NP-119 Science in Orbit: The Shuttle & Spacelab Experience, 1981-1986

 

Chapter 6

Sampling the Atmosphere: Atmospheric Science

 


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aft view of the space shuttle cargo bay with the earth in the background and a sun rise or sunset.

 

[71] Present knowledge of the atmosphere is immense compared to what we knew when the space age began three decades ago, but what we have yet to learn is still great. More over, we do not fully understand the roles we play in altering our atmosphere as we burn fossil fuels, use spray cans, and test nuclear weapons. Scientists worry about a multitude of factors that may turn our planet into a hothouse or an icebox.

The atmosphere is far more than oxygen and nitrogen; that familiar mix is roughly constant only to an altitude of about 100 kilometers (60 miles). As temperature changes with altitude, the pace at which some chemical reactions occur changes, and intensified sunlight causes new reactions like the splitting of oxygen molecules and the formation of ozone. Above this homosphere is the heterosphere where the chemical ratios change radically with altitude. Chemicals considered to be trace compounds are present at higher altitudes in greater ratios, although the total is still small.

Atmospheric chemistry, driven by light and a bewildering array of products which themselves modulate the light passing to Earth, becomes more complex and our understanding becomes less certain. Eliminating that uncertainty requires a global view and an inventory not only of the relative abundance of chemicals at various altitudes in the atmosphere but also of their energy states, which dictate the reactions in which they may take part.

Atmospheric chemistry is a complex, interactive process with seemingly small changes leading to extensive chain reactions. When an atom captures a photon of the right wavelength (i.e., energy), its energy state is raised. Usually within millionths or thousandths of a second, the photon is released as the atom returns to its ground state. The wavelength of this...

 


Chemical constituents and reactions change with altitude in different atmospheric layers.

Chemical constituents and reactions change with altitude in different atmospheric layers.

 

[72] ...emitted photon is a unique atomic or molecular signature. With such spectral signatures, the presence and energy states of chemicals can be detected at great distances. Spacelab has carried several instruments that have detected these signatures and started detailed analyses of our atmosphere's energy, chemistry, and movement.

The Shuttle and Spacelab offer atmospheric scientists a platform for global viewing over a broad latitude and altitude range. From this well-situated observatory, it is possible to make a complete chemical inventory of the different atmospheric regions and study the entire atmosphere as a system. Larger, more capable instruments can be carried on the Shuttle than on other satellites, and the Shuttle's resources (power, telemetry, crew) support advanced observational techniques. A variety of experiments to date proved the merits of the Shuttle and Spacelab as host observatories for atmospheric imaging and spectral measurement devices.

Energy: As the sun warms Earth, it prompts a chain of chemical reactions in the middle and upper atmosphere. These reactions change the transparency of the atmosphere, causing other changes at lower altitudes; greater fluxes of damaging ultraviolet radiation may pass to the ground, or infrared radiation (heat) emitted by the ground may be trapped rather than emitted, as in a greenhouse. The first concern is the total energy flow since life on Earth is so dependent on the constant sun emitting energy within a narrow range. Even a 0.1 percent shift in either direction could have a noticeable effect on the average temperature of the Earth and hence its climate. Yet measurements made to date vary by as much as 5 percent because of differences among and within instruments. Since the atmosphere is an unpredictable filter, these measurements can be made accurately only from orbit.

 


The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) measured solar ultraviolet output in the wavelength band from 120 to 400 nanometers. The blue line records high resolution measurements, and the red line marks low resolution measurements.

The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) measured solar ultraviolet output in the wavelength band from 120 to 400 nanometers. The blue line records high resolution measurements, and the red line marks low resolution measurements.

 

For the first time, solar ultraviolet radiation measurements by two independent instruments agree within a few percent. The thin black line on the upper part of the chart is the Spacelab 2 SUSIM spectrum and the thick red line is the Spacelab 1 Solspec spectrum. Both were made at 10 nanometers resolution.

For the first time, solar ultraviolet radiation measurements by two independent instruments agree within a few percent. The thin black line on the upper part of the chart is the Spacelab 2 SUSIM spectrum and the thick red line is the Spacelab 1 Solspec spectrum. Both were made at 10 nanometers resolution.

 

This drawing depicts large instruments flown on Spacelab 1 for atmospheric and plasma physics research. Larger, more capable instruments can be carried on the Shuttle than on other satellites, and the Shuttle's power and data processing support advanced observation techniques.

