SO LITTLE WAS KNOWN of Mercury before the epic voyage of Mariner that the mission was virtually man's first look at this innermost planet of the Solar System. The science objectives for the mission were to explore Mercury as thoroughly as possible with seven experiments: television imaging, infrared radiometry, extreme ultraviolet spectroscopy, magnetometer, plasma, charged particles, and radio wave propagation.
The same experiments were used to explore Venus, adding to knowledge gained from earlier U.S. and U.S.S.R. flights. Exploration of Venus was restricted somewhat by the trajectory requirements for reaching the prime target, Mercury. These requirements made it necessary, for example, for Mariner to follow a trajectory that did not produce a Sun occultation at Venus, so the ultraviolet occultation experiment (see page 24) could not be conducted at that planet.
To obtain best science results, the objectives of each experiment were established and the space near Mercury was evaluated for aiming points and trajectories that would satisfy them. Of major importance was a flight path that would place the planet between the spacecraft and the Sun, and also between the spacecraft and the Earth, i.e., solar and Earth occultation, respectively.
Study of the planet's effect on the Sun's plasma gas and magnetic fields ("solar wind") required a solar occultation, as did the sounding of Mercury's atmosphere by the ultraviolet occultation experiment. By observing the decrease in intensity of solar ultraviolet radiation as Mercury and its atmosphere blocked it out, a measure of this atmosphere could be obtained. Earth occultation was needed to observe the passage of radio signals from the spacecraft to Earth until cut off by the planet, and again on emergence from behind the planet. This would provide information concerning the radius of the planet, its atmosphere and ionosphere.
To provide the greatest amount of information obtainable with remote sensing devices, Mariner Venus/Mercury carried more science instruments (Fig. 3-1) than most previous Mariner spacecraft. A magnetometer measured magnetic fields, a plasma analyzer measured the ions and electrons of the solar wind, and cosmic ray telescopes provided information on solar and galactic cosmic rays. The main objective of these instruments was to learn about a planet by studying its effects on the interplanetary medium.
An infrared radiometer measured temperatures of the clouds of Venus and the surface of Mercury. Two independent ultraviolet instruments (measuring light beyond the violet end of the spectrum) analyzed the planetary atmospheres. One instrument was fixed to the body of the spacecraft and was used at Mercury to search for traces of atmosphere along the edges of the visible disc of the planet. A second instrument, mounted along with the television cameras on a....
....scan platform, could be pointed on command. This "airglow " spectrometer was used to scan both of the planets, searching for evidence of hydrogen, helium, argon, neon, oxygen, and carbon. At Venus, it searched for specific gases, and during the cruise phase it looked for sources of ultraviolet radiation coming from hot stars and gas clouds in the galaxy. Measurements were also made of the gaseous envelope surrounding the comet Kohoutek.
A complex of two televisor cameras with eight filters was the basis of the imaging experiment. These cameras were capable of taking both narrow- and wide-angle views of Venus and Mercury. Sharing the scan platform with the airglow spectrometer, the imaging complex was directed by command from Earth. As well as taking pictures in different colors of light, these cameras also measured how the light was polarized, observations intended to provide information on the composition of the clouds of Venus and the surface of Mercury.
A radio experiment used the signals transmitted from the spacecraft to Earth. By tracking the spacecraft signals, experimenters determined how the spacecraft was affected by the gravitational fields of the planets. From this information they determined the shape of each planet and whether there were anomalies in its gravitational field.
By analysis of what happened to the radio signals as they passed close to the limb or edge of the planet, experimenters were able to probe the  atmosphere of Venus and check for an atmosphere of Mercury. To take full advantage of the Venus occultation, which bent the radio signals appreciably, the high-gain antenna on the spacecraft was steered so as to compensate partially for the bending of the radio signal. In this way, information was obtained at deeper levels of the atmosphere than was possible with earlier flyby spacecraft.
The science experiments were selected from proposals submitted to NASA in response to the announcement of the Mariner Venus/Mercury flight opportunity.
This instrument measured temperatures on the surface of Mercury and the clouds of Venus by sampling thermal ( infrared ) radiation. Observations of thermal emission from Mercury were expected to provide information on the average thermal properties, large-scale and small-scale surface anomalies, and surface roughness. It was known that temperature variations on Mercury would be large, owing to intensive heating of the day side and the slow rotation period of 58.6 days, which allows the night side to radiate away most of its heat. Measurement of heat absorption and loss across the terminator ( shadow line ) regions could provide indirect evidence of the nature of the surface material: such as whether it is sand, gravel, or rock. At Venus the instrument was expected to provide cloud top brightness temperatures at higher resolution than can be achieved from Earth or had been achieved by earlier spacecraft.
