OVERVIEW OF THE NEW HORIZONS SCIENCE PAYLOAD
H. A. Weaver (a), W. C. Gibson (b), M. B. Tapley (b), L. A. Young (c), and
S. A. Stern (c)
a. Johns Hopkins University Applied Physics Laboratory, 11100 Johns
Hopkins Road, Laurel, MD 20723
b. Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238
c. Southwest Research Institute, 1050 Walnut St., Suite 400, Boulder, CO 80302
Abstract
The New Horizons mission was launched on 2006 January 19, and the
spacecraft is heading for a flyby encounter with the Pluto system in
the summer of 2015. The challenges associated with sending a
spacecraft to Pluto in less than 10 years and performing an ambitious
suite of scientific investigations at such large heliocentric
distances (> 32 AU) are formidable and required the development of
lightweight, low power, and highly sensitive instruments. This paper
provides an overview of the New Horizons science payload, which is
comprised of seven instruments. Alice provides moderate resolution
(~3-10 Angstroms FWHM), spatially resolved ultraviolet (~465-1880 Angstroms)
spectroscopy, and includes the ability to perform stellar and solar
occultation measurements. The Ralph instrument has two components: the
Multicolor Visible Imaging Camera (MVIC), which performs panchromatic
(400-975 nm) and color imaging in four spectral bands (Blue, Red, CH4,
and NIR) at a moderate spatial resolution of 20 microrad/pixel, and the
Linear Etalon Imaging Spectral Array (LEISA), which provides spatially
resolved (62 microrad/pixel), near-infrared (1.25-2.5 microns), moderate
resolution (lambda/del(lambda) ~ 240-550) spectroscopic mapping
capabilities. The Radio Experiment (REX) is a component of the New Horizons
telecommunications system that provides both radio (X-band) solar
occultation and radiometry capabilities. The Long Range Reconnaissance
Imager (LORRI) provides high sensitivity (V < 18), high spatial
resolution (5 microrad/pixel) panchromatic optical (350-850 nm) imaging
capabilities that serve both scientific and optical navigation
requirements. The Solar Wind at Pluto (SWAP) instrument measures the
density and speed of solar wind particles with a resolution delta(E)/E < 0.4
for energies between 25 eV and 7.5 keV. The Pluto Energetic Particle
Spectrometer Science Investigation (PEPSSI) measures energetic
particles (protons and CNO ions) in 12 energy channels spanning 1-1000
keV. Finally, an instrument designed and built by students, the
Venetia Burney Student Dust Counter (VB-SDC), uses polarized
polyvinylidene fluoride panels to record dust particle impacts during
the cruise phases of the mission.
1. Introduction
New Horizons was the first mission selected in NASA's New Frontiers
series of mid-sized planetary exploration programs. The New Horizons
spacecraft was launched on 2006 January 19 and is now on a 3 billion
mile journey to provide the first detailed reconnaissance of the Pluto
system during the summer of 2015. Assuming that this primary objective
is successful, NASA may authorize an extended mission phase that will
permit a flyby of another Kuiper belt object (KBO), as yet
unidentified, probably within 3 years of the Pluto encounter. The
genesis and development of the New Horizons mission is described by
Stern (2007). The scientific objectives of the mission are discussed
by Young et al. (2007). Here we provide a high level overview of the
scientific payload. Detailed descriptions of individual instruments
are given elsewhere in this volume, as referenced below.
The New Horizons mission is an ambitious undertaking that required the
development of lightweight, low power, and highly sensitive
instruments. Pluto will be nearly 33 AU from the sun at the time of
the encounter in 2015, and a launch energy (C3) of nearly 170 km2 s-2
was needed to reach this distance within the 9.5 year transit to the
Pluto system. Even using the powerful Lockheed-Martin Atlas 551
launcher in tandem with its Centaur second stage and a Boeing Star48
third stage, the entire spacecraft mass had to be kept below 480 kg,
of which less than 50 kg was allocated to the science payload. At
Pluto's large heliocentric distance, the use of solar photovoltaic
cells was not an option, so the New Horizons mission relies on a
radioisotope thermoelectric generator (RTG) for all of its power
needs. The mission requirement on the total power available at the
Pluto encounter is only 180 W, of which less than 12 W can be used at
any one time by the scientific instruments. The solar output (light
and particle) at Pluto is approximately 1000 times smaller than at the
Earth, which means that the instruments attempting to measure
reflected sunlight or the solar wind during the Pluto encounter must
be extremely sensitive. Finally, we note that the long mission
duration imposes strict reliability requirements, as the spacecraft
and science payload must meet their performance specifications at
least 10 years after launch.
Fortunately, all of the New Horizons instruments successfully met
these daunting technical challenges without compromising any of the
mission's original scientific objectives. Below we provide a
high-level description of all the instruments on New Horizons, discuss
their primary measurement objectives, and summarize their observed
performance, which has now been verified during in-flight testing. But
first we begin by briefly describing the spacecraft pointing control
system as it relates to the science payload.
2. Payload Pointing Control
The New Horizons spacecraft does not have enough power to support a
reaction wheel based pointing control system and instead relies on
hydrazine thrusters to provide slewing capability and attitude
control. The positions of stars measured by one of two star trackers
(the second star tracker provides redundancy) are used to determine
the absolute orientation of the spacecraft (i.e., the RA and DEC
locations of some reference axis on the spacecraft), and the drift
rate is monitored by a laser-ring gyro system (the inertial
measurement unit, or IMU). The attitude data from the star tracker and
IMU are used in a feedback loop to set the pointing within prescribed
limits in both absolute position and drift rate. The spacecraft IMUs,
star trackers, sun sensors, and guidance computers are all redundant.