This drawing depicts large instruments flown on Spacelab 1 for atmospheric and plasma physics research. Larger, more capable instruments can be carried on the Shuttle than on other satellites, and the Shuttle's power and data processing support advanced observation techniques.

 

[73] The Solar Constant (SolCon) and the Active Cavity Radiometer (ACR) instruments are designed to monitor the total solar radiation output. Each uses the same basic principle: a cavity is alternately exposed to the sun and then concealed while an identical one is kept concealed. Both cavities are heated to the same temperature, so the difference in power consumption corresponds to the total incoming solar energy.

SolCon, one of three radiometers used as a World Radiation Reference, measured the solar output at 1,365 watts per square meter. This concentration is slightly less than all the energy of a 100 watt light bulb falling on a sheet of legal paper. The ACR had some equipment problems that compromised the Spacelab 1 measurements, but a similar unit on the Solar Maximum satellite is operating well. A single set of measurements from either instrument is only a start, as the data necessary for an accurate measurement must be gathered over years and must be compared both with instruments that stay in orbit and with laboratory test data.

It is not enough to know the total energy output of the sun; we must also know how it is distributed across its spectrum of light emissions and how that varies with solar activity. The Solar Spectrum (SolSpec) instrument and the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) measured this distribution. These solar instruments are designed for recalibration in terrestrial laboratories to assure their continued accuracy on reflights.

SolSpec comprises three spectrometers to cover the spectrum from 170 nanometers (1,700 Angstroms, tar ultraviolet) to 3,200 nanometers (32,000 Angstroms, infrared). Operating at or near its planned accuracy, SolSpec obtained 35 high-quality solar spectra sets. SUSIM measured ultraviolet intensities in the 120 to 400 nanometer (1,200 to 4,000 Angstroms) region, which represents less than 1 percent of the solar output but varies widely and affects the balance of ozone and other chemicals in the stratosphere. It comprises two spectrometers, one for continual measurement and the other for regular calibration. SUSIM recorded spectra at high resolution with great accuracy. The SUSIM and SolSpec data were compared and for the first time two independent instruments have made measurements that agree within a few percent. These spectra together with repetition of these measurements over a solar cycle will answer questions regarding solar variability in the ultraviolet and will help scientists understand what energies are available to drive chemical reactions in the atmosphere.

 

Chemistry: Three Spacelab instruments - the Imaging Spectrometric Observatory (ISO), the Atmospheric Trace Molecules Spectroscopy (ATMOS), and the Grille Spectrometer - have assayed the makeup of the middle and upper atmosphere by observing how chemical species emit or absorb radiation.

ISO, actually five spectrometers in one facility, covers the spectrum from 30 to 1,270 nanometers (300 to 12,700 Angstroms). Each spectrometer focuses light from a narrow strip of the atmosphere - 20 kilometers (12 miles) - on solid-state detectors through a spectral grating that breaks a band of light into its colors. Pictures of portions of the atmosphere's structure can be generated in specific spectral lines or colors.

 


From space, scientists can study atmospheric emissions that are invisible from the ground.

From space, scientists can study atmospheric emissions that are invisible from the ground. The Imaging Spectrometric Observatory (ISO) measured several constituents simultaneously over an altitude range of 20 kilometers (12 miles).

 

[74] ISO (Spacelab 1 ) obtained a wealth of information about emissions from the middle atmosphere (or mesosphere) and the thermosphere extending above it. ISO also compiled the first comprehensive spectral atlas of the upper atmosphere, a data base rich in information on several chemical processes. Many unexpected effects were observed that may require years of analysis to be understood. In addition to surveying the natural atmosphere, ISO gathered data on the induced atmosphere around the Shuttle.

Outstanding simultaneous spatial and spectral images were recorded of several bright emission bands of oxygen, nitrogen, and sodium at around 80 to 100 kilometers (50 to 60 miles) altitude, forming a unique data set for studying the photochemistry of the mesosphere. At higher altitudes, anomalous spectral distributions from molecular nitrogen ions were detected, indicating that photochemical activity may be raising them to high vibrational states. What role this has in atmospheric chemistry is not yet known.

While ISO measures direct light emissions from the atmosphere, ATMOS measures elements illuminated by sunlight. Based on the interferometer principle, ATMOS is designed so that all incoming light except that of the desired wavelength cancels itself out. In 1 second, ATMOS takes 400,000 samples for a single interferogram covering the spectrum from 2,000 to 16,000 nanometers (20,000 to 160,000 Angstroms, near to far infrared). During the Spacelab 3 mission, ATMOS obtained approximately 1,200 atmospheric spectra, each of which contained information on the prime molecular species being studied by investigators. In addition, almost 1,500 full solar spectra were collected and are being used to make a high-resolution solar spectral atlas.

