Possible Science Instruments on a Spacecraft
This document contains basic descriptions of the purpose of some possible science instruments on a spacecraft. In addition, a
detailed description of each instrument from Basics of Space Flight by the
Jet Propulsion Laboratory, from Chapter 12. Typical Science
Instruments is included.
Direct Sensing Instruments
Direct-sensing instruments interact with phenomena in their immediate
vicinity, and register characteristics of them. The Heavy Ion
Counter on Galileo uses direct sensing; it registers the characteristics
of ions in the spacecraft's vicinity which enter the instrument.
It does not attempt to form any image of the ions' source.
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- Purpose: counts how much charge high-energy
(usually at least 20,000 electron Volts (20keV)) particles have and how many particles
it encounters. Used to measure radiation belts around planets.
Manned spacecraft must avoid radiation belts. Radiation in these belts will kill people
quickly, even if they are inside a spacecraft. Unmanned craft are also affected by these radiation belts
and being in them limit their lifespans. In addition, scientists
do not know how magnetic fields of planets interact with particles
to be able charge them at such high energies.
- Detailed description: High-energy Particle Detector instruments measure the energy spectra
of trapped energetic electrons, and the energy and composition
of atomic nuclei. They may employ several independent solid-state-detector
telescopes. The Cosmic Ray instrument on Voyager measures the
presence and angular distribution of electrons of 3-110 Million
electron Volts (MeV) and nuclei 1-500 Million electron Volts (MeV) from hydrogen
to iron. The Energetic Particle Detector on Galileo is sensitive
to the same nuclei with energies from 20,000 electron Volts to
10 million electron Volts.
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- Purpose: Detects low-energy particles by assuming plasma has a certain shape
and then computes measurements based on that. However, many plasmas
do not have that shape.
- Detailed description: Plasma detectors serve the low-end of particle energies. They
measure the density, composition, temperature, velocity and three-dimensional
distribution of plasmas, which are soups of positive ions and
electrons, that exist in interplanetary regions and within planetary
magnetospheres. Plasma detectors are sensitive to solar and planetary
plasmas, and they observe the solar wind and its interaction with
a planetary system.
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- Purpose: Tells about evolution of solar systems.
One stage of solar system formation is the production of dust.
A theory of solar system evolution is that this dust then clumps
together to form asteroids, then planets. Studying the dust's
charge tells whether the dust will avoid each other (like charged)
or if it can clump together. Shows how plasma has or is affecting the dust by the dust's distribution in a magnetosphere.
- Detailed description: Some spacecraft carry a dust detector which measures the velocity,
mass, charge, flight direction and number of dust particles striking
the instrument. Galileo's instrument can register up to 100 particles
per second and is sensitive to particle masses of between 10 and
10 g.
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- Purpose: Shows changes in magnetic fields around
planets over time. Shows composition of objects - if it has a
magnetic field, it must have magnetic materials (e.g. sand
would not have a magnetic field.) Composition tells us how the
object might have been formed.
- Detailed description: Magnetometers are direct-sensing instruments which detect and
measure the interplanetary and solar magnetic fields in the vicinity
of the spacecraft. They typically detect the strength of magnetic
fields in three planes. As a magnetometer sweeps an arc through
a magnetic field when the spacecraft rotates, an electrical signature
is produced proportional to the strength and structure of the
field.
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- Purpose: Scientists do not know how extremely
high-energy particles are formed. They think plasma waves may
be responsible, and study them to understand the astrophysics
of them. Shows the low-frequency plasma waves.
- Detailed description: Plasma wave detectors typically measure the electrostatic and
electromagnetic components of local plasma waves in three dimensions.
Plasma wave data provides key information on phenomena related
to the interaction of plasma and particles that control the dynamics
of a magnetosphere. The instrument functions like a radio receiver
sensitive to the wave lengths of plasma in the solar wind, from
about 10 Hz to about 60 kHz. When within a planet's magnetosphere,
it can be used to detect atmospheric lightning, and events when
dust and ring particles strike the spacecraft. Voyager's Plasma
Wave data has been used to produce digital sound recordings of
the particle bombardment the spacecraft experienced as it passed
through the ring planes of the outer planets.
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Remote-Sensing Science Instruments
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 radar waves which were
emitted by the spacecraft itself. When an instrument provides
the illumination, as does radar, it is referred to as an active
remote sensing instrument. If the illumination is not provided
by the instrument, as in the case of cameras observing planets
in sunlight, it is passive remote sensing.
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- Purpose: Same purpose as a plasma wave detector,
but it's remote sensing. Shows the high-frequency plasma waves.
- Detailed description: A planetary radio astronomy instrument measures radio signals
emitted by a target such as a Jovian planet. The instrument on
Voyager is sensitive to signals between about 1 kHz and 40 MHz,
and uses a dipole antenna 10 m long, which it shares with the
plasma wave instrument. The planetary radio astronomy instrument
detected emissions from the heliopause in 1993. Ulysses carries a similar instrument.
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- Multi-Spectral Imager (~1 degree field of
view)
- Purpose: Shows shape, surface morphology, and
color of object. Shape tells origin of the object. Surface morphology
tells about surface processes that have occurred/are occurring
like impacts. Color shows that the different rock types. Younger
rock types are on top of older. Tells how it got there, where
it came from.
- Wide-angle camera (~30 degree field
of view)
- Purpose: Shows low-resolution images covering
large areas. Good for monitoring weather.
