Remote Scientific Sensors

The First Instrumented Spacecrafts

The first space-radiation detectors onboard a spacecraft included two gas-discharge detectors (Geiger counters), designed by S.N. Vernov. Three photomultiplier tubes were installed by S.L. Mandel'shtam, to study the Lyman-α wavelength of atomic hydrogen and solar X-ray emissions. A wheel of filters included beryllium foil (0.1 - 1 nm), aluminum foil (1 - 2.2 nm), polyethylene (4.4 - 12 nm), lithium fluoride (121.6 nm) and an unfiltered opening. The photomultipliers and filter wheels can be seen above spaced apart by 120°, with the geiger counters and their solid-state electronics spanning two edges of the triangle.

The most sophisticated experiment on Sputnik-2 was the life support system and biometric sensors for the dog Laika. The compartment contained an oxygen supply, carbon-dioxide scrubbers, a feeding trough and a waste collecting system. Sensors transmitted electrocardiograms, pulse rate, blood pressure, respiration rate, body temperature, reflex response, and general activity. Some thermal insulation was torn loose by the ejection of the protective shell, and overheating resulted in the premature death of Laika two days after launch. Contrary to some reports, Sputnik-2 did not contain a camera.

Sputnik-3 set the bar for scientific instrumentation on spacecrafts. Overseen by M.V. Keldysh, the committee for Object-D worked from 1954, planning and building the scientific experiments onboard the satellite. Many of these experiments became part of a canonical set of sensors installed on virtually every Soviet spacecraft: micrometeorite detectors, cosmic-ray counters, ion traps and magnetometers. As sophisticated as Sputnik-3 was, its operation was hampered by the failure of its onboard tape recorder, and only real-time telemetry over the Soviet Union could be received.

The first major discovery of the space age was the existence of Earth's radiation belts, where charged-particle counts increase to 1000 times the normal rate. Geophysicists had suspected their existence since the theoretical work of Carl Størmer in 1906 and simulations with magnetic spheres in vacuum chambers. But this work was not widely known among astronomers, and Explorer-1's first measurements of the belts came as a surprise. An outer radiation belt was first observed by Sputnik-3, and a complete mapping of the Earth's magnetosphere gradually formed as a result of numerous American and Soviet probes.

Following the three Sputniks, the Soviet Union continued nonstop, launching hundreds of scientific satellites, including the heavy Spacecraft-Sputniks, the Electron and Proton satellites, and countless Kosmos satellites. Unlike the American program, scientific and military missions sometimes shared the same vehicles.

Micrometeorite Detectors

Impacts proved to be rare (about once a day on a square meter of surface), but during meteor showers this rate can increase by 1000 or more. Deep space probes like Mars-1 and Zond-3 found regions of high dust concentration, also associated with meteor showers and orbits of comets.

Early spacecraft containing published results from Nazarova's detectors were: Sputnik-3, Luna-1,2,3,10,11,12, Venera-1,2,3 Mars-1, Zond-2,3. Beginning with Mars-1, sensors were attached to the rear of the solar panels, using 1.5 square meters of their area as a detector. Nazarova's micrometeorite detectors were installed on nearly all planetary probes.

Magnetometers

Dolginov's Magnetometers on Luna-1 to Luna-3 discovered that the Moon has almost no field. Luna-2 data suggested a field of less than 50 nT (its dipole field is today known to be zero). Sputnik-3 had the first spacecraft magnetometer, but it was used primarily to measure the satellite's orientation. His devices were included on virtually all Soviet spacecrafts, including Sputnik-3, Luna-1,2,10, Venera-1,2,3, Zond-3, Mars-1 and later missions. Kosmos-26 and 49 were used to make detailed maps of the Earth's field.

The first planetary probe, Venera-1, had a triaxial fluxgate magnetometer, to measure the field of Venus (if contact had not been lost). It also had two parallel magnetic variometers to measure the magnitude of the faint interplanetary field. Readings from Venera-1 at a distance of 1.9 million km, and from Mars-1 at far greater distances indicated interplanetary fields of 3 to 4 nT. Dolginov's results from the Mars-3 orbiter suggested a weak Martian magnetic field, but recent measurements suggest it is from solar wind, not the planet's core.

Ion Traps

Sputnik-3 carried spherical ion traps on the end of two booms, designed by K.I. Gringauz. These varied grid voltage to return "retardation spectra" of plasma energy. In 1959, Luna-1 to Luna-3 had four hemispherical ion traps with outer grid voltages set from -10 to +15. The presence of a uniform plasma wind of about 400 km/sec was indicated by periodic readings as the probe rotated. This was first observed on Luna-1, and Gringauz made improvements to confirm the solar wind with Luna-2 (e.g., changing the design and arrangement of traps, from coplanar to tetrahedral).

In 1961, Venera-1 carried two ion traps on its sunward side, with outer grid voltages of 0 and +50. Data from the three successful telemetry downloads verified the solar wind's existence to a distance of 1.9 million km. Mars-1 verified the solar wind to a distance of over 100 million km. Gringauz' ion traps were on nearly every Soviet planetary probe and numerous satellites.

Mars-1 and later probes also carried flat modulation traps, which could measure the energy spectrum of solar plasma, from 0 to 3600 eV, by sweeping a range of retardation potentials. Hemispherical and modulation traps were typically mounted on the side of the orbital module, below the astrosensor dome.

More sophisticated devices were deployed to measure the direction and energy spectrum of the interplanetary plasma. Electrostatic analyzers were first flown on the Elektron satellites, and compound plasma spectrometers were included on later spacecraft like Venera-11.

Charged-Particle Detectors

In the scintillation counter, radiation striking a crystal of sodium iodide (NaI) or cesium iodide (CsI) causes a flash of light, measured by a photomultiplier tube. Zinc Sulphide (ZnS) scintillators were used to measure low-energy electrons. Scintillators have many excellent qualities, but they are heavy and require high-voltage power supplies. Only the Soviet Union routinely carried them into space, on their massive spacecrafts. In the mid 1960s, scintillation counters were generally replaced with silicon solid-state detectors, which were small and could be mounted outside the vehicle, often on the Solar and anti-Solar sides of the spacecraft for comparisons. S.N. Vernov and his associates at Moscow State University played a major role in the design of these devices.

Detectors were typically clustered, as in the KS-18 unit seen above. Scintillators and gas-discharge detectors respond in different ways to charge and energy, and shielding can be used to discriminate particle energies. The KS-18 consists of a scintillation counter (1), two STS-5 gas-discharge tubes (2,3) and a solid-state electronics to amplify and count events (4). Luna-2 contained a CsI and two NaI scintillation counters, and six gas-discharge tubes shielded with different thickness of copper or lead. Mars-2 contained 11 detectors, including a Cherenkov counter, four gas discharge detectors and seven silicon solid-state detectors.

Cosmic rays striking the body of the spacecraft can produce a shower of secondary particles, interfering with the measurement of primary radiation. In 1964, Zond-1 carried the L-1A sensor, an STS-5 gas-discharge tube surrounded by an anti-coincidence shield of seven tubes. The device was also included in the 2MV and 3MV Mars and Venus (Zond-1, Venera-3) landing capsules. Charged particles create a trail of ionization, setting off the center tube and some of the surrounding shield tubes. An event in the center tube with no shield-tube signal indicates a gamma ray, which is absorbed at a single point. Used in flight, the device was contained in the landing capsule to measure the total amount of potassium, uranium and thorium in a planetary crust.

In 1962, A.I. Lebedinskii and V.A. Krasnopol'skii built the DAS-1 anti-coincidence counter for the 2MV Venus lander (in addition to the L-1A). It contained a 3×3 array of gas-discharge tubes, with the center detector actively shielded by the surrounding 8. The unit contained a circuit which alternately counted gamma-ray events from this detector and motion-changing events from a mercury switch. The latter device was installed to detect rocking frequency in the event of a hypothetical ocean landing on Venus. On 3MV landers the DAS-2 replaced the gamma detector with a photometer, and the L-1A remained for gamma-ray measurements.

The proton telescope on Venera-11 used a thick hemisphere of plexiglas as a Cherenkov detector. An aluminum shield kept out electrons. Higher energy protons produce a stronger signal, but so also do protons that travel through a longer length of plastic. To reduce the variation in path length, a solid state detector is placed at the center, and a coincidence detector records only particles that strike both the Cherenkov detector and the SSD. This limits the events to ones entering perpendicular to the plexiglas hemisphere.

When one particle triggers two different styles of detector, one can solve for two unknown quantities: the charge and energy. Soviet scientists S.N. Vernov and A.E. Chudakov pioneered the use of gas discharge detectors combined with crystal scintillator or Cherenkov detectors. The latter devices required high-voltage power supplies and photomultiplier tubes and were too heavy to be carried by the first American satellites and probes. Vernov and Chudakov were the first to measure the makeup and energy spectra of the Earth's radiation belts and the first to detect the outer radiation belt. Called the "Van Allen-Vernov belts" in Russian literature, Soviet discoveries have rarely been acknowledged by Western scientists.

Hard Electromagnetic Radiation

Solar X-rays and hard UV-radiation has been studied by Soviet spacecraft since Sputnik-2. Since hard radiation is difficult to refract or diffract, telescopes and spectrometers have been a challenge. S.L. Mandel'shtam used filter banks and photomultiplier tubes. Diffraction by crystal lattice has also been used to study x-ray spectra. Mandel'shtam installed a Solar x-ray/UV sensors on the 3MV-1A and 3MV-4A planetary test vehicles, but all were lost in launch mishaps. In the 1980s, the Astron and Granat orbiting x-ray and ultraviolet telescopes functioned for many years.

The anti-coincidence counters carried carried by Zond-1 and Venera-3 (described above) were used in flight to measure primary cosmic-ray protons, but their main purpose was to measure gamma-ray emissions from the surface of Venus and Mars (a model 3MV Mars lander was never sent).

In the early 1970s, galactic gamma-ray bursts were observed by American and Soviet satellites, deployed to monitor nuclear-weapons testing. The Konus gamma-ray detectors were first carried on Venera-11 and Venera-12, as well as a Prognoz satellite, to triangulate on the source of these rare events. Over 140 gamma bursts were recorded, and the experiment was repeated by Venera-13 and 14. The Konas apparatus consisted of 6 perpendicular, directional gamma-ray detectors, consisting of shielded NaI scintillation counters. V.G. Kurt played an important role in Astron, Granat and Konus experiments.

Three natural elements are radioactive: uranium, thorium and potassium. A gamma-ray spectrometer uses a scintillation counter and forms a histogram of counts in different energy ranges. Each element emits a gamma ray of a particular wavelength, and the abundances of K,U,Th is known for common rocks and minerals. This technique was employed by Iu.A. Surkov on many Soviet spacecrafts. Since it has no atmosphere to block the rays, the gamma spectrum of the Moon was measured from Earth satellites (Kosmos-60) and from Luna-10, the first Lunar satellite. The Kosmos-60 detector consisted of a CsI scintillator surrounded by a plexiglas Cherenkov counter. The Scintillator responds to both gamma rays and charged particles (cosmic rays), while the Cherenkov counter only responds to charged particles and acts as an anti-coincidence shield.

Mars was also analyzed by the Mars-3 and Mars-5 orbiters, compensating for the thin atmosphere. Seen above, the Venera gamma-ray spectrometer used a CsI scintillator with FEU-118 photomultiplier tube. The surface of Venus was analyzed at several locations by Venera-8,9,10 and Vega-1,2. Most readings have indicated a basalt-like volcanic rock, but Venera-8 readings appeared more like granite.

Temperature, Pressure and Density Sensors

Venera landers carried platinum wire resistance thermometers, enameled to resist chemical corrosion, absorption of gas into metal or even the effects of immersion in liquid. The electrical resistance of a spool of fine (0.05 mm) wire would increase with temperature; for example, from 27 to 85 Ω in a range of temperature from -50° to 400° C. This was measured with an AC balanced bridge circuit and stabilized amplifier. The enameling of the thermometer was important; bare platinum-wire thermometers on the American Pioneer Venus probes all shorted out in the lower atmosphere, as the result of being coated with an unknown conducting substance.

Pressure measurements span the range from the near vacuum of the ionosphere and exosphere of planets to the almost 100-atmosphere pressure of the surface of Venus. No single instrument can handle that range. Higher pressures are measured with a battery of aneroid and capacitor barometers that measure pressure in different ranges. More rarified atmospheres are measured with hot-wire (Pirani) gauges, which measure the rate at which heat is carried away by gas conduction. Extremely low pressures (such as the Sputnik-3 sensors) are measured with ionization gauges, using hot cathodes or cold cathodes. The latter are often called "magnetic pressure gauges" in early Soviet literature.

Venera-4 Densitometer/Thermometer Sensor and Internal Electronics Unit

The early Venera landers measured density directly. The Venera-4 sensor was an ionization chamber, coated on the inside with 1-millicurie of Strontium-90, it measured the minute electric current from ions swept up by an electric field. One of the thermometers and a densitometer were integrated in the unit shown above. It provided information at higher altitudes, in a density range of 0.0005 to 0.015 g/cc with a 1.5% error. Venera-5 and 6 used a 160 Hz tuning-fork densitometer with a range of 0.0005 to 0.040 g/cc and 3% error. Atmospheric density was also measured by onboard accelerometers and radio Doppler measurement of the descent capsule's velocity.

V.V. Mikhnevich designed and built the early P,T,ρ sensors, from Sputnik-3 to Venera-4. Her skill as a builder of precision instrumentation has been remarked on by colleagues. Later missions and landers contained pressure and temperature sensors built by Lavochkin engineers, with resulting data and analysis published by V.S. Avduevskii and M.Ia. Marov.

Infrared Spectrometry

A.I. Lebedinskii designed the IR spectrometer to measure the spectrum of Mars from 2.4 to 4.1 μm. It consists of cassegrain optics, a mechanically scanning diffraction grating and a lead sulphide (PbS) detector cooled to -78° C by exposure to space. The spectrometer is seen above, flanked by boxes containing internally mounted electronics and externally mounted preamplifiers. It looked coaxially with the Mars-1 and Zond-2 cameras, mounted outside the special instrument module, and a small photometer mounted on the side of the spectrometer provided a reference reading at visible wavelengths.

This is Lebedinskii's "Martian Life Detection" experiment, which rocket engineer Boris Chertok criticizes in his book and in an interview on the BBC's Planets program. However, the design and scientific motivation of the device are perfectly sound, and it operated successfully in later testing onboard Zond-3. A similar experiment was carried out years later by the American Mariner-6 mission. What actually transpired between Chertok and Lebedinskii may have been a misunderstanding or a conflict of strong personalities.

Mars-3 contained IR photometers designed by V.I. Moroz to measure absorption bands of water and carbon dioxide and the planets brightness temperature. The narrow-band carbon dioxide photometer measured altitude of terrain from the thickness of the atmosphere. In 2004, V.A. Krasnopol'skii, who worked with Lebedinskii in the 1960s, completed the long Russian quest to detect methane on Mars.

IR Spectrometer on Venera-2.

For Venus, Lebedinskii designed an spectrometer to measure the thermal radiation of the atmosphere and surface. It consisted of two spectrometers, measuring the ranges from 7 to 20 μm and 14 to 38 μm, each at 150 positions. A two-sided gold chopper mirror alternately exposed the spectrometers to Venus or to deep space, so a differential signal could be measured. The detectors were semiconductor bolometers (sensitive thermometers). To eliminate wavelengths outside the range of interest, the first spectrometer contained an indium antimonide window, and the second used a lithium fluoride mirror.

The device shown above was flown onboard Venera-2 in 1965. Like the Martian IR spectrometer, it was located outboard, attached to the side of the special instrument module, a 13-15 kg unit about 50 cm in size. It was oriented coaxially with the camera and also contained a coaxial photometer for a visible-light reference signal. Programmed by shaped cams, the device was designed to make a spectral scan of the range of wavelengths, and then perform a spatial scan along the disk of the planet at fixed wavelengths (9.5 and 18.5 μm). The spectrometer was successfully tested on Earth orbiting satellites, but the Venera-2 spacecraft failed shortly before planetary encounter.

It is believed that Venera-1 contained an IR radiometer, probably designed by Lebedinkskii. The 1962 2MV photo-flyby attempt (which was stranded in parking orbit) also contained an IR sensor, either a radiometer or an earlier version of his spectrometer.

In 1983 and 84, the most complete infrared spectral data for Venus was measured using Fourier spectrometers on Venera-15 and 16. This measured a 256-point spectrum in the range of 6 to 36 μm, sampling about 60 points per orbital pass over the north polar region. The device was built by Dieter Oertel's team at the Berlin Space Research Institute in East Germany. A Fourier-transform spectrometer uses a beam splitter and mirrors to mix the incoming light with itself and produce constructive or destructive interference depending on wavelength. By moving one mirror and changing the path length, varying interference is measured, which happens to measure the Fourier transform of the IR spectrum. This elegantly simple idea was realized in the 19th century, but was useless before the invention of computers and efficient algorithms for inverting the Fourier transform.

The Venera-15 spectrometer identified carbon dioxide, water vapor, sulphuric acid and sulphur dioxide. A strong carbon-dioxide band was used to measure the altitude profile of temperature. An interesting result was the variation of spectra across the north pole, indicating complex atmosphere structure in the region.

Ultraviolet Spectrometry

Later Mars and Venus missions contained ultraviolet photometers which were used to measure spatial profiles across the disk of the planets. In 1975, Venera-9 and 10 returned scans at 352 and 345 nm with 16 arc-seconds of spatial resolution. Similar devices on the 1973 Mars-5 orbiter measured 260 and 280 nm. As with IR scans, these UV measurements were used to study the structure of the atmosphere, clouds and ozone layers. Some of these devices sent to Mars and Venus were also tested on Soviet meteorological satellites.

Venera-13 and 14 descent vehicles carried gallium phosphide photometers and the penetration of radiation in the range of 320 to 390 nm. The Vega mission landed two vehicles on the night side of Venus. During descent, atmospheric gasses flowed through the long pipe seen above, and ultraviolet light from a Xenon lamp was passed through it. A 512-point absorption spectrum from 230 to 400 nm was measured using a diffraction grating and an integrated linear array of photodiodes.

Visible and Near-Infrared Spectrometry

Prior to Venera-8, landers targeted the night side, but some contained photometers to detect airglow or lightning. Zond-1 and Venera-3 contained photometers built by A.I. Lebedinskii and V.A. Krasnopol'skii. Landing on the night side of the planet, these sensors were meant to detect airglow. They were sensitive to a wide range from 0.001 - 100,000 lux, encoding their output logarithmically. The unit also included a mercury level sensor, similar to the one used on 2MV landers, to detect rocking motion in the event of a water landing.

Venera-5,6 also contained photometers to detect nightside airglow. One bright signal was returned by Venera-5, which may have been a lightning flash. These devices were built by A.P. Ekonomov and B.E. Moshkin.

Venera-9 and 10 contained an improved model, the IOV-75, which used three fiber-optic pipes to view upward (narrow and wide angles) and downward. A wheel of five glass filters provided color and radiance measurements at wavelengths between 440 and 1160 nm (green, yellow, red, IR1, IR2). It was correctly assumed that blue light would be extinguished by Rayleigh scattering. This instrument, along with Marov's nephelometers, revealed for the first time that the Venusian clouds consist of three layers.

In 1978, Venera-11 and 12 contained the more advanced IOAV, a combined scanning photometer and spectrometer. The scanning photometer used a 360° rotating prism mounted on the aerobrake to scan from zenith to nadir. This light passed through an inner ring of four color filters on a rotating filter wheel (green, red, IR1, IR2). The spectrometer measured the zenith sky radiance throught an outer ring on the filter wheel, containing a ramp interference filter. This measured a spectrum from 430 to 1170 nm with a resolution of about 20 nm. In 1981, Venera-13 and 14 carried the IOAV-2, which measured full spectra (470-1200 nm) from a wide-angle sky view and six narrow-angle directional views.

Planetary spectroscopy was also done onboard orbiting spacecraft. After Venera-3 and the untimely death of Lebedinskii, Keldysh created the Space Research Institute (IKI) and reorganized the responsibilites for scientific instrumentation. The astronomer and spectroscopist, V.I. Moroz headed the development of IR, UV and Visible spectroscopy. He was joined by V.A. Krasnopol'skii (who had worked with Lebedinskii), L.V. Ksanfomaliti, A.P. Ekonomov and B.E. Moshkin.

In 1973, the Mars-5 orbiter contained an important innovation, the first photopolarimeter onboard a planetary probe. The measurement and analysis of polarization was pioneered by French astronomer Audouin Dollfus, who joined Ksanfomaliti and redesigned his optical photometer to measure polarization through seven different color filters. This instrument yielded new information about high altitude haze layers in the atmosphere and about the size and shape of dust and sand grains on the surface. A third version of this instrument was flown on the Venera-9 and Venera-10 orbiters in 1975.

Nephelometry

Remote Chemical Analysis

Manometers were used to measure abundant gases. The G-8 unit, seen above, contained five manometer analysis cells, a moisture gauge and control electronics in a 10 cm diameter cylindrical package. At a predetermined time after probe activation, a glass window was broken, the cells filled with atmospheric gas, and then were sealed again. On one or both sides of a membrane dividing a cell, specific chemicals absorbed particular atmospheric gases, and a pressure difference was measured. For example, potassium hydroxide (KOH) absorbs carbon dioxide and water vapor, calcium chloride (CaCl) absorbs water vapor, so the pressure difference measures carbon dioxide. A zirconium wire, heated to ignition, absorbs oxygen, carbon dioxide and nitrogen (zirconium will burn even in pure nitrogen). White phosphorus was used to react specifically with oxygen, but the resulting oxide also absorbs water.

A second analysis unit, the G-10, contained two manometers and three miscellaneous experiments designed to detect trace amounts of gases. In the case of oxygen and water vapor, it was not initially known if the amounts would be small or large, so both manometer and small-trace experiments were devoted. For example, measuring the electrical conductivity of a moisture absorbing film (phosphorus pentoxide) was well suited to dry atmospheres with just a trace of water. This unit may have been a consolidation of several separate experiments designed for Zond-1 and Venera-3.

The G-9 unit, planned for use on Mars, was probably never built, since no model 3MV Mars landing missions were ever launched.

Before Venera-4, little was known about the abundance of major gases, so these experiments were designed to deal with many possibilities. Some would return an indication that a threshold was exceeded. Some indicated the amount of a gas, within some working dynamic range. The analysis units on Venera-5 and 6 were redesigned to more accurately measure within the known ranges of abundance.

With water and ice ruled out as cloud materials, theories of ammonium compounds were briefly popular. For this reason, Venera-8 contained a sensor to measure ammonia by color change of a specialized indicator (tetrabromine-phenolsulphaphtalein). This turns dark blue in the presence of ammonia, and indicated 0.1 to 0.01 percent; however, it was later discovered that concentrated sulphuric acid turns this indicator dark red. It is currently believed that ammonia is 10 to 1000 times more scarce than the Venera-8 reading.

The Vega landers carried carefully designed devices to measure water vapor by two methods. Both were held inside insulated domes, temperature stabilized with a heat-absorbing phase-change material (lithium nitrate trihydrate). The right dome carried an electroconductivity sensor based on the moisture absorbing salt lithium chloride. A similar experiment was carried by Venera-13 and 14, but not as well stabilized in temperature. The left dome carried a thermoelectrolytic sensor that detected hydrogen gas generated by electrolysis of another moisture-absorbing material. Results indicated 0.1 to 0.2 percent water vapor in the upper atmosphere (around 50 km) and drier conditions at lower levels.

Mass Spectrometry

The first mass spectrometers on Venus were onboard Venera-9 and 10. These were model P-11 devices built by the Low-Temperature Institute in Kiev and the Vernadskii Institute. A pressure regulator was designed to handle input pressures from 0.1 to 10 atmospheres. The gas sample was ionized between a cold cathode and a conical tungsten screen. These devices may have detected carbonyl sulphide (COS), but the results were inconsistant with later mass spectra. it is believed they had problems with the gas intake or pressure regulators.

A few days later, Venera-11 and 12 deployed model MKh-6411 mass spectrometers developed by V.G. Istomin. To avoid the problem of clogged micro-pore intakes, a pulsed piezoelectric valve opened a relatively large hole for a brief instant. They also avoided the cloud layer by switching on at an altitude of 25 km. The batch of gas was then analyzed in a series of 8 second frequency sweeps, and the device pumped down to hard vacuum for a while before processing the next sample. Taking advantage of Venera's telemetry relay through the bus's high-gain antenna, a mass-spectrometer data rate 100 times greater than the Pioneer Venus large probe's was available. 176 full spectra were transmitted from 22 gas samples during the descent to the surface.

