Chapter 12: Science Instruments

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 (see the illustration in Chapter 1). Ulysses carries a similar instrument.

An imaging instrument uses optics such as lenses or mirrors to project an image onto a detector, where it is converted to digital data. Natural color imaging requires taking three exposures of the same target in quick succession through different color filters, typically selected from a filter wheel. Earth-based processing combines data from the three black and white images, reconstructing the original color by utilizing the three values for each picture element (pixel). Movies are produced by taking a series of images over an extended period of time.

In the past, the detector that created the image was a vacuum tube resembling a small CRT (cathode-ray tube), called a vidicon. In a vidicon, signals applied to deflection coils and focus coils sweep an electron beam from a heated cathode (electron source) across a photoconductor coating inside the tybe's glass front where the image is focused. Light striking the photoconductor causes its grains to leak their electric charge in proportion to the light's intensity. The sweeping beam's current would vary as it recharges depleted grains' charges to the point they repel the beam's electrons. This current becomes the basis for the digital video signal produced. Viking, Voyager, and Mariner spacecraft used vidicon-based imaging systems. A vidicon requires a fairly bright image to detect.

Modern spacecraft use charge-coupled devices (CCD). A CCD is usually a large-scale integrated circuit having a two-dimensional array of hundreds of thousands, or millions, 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 light. The CCD detects and stores accumulated electrical charge representing the light level on each well over time. These charges are 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, require less energy, and they interface more easily with digital circuitry. It is typical for CCDs to be able to detect single photons.

Galileo's Solid State Imaging instrument (SSI), which pioneered the technology, contains a CCD with an 800 x 800 pixel array. The optics for Galileo's SSI, inherited from Voyager, consist of a Cassegrain telescope with a 176.5-mm aperture and a fixed focal ratio of f/8.5. Since the SSI's wavelength range extends from the visible into the near-infrared, the experimenters are able to map variations in the satellites' color and reflectivity that show differences in the composition of surface materials.

Not all CCD imagers have two-dimensional arrays. The HIRISE instrument, or High-Resolution Imaging Science Experiment, on the Mars Reconnaissance Orbiter (MRO) spacecraft has detectors made of single lines of CCD sensors. A two-dimensional image is built up as the image of the Martian surface moves across this one-dimensional detector while the spacecraft moves in orbit, much as a page moves across the detector in a fax machine. HIRISE often achieves a resolution of less than 25cm per pixel. The Mars Orbiter Camera (MOC) on the Mars Global Surveyor spacecraft, also used a single-dimension detector.

Cassini carried a unique instrument that had never been flown before in the outer solar system. The Magnetospheric Imaging Instrument (MIMI) Ion and Neutral Camera (INCA) could form images of the giant magnetic envelopes of Jupiter and its main objective, Saturn, as well as fields associated with Saturn's moons.

MIMI INCA doesn't use CCDs to make images. In fact it doesn't even use light at all. MIMI INCA is more like a particle detector, although unlike most particle detectors, it is actually a remote sensing instrument. MIMI INCA senses ions and neutral atoms that have been flung out of a planet's magnetosphere, forming an image of the source of the particles.

Polarimeters are optical instruments that 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 analysis 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, rings, and satellite surfaces, since they reflect light with differing polarizations. A polarimeter's function may be integrated with another instrument, such as a camera, or the Voyager photopolarimeter that combines functions with a photometer.

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.

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. One of the three instruments on the Spitzer Space Infrared Facility (SIRTF) is a photometer designed to measure the intensity of stars in the infrared.

Spectroscopy (see Chapter 6) provides a wealth of information for analysis of observed targets. The field of spectroscopy includes methods for analysis of virtually every part of the electromagnetic spectrum. Most are differentiated as either atomic or molecular based methods, and they can be classified according to the nature of their interaction:

Spectroscopic instruments are optical remote-sensing instruments that split the light received from their targets into component wavelengths for measuring and analyzing. In general, a spectroscope provides spectral results visible to the eye, for example through an eyepiece on the instrument. Spectrometers and spectrographs use electronic detectors, such as CCDs, and return data representing the intensity of various parts of the spectrum observed. Instruments on spacecraft typically employ a diffraction grating to disperse the incoming light signal, while a spectroscope might employ either a prism or a grating.

A Compact Disc (CD) or Digital Versatile Disk (DVD) offers an example of a spectroscopic diffraction grating. Observing a bright light shining on the microscopic data tracks on its surface demonstrates the effect diffraction gratings produce, separating light into its various wavelength, or color, components. The apparent separation of wavelengths is due to constructive and destructive interference of light waves reflecting at varying angles from the grating lines according to their wavelengths. A similar effect can be seen from a thin layer of oil floating on water.