The New Horizons spacecraft spends much of its time spinning at ~5 RPM
around the Y-axis. In this mode, useful data can be obtained by REX,
SWAP, PEPSSI, and the VB-SDC, but typically not by any of the other
instruments.
For virtually all observations made by the imaging instruments, 3-axis
pointing control mode is required. In 3-axis mode, the spacecraft can
be slewed to a targeted location to an accuracy of +/-1024 microrad
(3sigma) and controlled to that location within a typical "deadband" of +/-500
microrad. For some Alice observations, when the target must be kept near
the center of its narrow slit, the deadband can be reduced to +/-250
microrad. The drift rate is controlled to within +/-34 microrad/sec
(3sigma) for both fixed and scanning observations. The post-processing
knowledge of the attitude and drift rate derived from the star tracker
and IMU data are +/-350 microrad (3sigma) and +/-7.5 microrad/sec (3sigma),
respectively. Ralph observations usually require the spacecraft to
scan about its Z-axis. The nominal scan rate for Ralph/MVIC is
1.1mrad/sec, and the nominal scan rate for Ralph/LEISA is 0.12 mrad/sec.
Further details about the New Horizons guidance and control system can
be found in Rogers et al. (2006).
3. Science Payload
3.1 OVERVIEW
All of the fundamental ("Group 1") scientific objectives for the New
Horizons mission (Stern 2007; Young et al. 2007) can be achieved with
the core payload comprised of: (i) the Alice ultraviolet (UV) imaging
spectroscopy remote sensing package, (ii) the Ralph visible and
infrared imaging and spectroscopy remote sensing package, and (iii)
the Radio Experiment (REX) radio science package. The supplemental
payload, which both deepens and broadens the mission science, is
comprised of the Long Range Reconnaissance Imager (LORRI), which is a
long-focal-length optical imaging instrument, and two plasma-sensing
instruments: the Solar Wind Around Pluto (SWAP) and the Pluto
Energetic Particle Spectrometer Science Investigation (PEPSSI). The
supplemental payload is not required to achieve minimum mission
success, but these instruments provide functional redundancy across
scientific objectives and enhance the scientific return by providing
additional capabilities not present in the core payload. The Venetia
Burney Student Dust Counter (VB-SDC), which was a late addition to the
supplemental payload approved by NASA as an Education and Public
Outreach (EPO) initiative, also provided a new capability to New
Horizons, namely, an interplanetary dust detection and mass
characterization experiment.
Drawings of all seven instruments are displayed in Figure 1, which
also lists the mass and power consumption of each instrument. The
locations of the instruments on the New Horizons spacecraft are
displayed in Figure 2.
Fig 1: The three instruments comprising the New Horizons core payload
are shown along the top row, and the instruments comprising the
supplemental payload are displayed along the bottom row. The
approximate mass and power consumption are shown just below the
picture of each instrument. The total mass of the entire science
payload is less than 30 kg, and the total power drawn by all the
instruments is less than 30 W.
Fig. 2: This drawing shows the locations of the instruments on the New
Horizons spacecraft. The VB-SDC is mounted on the bottom panel, which
is hidden from view. The boresights of LORRI (sketched in figure),
Ralph, and the Alice airglow channel are all approximately along the
-X direction. The boresights of the Alice solar occultation channel
and the antenna are approximately along the +Y direction. SWAP covers
a swath that is ~200 degrees in the XY plane and ~10 degrees in the YZ
plane. PEPSSI's field-of-view is a ~160 degrees by ~12 degrees swath
whose central axis is canted with respect to the principal spacecraft
axes to avoid obstruction by the backside of the antenna. The black
structure with fins located at +X is the RTG, which supplies power to the
observatory. The star trackers, which are used to determine the
attitude, can also be seen. The antenna diameter is 2.1 m, which
provides a scale for the figure.
As discussed further below, Ralph is essentially two instruments
rolled into a single package: the Multispectral Visible Imaging Camera
(MVIC) is an optical panchromatic and color imager; the Linear Etalon
Imaging Spectral Array (LEISA) is an infrared imaging
spectrometer. The boresights of MVIC, LEISA, LORRI, and the Alice
airglow channel are aligned with the spacecraft -X axis (Fig. 2)
except for minor tolerancing errors. The projections of the fields of
view of those instruments onto the sky plane are depicted in Figure 3.
Fig. 3: The fields of view (FOVs) of the MVIC, Ralph, Alice airglow,
and LORRI instruments are projected onto the sky plane; the listed
boresights are measured in-flight values. The angular extent of each
instrument's FOV is also listed. The spacecraft +X direction is out of
the page, the +Y direction is up, and the +Z direction is to the
left. The LORRI field FOV overlaps the narrow portion of the Alice
airglow channel, and the MVIC FOV overlaps the wide portion. The LEISA
FOV overlaps the MVIC FOV.
The types of observations performed by the New Horizons instruments
are depicted in Figure 4. None of the instruments have their own
scanning platforms, so the entire spacecraft must be maneuvered to
achieve the desired pointings. As described below, the guidance and
control system uses hydrazine thrusters to point the spacecraft at the
desired target.
