| INSTRUMENT_HOST_DESC |
This description is based on several sources used with the
permission of the New Horizons project, SWRI and JHU/APL:
- Stern & Spencer, New Horizons: The First Reconnaissance Mission to
Bodies in the Kuiper Belt, 2004 [STERN&SPENCER2004A]
- The New Horizons web page originally at
http://pluto.jhuapl.edu/
Overview
========
The New Horizons spacecraft observatory includes propulsion,
navigation, and communications systems, plus the payload. The
spacecraft is roughly 2.5 meters across and its mass is 465 kg
including propellant. Design features include 64 Gbits of redundant
solid-state data storage, a 290 m/s propulsion budget, and the
capability to transmit data from 32 AU at almost 1 kilobit/second.
The instrument payload [Stern & Cheng, 2002, STERN&CHENG2002]
comprises the two-sensor RALPH Vis-IR remote sensing package, the
ALICE UV imaging spectrograph, the REX radio/radiometry experiment,
the two-sensor PEPSSI/SWAP plasma suite, the LORRI long-focal-length
imager, and the SDC student-built dust counter.
Payload
=======
The New Horizons team selected instruments that not only directly
measure NASA-specified items of interest (NASA AO 01-OSS-01, 2001,
[NASAAO2001]), but also provide backup to other instruments on the
spacecraft should one fail during the mission.
The payload comprises seven instruments:
RALPH
-----
The main objectives for the RALPH instrument are to obtain high
resolution color maps and surface composition maps of the surfaces
of Pluto and Charon. The instrument has two separate channels: the
Multispectral Visible Imaging Camera (MVIC) and the Linear Etalon
Imaging Spectral Array (LEISA). A single telescope with a 3-inch
(6-centimeter) aperture collects and focuses the light used in both
channels.
RALPH/MVIC operates at visible wavelengths and has 4 different
filters for producing color maps. One filter allows measurement of
the methane frost distribution over the surface (860-910nm), while
the others are more generic and cover blue (400-550nm), red
(540-700nm) and near-infrared colors (780-975nm), respectively. MVIC
also has two panchromatic filters that pass essentially all visible
light (400-975nm). This will be useful for low-light level
observations requiring maximum sensitivity. In all cases, the light
passes from the telescope through the filters and is focused onto a
charge coupled device (CCD).
RALPH/LEISA operates at infrared wavelengths (1.25-2.5 micron,
plus a separate section of higher resolving power covering 2.1 to
2.25 micron); its etalon (wedged filter with a narrow spectral
bandpass that varies linearly in one dimension) is bonded to the
illuminated side of the IR detector. As a result, each row of
detector pixels receives only light of a particular wavelength.
Spectral maps are produced by sweeping the FOV of the instrument
across a scene, sequentially sampling each point in the scene at each
wavelength. LEISA maps the distribution of frosts of methane (CH4),
molecular nitrogen (N2), carbon monoxide (CO), and water (H2O) over
the surface of Pluto and the water frost distribution over the
surface of Charon. LEISA data may also reveal new constituents on
the surfaces that have never before been detected.
ALICE
-----
Alice is an ultraviolet imaging spectrograph that probes the
atmospheric composition of Pluto.
The Alice wavelength range is from 520 - 1870 Angstroms. Alice
has two modes of operation: an airglow mode, which measures
emissions from atmospheric constituents, and an occultation mode,
which views either the Sun or a bright star through the atmosphere
producing absorption by the atmospheric constituents. The Alice
occultation mode occurs just after New Horizons passes behind Pluto
and looks back at the Sun through the Pluto atmosphere.
REX
---
REX is an acronym for Radio EXperiment. It is integrated into the
New Horizons radio telecommunications system.
Using an occultation technique similar to that described above for
the Alice instrument, REX probes the Pluto atmosphere. After New
Horizons flies by Pluto, its 2.1 meter radio antenna points back at
Earth. On Earth, powerful radio transmitters in the NASA Deep Space
Network (DSN) point at New Horizons and send radio signals to the
spacecraft. As the spacecraft passes behind Pluto, the atmosphere
bends the radio waves by an amount that depends on the average
molecular weight of the gas in the atmosphere, the atmospheric
temperature, and the closest approach distance of the raypath at
each instant of time. REX samples the received radio signal and
sends the data back to Earth for analysis
REX also has a radiometry mode, which measures the weak radio
thermal emission from Pluto itself. When REX looks back at Pluto
following the flyby, radiometry data are taken to derive a value for
the Pluto nightside temperature.