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- Detailed description: Optical imaging is performed by two families of detectors: vidicons
and the newer charge coupled devices (CCDs). Although the detector
technology differs, in each case an image is focused by a telescope
onto the detector, where it is converted to digital data. Color
imaging requires three exposures of the same target, through three
different color filters selected from a filter wheel. Ground processing
combines data from the three black and white images, reconstructing
the original color by utilizing the three values for each picture
element (pixel). A vidicon is a vacuum tube resembling a small
CRT. An electron beam is swept across a phosphor coating on the
glass where the image is focused, and its electrical potential
varies slightly in proportion to the levels of light it encounters.
This varying potential becomes the basis of the video signal produced.
Viking, Voyager, and many earlier spacecraft used vidicon-based
imaging systems. A CCD is typically a large-scale integrated circuit
which has a two-dimensional array of hundreds of thousands of
charge-isolated wells, each representing a pixel. Light falling
on a well is absorbed by a photoconductive substrate, such as
silicon, and releases a quantity of electrons proportional to
the intensity of the light. The CCD detects and stores an accumulated
electrical charge representing the light level on each well. These
charges are subsequently read out for conversion to digital data.
CCDs are much more sensitive to light of a wider spectrum than
vidicon tubes, they are less massive, they require less energy,
and they interface more easily with digital circuitry. Galileo's
Solid State Imaging instrument (SSI) contains a CCD with an 800
x 800 pixel array. The cameras on the Mars Observer spacecraft
were unique in that they employed a single-dimensional CCD array.
The orbital motion of the vehicle over the surface of Mars supplied
the second dimension required for image formation.
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- Purpose: Tells about texture, e.g. bare rock
vs. loose stone. (not used much)
- Detailed description: Polarimeters are optical instruments which measure the direction
and extent of the polarization of light reflected from their targets.
Polarimeters consist of a telescope fitted with a selection of
polarized filters and optical detectors. Careful analyses of polarimeter
data can infer information about the composition and mechanical
structure of the objects reflecting the light, such as various
chemicals and aerosols in atmospheres, ring arcs, and satellite
surfaces reflect light with differing polarizations. The molecules
of crystals of most materials are optically asymmetrical; that
is, they have no plane or center of symmetry. Asymmetrical materials
have the power to rotate the plane of polarization of plane-polarized
light.
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- Purpose: Measures light intensity at one point
at a time. Intensity of light tells composition of object at that
point.
- Detailed description: Spectral photometers are optical instruments that measure the
intensity of light from a source. They may be directed at targets
such as planets or their satellites to quantify the intensity
of the light they reflect, thus measuring the object's reflectivity
or albedo. Also, photometers can observe a star while a planet's
rings or atmosphere intervene during occultation, thus yielding
data on the density and structure of the rings or atmosphere.
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Combinations
Sometimes various optical functions are combined into a single
instrument, such as photometry and polarimetry combined into a
photopolarimeter, or spectroscopy and radiometry combined into
a radiometer-spectrometer instrument.
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Scan Platforms
Optical instrument are sometimes installed on an articulated,
powered appendage to the spacecraft bus called a scan platform,
which points in commanded directions, allowing optical observations
to be taken independently of the spacecraft's attitude.
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Active Sensing Science Instruments
- Purpose: Gives a not-so-great image and distance
to object when camera won't work due to clouds or haze.
- Detailed description: Some solar system objects that are candidates for radar imaging
are covered by clouds or haze, making optical imaging difficult
or impossible. These atmospheres are transparent to radio frequency
waves, and can be imaged using Synthetic Aperture Radar (SAR)
instruments, which provide their own penetrating illumination
with radio waves. SAR synthesizes the angular resolving power
of an antenna many times the size of the antenna aperture actually
used. A SAR illuminates its target to the side of its direction
of movement, and travels a distance in orbit while the reflected,
phase shift-coded pulses are returning and being collected. This
provides the basis for synthesizing an antenna (aperture) on the
order of kilometers in size, using extensive computer processing.
For a SAR system to develop the resolution equivalent to optical
images, the spacecraft's position and velocity must be known with
great precision, and its attitude must be controlled tightly.
This levies demands on the spacecraft's AACS and requires spacecraft
navigation data to be frequently updated. SAR images are constructed
of a matrix where lines of constant distance or range intersect
with lines of constant Doppler shift. Magellan's radar instrument
alternated its active operations as a SAR imaging system and radar
altimeter, with a passive microwave radiometer mode several times
per second in orbit at Venus.
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- Laser Rangefinder
- Purpose: Gives very accurate distance to the
object. Used to get topography if keep a constant orbit. Also
used for navigation.
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- Radar Rangefinder
- Purpose: Older, less accurate version of laser
rangefinder.
- Detailed description: Radar pulses may be directed straight down to a planet's surface,
the nadir, from a spacecraft in orbit, to measure variations in
the height of terrain being overflown. The coded, pulsed signals
are timed from the instant they leave the instrument until they
are reflected back, and the distance is obtained by dividing by
the speed of light. Terrain height is then judged based upon knowledge
of the orbital position of the spacecraft. The Pioneer 12 spacecraft
and the Magellan spacecraft used radar altimeters at Venus. Laser
light may also be used in the same manner for altimetry. Laser
altimeters generally have a smaller footprint, and thus higher
spatial resolution, than radar altimeters. They require less power.
The Mars Global Surveyor spacecraft carries a laser altimeter
which used a small cassegrain telescope.
- Data Processing Unit/Power Supply
The Data Processing Unit on the spacecraft hold the data that will be collected from the instruments
and hold the software that runs the spacecraft. The power supply is needed to power the instruments.
You will need one DPU/PS for every three science instruments.
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