Fig. 4: Types of New Horizons observations. Typical Ralph MVIC Time
Delay Integration (TDI) and LEISA observations (upper left) are
performed by rotating the spacecraft about the Z-axis. Typical Ralph
MVIC frame, LORRI, and Alice airglow observations (lower left) are
made with the spacecraft staring in a particular direction. The Alice
and REX occultation observations (upper right) are performed by
pointing the antenna at the Earth and the Alice occultation channel at
the sun, so that radio signals from the DSN on Earth can be received
by REX at the same time that Alice observes the Sun. Observations by
the particle instruments (SWAP, PEPSSI, and VB-SDC; lower right) can
occur essentially anytime, in either spinning or 3-axis mode. However,
most of the VB-SDC data will be collected during cruise mode, when the
other instruments are in hibernation mode and the spacecraft is
passively spinning, because thruster firings add a large background
noise level to the VB-SDC's data.
The principal measurement objectives and the key characteristics of
the New Horizons science payload are summarized in Table I, which also
includes the names and affiliations of the instrument Principal
Investigators (PIs) and the primary builder organization for each
instrument. The measurement objectives that are directly related to
the mission Group 1 scientific objectives are highlighted in
boldface. In the following subsections, we provide further discussion
of each of the New Horizons instruments.
TABLE I
New Horizons Instruments: Pluto System Measurement Objectives and
Characteristics (PI=Principal Investigator; Instrument Characteristics
are summary values with details provided in the individual instrument
papers)
TABLE I (continued)
New Horizons Instruments: Measurement Objectives and Characteristics
In the following subsections, we provide further discussion on each of
the New Horizons instruments. We attempt to provide a high-level
summary of the instruments' capabilities, with detailed descriptions
left to the individual instrument papers, which are referenced in each
subsection.
3.2 ALICE
The Alice instrument aboard New Horizons is an ultraviolet (UV)
imaging spectrometer that provides moderate spectral and spatial
resolution capabilities over the wavelength range ~465-1880 Angstroms with a
peak effective area of ~0.3 cm^2. Light enters Alice's f/3 telescope
via either the main entrance aperture (called the Airglow Aperture,
co-aligned with the Ralph and LORRI apertures), or, via a small, fixed
pickoff mirror, through the Solar Occultation Channel (SOCC,
co-aligned with the New Horizons high-gain antenna). Light from either
aperture is reflected off the 4 cm x 4 cm primary mirror, passes
through a single slit, is reflected off a holographic grating, and
finally is detected using a photon-counting, microchannel plate double
delay line device, read out as a 32 x 1024 element digital array. The
SOCC aperture is stopped down by a factor of 6400 relative to the
Airglow Aperture to allow Alice to look directly at the Sun for solar
occultations of Pluto's and Charon's atmospheres. The Alice entrance
slit is a "lollipop" (see Fig. 3) with a 0.1 x 4 degrees "slot" used
primarily for airglow observations and a 2 degrees x 2 degrees "box"
used mainly during solar occultation observations. The point source spectral
resolution is 3-6 Angstroms, depending on wavelength, and the plate scale in
the spatial dimension is 0.27 degrees per pixel. During the Pluto and Charon
occultation observations, the Sun has an apparent diameter of ~1', and
the spectral resolution is 3-3.5 Angstroms. During filled-slit airglow
aperture observations, the spectral resolution is ~9-10 Angstroms.
Alice is a name, not an acronym, taken from one of the main characters
of the American television show The Honeymooners. Alice is sometimes
called Pluto-Alice (P-Alice) to distinguish it from its predecessor,
Rosetta-Alice (R-Alice), which is a similar instrument being flown on
the European Space Agency (ESA) Rosetta mission to comet
67P/Churyumov-Gerasimenko. Compared to R-Alice, P-Alice has a somewhat
different bandpass and various enhancements to improve
reliability. P-Alice also includes a separate solar occultation
channel, which is not available on R-Alice. Both P-Alice and R-Alice
are significantly improved versions of the Pluto mission "HIPPS" UV
spectrograph (HIPPS/UVSC), which was developed at Southwest Research
Institute (SwRI) in the mid-1990s with funds from NASA, JPL, and
SwRI.
Alice's principal measurement objectives and its key characteristics
are summarized in Table I. Alice was designed to measure Pluto's upper
atmospheric composition and temperature, which is a New Horizons Group
1 scientific objective. Alice will also obtain model-dependent escape
rate measurements from Pluto's atmosphere, and it will provide some
limited surface mapping and surface composition capabilities in the
UV. Alice's spectral bandpass includes lines of CO, atomic H, Ar, and
Ne, which may be detectable as airglow, and the electronic bands of
N2, CH4, and other hydrocarbons and nitriles, which are detectable
during solar and stellar occultation observations. Young et al. (2007)
provide a detailed discussion of Alice's scientific objectives. Stern
et al. (2007) should be consulted for further details on Alice's
design and performance.
3.3 RALPH: MVIC AND LEISA
Ralph and Alice together comprise the primary remote sensing payload
on New Horizons. Ralph is named after Alice's husband in The
Honeymooners. It is a combined visible/NIR imager (called MVIC) and
imaging IR spectrograph (called LEISA). Both of these two focal planes
are fed by a single telescope assembly. MVIC (Multi-spectral Visible
Imaging Camera) is an optical imager employing CCDs with panchromatic
and color filters. LEISA (Linear Etalon Imaging Spectral Array,)is a
near infrared (IR) imaging spectrograph employing a 256 x 256 mercury
cadmium telluride (HgCdTe) array. In addition to its scientific
capabilities, MVIC also serves as an Optical Navigation camera for New
Horizons.