Inexpensive "diffraction glasses" provide another effective way to explore spectroscopy at home or in the classroom.

Spectrometers carried on spacecraft are typically sensitive in the infrared, visible, and ultraviolet wavelengths. The Near-Infrared Mapping Spectrometer, NIMS, on Galileo mapped the thermal, compositional and structural nature of its targets using a two-dimensional array of pixels.

An imaging or mapping spectrometer provides spectral measurements for each of the many the pixels of an image the instrument obtains, thus supplying spectral data for many different points on a target body all at once.

Cassini's ultraviolet instrument is the Ultraviolet Imaging Spectrograph, UVIS. Its infrared instrument is the Composite Infrared Mapping Spectrometer, CIRS. Cassini's Visible and Infrared Mapping Spectrometer, VIMS, produce images whose every pixel contains spectral data at many different wavelengths. It is as though the instrument returns a whole stack of images with each observation, one image at each wavelength. Such data units are frequently called "cubes."

An infrared radiometer is a telescope-based instrument that measures the intensity of infrared (thermal) energy radiated by the targets. One of its many modes of observing is filling the field of view completely with the disc of a planet and measuring its total thermal output. This technique permits the planet's thermal energy balance to be computed, revealing the ratio of solar heating to the planet's internal heating. Another application measures Mars's atmospheric and surface thermal properties.

Follow the links below for additional instrument descriptions:

As mentioned above, sometimes various optical instrumentation 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. One example was Galileo's Photopolarimeter Radiometer instrument, PPR. Another example is the Voyagers' infrared interferometer spectrometer and radiometers, IRIS.

CCD detectors in imagers and some spectrometry instruments perform best when they are cool. In infrared instruments, cooling essential for maintaining a signal-to-noise ratio that permits useful observations. Fortunately, it is easy to cool some detectors in flight. On Cassini, for example, each of the optical detectors is mounted to a thermally conductive metal part, that thermally connects to a radiator facing deep space. As long as flight operations keeps Sunlight off the radiator, the detector's heat will radiate away into space. Detector temperatures in the neighborhood of 55 K can easily be maintained in this manner.

Observatory spacecraft instruments dedicated to sensing infrared or longer wavelengths need more elaborate cooling. The Hubble Space Telescope's NICMOS (Near Infrared Camera and Multi-Object Spectrometer) originally used solid nitrogen coolant to reduce the detector temperature to 40 K, although it proved to be problematic. The Spitzer Infrared Telescope Facility (SIRTF) achieves a temperature of 1.4 K for its detectors by vapor cooling with helium effluent. BICEP (Background Imaging of Cosmic Extragalactic Polarization), is a ground-based instrument being developed to measure the polarization of the cosmic microwave background from Antarctica. Its detectors are cooled to 250 milli-Kelvens using a helium refrigeration system.

Optical instruments 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. This is the case on Voyager and Galileo. Most newer spacecraft are designed without scan platforms, since a scan platform adds to a spacecraft's complement of mechanical and other subsystems, increasing mass and potential for failure. The alternative is to mount optical instruments directly to the spacecraft and rotate the entire spacecraft to point them. This approach is feasible due to the availability of large-capacity data storage systems that can record data while the spacecraft is off Earth-point carrying out observations.

A spacecraft's altimeter sends coded radio pulses, or laser-light pulses, straight down to a planet's surface (the nadir) to measure variations in the height of the terrain below. The signals are timed from the instant they leave the instrument until they are reflected back, and the round-trip distance is obtained by dividing by the speed of light, and factoring in known equipment processing delays. Dividing by two then approximates the one-way distance between the instrument and the surface. Actual terrain height is then deduced based upon precise knowledge of the spacecraft's orbit.

The Pioneer 12 spacecraft and the Magellan spacecraft used radar altimeters at Venus. Laser altimeters generally have a smaller footprint, and thus higher spatial resolution, than radar altimeters. They also require less power. The Mars Global Surveyor spacecraft carried a laser altimeter that used a small Cassegrain telescope. Known as MOLA for Mars Orbiter Laser Altimeter, its technology formed the basis for an experiment flown in Earth orbit in 1997 by the Space Shuttle.

Each of these linked pages is a complete list of all of the science instruments on a spacecraft, with links to information about each instrument. Among them you'll find direct- and remote-sensing instruments, and active and passive sensing instruments.

For additional information:

NASA: