice director of the Paris
Observatory. Although Mathilde’s existence has been
known since 1885, it was only following the announcement
of NEAR’s possible flyby that extensive
physical observations were carried out using telescopes
on Earth. These showed that Mathilde was an unusual
object, especially because of its rotation, which is at
least an order of magnitude slower than typical mainbelt
asteroids.
Using a series of observations of this asteroid made
in the first half of 1995, Stefano Mottola and his
colleagues19 determined that Mathilde’s rotation period
is an extremely long 17.4 days. Only two asteroids,
288 Glauke and 1220 Clocus, have longer periods (48
and 31 days, respectively), and there is no obvious
mechanism that can account for these extremely long
asteroid “days.”
The only previous spacecraft encounters with asteroids,
as noted earlier, had been the Galileo flybys of 951
Gaspra in October 1991 and 243 Ida in August 1993.
Recall that both of these objects, as well as Eros, are
S-type asteroids. However, the most common type of
asteroid in the outer asteroid belt, the dark and primitive
C-type objects, had not yet been investigated.
Spectral observations of Mathilde20 showed that its
spectrum was consistent with those of C-type asteroids
and that it was similar to those of the large carbonaceous
asteroids 1 Ceres and 2 Pallas (the two largest
asteroids). (Mathilde is about twice the size of Ida and
four times the size of Gaspra.) Before the NEAR spacecraft
executed its flyby of Mathilde on 27 June 1997,
these additional facts were known about the asteroid:
estimated diameter, 61 km; H magnitude (a measure
of absolute visual brightness), 10.30; perihelion, 1.94
AU; aphelion, 3.35 AU; and orbital inclination, 6.71°.
NEAR’s encounter with Mathilde is summarized in
Fig. 4. As noted earlier, the flyby occurred at about 2
AU from the Sun, where the available power from the
spacecraft’s solar panels was reduced to about 25% of
its maximum mission level. Furthermore, a requirement
to point the solar panels about 50° away from the
optimal solar direction during the encounter reduced
the available power by another 36%. Because of this
power constraint, the only science instrument operated
during the encounter period was the MSI.21 However,
spacecraft tracking data for the radio science
experiment were obtained for an asteroid mass
determination.22
The imaging experiment during the flyby had three
major objectives:
1. Most importantly, to obtain at least one image of
Mathilde near closest approach to provide the highest-
spatial-resolution view of the surface
2. To obtain an image of the complete illuminated
portion of the asteroid visible during the flyby
3. To acquire images of the sky around the asteroid to
search for possible satellites
The entire imaging sequence was accomplished in
about 25 min around closest approach (1200 km) at
a speed of 9.93 km/s (Sun distance, 1.99 AU; Earth
distance, 2.19 AU). A total of 534 images (24 highphase,
144 high-resolution, 188 global imaging, 178
satellite search) were obtained during this interval.
The whole illuminated portion of the asteroid was
imaged in color at about a 500-m resolution before and
after closest approach at phase angles of 125 and 140°,
respectively. The best partial views were also in color,
with a resolution of 200 to 350 m.
Mathilde’s mass was determined by accurately
tracking NEAR before and after the encounter. Apart
from an interval of 1 to 2 h during the closest approach
period, when imaging experiments were conducted,
continuous tracking of the spacecraft was conducted
for 3 days on either side of closest approach. During
the flyby, Mathilde exerted a slight gravitational tug
on NEAR. The corresponding gravitational tugs on
the Galileo spacecraft at Gaspra and Ida were too small
to allow mass determinations. However, because
Mathilde’s mass is so much larger than either Gaspra
or Ida, its effects on NEAR’s path were detectable in
the spacecraft’s radio tracking data.
JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998) 101
NEAR OVERVIEW
Earth Swingby
The next critical phase of NEAR’s flight profile was
scheduled for January 1998, when the spacecraft would
pass by the Earth at an altitude of only 532 km (Fig.
5). This maneuver was expected to drastically alter
NEAR’s heliocentric trajectory, changing the inclination
from 0.52 to 10.04°, and reducing the aphelion
distance from 2.18 to 1.77 AU and perihelion distance
from 0.95 to 0.98 AU. An interesting consequence of
the Earth flyby was that the post-swingby trajectory
remained over the Earth’s south polar region for a
considerable time.
Eros Encounter
The NEAR mission target, 433 Eros, is the second
largest asteroid and is intermediate in size between
Gaspra and Ida. Eros is one of only three near-Earth
asteroids with maximum diameter above 10 km, and
it is the only one whose heliocentric orbit is accessible
enough to permit a rendezvous mission using the Delta
II launch vehicle. The mean radius of Eros, at about
10 km, is an order of magnitude larger than that of
typical known near-Earth asteroids.
Eros was discovered in 1898. It was the subject of
a worldwide ground-based observing campaign in 1975
when it passed within 0.15 AU of Earth. Visible, infrared,
and radar observations determined the approximate
size, shape, rotation rate, and pole position of
Eros (Table 1) and showed that a regolith (fragmentary
material produced by impacts) was present on its surface.
23–25 Eros is presently in a Mars-crossing (but not
Earth-crossing) orbit; however, numerical simulations
suggest that it may evolve into an Earth crosser within
2 million years.26
More recent spectroscopic analyses have found a
hemispherical heterogeneity in the near-infrared spectra
of Eros.9 One side of the asteroid has a spectrum
consistent with higher pyroxene content and a radar
signature consistent with a facet-like surface; the
other side displays higher olivine content and a
convex-shaped surface. The detection of variations in
disk-averaged data with rotation phase suggests that
substantial geologic and compositional complexity
–5.2 min
Closest
approach
Mathilde To Sun
High
phase
–2.8 min
High
resolution
+3 min
Global
imaging
+10 min
Satellite
search
+20 min
Figure 4. Timeline of the Mathilde flyby.
A. F. CHENG, R. W. FARQUHAR, AND A. G. SANTO
102 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998)
Operations
Beginning on 20 December
1998, a sequence of rendezvous
maneuvers will be used to decrease
the relative velocity between
NEAR and Eros to only 5 m/s. On
10 January 1999, NEAR will fly by
Eros on its sunward side at a distance
of about 500 km. In addition
to gathering important scientific
data, this first pass is expected to
provide improved estimates of the
asteroid’s physical parameters (Table
2), which are needed for navigational
purposes. Goals for the
first pass include a mass determination
to ±1% accuracy, identification
of several hundred surface
landmarks, and a vastly improved
estimate of Eros’s spin vector. As
the spacecraft is maneuvered closer
to Eros, estimates of its mass,
moments of inertia, gravity harmonics, spin state,
landmark locations, etc., will be determined with
increasing precision.
The prime science phase at Eros will begin around
15 March 1999. During this phase, NEAR will be
operating in orbits that come as close as 15 km to the
asteroid’s surface. The evolution of low-altitude orbits
around Eros will be strongly influenced by its irregular
gravity field. Orbits exist that are quite unstable, and
the spacecraft could crash into Eros in a matter of days.
Therefore, safe operation of NEAR during its 11-
month prime science phase will require close coordination
between the science, mission design, navigation,
and mission operations teams.21
Figure 6 depicts the NEAR spacecraft in a 35-km
circular orbit around Eros as viewed by an observer on
the Sun. The orbit and Eros are drawn to scale, but
obviously the spacecraft is not. This is a convenient
reference frame to show NEAR’s orbit because the
orbital plane will be controlled so that it is always
within 30° of a plane that is normal to the Sun–Eros
line. In this configuration, NEAR’s fixed solar panels
are oriented toward the Sun. The science instruments
are pointed at Eros’s surface by slowly rolling the spacecraft
as it orbits the asteroid.
Two fundamentally different orbital geometries are
shown in Fig. 6. In Fig. 6a, Eros’s rotation axis is aligned
with the Sun–Eros line. Its South Pole points toward
the Sun, which means that its Northern Hemisphere
will be shadowed. Approximately 4 months later (Fig.
6b), as the asteroid’s orbital position around the Sun
changes, its rotation axis is perpendicular to the Sun–
Eros line. Coverage of Eros’s surface by NEAR’s science
instruments will vary considerably throughout the
6:30 6:45 7:00
7:15
7:30
7:45
8:00
8:15
Perigee
altitude,
532 km
Sunset
Figure 5. Earth swingby of 23 January 1998.
may be found at higher spatial resolution by NEAR.
Such diversity may arise in a number of ways. One
possibility is that Eros is a fragment of a large, differentiated
parent body that did not survive intact to the
present; this body may have had volcanic activity, for
example, and Eros may preserve material from the
surface or the interior of such a body. Alternatively,
present-day Eros may have been assembled from fragments
of many different parent bodies, and a detailed
study of variations across the asteroid’s surface may
reveal evidence of such diverse origins.
Table 1. Features of 433 Eros.
Parameter Measurement
Diameter of triaxial
ellipsoid model 40 ´ 14 ´ 14 km
Heliocentric orbit period 1.76 years
Inclination of heliocentric
orbit 10.8°
Perihelion, aphelion 1.133 AU, 1.784 AU
Rotation period 5.27011 h
Rotation pole
Ecliptic longitude 16°
Latitude 11°
Spectral type S (pyroxene, olivine,
Fe–Ni metal)
Geometric albedo 0.16
JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998) 103
NEAR OVERVIEW
S S
N N N
Initial orientation Rotated 90° Rotated 180°
Rotated 180° Rotated 90° Initial orientation
S
(a)
(b)
Figure 6. NEAR orbit around Eros as viewed from the Sun (35 ´ 35 km orbit; period of about 11.4 h). (a) Rotation axis aligned with Sun–
Eros line (29 April 1999). (b) Rotation axis perpendicular to Sun–Eros line (2 September 1999). (Reprinted from Ref. 18 by permission.)