The common telescope assembly for Ralph has a three-mirror, off-axis
anastigmat design with a 7.5 cm primary mirror. A dichroic reflects
the optical light to the MVIC focal plane and transmits the IR light
to the LEISA focal plane. Only one focal plane is active at a time,
with a relay used to select either MVIC or LEISA. Owing to Ralph's
critical role in achieving the New Horizons Group 1 scientific
objectives, all of its electronics and some of its focal plane CCDs
are redundant.
The MVIC focal plane has seven independent CCD arrays mounted on a
single substrate. Figure 3 shows the relative positions of the arrays,
as projected on the sky. Six of the arrays have 5000 (columns) x 32
(rows) photosensitive pixels and operate in time-delay integration
(TDI) mode. Two of the TDI arrays provide panchromatic (400-975 nm)
images, and the other four TDI arrays provide, respectively, color
images in blue (400-550 nm), red (540-700 nm), near IR (780-975 nm),
and narrow band methane (860-910 nm) channels. The frame transfer
array has 5000 (columns) x 128 (rows) pixels and provides panchromatic
images (400-975 nm). All of the MVIC arrays have square pixels that
are 20 microrad on a side. Thus, the TDI arrays have a field of view of
5.7 degrees x 0.037 degrees, and the frame transfer array has a field
of view of 5.7 degrees x 0.15 degrees. To obtain MVIC TDI images, the
spacecraft scans the TDI arrays across the target (Fig. 4) at the same
rate that charge is shifted from one row to the next, so that the
effective exposure time is 32 times the row transfer time. The two TDI
panchromatic arrays are sized to meet the 0.5 km/pixel Group 1 mapping
requirement near closest approach when Pluto's diameter subtends ~5000
pixels. Each panchromatic array can be operated independently, for
redundancy. The four color arrays are operated in tandem. The primary
measurement objectives and key characteristics of MVIC are summarized
in Table I.
MVIC images in the three broadband colors will provide information on
spectral slopes of Pluto's surface and on its atmospheric
properties. The narrow band filter permits mapping of the surface
methane abundance, as the well-known 890 nm absorption band is the
strongest methane feature available at optical wavelengths. The near
IR filter doubles as the continuum comparison for this methane
mapping. MVIC's framing array is operated in stare, not scanning,
mode, and is used when geometrical fidelity is important (e.g., for
optical navigation) or when scanning is not practical (e.g., observing
Pluto at closest approach when the apparent motion is too fast). The
700-780 nm gap between the red and near IR bandpasses overlaps another
methane band at 740 nm; combining data from the panchromatic, blue,
red, and near IR filters can provide some information about band depth
in this "virtual" filter. Young et al. (2007) discuss MVIC's
scientific objectives in more detail. Further details on MVIC and its
performance can be found in Reuter et al. (2007).
LEISA's dispersive capability is provided by its wedged etalon (a
linear variable filter, or LVF), which is mounted ~100 micron above its
256x256 pixel HgCdTe PICNIC array. The etalon covers 1.25-2.5 microns, a
spectral region populated with many absorption features of N2, CH4,
H2O, NH3, CO, and other molecules, at a resolving power of ~250. A
higher-resolution sub-segment, covering 2.10-2.25 microns at a resolving
power of ~560, will be used to discern grain sizes, mixing states, and
pure versus solid-solution abundances (Quirico et al. 1999). The
higher-resolution segment is also critical for taking advantage of the
temperature-sensitive N2 bands (Grundy et al. 1993, 1999), and the
symmetric, doubled v2 + v3 CH4 band that is diagnostic of pure versus
diluted CH4 abundances (Quirico & Schmitt 1997). As was the case for
MVIC, LEISA images are obtained by scanning its field of view across
the target (Fig. 4) with the frame transfer rate synchronized with the
scan rate. The LVF is oriented so that wavelength varies along the
scan direction. Thus, scanning LEISA over a target produces images at
different wavelengths (this is unlike the case for MVIC where the
scanning simply increases the signal 32-fold). LEISA builds up a
conventional spatial-spectral data cube (256 monochromatic images) by
scanning the FOV across all portions of the target at a nominal scan
rate of 125 microrad/sec. A nominal framing rate of 2 Hz is currently
planned to maintain <1 pixel attitude smear and provide good
signal-to-noise ratio in the Pluto system. The primary measurement
objectives and key characteristics of LEISA are summarized in Table
I. Reuter et al. (2007) provide further details on LEISA's design and
performance, and Young et al. (2007) provides a more in-depth
discussion of LEISA's scientific objectives.
3.4 REX
REX is the radio science package on New Horizons. REX stands for Radio
EXperiment. The REX instrument is unique among the suite of
instruments comprising the New Horizons payload in that it is
physically and functionally incorporated within the spacecraft
telecommunications subsystem. Because this system is entirely
redundant, two REX's are carried on New Horizons. They can be used
simultaneously to increase SNR.
The REX principle of operations for radio science is as follows: the
2.1 m High Gain Antenna aboard New Horizons (see Fig. 1) receives
radio signals from NASA's Deep Space Network (DSN) at a carrier
frequency of 7.182 GHz. New Horizons transmits radio signals via the
antenna to the DSN at a carrier frequency of 8.438 GHz. By measuring
phase delays in the received signal as a function of time, the
instrument allows one to invert a radio occultation profile into a
temperature, number density profile of the intervening atmosphere, if
it is sufficiently dense. REX can also operate in a passive radiometry
mode to measure radio brightness temperatures at its receiver
frequency.
The heart of the REX instrument is an ultra-stable oscillator (USO),
which operates at 30 MHz for the down-conversion to an intermediate
frequency (IF). An Actel Field Programmable Gate Array (FPGA) takes
samples of the IF receiver output and generates wideband radiometer
and narrowband sampled signal data products. The REX hardware also
includes an analog-to-digital converter (ADC) and other electronics
interface components in the telecommunications system. As noted above,
there are two copies of the entire telecommunications system (except
for the High Gain Antenna), which means that there is full system
redundancy in the REX capabilities. The primary measurement objectives
and key characteristics of REX are summarized in Table I. Tyler et
al. (2007) discuss REX and its performance in much greater detail.
REX addresses the Group 1 scientific objective of obtaining Pluto's
atmospheric temperature and pressure profiles down to the surface
using a unique uplink radio occultation technique. REX detects the
changes induced by Pluto's atmosphere in the radio signal transmitted
to the spacecraft from the DSN. This differs from the typically used
downlink method, in which the spacecraft transmits to receivers on
Earth. REX will also address Group 2 and Group 3 scientific
objectives, including probing Pluto's ionospheric density, searching
for Charon's atmosphere, refining bulk parameters like mass and
radius, and measuring the surface emission brightness at a wavelength
of 4.2 cm, which permits the determination of both the dayside and
nightside brightness temperatures with an angular resolution of ~1.2 degrees
(full-width between the 3 dB points). Young et al. (2007) provide
further discussion of the REX's scientific objectives.
3.5 LORRI
The Long Range Reconnaissance Imager (LORRI) is a narrow angle (field
of view=0.29 degrees x 0.29 degrees), high resolution (4.96 microrad/pixel),
panchromatic (350-850 nm) imaging system. It was placed on New
Horizons to augment and also back up Ralph's panchromatic imaging
capabilities. LORRI's primary function is to provide higher resolution
imagery.
LORRI's input aperture is 20.8 cm in diameter, making LORRI one of the
largest telescopes flown on an interplanetary spacecraft. The large
aperture, in combination with a high throughput (QE_peak = 60%) and
wide bandpass, will allow LORRI to achieve a signal-to-noise ratio
exceeding 100 during disk-resolved observations of Pluto, even though
exposure times must be kept below 100 ms to prevent smearing from
pointing drift. A frame transfer 1024 x 1024 pixel (optically active
region), thinned, backside-illuminated charge-coupled device (CCD)
detector records the image in the telescope focal plane. The CCD
output is digitized to 12 bits and stored on the spacecraft's solid
state recorder (SSR).
Raw images can be downlinked, but typically the images will be either
losslessly or lossy compressed before transmission to the ground in
order to minimize the use of DSN resources. LORRI image exposure times
can be varied from 0 ms to 29,967 ms in 1 ms steps, and images can be
accumulated at a maximum rate of 1 image per second. LORRI's large
dynamic range allows it to be an imaging workhorse during the Jupiter
encounter, when saturation limits MVIC observations to relatively
large solar phase angles.
LORRI operates in an extreme thermal environment, mounted inside the
warm spacecraft and viewing cold space, but the telescope's
monolithic, silicon carbide construction allows the focus to be
maintained over a large temperature range (-120 C to 50 C) without any
focus adjustment mechanisms. Indeed, LORRI has no moving parts making
it a relatively simple, reliable instrument that is easy to operate. A
one-time deploy aperture door, mounted on the spacecraft structure,
protected LORRI from the harsh launch environment. Cheng et al. (2007)
provide a detailed description of LORRI and its performance.
Owing to its higher spatial resolution, higher sensitivity, and lower
geometrical distortion (< 0.5 pixel across the entire field of view)
compared to Ralph/MVIC, LORRI is also serving as the prime optical
navigation instrument on New Horizons. During a typical 100 ms
exposure using the full format (1024 x 1024) mode, LORRI can achieve a
signal-to-noise ratio of ~5 on V=13 stars. On-chip 4x4 binning, used
in conjunction with a special pointing control mode that permits
exposing up to 10 s while keeping the target within a single rebinned
pixel, allows imaging of point sources as faint as V=18, which will
permit LORRI to detect a 50 km diameter KBO ~7 weeks prior to
encounter, thereby enabling accurate targeting to the KBO.
LORRI's primary measurement objectives and key characteristics are
summarized in Table I. LORRI has first successfully detected Pluto (on
September 21, 2006 at a distance of 28 AU), and its resolution at
Pluto will start exceeding that available from the Hubble Space
Telescope approximately 3 months prior to closest approach. En route
to Pluto, LORRI will obtain rotationally resolved phase curves of
Pluto and later Charon, once the two can be separately
resolved. LORRI will obtain panchromatic maps over at least 10 Pluto
rotations during approach, with the final complete map of the sunlit
hemisphere exceeding a resolution of 0.5 km/pixel. LORRI will map
small regions near Pluto's terminator with a resolution of ~50 m/pixel
near the time of closest approach, depending on the closest approach
distance selected. LORRI will also be heavily used for studies
requiring high geometrical fidelity, such as the determining the
shapes of Pluto, Charon, Nix, and Hydra and refining the orbits of all
these objects relative to the system barycenter. LORRI observations at
high phase angles will provide a sensitive search for any particulate
hazes in Pluto's atmosphere. Young et al. (2007) provides a more
detailed discussion of the scientific objectives addressed by LORRI
observations.
3.6 SWAP
The Solar Wind Around Pluto (SWAP) instrument is one of two particle
detection in situ instruments aboard New Horizons. It is comprised of
a retarding potential analyzer (RPA), a deflector (DFL), and an
electrostatic analyzer (ESA). Collectively, these elements are used to
select the angles and energies of solar wind ions entering the
instrument. The selected ions are directed through a thin foil into a
coincidence detection system: the ions themselves are detected by one
channel electron multiplier (CEM), and secondary electrons produced
from the foil are detected by another CEM. SWAP can measure solar wind
particles in the energy range from 25 eV up to 7.5 keV with a
resolution of delta(E)/E <0.4. SWAP has a fan-shaped field of view that
extends >200 degrees in the XY-plane of the spacecraft by >10 degrees
out of that plane (see Fig. 2). For typical observations, SWAP
measures solar wind speed and density over a 64 sec measurement
cycle. The principal measurement objectives and key characteristics of
SWAP are summarized in Table I. Further details on SWAP and its
performance can be found in McComas et al. (2007).
SWAP was designed to measure the interaction of the solar wind with
Pluto, which addresses the Group 1 scientific objective of measuring
Pluto's atmospheric escape rate. Additionally, SWAP has a specific
goal of characterizing the solar wind interaction with Pluto as a
Group 2 objective. SWAP also addresses the Group 3 objectives of
characterizing the energetic particle environment of Pluto and
searching for magnetic fields, which it does indirectly. For more
details on SWAP's scientific objectives, see McComas et al. (2007) and
Young et al. (2007).
3.7 PEPSSI
The Pluto Energetic Particle Spectrometer Science Investigation
(PEPSSI) is the other in situ particle measurement instrument aboard
New Horizons. It is a compact, radiation-hardened particle instrument
comprised of a time-of-flight (TOF) section feeding a solid-state
silicon detector (SSD) array. Each SSD has 4 pixels, 2 dedicated to
ions, and 2 for electrons. PEPSSI's field of view (FOV) is fan-like
and measures 160 degrees x 12 degrees, divided into six angular
sectors of 25 degrees x 12 degrees
each. Ions entering the PEPSSI FOV generate secondary electrons as
they pass through entrance and exit foils in the TOF section,
providing "start" and "stop" signals detected by a microchannel plate
(MCP). Particle energy information, measured by the SSD, is combined
with TOF information to identify the particle's composition. Each
particle's direction is determined by the particular 25 degrees sector in
which it is detected. Event classification electronics determine
incident mass and energy, with 12 channels of energy
resolution. Protons can be detected in the energy range 40-1000 keV,
electrons in the range 25-500 keV, and CNO ions in the range 150-1000
keV. TOF-only measurements extend to <1 keV for protons, to 15 keV for
CNO ions, and to 30 keV for N2+. TOF measurements are possible in the
range 1-250 ns to an accuracy of +/-1 ns. The geometrical factor for
ions is slightly larger than 0.1 cm2 sr. A typical measurement
includes 8-point spectra for protons and electrons and reduced
resolution energy spectra for heavier ions for all six look
directions. The mass resolution of PEPSSI varies with energy: for CNO
ions, it is <5 AMU for >1.7 keV/nucleon, and <2 AMU for >5
keV/nucleon. The principal measurement objectives and key instrument
characteristics of PEPSSI are summarized in Table I. McNutt et
al. (2007) provide a detailed discussion of PEPSSI and its
performance.
The PEPSSI design is derived from that of the Energetic Particle
Spectrometer (EPS), which is flying on the MESSENGER mission to
Mercury. PEPSSI has thinner foils than EPS, which enables measurements
down to smaller energy ranges. PEPSSI also has a slightly increased
geometric factor and draws less power than EPS. Both EPS and PEPSSI
trace back their heritage to a NASA PIDDP program in the 1990s to
develop a particle instrument for use on a Pluto flyby mission.
By measuring energetic pickup ions from Pluto's atmosphere, PEPSSI
provides information related to the atmospheric escape rate on Pluto,
which is a New Horizons Group 1 scientific objective. PEPSSI's primary
role, however, is to address the Group 3 objective of characterizing
the energetic particle environment of Pluto. Fluxes of energetic
pickup ions may be measured as far as several million kilometers from
Pluto (see Bagenal et al. 1997), and PEPSSI observations will be used
to determine the mass, energy spectra, and directional distributions
of these energetic particles (Bagenal & McNutt 1989). Secondarily,
PEPSSI will also provide low-resolution, supporting measurements of
the solar wind flux, complementing SWAP. Young et al. (2007) provides
a more detailed discussion of PEPSSI's scientific objectives.
3.8 VB-SDC
The Student Dust Counter (SDC), also known as the Venetia Burney SDC
in honor of the student who named Pluto in 1930, is an impact dust
detector that will be used to map the spatial and size distribution of
interplanetary dust along the trajectory of the New Horizons
spacecraft from the inner solar system to and through the Kuiper
Belt.
Unlike all of the other instruments, the VB-SDC was not part of the
original New Horizons proposal and was added by NASA as an Education
and Public Outreach (EPO) experiment. For the first time ever,
students were given the opportunity to design, build, and operate an
instrument for an interplanetary mission. (NASA-certified personnel
performed all quality assurance inspections and supervised the final
assembly.) Approximately 20 undergraduate physics and engineering
students at the University of Colorado worked on the VB-SDC and,
despite getting a rather late start, their instrument was the first to
be delivered to the New Horizons spacecraft.
The VB-SDC's sensors are thin, permanently polarized polyvinylidene
fluoride (PVDF) plastic films that generate an electrical signal when
dust particles penetrate their surface. The SDC has a total sensitive
surface area of ~0.1 m^2, comprised of 12 separate film patches, each
14.2 cm x 6.5 cm, mounted onto the top surface of a support panel. In
addition, there are two reference sensor patches mounted on the
backside of the detector support panel, protected from any dust
impacts. These reference sensors, identical to the top surface
sensors, are used to monitor the various background noise levels, from
mechanical vibrations or cosmic ray hits.
The entire support panel is mounted on the exterior of the New
Horizons spacecraft, outside the spacecraft multi-layer insulating
(MLI) blanket, facing the ram (-Y) direction. The VB-SDC observations
are most useful during the cruise phases of the mission, when the
spacecraft is spinning and the other instruments are turned
off. Thruster firings during 3-axis operations generate large VB-SDC
background signals, which make it very difficult to detect true IDP
impacts.
The VB-SDC was designed to resolve, to within a factor of ~2, the
masses of interplanetary dust particles (IDPs) in the range of 10-12 <
m < 10-9 g, which corresponds roughly to a size range of 1-10 microns in
particle radius. Bigger grains are also recorded, but their masses
cannot be resolved. With the characteristic spacecraft speed during
cruise of ~13 km/s, current models of the dust density in the solar
system (Divine, 1993) suggest that the VB-SDC should record
approximately 1 IDP hit per week.
The principal measurement objectives and key instrument
characteristics of the VB-SDC are summarized in Table I. Horanyi et
al. (2007) provide a detailed discussion of the VB-SDC and its
performance.
4. Science Payload Commissioning Overview
The New Horizons instrument commissioning activities began shortly
after the nominal performance of the spacecraft subsystems was
verified; this was approximately 1 month after launch. Over a period
of about 8 months, each instrument team developed a detailed set of
science activity plans (SAPs) to characterize the in-flight
performance and functionality, and to verify that their measurement
objectives could be achieved. Functional tests were executed first to
demonstrate that critical engineering parameters (e.g., currents,
voltages, temperatures, etc.) fell within their expected ranges. After
nominal functional performance was verified, a series of performance
tests were executed for each instrument. All of the commissioning
tests discussed below took place during calendar year 2006. A small
subset of commissioning activities (about 10% of the total) remain to
be completed during and after the Jupiter encounter in early 2007.
The instruments completed their functional tests during February-March
2006. The first observations of an external target are termed "first
light" observations, and these were staggered throughout the May to
September 2006 period for the various instruments. Alice detected
interplanetary hydrogen Lyman-alpha and Lyman-beta emission during its first
light observations on May 29. Alice then observed two UV calibration
stars, gamma Gruis and rho Leonis, on August 31. Owing to safety reasons,
the Alice SOCC door will not be opened until at least March 2007,
after the Jupiter encounter. Ralph/MVIC first light occurred during
observations of its stellar calibration targets (the M6 and M7
galactic open clusters) through its windowed door on May 10, and then
through the opened door on May 28. Both Ralph/MVIC and Ralph/LEISA
observed the asteroid 2002 JF56 in a moving target tracking test
during May 11-13. Ralph/LEISA made the first observations of its
calibration star (Procyon) on June 29. LORRI first light occurred when
it opened its aperture door on August 29 and observed M7. LORRI
observed M7 for an extensive set of calibration observations on
September 3, including a simultaneous observation with MVIC to measure
the relative alignments of those two instruments. Both Ralph and LORRI
observed Jupiter on September 4 as test observations in preparation
for the Jupiter encounter in February 2007. Ralph and LORRI also
observed Uranus and Neptune in September for optical navigation
testing. LORRI observed Pluto during observations on September 21 and
23, and the Jovian irregular satellite Himalia on September 22, again
as part of optical navigation testing. The first use of REX mode by
the telecommunications system took place on April 19. REX scanning
observations to measure the high gain antenna (HGA) beam pattern were
performed on June 20. Two radio calibration sources (Cass A and Taurus
A) and "cold sky" were observed on June 29 to measure the REX
radiometry mode performance. All REX calibration observations in 2006
were performed on side-A; side-B calibration observations were
executed in early January 2007. SWAP's door was opened on March 13,
but the first solar wind observations started in late-September and
continued through December. PEPSSI's door was opened on May 3, but its
ability to measure particles was first tested in June. The VB-SDC
attempted to take science data in early-March, but the spacecraft was
in 3-axis mode and the high VB-SDC background rate produced by the
nearly continual thruster firings made it essentially impossible to
detect real dust particle events. The VB-SDC had its first real chance
to detect dust particles while the spacecraft was in "passive" spin
mode (thruster firings still occur during "active" spin mode) in
April, but the relatively low count rate expected requires that the
instrument be well-calibrated and the data carefully analyzed,
Additional VB-SDC data was then taken from October through December
2006, while the spacecraft remained in spin mode.
5. In Flight Hibernation, Annual Checkouts, and Encounter Rehearsals
The New Horizons mission is exceptionally long in duration, with the
primary mission objective not being completed until nearly 10 years
after launch. Activities during the mission are generally either
front-end or back-end loaded, with the first 14 months busy with
instrument commissioning and the Jupiter encounter, and the last year
of the mission devoted to intensive observations of the Pluto
system. For most of the time during the 8 years between the encounter
phases (2007-2014, inclusive), the spacecraft will be placed into a
"hibernation" mode, with all non-essential subsystems, including the
scientific payload, powered off. This preserves component life.
During the hibernation period, beacon radio tones are sent
periodically from the spacecraft to the Earth that allow flight
controllers to verify the basic health and safety of the
spacecraft. Additionally, monthly telemetry passes are scheduled to
collect engineering trend data.