LORRI
-----
The instrument that provides the highest spatial resolution on New
Horizons is LORRI - short for LOng Range Reconnaissance Imager -
which comprises a telescope with a 20.8cm aperture that focuses
visible light (350 - 850nm) onto a charge coupled device (CCD).
LORRI has a very simple design; there are no filters or moving
parts. Near the time of closest approach, LORRI takes images of
the Pluto surface at 100m resolution.
SWAP
----
The Solar Wind Analyzer around Pluto (SWAP) instrument measures
charged particles from the solar wind near Pluto to determine
whether Pluto has a magnetosphere and how fast the atmosphere is
escaping.
PEPSSI
------
Another plasma-sensing instrument, the Pluto Energetic Particle
Spectrometer Science Investigation (PEPSSI), searches for neutral
atoms that escape the Pluto atmosphere and subsequently become
charged by their interaction with the solar wind.
SDC
---
The Student Dust Counter, which was later re-named The Venetia
Burney Student Dust Counter (SDC), is an Education and Public
Outreach project. SDC measures the dust density of the
Interplanetary Dust Particles (IDP) by measuring the charge
generated in the SDC sensor from dust impact events. From this may
be inferred the size and distribution of dust particles along the
entire New Horizons trajectory, including regions of interplanetary
space never before sampled. Such dust particles are created by
comets shedding material and Kuiper Belt Objects (KBOs) colliding
with other KBOs. The SDC is managed and was built primarily by
students at the University of Colorado in Boulder, with supervision
from professional space scientists and engineers.
The SDC is located on the -Y side of the spacecraft near the -X edge
of that side, near the star trackers, so it will be near the
direction-of-flight side of the spacecraft during most cruise, spin
and hibernation activities.
Spacecraft reference frame (a.k.a. Coordinate system)
=====================================================
During hibernation and other periods of inactivity, the spacecraft is
designed to spin about its +Y axis, which is also the nominal
boresight of the High Gain Antenna (HGA) and REX. Imaging instruments
have nominal boresights pointing along the -X spacecraft axis. The
RTG (see Power below) is a cylinder extending out along the +X
spacecraft axis to keep it away from the instruments. The +Z axis
completes a right-handed three-dimensional Cartesian coordinate
system. Note that each instrument has its own reference frame.
The following two sketches, extracted from the SPICE Frames kernel,
represent the spacecraft as viewed from the spacecraft +X and +Y
directions. The instrument locations are approximate; refer to
[STERNETAL2008] and [FOUNTAINETAL2008] for more detail.
Spacecraft sketches
===================
+X view:
--------
o
/|\
/ | \
/ | \
/ | \
/ | \
/ | \
___________________/______|______\__________________
`-. HGA(REX) ,-'
`-. ,-'
`-. ,-' __
`-.____________________________,-' / / PEPSSI
__________/_\________________________/_\_____|___|
.-| | | |______
Alice | | | RTG | | ||
'-| | .-*-. | |_____|| SWAP
| | / \ | | ||
|----| | \ / | | ||
| | | '-.-' | |
Ralph |___ | | | |
| |________________|_______________|________________|
[_________|_ _ _ _] +X (out of page)
SDC* /__<------o_________\
+Zsc | adapter ring
|
|
V
-Ysc
* N.B. In the graphic above, SDC
is behind, i.e. in the -X direction
from, the adapter ring
+Y view:
--------
______
------
|| SWAP
----
_|__|______ __..---..__
| | \ _`-' ``-. HGA(REX)
PEPSSI | ---- _' `-_ `-.
| .' `-_ `.
.-| , `-_ `.
LORRI : | . `-_ `.
: | / `-_ \
'-|. `-_ . _______ _______
|' .-*-. `-_ ||+|+|+|+| |+|+|+|+|
| SDC** / \ `|--`-------------------|
+ - - + ! o-----> +X | | |
| \ | / _,|--.-------------------|
ASTR 1 \ |. | '-|-' _,- ||+|+|+|+| |+|+|+|+|
\\|' | _,- ' ------- -------
Star \| ' | V _,- / RTG (Radioisotope
Trackers | ` +Z _,- . Thermoelectric
/+ - - + _,- - Generator)
//| `. _,- .'
ASTR 2 / | '. _,- _.-'
|__________',-__ __,,,''
| | '' --- ''
| |
`-----' Alice and Ralph
** N.B. In the graphic above, SDC
is behind, i.e. on the -Y side
of, the spacecraft
Communications
==============
The spacecraft has three antenna systems: Low-, Medium- and High-Gain
Antennas (LGA, MGA, HGA). The New Horizons mission operations team
communicates with the spacecraft through the Deep Space Network (DSN).