 


These ISO spectra show how atmospheric emissions vary with altitude.

These ISO spectra show how atmospheric emissions vary with altitude.

 

This diagram summarizes the vertical range of detection for some of the chemicals observed by Atmospheric Trace Molecules Spectroscopy (ATMOS).

This diagram summarizes the vertical range of detection for some of the chemicals observed by Atmospheric Trace Molecules Spectroscopy (ATMOS). The colors denote different groups of atmospheric constituents. The first group is minor gases commonly found in the atmosphere; the rest are trace gases grouped by chemical families. Notable among the results are the detection of trace species that had not been observed previously and the first measurement by remote sensing techniques of the principal natural halocarbon, methylene chloride (CH3CI)

 

ATMOS extended the altitude ranges over which some 30 molecular species are known. At least five molecules -dinitrogen pentoxide, chlorine nitrate, carbonyl fluoride, methyl chloride [75], and nitric acid were found in the stratosphere where their presence only had been suspected. Measurements of other known molecular species in the stratosphere were three to tour times more precise than previous data.

The new data show all the nitrogen species at the same time so they can be added to the family of nitrogen-oxygen compounds that figure prominently in much of atmospheric chemistry. Equally important, by not detecting other gases, ATMOS effectively ruled them out as major actors in atmospheric chemistry. Measurements of the mesosphere showed this layer of the atmosphere to be more active than expected, with many minor gases being split by sunlight to start other chemical reactions. The distribution of many compounds, particularly methane and water, and of molecules in the polar atmospheres differed from prediction.

The Grille Spectrometer (Spacelab 1) was designed to observe the atmosphere's constituents from 15 to 150 kilometers (10 to 95 miles) altitude in the 2,500 to 10,000 nanometer (25,000 to 100,000 Angstrom) band. Its name comes from a special grille used as a window for one leg of its optical system and as a mirror for the other to overcome the limitations of many conventional instruments.

The Grille discovered methane in the mesosphere from 50 kilometers (30 miles) up, a higher altitude than previously observed or expected. Methane traces the vertical migration of gases because it comes largely from biological decay and, to a lesser extent, fossil fuel burning. The Grille also observed ozone, water vapor and nitrous oxide in the mesosphere, and carbon monoxide and carbon dioxide in the thermosphere above 85 kilometers (55 miles).

While these instruments were designed to survey the entire makeup of the atmosphere, the Measurement of Air Pollution from Space (MAPS) instrument looked for just one component, carbon monoxide. Its source,...

 


ATMOS confirmed the presence of the trace species (CIONO2) in the stratosphere by acquiring several spectra in large sets.

ATMOS confirmed the presence of the trace species (CIONO2) in the stratosphere by acquiring several spectra in large sets. One advantage of Shuttle/Spacelab flights is that several data sets can be acquired and averaged together to accent marginal features that are often overlooked.

 

This ATMOS figure demonstrates that dramatic atmosheric effects occur throughout the electromagnetic spectrum.

This ATMOS figure demonstrates that dramatic atmospheric effects occur throughout the electromagnetic spectrum. The feature marked by A is Freon-11; B and C are two transitions of nitric acid (HNO3); D is a Freon-12 absorption feature: E is carbon dioxide (CO2); and beyond F, tightly packed lines of ozone (O3) dominate the spectrum. (This spectrum does not represent the best resolution of the ATMOS instrument.)

 

As shown in this spectrum, the Spacelab 1 Grille Spectrometer measured methane in the mesosphere, where it had not been measured before.

As shown in this spectrum, the Spacelab 1 Grille Spectrometer measured methane in the mesosphere, where it had not been measured before. Since its only atmospheric source is at ground level, methane has been used by atmospheric physicists to model how constituents are transported upward through atmospheric layers.

 

[76] ....surprisingly, is largely natural - the decay of organisms. But man's industrial contribution is believed to be approaching nature's output, and "sinks" that absorb carbon monoxide are not well known. Using a small carbon monoxide gas cell to filter out unwanted signals, MAPS measured carbon monoxide at levels of a few parts per billion in the middle and upper atmosphere and as high as 114 parts per billion in the region over central Africa. Data from the second mission look equally precise.

 

Dynamics: The location of atmospheric chemicals is not static but ever changing in ways not studied by weather satellites. Two Spacelab instruments were designed to observe unique aspects of this motion, and a third modeled stellar and planetary atmospheres.

The upward migration of gases through the atmosphere can be traced with deuterium (heavy hydrogen). The Atmospheric Lyman-Alpha Emissions detector (ALAE, Spacelab 1), in a manner similar to MAPS, used small hydrogen gas cells as filters for the slightly different wavelengths of Lyman-alpha, a "color" emitted by hydrogen and deuterium. ALAE made the first measurements of atomic deuterium in the atmosphere and saw the auroras in the Northern and Southern hemispheres. It also detected the glow of hydrogen atoms and free protons (hydrogen nuclei) colliding and exchanging electrical charges in the corona of hydrogen gas that envelops Earth.

 


As the sun set, the GRILLE Spectrometer obtained these spectra of water vapor.

As the sun set, the GRILLE Spectrometer obtained these spectra of water vapor. The uppermost spectrum is an average of water vapor measured between 200 and 250 kilometers (125 to 155 miles), while the rest of the spectra were obtained at lower altitudes. The arrows indicate the strongest water vapor signatures.

 

This global chart of the Measurement of Air Pollution from Space data shows carbon monoxide levels measured at a few parts per billion (ppbv) in the middle and upper atmosphere.

This global chart of the Measurement of Air Pollution from Space data shows carbon monoxide levels measured at a few parts per billion (ppbv) in the middle and upper atmosphere. Measurements as high as 114 ppbv (light pink) were recorded in the region over central Africa.

 

[77] The motion of atmospheres on a planetary scale was studied with the Geophysical Fluid Flow Cell (GFFC, Spacelab 3), a simulated planet. Tabletop circulation models of the atmosphere have been used for decades but are limited because, in effect, they have to be flat, which precludes laboratory study of atmospheric dynamics on a full sphere or hemisphere. Only in the microgravity environment of space can scientists generate true three-dimensional experiment models on mathematical scales that exceed ground tests and computer simulations.

The GFFC sandwiched a silicone oil "atmosphere" in a hemispherical capacitor formed by a rotating sapphire dome and a metal sphere. Electrical force fields provided "gravity" and the inner sphere was heated to mimic planetary atmospheres and the solar interior. A 16-mm movie camera with an inverted fisheye lens photographed global flow patterns (as revealed by dyes and schlieren patterns) resulting from fluid density changes.

More than 50,000 images were taken in 103 hours of simulations. Among the expected features were longitudinal "banana" cells like those believed to exist beneath the surface of the sun. What u as not expected was that the tips of the banana cells seemed to interact with standing waves encircling the pole. Under different conditions, new phenomena were seen such as spiral waves emanating from the pole; these may be similar to gas flow on Uranus. More discoveries are anticipated as the pictures are analyzed in greater detail.

 


In space, scientists can generate more accurate models of atmospheric features such as the great red spot on Jupiter.

In space, scientists can generate more accurate models of atmospheric features such as the great red spot on Jupiter.

 

Microgravity allows scientists to generate true three-dimensional models of atmospheric convection patterns on planets, the sun, and other stars.

Microgavity allows scientists to generate true three-dimensional models of atmospheric convection patterns on planets, the sun, and other stars. The image above made in the Geophysical Fluid Flow Cell reveals long "banana" cells that may be similar to convection cells on the sun. As the parameters were changed, a more turbulent convection pattern (below) evolved.

 

A Global Survey of the Atmosphere: The earl Spacelab missions have given atmospheric physicists detailed views of slices of the atmosphere. New species have been detected at various altitudes, and the impacts of natural and human activity are evident; however, the atmosphere changes quickly with effects [78] rippling from one atmospheric layer to the next. Continuous observation of the entire atmosphere is needed to study with accuracy these dynamic processes as they unfold.

To achieve this goal, instruments will be deployed on platforms that can be controlled from the Space Station or the ground. The Shuttle/Spacelab has carried large and complex instruments into low-Earth orbit; these will be used to design even more sophisticated instruments for platforms.

As on Spacelab missions, instruments attached to the platforms and the Space Station will use remote sensing techniques to detect atmospheric phenomena. Middle and upper atmospheric interactions vary greatly with latitude; therefore, the platforms will be in polar orbits, allowing them to measure the detailed physics of the atmosphere at different latitudes.

Continuous observations will allow atmospheric scientists to study how the atmosphere responds to variations in the solar cycle and to solar stimuli. Campaigns to study the sun-Earth system can be coordinated with solar and plasma physicists working at the Space Station. This teamwork will provide an understanding of the relationship between changes in the sun and the resulting changes in Earth's atmosphere.

Several types of instruments are needed to study the interactive atmosphere. Observatory class instruments will provide a data base for a broad range of investigations from single samples of atmospheric processes to long term studies of diurnal, seasonal, and solar cyclic responses. Instruments can be programmed to operate at high data rates for collecting sets of measurements on natural events, such as solar flares or the solar wind, as they affect the atmosphere. They also can operate in a "sentry" mode at low data rates to record temperature features and the subtle changes that trigger major events.

Most instruments will be attached to the polar platform operated from the ground, but some can be attached to the Space Station. The Space Station will be important for calibrating sensi-....

 


The Atmospheric Laboratory for Applications and Science (ATLAS) mission aboard the Shuttle will produce an even more precise atlas of atmospheric constituents and more accurate measurements of solar output.

The Atmospheric Laboratory for Applications and Science (ATLAS) mission aboard the Shuttle will produce an even more precise atlas of atmospheric constituents and more accurate measurements of solar output.

 

[79] ...-tive instruments. This is especially needed for instruments measuring solar output because they must be very accurate. The Space Station crew will be needed to check out new instruments and repair and refurbish existing ones.

The next step beyond Space Station will be to deploy a platform in a higher orbit; this will enable the atmosphere to be studied simultaneously and continuously. While low-Earth orbit platforms provide greater coverage, it is only by getting higher above Earth that the whole atmosphere can be viewed at once. From higher orbits, scientists will be able to investigate the effects of sudden changes such as magnetic storms or solar flares quickly and globally. It will be possible to make global maps of constituents such as ozone and measure atmospheric features at all latitudes simultaneously.

To add to the catalogue of existing data and prepare for future operations, more flights of the Shuttle/Spacelab are planned. The Atmospheric Laboratory for Applications and Science (ATLAS) will be a comprehensive environmental observatory built around instruments from Spacelabs 1, 2, and 3: the Space Experiments with Particle Accelerators (SEPAC), the Atmospheric Emissions Photometric Imager (AEPI), the Imaging Spectrometric Observatory (ISO), the Atmospheric Trace Molecules Spectroscopy (ATMOS), and the solar constant and solar ultraviolet monitors. New instruments planned for the ATLAS series include a backscatter instrument to measure that portion of the sun's ultraviolet output which is reflected back into space and a scanning microwave radiometer to monitor rainfall locations and intensities from space. This series of missions will measure changes in solar energy output and the distribution of key molecular species in the middle atmosphere. These investigations will reveal new areas of study to be probed as operations are expanded for continuous, global coverage.

 

Atmospheric Science Investigations

OSTA-1/STS-2

OSTA-3/41-C

Measurement of Air Pollution from Space (MAPS) *

H. G. Reichle, NASA Langley Research Center, Hampton, Virginia

.

Night/Day Optical Survey of Lightning (NOSL) *

B. Vonnegut, State University of New York, Albany, New York

.

OSS-1/STS-3

Spacelab 2/51-F

Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) *

G.E. Brueckner, Naval Research Laboratory, Washington, D.C.

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Spacelab 1/STS-9

Active Cavity Radiometer (ACR)

R.C. Willson, NASA Jet Propulsion Laboratory, Pasadena, California

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Grille Spectrometer

M. Ackerman, Space Aeronomy Institute, Brussels, Belgium

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Imaging Spectrometric Observatory, (ISO)

M.R. Torr, NASA Marshall Space Flight Center, Huntsville, Alabama

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Investigation of Atmospheric Hydrogen and Deuterium through Measurement of Lyman-Alpha Emission (ALAE)

J.L. Bertaux, National Center for Scientific Research, Paris, France

.

Solar Constant (SolCon)

D. Crommelynck, Royal Meteorological Institute, Brussels, Belgium

.

Solar Spectrum (SolSpec)

G. Thuillier, National Center for Scientific Research, Paris, France

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Waves in the OH Emissive Layer

M. Herse, National Center for Scientific Research, Paris, France

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Spacelab 3/51-B

Atmospheric Trace Molecules Spectroscopy (ATMOS)

C.B. Farmer, NASA Jet Propulsion Laboratory, Pasadena, California

.

Geophysical Fluid Row Cell (GFFC)

J.E. Hart, University of Colorado, Boulder, Colorado

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* Reflight


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