Although the spacecraft is kept in hibernation to reduce component use
prior to the Pluto-system encounter, it is also important to verify
periodically the performance of the spacecraft subsystems and
instruments, and to keep the mission operations team well trained and
prepared for the Pluto encounter activities. Therefore, the spacecraft
will be brought out of hibernation each year for roughly 60 days,
called "annual checkouts" (ACOs), during which time the performance of
the spacecraft subsystems and instruments can be verified. Generally,
the ACO instrument activities are comprised of a subset of the
commissioning activities that focus on the instrument's performance
(e.g., stellar calibration observations). ACOs are also the
opportunity for annual cruise science observations to be collected,
such as interplanetary charged particle measurements, studies of the
hydrogen distribution in the interplanetary medium, and extensive
phase curve studies of Pluto, Charon, Uranus, Neptune, Centaurs, and
KBOs, none of which can be obtained from spacecraft near Earth.
In addition, two full rehearsals of the Pluto encounter will be
conducted, during the summers of 2012 and 2014, respectively, that
will serve both to verify that the Pluto encounter sequence will work
and to provide essential training for the mission operations team in
preparation for the actual encounter.
6. Current Status of the Science Payload
All seven of the instruments comprising the New Horizons science
payload have essentially completed their in-flight commissioning
activities, with only a few tests remaining to be executed. In all
cases, the in-flight performance verifies that the science payload can
meet its measurement objectives, thereby accomplishing all of the
scientific objectives of the New Horizons mission.
The Ralph instrument was used to observe asteroid 2002 JF56 during a
serendipitous flyby at a closest approach distance of 100,000 km on
2006 June 13 (Olkin et al. 2006), which verified the spacecraf's
ability to track reliably a fast-moving target. The SWAP, PEPSSI, and
VB-SDC instruments also began taking scientific data, in addition to
commissioning data, during 2006.
All of the instruments will participate in the upcoming encounter with
Jupiter (with closest approach on 2007 February 28), which will be
considerably more ambitious than any of the activities executed to
date. In fact, the Jupiter encounter will likely have approximately
double the number of observations, and double the data volume,
compared to what is currently planned for the Pluto encounter in
2015. Most importantly, however, the Jupiter encounter provides an
invaluable and unique opportunity to test the mission's
capabilities. Even if some of the observations taken in the Jovian
system fail, the lessons learned from that encounter will undoubtedly
improve the prospects for a successful Pluto system encounter in 2015,
which is the most important activity of the New Horizons mission.
Acknowledgments
We thank all of the New Horizons Instrument Teams for their
extraordinary efforts in designing, developing, testing, and
delivering a highly capable science payload that promises to
revolutionize our understanding of the Pluto system and the Kuiper
belt. We also thank the numerous contractors who partnered with the
Instrument Teams for their outstanding work and dedication. Partial
financial support for this work was provided by NASA contract
NAS5-97271 to the Johns Hopkins University Applied Physics Laboratory.
References
Cheng, A.F. et al. 2007. Long Range Reconnaissance Imager on New
Horizons. Space Sci. Rev., this volume.
Divine, N. 1993. Five populations of interplanetary
meteoroids. J. Geophys. Res., 98, 17029-17051.
Grundy, W.M., Schmitt, B., and Quirico, E. 1993. The temperature
dependent spectra of alpha and beta nitrogen ice with application to
Triton. Icarus 105, 254-258.
Grundy, W.M., Buie, M.W., Stansberry, J.A., Spencer, J.R., and
Schmitt, B. 1999. Near-infrared spectra of icy outer solar system
surfaces: Remote determination of H2O ice temperatures. Icarus 142,
536-549.
Horanyi, M. et al. 2007. The Student Dust Counter on the New Horizons
mission. Space Sci. Rev., this volume.
McComas, D. et al. 2007. The Solar Wind Around Pluto (SWAP) instrument
aboard New Horizons. Space Sci. Rev., this volume.
McNutt, R. E. et al. 2007. The Pluto Energetic Particle Spectrometer
Science Investigation (PEPSSI) on New Horizons. Space Sci. Rev., this
volume.
Olkin, C.B., Reuter, D., Lunsford, A., Binzel, R. P., and Stern,
S. A. 2006. The New Horizons distant flyby of asteroid 2002 JF56. DPS
Meeting #38, abstract #59.22.
Quirico, E. and Schmitt, B. 1997. A spectroscopic study of CO diluted
in N2 ice: Applications for Triton and Pluto. Icarus 128, 181-188.
Reuter, D. et al. 2007. Ralph: A visible/infrared imager for the New
Horizons mission. Space Sci. Rev., this volume.
Rogers, G.D., Schwinger, M.R., Kaidy, J.T., Strikwerda, T. E., Casini,
R., Landi, A., Bettarini, R., and Lorenzini, S. 2006. Autonomous star
tracker performance, in Proc. The 57th IAC Congress, Valenica, Spain.
Stern, S.A. 2007. The New Horizons Pluto Kuiper belt Mission: An
Overview with Historical Context. Space Sci. Rev., this volume.
Stern, S.A. et al. 2007. Alice: The ultraviolet imaging spectrometer
aboard the New Horizons Pluto-Kuiper belt mission. Space Sci. Rev.,
this volume.
Tyler, L. et al. 2007. The Radio EXperiment (REX) on New
Horizons. Space Sci. Rev., this volume.
Young, L.A., et al. 2007, New Horizons: Anticipated scientific
investigations at the Pluto system. Space Sci. Rev., this volume.