The readings from Venera-13 and 14 were the last and best analysis of the mass spectra of the Venusian atmosphere. The mass spectrometer was improved to yield 2 to 40 times finer resolution than Venera-11's. 250 full spectra were transmitted. The anomalous high Krypton reading from Venera-11 was explained and the problem corrected. A major disagreement between Venera and Pioneer was the abundance of argon. Istomin performed laboratory simulations that indicated that the Pioneer mass spectrometer may have been contaminated by helium

Istomin placed earlier models of his mass spectrometer on the Mars-3 and Mars-6 landers, which unfortunately were not able to transmit significant telemetry.

Gas Chromatography

Chromatography was invented in 1903 by the Russian biochemist Mikhail Semenovich Tsvet. He coined the name chromatography, and it is amusing to note that his name, Tsvet, is also the Russian word for color.

While chromatography is not always quantitatively accurate, it can be a sensitive detector of substances. It complements mass spectroscopy by resolving chemically reactive gases, and it provides an independent check, with different types of ambiguity. For example, a mass spectrometer may not be able to differentiate O+ from S++ ions, since they have the same mass over charge. Mukhin's chromatograph discovered an unexpected peak which could be sulphur hexafluoride or nitreous oxide. Both have the same retention time, but detector sensitivity (ionization rate) ruled out the second, which would have required an unrealistically high abundance.

The Vega missions focused attention on the analysis of the cloud droplets of Venus. The "Sigma 3" chromatograph was designed to capture cloud aerosol in a filter and then vaporize the material for analysis.

X-Ray Fluorescence Spectroscopy

Venera-12,14 Cloud-Material X-Ray Fluorescence Spectrometer

Venera-12,14 and Vega-1,2 carried spectrometers designed to analyze the makeup of the Venusian clouds. One or two cubic meters of atmospheric gas were drawn through a 0.4μm-pore filter of ultrapure cellulose acetate. Gamma rays from the radioactive isotope Fe-55 were used to excite fluorescence in atoms in the range of 13 ≤ Z ≤ 25, and on Venera-12 a Cd-109 source excited heavier elements. Gas proportional counters were used to measure the energy of resulting x-ray emissions and return a series of spectra as material accumulated. On Venera-12, the device operated from an altitude of 64km to 49km, at which point the filter was destroyed by the increasing temperature.

X-ray spectroscopy is imprecise about amounts of material, but it has yielded unique and important information about the Venusian clouds. The Vega probes revealed large amounts of phosphorus in the lower cloud layer, and am image is emerging about the structure of the clouds and why there are three layers. V.A. Krasnopol'skii has written extensively on the chemical and physical equilibrium of possible cloud systems, including the interaction of sulphuric acid, sulphur, phosphorus oxides, aluminum chloride and iron chloride. Heavy metal chlorides have low boiling points and are gaseous in the hot lower atmosphere of Venus. At higher altitudes, they may exist as crystals or dissolved in the sulphuric acid droplets of the clouds.

Penetrometers

The Mars landers included a small mini rover, PROP-M, on a 15-meter tether, which ski-walked and avoided obstacles. It carried a conical penetrometer and a gamma-ray densitometer, which it was programmed to activate every 1.5 meters. Contact with Mars-3 was lost shortly after landing. Venera-11,12,13,14 and Vega-1,2 carried rotating conical penetrometers, PROP-V, that also performed electrical resistance measurements. On Venera-11 and 12, this device was damaged by the rough descent and landing. Venera-14's penetrometer landed inside the ejected lens cap, but it may have still penetrated the Venusian rock, instead of the cap.

Venera-13 and 14 carried BISON-M accelerometers, which measured a high resolution profile of impact deceleration. This was stored in memory and transmitted after landing. The experiment was calibrated by drop testing the lander on a large variety of surfaces, and the results on Venus supported the finding that the surface was somewhat porous and crumbly.

The first penetrometer experiment on Venus was the Venera-7 descent capsule itself. After its parachute failed, the probe landed at a considerable speed, but still survived and continued to transmit. From its doppler shift at the moment of impact, and from the fact that the probe was not destroyed, Soviet engineers determined that the surface was harder than sand but no harder than pumice.

Biographies

Acknowledgements

Special thanks to Vladimir Krasnopolskii, Vladimir Kurt for answering many questions about their onboard experiments.