Spacecraft Instrument Considerations
SPACECRAFT SYSTEMS from
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Types of Spacecraft
A spacecraft is any piloted or unpiloted vehicle designed for travel in space. The systems and instruments
a spacecraft must carry depends upon the data it will gather and the functions it will carry out. Their
complexity varies greatly but all must endure the hostile environment of space. Spacecraft may be broadly
categorized according to the missions they are intended to fly.
Manned spacecraft: A piloted spacecraft designed to carry astronauts into space. Unlike an unmanned
probe, it requires a crew compartment and life support systems. Manned spacecraft are either reusable,
such as the Space Shuttle, or designed for one time use, such as Soyuz. The latter type is generally
modular, such as consisting of a reentry module which houses the crew and a service module which
contains propulsion, power supply and life support. Only the reentry module returns to Earth.
Flyby spacecraft: A spacecraft which follows a continuous trajectory past a target object, never to be
captured into an orbit. It must carry instruments that are capable of observing passing targets by
compensating for the target's apparent motion. The spacecraft must be able to survive long periods of
interplanetary cruise and must be able to downlink data at high rates. Examples of flyby spacecraft include
Pioneers 10 and 11, and Voyagers 1 and 2.
Orbiter spacecraft: A spacecraft designed to travel to a distant planet or moon and enter orbit. It must
carry a substantial propulsive capability to decelerate it at the right moment to achieve orbit insertion. An
orbiter spacecraft must endure periods during which it is shaded from sunlight, thus it must be resistant to
extreme thermal variation and will require power storage capacity if equipped with solar panels. Examples
of orbiter spacecraft include Magellan, Galileo and Mars Global Surveyor.
Atmospheric probe: A small instrumented craft which separates from the main spacecraft prior to
closest approach to a planet to study the gaseous atmosphere of the body as it drops through it. It is
equipped with an aeroshell to protect it during atmospheric entry and a parachute to slow its decent. Data
is typically telemetered to the mother craft where it is recorded for later transmission to Earth.
Atmospheric balloon package: An instrumented package suspended from a buoyant gas bag, deployed
in a planet's atmosphere to study wind circulation patterns. It has a limited complement of spacecraft
systems aboard. The minimum requirements include a power supply and telecommunications equipment
to permit tracking.
Lander spacecraft: A spacecraft designed to reach the surface of a planet or moon and survive long
enough to telemeter data back to Earth. It may perform a powered descent and landing or may descend
to the surface suspended from a parachute. Examples of lander spacecraft include Surveyor, Viking, Mars
Pathfinder and the Soviet Venera.
Surface penetrator: A probe designed to penetrate the surface of a body, surviving an impact of
hundreds of g's, measuring and telemetering the properties of the penetrated surface. Data is typically
telemetered to the mother craft for re-transmission to Earth. The Mars Polar Lander will deploy two
penetrators upon its arrival at Mars.
Surface rover: A semi-autonomous roving vehicle, steerable from Earth, deployed on the surface of a
planet or other body, taking images and soil analyses for telemetering back to Earth. The Mars Pathfinder
mission included a solar-powered, wheeled microrover named Sojourner.
Structural Subsystem
The spacecraft bus is a major part of the structural subsystem of a spacecraft which provides a place to
attach components internally and externally, and to house delicate modules requiring a measure of
thermal and mechanical stability. It is an integral card chassis for supporting the circuit boards of radio
equipment, data recorders, computers, gyroscopes and other components. The bus also establishes the
basic geometry of the spacecraft, and it provides the attachment points for appendages such as booms,
antennas and scan platforms.
Power Supply and Distribution
Power supply systems are those that produce electricity for use by other onboard systems and
instruments. The type of power system used on a spacecraft depends on factors such as the duration of
the mission and location at where the spacecraft must operate.
A spacecraft's electrical components can be switched on or off via command by using relays that connect
or disconnect the component from the common distribution circuit, called a main bus. On some spacecraft
it is necessary to power off some components before switching others on to keep the electrical load within
the limits of the supply. Typically a shunt-type regulator maintains a constant voltage from the power
source. The voltage applied as input to the regulator is generally variable but higher than the spacecraft's
required constant bus voltage. The regulator converts excess electrical energy into heat which is radiated
away into space.