11-month prime science phase.
However, careful planning of the
mission and science operations
should yield satisfactory global coverage
by all NEAR instruments.
To simplify science operations,
the rendezvous will be divided into
distinct phases.22 During each
phase, particular aspects of the science
will be emphasized (Table 2).
The highest-priority science will
vary by mission phase and during
the course of the year in rendezvous
orbit around Eros. The actual dates
and orbit parameters during the
rendezvous phase are still to be
determined because of the uncertain
mass of Eros.
The first detection of Eros by the
imager is expected some 200 days
before the unperturbed flyby closest
approach date (i.e., the date that
flyby would occur in the absence of
rendezvous burns). Following this
Table 2. Sumary of science operations at Eros.
Phase Goal
Approach/initial flyby Mass determination (radio science)
Shape/rotation state determinations (MSI)
Color map production at low phase angle
(NIS/MSI)
Satellite search
Rendezvous Imaging to 3 ´ 5 m resolution in seven colors
(MSI)
Light element map production at about 2-km
resolution (XRS)
Elemental composition analysis in four
quadrants of Eros (GRS)
Spectral map production at about 200 ´ 400 m
resolution (NIS)
Gravity field determination (radio science)
Shape and topography determination (NLR/MSI)
Determination of density homogeneity/
heterogeneity (various)
Magnetic field mapping (MAG)
A. F. CHENG, R. W. FARQUHAR, AND A. G. SANTO
104 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998)
Figure 7. Mission phases at Eros.
first detection, imaging of Eros will be undertaken for
optical navigation and for initial shape and rotation
studies.
The rendezvous burn sequence is targeted to put
NEAR into an initial slow flyby trajectory past Eros,
which, at a nominal speed of 5 m/s and a miss distance
of 500 km, will take the spacecraft through a zero solar
phase angle (Sun-asteroid-spacecraft angle). The purpose
of this flyby will be to enable critical initial science
observations. Since the nominal rendezvous orbit
plane will be near the Eros terminator (plane dividing
the dayside from the nightside), most of the observations
during this period will be made at large phase
angles that are favorable for imaging but unfavorable
for infrared spectral mapping. The initial flyby will
allow more than 30 h of observation at low phase
angles that are not accessible within the nominal rendezvous
geometry. Hence, it will provide an important
opportunity to obtain global infrared spectral maps
under optimal lighting conditions.
The approach to Eros and the initial flyby will also
be used to perform an optical search for satellites of
Eros. The satellite Dactyl of 243 Ida was discovered by
Galileo.24 If a satellite like Dactyl is present at Eros,
it will not prevent NEAR from entering low-altitude
orbit and achieving its primary science goals. Needless
to say, if an Eros satellite is discovered, mission plans
will be modified to include appropriate studies.
NEAR will remain in a bound orbit around Eros for
more than 10 months. The spacecraft will spend at
least 120 days in a 35 ´ 35 km orbit, during which time
the highest-priority science will be measurement of
elemental composition, although every instrument
will be in operation. Much of the remaining time in
orbit around Eros will be spent at semimajor axes of
50 km or less. Again, all instruments will be operating
during these periods, but imaging and spectral mapping
will have increased priority. The various mission phases
at Eros are summarized in Fig. 7.
Launch
(17 Feb 1996)
Mathilde
(27 Jun 1997)
Earth
(23 Jan 1998)
Eros
initial flyby
(10 Jan 1999)
Orbit
insertion
(12 Jan 1999)
10 Apr 1999
19 Jun 1999
10 Nov 1999
End of
mission
(6 Feb 2000)
Cruise
Far
encounter
and
approach
Eros
rendezvous
Lowering
of orbit
35-km orbit High inclination
and polar orbits
35-km orbit
Data Flow
All data from the NEAR mission will be downlinked
to the NASA Deep Space Network and then
forwarded to the Mission Operations Center (MOC)
at APL. Doppler and ranging data from the spacecraft
will be analyzed primarily by the NEAR navigation
team at the Jet Propulsion Laboratory (JPL) and processed
to determine the spacecraft ephemeris as well
as to perform radio science investigations. The entire
spacecraft telemetry stream, including spacecraft and
instrument housekeeping data and all science data,
will be forwarded to the APL MOC together with the
radiometric Doppler and range data along with results
of navigation solutions. Navigation data will be forwarded
to MOC in the form of SPICE kernels. (SPICE
is an information system developed by the Navigation
Ancillary Information Facility at JPL. It consists of
data files and software for managing navigation-related
data including spacecraft and planetary ephemerides,
spacecraft pointing, timekeeping, gravity data, etc.)
From the APL MOC the spacecraft telemetry
stream will be passed to the Science Data Center
(SDC), the project facility responsible for low-level
processing of spacecraft telemetry, data distribution,
and data archiving. As such, the SDC supports the
activities of the science team in data analysis and
mission planning. The SDC will create and maintain
an archive, which will be the central project repository
for science data products such as images, asteroid
models, and asteroid maps that will be stored in standard
formats agreed upon with the science team. The
SDC will enable easy access to mission data sets by
members of the science team and by others, and it will
collect observing requests and science priorities from
the science team. It will maintain a telemetry archive,
a record of instrument and spacecraft commands as
executed, and records of science sequences as requested
and as executed. It will provide ancillary data
(spacecraft and planetary ephemerides, spacecraft and
JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998) 105
NEAR OVERVIEW
planetary attitudes, shape and gravity files, and spacecraft
clock files) in the form of SPICE kernels to the
science team.
Moreover, SDC will create and maintain a database,
called the Science Data Catalog, to facilitate
access to science data. The catalog will provide pointers
to data that satisfy specified criteria such as observing
conditions, instrument status or mode, mission
phase, target of observation, etc. The SDC will also be
responsible for providing access to NEAR data for the
scientific community at large and for the public.
Uncalibrated data, including images, will be released
publicly over the Internet as soon as they are validated.
The Internet World Wide Web will be used for data
released by the project at http://sd-www.jhuapl.edu/
NEAR/.
CONCLUSION
NEAR will substantially increase our knowledge of
primitive bodies in the solar system by providing a
long, up-close look at the S-type asteroid 433 Eros and
the first resolved images of the C-type asteroid 253
Mathilde. NEAR is the first mission to a near-Earth
asteroid and a C-type asteroid, and it will be the first
spacecraft to orbit a small body.
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ACKNOWLEDGMENTS: We thank the many members of the NEAR team at
APL, NASA Headquarters, NASA centers, universities, and industry sites for
their hard work and dedication, but especially Thomas Coughlin, the NEAR
Project Manager. This work was supported under contract N00039-95-C-0002
with the U.S. Navy.
THE AUTHORS
ANDREW F. CHENG received an A.B. in physics from Princeton University
in 1971 and a PhD. in physics from Columbia University in 1977. He joined
APL in 1983. Dr. Cheng is a Principal Professional Staff member and is
currently supervisor of the Planetary Science Section. His scientific interests
include space plasma and astrophysical fluid dynamics, interactions of plasmas
with atmospheres and solid surfaces, and magnetospheric physics. He is the
project scientist for the NEAR mission, an interdisciplinary scientist on the
Galileo mission to Jupiter, and a co-investigator on the Cassini mission to
Saturn. Dr. Cheng is a fellow of the American Physical Society. He has served
as an editor of Eos Transactions of the American Geophysical Union and as an
associate editor for The Journal of Geophysical Research and Geophysical Research
Letters. His e-mail address is andrew.cheng@jhuapl.edu.
A. F. CHENG, R. W. FARQUHAR, AND A. G. SANTO
106 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998)
ROBERT W. FARQUHAR is a member of APL’s Principal Professional Staff.
He received a B.S. in aeronautical engineering from the University of Illinois in
1959, an M.S. in engineering from UCLA in 1961, and a Ph.D. in astronautical
sciences from Stanford University in 1969. Before coming to APL, Dr. Farquhar
worked at several NASA facilities including Goddard Space Flight Center and
NASA Headquarters from 1966 to 1990. Since joining APL in 1990, he has
served as the mission director for the NEAR Program, playing a major role in
the development of the flight profiles for the Mathilde flyby and the Eros
rendezvous opportunity. Dr. Farquhar organized the technical programs for the
first and second IAA Conferences in Low-Cost Planetary Missions. His e-mail
address is robert.farquhar@jhuapl.edu.
ANDREW G. SANTO is a member of APL’s Principal Professional Staff. He
received a B.S. in engineering science (magna cum laude) from Pennsylvania
State University in 1983 as well as an M.S. in electrical engineering and
computer science (magna cum laude) in 1985 and an M.S. in technical
management in 1994, both from The Johns Hopkins University. Before
employment at APL, Mr. Santo worked at IBM (1983) and the Lawrence
Livermore Laboratory (1984). Since joining the Laboratory in 1985, he has
worked in the Space Department. Mr. Santo is the spacecraft system engineer
and mission systems engineer for the NEAR spacecraft. He has also worked as
the spacecraft system engineer for ALTAIR, spacecraft ground system lead for
the UVISI instrument on the MSX spacecraft, launch vehicle interface lead for
the Delta 183 spacecraft, and ground system lead for the Delta 180 and 181
spacecraft. His e-mail address is andrew.santo@jhuapl.edu.