The DSN comprises facilities in the Mojave Desert in California; near
Madrid, Spain; and near Canberra, Australia.
Power
=====
Electrical power for the New Horizons spacecraft and science
instruments is provided by a single radioisotope thermoelectric
generator, or RTG, supplied by the Department of Energy. The New
Horizons trajectory takes it into the Kuiper Belt and more than six
billion kilometers from Earth, where light from the Sun is over
1,800 times fainter than at Earth. An RTG is used on missions, such
as New Horizons, that can not use solar power yet require a proven,
reliable power supply that can produce up to several kilowatts of
power and operate under severe environmental conditions for many
years.
Carrying out the New Horizons mission safely is a top priority at
NASA. As part of that focus, NASA informed the public about use by New
Horizons of an RTG by publishing a detailed Environmental Impact
Statement - or EIS - and several fact sheets. The Final EIS, which
includes public comments on the Draft EIS and the NASA responses to
those comments, was released in July 2005.
Propulsion
==========
The propulsion system (see [FOUNTAINETAL2008], section 3) includes
twelve 0.8N thrusters, four 4.4N thrusters, and the hydrazine
propellant tank and associated control valves. The titanium
propellant/pressurant tank feeds the thrusters through a system
filter, a flow control orifice, and a set of latch valves that prevent
flow of the fuel until commanded to the open position after launch.
Helium was selected as the tank pressurant instead of nitrogen to
allow the loading of an additional kilogram of hydrazine. Measurements
of tank pressure and temperatures at various points in the system
allow the mission operations team to monitor system performance and
the amount of fuel remaining in the tank.
The 16 rocket engine assemblies (REAs) are organized into 8 sets and
placed on the spacecraft as shown in Figure 5 of [FOUNTAINETAL2008].
Pairs of the 0.8N thrusters (each thruster from a different set) are
usually fired to produce torques and control rotation about one of the
three spacecraft axes. The one exception to the use of coupled thruster
firings to control spacecraft rates is that of controlling rates about
the spacecraft X axis during science observations, where uncoupled
thruster firings are required to meet the maximum spacecraft drift
rates allowed during this operation mode. Control rates for each of
the spacecraft axes are shown in [FOUNTAINETAL2008] Table 2. One pair
of the 4.4N thrusters is aligned along the -Y spacecraft axis to
provide delta-V for large propulsive events such as trajectory
correction maneuvers (TCMs). The second pair of 4.4N thrusters is
aligned to produce thrust along the +Y axis. These thrusters are
rotated 45 degrees in the YZ plane to minimize the plume impingement
on the HGA dish. The net propulsive effect of these thrusters is
therefore reduced. They still provide the required redundancy and the
ability to generate thrust in both directions without a 180-degree
rotation of the spacecraft.
Each thruster requires a heater to warm its catalyst bed to a minimum
temperature prior to use. Each thruster catalyst bed has both a
primary and a secondary heater element, with each element drawing
approximately 2.2 W of power. Control of the catalyst bed heater
circuits is grouped functionally by pairs (to minimize the number of
switches required), so that a total of 16 switches control the heater
elements, allowing great flexibility to operate the spacecraft safely
while drawing the minimum required power.
The pulse duration and total on-time of each thruster are commanded
very precisely, providing accurate control of the total impulse
generated during a maneuver. The 0.8N thrusters can be turned on for
periods as short as 5 ms. The initial propellant load was allocated
between primary mission TCMs, attitude control (including science and
communication operations), and primary mission margin. At the end of
the primary mission, sufficient margin may allow for an extended
mission to one or more objects in the Kuiper Belt. The original margin
was augmented during the final mission preparations when the unused
dry mass margin was converted to additional propellant.
Given the mass and moments of inertia at launch, the delta-V
propellant cost is approximately 4.9 m/s/kg. A change in spin rate of
5 rpm (i.e., the change from the nominal spin rate to zero rpm for
3-axis control mode) requires approximately 0.125 kg of hydrazine.
Propellant budget allocations
-----------------------------
delta-V Propellant
Description m/s kg
----------------------------- ------- ----------
Primary mission TCM 110 22.3
Attitude control N/A 29.3
Primary mission margin 132 25.2
- original margin allocation ( 91) (17.5)
- Additional margin obtained ( 41) (29.3)
from unused spacecraft dry
mass allocation
Total navigation delta-V 242
Total propellant load 76.8
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