Batteries: Devices with two or more connected cells that produce a direct current by converting chemical
energy into electrical energy. Due to their short lifetime, batteries are only used when a very short
operating life is required. They are often used in combination with solar panels to store power for use
during times when the panels are shaded from sunlight.
Photovoltaic cells: Crystalline wafers, called solar cells, which convert sunlight directly into electricity
without moving parts. These cells are grouped into an array and cemented onto a substrate. The resulting
assemblies are called solar panels or solar arrays. Solar power is practical for spacecraft operating no
farther from the sun than about the orbit of Mars.
Radioisotope thermoelectric generator: A device that converts the heat produced by the radioactive
decay of plutonium-238 into electricity by an array of thermocouples made of silicon-germanium
junctions. They are used when a spacecraft must operate at significant distances from to sun (usually
beyond the orbit of Mars). The Pu-238 is contained within a crash resistant housing.
Fuel cells: Cells in which chemical reaction is used directly to produce electricity. The reactants are
typically hydrogen and oxygen which results in water as a by-product. The water can then be used for
cooling and human consumption. Fuel cells are generally used on manned spacecraft.
Telecommunications
Telecommunication components for a particular spacecraft are chosen in response to the requirements of
the mission it will perform. Anticipated distances, planned frequency bands, data rates and available onboard transmitter power are all taken into account.
High-gain antenna: A dish-shaped spacecraft antenna principally used for high rate communication with
Earth. This type of antenna is highly directionally and must be pointed to within a fraction of a degree of
Earth. It may be either steerable or fixed to the spacecraft bus. High-gain antennas are typically used on
interplanetary spacecraft.
Low-gain antenna: An omnidirectional spacecraft antenna that provides relatively low data rates at close
range, several astronomical units for example. Many spacecraft include both a low-gain antenna and a
high-gain antenna.
Medium-gain antenna: A spacecraft antenna that provides greater data rates than a low-gain antenna,
with wider angles of coverage than a high gain antenna, about 20-30 degrees.
Transmitter: An electronic device that generates and amplifies a tone at a single designated radio
frequency, called a carrier wave. The carrier wave can be sent from the spacecraft to Earth as it is, or it
can be modulated with a data-carrying subcarrier. The transmitter radiates the resulting signal, called
downlink, from an antenna.
Receiver: An electronic device that receives incoming radio signals, called uplink, and converts them to
perceptible forms. The received uplink is stripped of its command-data-carrying subcarrier which is
converted into binary code, which is then typically passed to the spacecraft's command and data
subsystem.
Frequently, transmitters and receivers are combined into one electronic device which is called a
transponder.
Data Handling
The onboard computer responsible for overall management of a spacecraft's activity is generally the same
one which maintains timing, interprets commands from Earth, collects, processes and formats the
telemetry data which is to be returned to Earth, and manages high-level fault protection and safing
routines. This computer is sometimes referred to as the command and data subsystem. For convenience
that term will be used here, recognizing that other names may apply to similar subsystems or sets of
subsystems which accomplish some or all of the same tasks.
Spacecraft clock: A counter maintained by the command and data subsystem. It meters the passing of
time during the life of the spacecraft, and regulates nearly all activity within the spacecraft systems. Many
types of commands uplinked to the spacecraft are set to begin execution at specific spacecraft clock
counts.
Telemetry: The system for radioing information from a spacecraft to the ground. Telemetry is typically a
mixture of science data from the experiments and spacecraft engineering or health data. Engineering or
health data is composed of a wide range of measurements, from switch positions and subsystem states to
voltages, temperatures and pressures. Telemetry may be transmitted in real time, or it may be written to
a data storage device until transmission is feasible.
Tape recorder: A mechanical device for recording digital information on magnetic tape and for playing
back the recorded material. The stored data can be played back for downlink when receiving station
resources are available.
RAM: Random access memory, the solid-state equivalent of a tape recorder. Banks of RAM can store large
quantities of digital information without any moving parts.
Fault protection: Algorithms, which normally reside in more than one of a spacecraft's subsystems, that
insure the ability of the spacecraft to both prevent a mishap and to reestablish contact with Earth if a
mishap occurs and contact is interrupted.
Attitude and Articulation Control
A spacecraft's attitude, its orientation in space, must be stabilized and controlled so that its high-gain
antenna may be accurately pointed to Earth, so that onboard experiments may accomplish precise
pointing for accurate collection and subsequent interpretation of data, so that the heating and cooling
effects of sunlight and shadow may be used intelligently for thermal control, and so that propulsive
maneuvers may be executed in the right direction.
Spin stabilization: Stabilization accomplished by rotating the spacecraft mass, thus using gyroscopic
action as the stabilizing mechanism. Thrusters are fired to make desired changes in the spin-stabilized
attitude. Spin-stabilized spacecraft provide a continuous sweeping desirable for fields and particle
instruments, but they may require complicated systems to de-spin antennas or optical instruments which
must be pointed at targets.
Three-axis stabilization: Stabilization accomplished by nudging a spacecraft back and forth within a
deadband of allowed attitude error, using small thrusters or reaction wheels. Three-axis controlled
spacecraft can point optical instruments and antennas without having to de-spin them, but they may have
to carry out rotation maneuvers to best utilized their fields and particle instruments.
Reaction wheels: Electrically-powered wheels mounted in three orthogonal axes aboard a spacecraft. To
rotate the vehicle in one direction, you spin up the proper wheel in the opposite direction. To rotate the
vehicle back, you slow down the wheel. Excess momentum that builds up in the system due to external
torques must be occasionally removed from the system via propulsive maneuvers.
Attitude and articulation control subsystem: The onboard computer that manages the tasks involved
in spacecraft stabilization via its interface equipment. For attitude reference, star trackers, star scanners,
solar trackers, sun sensors, and planetary limb trackers are used. Gyroscopes are carried for attitude
reference for those periods when celestial references are not being used. The AACS also controls the
articulation of a spacecraft's moveable appendages such as solar panels, high-gain antennas, and optical
instrument scan platforms.
Propulsion Subsystems
In order to maintain or restore three-axis stability, to control spin, to execute maneuvers and make minor
adjustments in trajectory, spacecraft are provided with sets of propulsive devices. These devices are
typically of either the hypergolic bi-propellant type or the mono-propellant type. Many if the activities of
the propulsion subsystems are routinely initiated by the attitude and articulation control subsystem. Some
or all may be directly controlled by or through the command and data subsystem.
Engines: The larger of a spacecraft's propulsive devices, perhaps producing a force of several hundred
Newtons, used to provide the large torques necessary to maintain stability during a solid rocket motor
burn, or they may be the rockets used for orbit insertion.
Thrusters: A set of small propulsive devices, typically generating between less than 1 N and 10 N, used
to provide the delta-V required for interplanetary trajectory correction maneuvers, orbit trim maneuvers,
reaction wheel desaturation maneuvers, or routine three-axis stabilization or spin control.
Propellant: The fuel and oxidizer burned to produce thrust in a rocket engine. Propellant subsystems
include propellant tanks, plumbing, valves, and helium tanks to supply pressurization for the propellant
tanks.
Pyrotechnic Subsystems
Pyrotechnics are electrically initiated explosive devices used to operate valves, ignite solid rocket motors,
and explode bolts to separate from or jettison hardware, or to deploy appendages. They obtain their
electrical power from a bank of capacitors which are charged from the main bus several minutes prior to
the planned detonation of a device.
Environmental Subsystems
A spacecraft must operate and survive in the hostile environment of space and, in most cases, it must
have a usable life of many years. Delicate Spacecraft components must be protected from this harsh
environment. Environmental subsystems are those designed to protect spacecraft components from
extreme thermal variations, micrometeoroid bombardment, and other space hazards.
Passive cooling: The use of painting, shading, reflectors and other techniques to cool a spacecraft.
Internal components are typically painted black to radiate heat more efficiently. White thermal blankets
are used to reflect infrared radiation, helping to protect a spacecraft from excess solar heating. Critical
components are generally shaded using gold or optical solar reflectors. Mechanical louvers are frequently
used to control thermal radiation from within parts of a spacecraft.
Active heating: The use of resistive electric heaters or radioisotope heaters to keep spacecraft
components above their minimum allowable temperatures. Electric heaters can be controlled either
autonomously or via command, while radioisotope heaters are used where it is necessary to provide
components with a permanent supply of heat.
Micrometeoroid protection: Shielding used to protect spacecraft components from micrometeoroid
impacts. Interplanetary spacecraft typically use tough blankets of Kevlar or other strong fabrics to absorb
the energy from high-velocity particles.
Life support: The subsystems aboard a manned spacecraft or space station responsible for maintaining a
livable environment within a pressurized crew compartment. Included are subsystems for providing
oxygen, drinking water, waste processing, temperature control, ventilation and CO 2 removal.
Landing Subsystems
A spacecraft designed to land on a planet or moon must be provided with subsystems to slow its descent
and guide it to a soft landing. The craft may be equipped with propulsion for making a powered descent,
or a parachute to retard free-fall. A piloted spacecraft, such as the Space Shuttle, may be equipped with
wings for a glided landing. If the craft is to pass through an atmosphere it must be equipped with
shielding to protect it from the high temperatures generated during atmospheric entry.
Heat shield: A device that protects crew and equipment from heat, such as an ablative shield in front of a
reentry capsule or atmospheric probe.
Parachute: An apparatus used to retard free fall, consisting of a light, usually hemispherical canopy
attached by cords and stored folded until deployed in descent.
Drogue: A small parachute used to slow and stabilize a spacecraft, usually preceding deployment of a
main landing parachute.
Descent engine: The rocket used to power a spacecraft as it makes a controlled landing on the surface of
a planet or moon.
Science Instruments
Spacecraft designed to carry out scientific missions are typically equipped with a large complement of
science instruments, covering large portions of the electromagnetic spectrum, and including both remote
and direct-sensing components.
Direct-sensing instruments interact with phenomena in their immediate vicinity, and register
characteristics of them. Remote-sensing instruments record characteristics of objects at a distance,
sometimes forming an image by gathering, focusing, and recording reflected light from the Sun, or
reflected radio waves emitted by the spacecraft. When an instrument provides the illumination, as does
radar, it is referred to as an active remote-sensing instrument.
Direct-Sensing Instruments
High-energy particle detector: A device for measuring the energy spectra of trapped energetic
electrons, and the energy and composition of atomic nuclei.
Low-energy charged particle detector: A device designed to characterize the composition, energies,
and angular distributions of charged particles in interplanetary space and within planetary systems.
Plasma detector: A device for measuring the density, composition, temperature, velocity and threedimensional distribution of plasmas that exist in interplanetary regions and within planetary
magnetospheres.
Dust detector: A device for measuring the velocity, mass, charge, flight direction and number of dust
particles striking the instrument.
Magnetometer: A device for measuring the strength and direction of the interplanetary and solar
magnetic fields. They typically detect the strength of magnetic fields in three planes.
Plasma wave detector: A device for measuring the electrostatic and electromagnetic components of
local plasma waves in three dimensions.
Remote-Sensing Instruments
Imaging instruments: Optical imaging is performed by two families of detectors: vidicons and the newer
charged coupled devices. Although the detector technology differs, in each case an image in focused by a
telescope onto the detector where it is converted to digital data.
Vidicon: An imaging device consisting of a vacuum tube, which produces a video signal based on the
varying electrical potential measured as an electron beam is swept across a phosphor coating on the glass
where the image is focused.
Charged coupled device: An imaging device consisting of a large-scale integrated circuit which has a
two-dimensional array of hundreds of thousands of charge-isolated wells, each representing a pixel.
Infrared radiometer: An optical instrument that measures the intensity of infrared energy radiated by
its targets.
Polarimeter: An optical instrument that measures the direction and extent of the polarization of light
reflected from its targets.
Photometer: An optical instrument that measures the intensity of light from a source.
Spectrometer: An optical instrument that splits the light received from an object into its component
wavelengths by means of a diffraction grating; then measuring the amplitudes of the individual
wavelengths.
Scan platform: An articulated, powered appendage to the spacecraft bus which points in commanded
directions, allowing optical observations to be taken independently of the spacecraft's attitude.
Active Sensing Instruments
Synthetic aperture radar: A radar imaging instrument which provides a penetrating illumination of radio
waves, and is capable of imaging surfaces covered by clouds and haze. SAR images are constructed of a
matrix where lines of constant distance or range intersect with lines of constant Doppler shift.
Altimeter: A device that measures altitude above the surface of a planet or moon. Spacecraft altimeters
work by timing the round trip of radio signals bounced off the surface.
Mars Science Laboratory Instruments
Science Instrument Details
Visit the MSL Science Corner to view technical information about the science instruments.
Science instruments are state-of-the-art tools for acquiring information about the geology, atmosphere,
environmental conditions, and potential biosignatures on Mars. Mars Science Laboratory will carry:
Cameras
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Mast Camera (Mastcam)
Mars Hand Lens Imager (MAHLI)
Mars Descent Imager (MARDI)
Spectrometers
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Alpha Particle X-Ray Spectrometer (APXS)
Chemistry & Camera (ChemCam)
Chemistry & Mineralogy X-Ray Diffraction/X-Ray Fluorescence Instrument (CheMin)
Sample Analysis at Mars (SAM) Instrument Suite
Radiation Detectors
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Radiation Assessment Detector (RAD)
Dynamic Albedo of Neutrons (DAN)
Environmental Sensors
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Rover Environmental Monitoring Station (REMS)
Atmospheric Sensors
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Mars Science Laboratory Entry Descent and Landing Instrument (MEDLI)
Mars Exploration Rovers
These are the primary science instruments carried by the rovers:
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Panoramic Camera (Pancam): for determining the mineralogy, texture, and
structure of the local terrain.
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Miniature Thermal Emission Spectrometer (Mini-TES): for identifying
promising rocks and soils for closer examination and for determining the
processes that formed Martian rocks. The instrument is designed to look
skyward to provide temperature profiles of the Martian atmosphere.
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Mössbauer Spectrometer (MB): for close-up investigations of the
mineralogy of iron-bearing rocks and soils.
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Alpha Particle X-Ray Spectrometer (APXS): for close-up analysis of the
abundances of elements that make up rocks and soils.
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Magnets: for collecting magnetic dust particles. The Mössbauer Spectrometer
and the Alpha Particle X-ray Spectrometer are designed to analyze the
particles collected and help determine the ratio of magnetic particles to nonmagnetic particles. They can also analyze the composition of magnetic
minerals in airborne dust and rocks that have been ground by the Rock
Abrasion Tool.
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Microscopic Imager (MI): for obtaining close-up, high-resolution images of
rocks and soils.
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Rock Abrasion Tool (RAT): for removing dusty and weathered rock surfaces
and exposing fresh material for examination by instruments onboard.
Mars Polar Lander Instruments
Mars Polar Lander
Spacecraft Description
 Mars Volatiles and Climate
Surveyor (MVACS) instrument
package which was comprised of a
robotic arm and attached camera,
mast-mounted surface stereo imager
and meteorology package, and a gas
analyzer.
 Mars Descent Imager (MARDI)
was planned to capture regional views from parachute deployment at about 8
km altitude down to the landing.
 The Russian Space Agency provided a laser ranger (LIDAR) package for the
lander, which would be used to measure dust and haze in the Martian
atmosphere.
 A miniature microphone was also on board to record sounds on Mars.
 Attached to the lander spacecraft were a pair of small probes, the Deep Space
2 Mars Microprobes, which were to be deployed to fall and penetrate beneath
the Martian surface when the spacecraft reached Mars.