The infrared radiometer was fixed to the body of the spacecraft on the sunlit side, with apertures shielded from the direct sunlight under a thermal blanket. The instrument ( Fig. 3-2 ) was based upon earlier radiometers flown on Mariner Mars 1969 and 1971, but instead of the reflecting optics of the earlier radiometers, the new instrument made use of two Cassegrain telescopes with special long-wavelength filters. This allowed observations at longer wavelengths and also increased sensitivity.
Two 1/2-deg fields of view separated by 120 deg were used to scan Mercury, the angular separation being obtained by a three-position scan mirror (see Fig. 3-3). The forward and aft....
 ....viewing beams thus ensured that there could be both a planet viewing beam and a black space reference beam for all the observations.
The instrument measured surface brightness temperature in the two spectral bands 34 to 55 and 7.5 to 14 micrometers, which correspond to temperature ranges of 80 to 340 and 200 to 700 K, respectively.
Observations of the velocity and the directional distributions of the normal solar wind constituents in the vicinity of Mercury were required to understand the interaction of the solar wind with the planet. Observations of the solar wind inside the orbit of Venus were also important, since no previous spacecraft had penetrated this region. Therefore, continuous measurements were planned from the orbit of the Earth to the orbit of Mercury. Additionally, an objective of the experiment was to verify and extend previous observations of the solar wind's interactions with Venus and to clarify the role of electrons in these interactions.
Instrumentation for the experiment consisted of two detectors on a motor-driven platform (Fig. 3-4). The principal detector, facing sunward,....
....consisted of a pair of electrostatic analyzers. The auxiliary detector, facing away from the Sun, was a single electrostatic analyzer. The forward looking device was called the scanning electrostatic analyzer while the backward looking device was called the scanning electron spectrometer. The former measured positive ions and electrons, the latter only electrons.
The importance of investigating the interaction of the solar wind with the planets and the variation of the wind with distance inside the orbit of Venus was evidenced by the large team of investigators selected from seven research organizations for this experiment.
The solar wind is an extension of the Sun's corona into interplanetary space. It is a fully ionized gas which consists of equal numbers of positively charged particles (mostly protons) and negative electrons. This ionized gas or plasma moves radially outward from the Sun at a very high velocity, hundreds of kilometers per second. The magnetic field of the Sun is carried outward by the plasma and is bent into a spiral configuration by a combination of the radial motion of the plasma and the rotation of the Sun. If one thinks of the plasma as a hot, ionized gas, the ions and electrons have two sorts of motions: a bulk velocity because they are both streaming outward from the Sun, and a thermal velocity because the gas is hot. For the protons, the bulk velocity is much higher, about a factor of 10, than the average thermal velocity; for electrons, the situation is exactly reversed. To an observer on the spacecraft, the positive ions appear to come almost directly from the Sun, whereas the electrons come almost uniformly from all directions. To study the properties of the plasma, the combined experiments were mounted at the end of a short boom, on a platform which allowed the plasma experiment to scan right or left through an angle of 60 deg above and below the spacecraft-Sun line.
Magnetic Field Experiment
The magnetic field experiment consisted of two 3-axis sensors located at different positions along a 6.1-m (20-ft) boom. Figure 3-5 shows a magnetometer mounted on the boom, together with a cutaway view of a sensor. The two sensors....
....carried on the boom were biaxial fluxgate magnetometers. Each sensor was protected from direct solar radiation by a sunshade and a thermal blanket. The purpose of the two sensors was to permit the simultaneous measurement (at different distances from the spacecraft ) of the magnetic field, which is the sum of the weak magnetic field in space (and near the planets) and the magnetic field of the spacecraft itself. The inboard magnetometer, being approximately twice as close to the spacecraft as the outboard sensor, was more sensitive to changes in the magnetic field of the spacecraft, with the result that these perturbations could be isolated and removed from the outboard sensor measurements.
 In interplanetary space, the magnetic field is typically about 6 gamma (compared with the strength at Earth's equator on the surface of 30,000 gamma ). By contrast, the field of the spacecraft, as measured at the outboard sensor, was observed to vary in direction and intensity quite considerably during the mission, swinging from 1 to 4 gamma. This variation in the spacecraft field demonstrated the importance of having two sensors to remove the spacecraft field from the measured field. In addition to the planetary observations, magnetic field observations were important in studying how the interplanetary plasma varies with distance from the Sun and how this plasma moves outward from the Sun. The measurements of plasma and magnetic fields were mutually supporting, and their correlation was an important and sensitive test of consistency between the two scientific instruments.
This experiment was designed to observe high-energy charged particles-atomic nuclei-over a wide range in energy and atomic number. The instrument had two parts, a main telescope and a low-energy telescope, both mounted on the body of the spacecraft. During cruise the charged particle experiment measured solar and galactic cosmic rays with the objective of determining the effect of the Sun's extended atmosphere (heliosphere ) on cosmic rays coming into the Solar System from elsewhere in the galaxy. During encounter with Mercury, the experiment was to search for charged particles in the vicinity of Mercury. The effect of solar flares on the flux of charged particles was correlated with measurements made from Pioneer spacecraft in the inner and outer Solar System as well as IMP (Interplanetary Monitoring Platform) spacecraft circling the Earth to determine how solar particles propagate in interplanetary space. The instrument is shown in Fig. 3-6. The two telescopes looked 45 to 50 deg west of the line from spacecraft to the Sun, with a 70-de" field of view. The low-energy telescope allowed the separate detection of relatively low-energy protons in the range 0.4 to 9 MeV (million electron volts) and alpha particles (helium nuclei) in the range 1.6 to 25 MeV.
The high-energy telescope detected electrons in the range 200 KeV (thousand electron volts) to 30 MeV, protons of energy greater than 0.55 MeV, and uniquely detected alpha particles with energy greater than 40 MeV. Both telescopes were able to detect energetic nuclei of atomic numbers up to oxygen.
The telescopes were very similar to those flown in Pioneer 10 and 11 to the outer Solar System. In fact, when Mariner reached Mercury for a first encounter, Pioneer 10 was more than five times the distance of the Earth from the Sun and Pioneer 11 3.5 times the distance from the Sun. Thus the three spacecraft provided an unprecedented range of radial measurements of the modulation of the cosmic ray flux by the heliosphere.
This experiment consisted of two independent instruments: a fixed solar-looking occultation spectrometer, mounted on the body of the spacecraft, and an airglow instrument, mounted on the scan platform. The aim of the experiment  was to analyze planetary atmospheres, and, during cruise, to measure distribution of hydrogen and helium Lyman-alpha radiation emanating from outside the Solar System.
The search for an atmosphere on Mercury represented a primary scientific objective of this experiment. The extreme ultraviolet spectrometers provided two approaches to this search. The first observed the occultation of the Sun by the disc of Mercury; the other scanned through the atmosphere on both bright and dark limbs in search of emission from the neutral constituents hydrogen, helium, carbon, oxygen, argon and neon, at wavelengths ranging from 304 to 1659 angstroms.
These elements were selected for study on the basis of theoretical prediction of the most likely constituents of the presumably tenuous atmosphere of Mercury.
The occultation spectrometer (Fig. 3-7) was set to be responsive at four spectral bands, centered at 475, 740, 810, and 890 angstroms, where the relatively high solar ultraviolet intensity and the large absorption cross section of all gases in this spectral region would combine to provide highly sensitive measurements of the atmosphere of Mercury, independent of its composition.
The airglow experiment (Fig. 3-8), in addition to providing a measurement of the relative....
 ....abundances of the constituents sought in the atmosphere of Mercury, also made important observations at Venus and during the cruise phase between the planets. The angular dimension of the field of view of the airglow instrument was selected to allow resolution to about one scale height of the heaviest expected atmospheric constituent at the limb of the planet (argon), thereby providing data on the structure as well as the composition of the planetary atmosphere.
Celestial Mechanics and Radio Science
These experiments relied upon mathematical analysis of the radio signals coming from the spacecraft, based upon radio tracking of the spacecraft and analysis of the effects of the planetary atmospheres on the radio signal.
In the celestial mechanics experiment the mass and gravitational characteristics of both Mercury and Venus were to be determined from the effect of each planet on the predicted trajectory of the spacecraft. These data would also provide estimates of the internal composition and density of the planets.
The occultation experiment (Fig. 3-9) observed changes to the radio waves from the spacecraft transmitter as they passed through the atmosphere of Venus and Mercury en route to the Earth-based receivers as Mariner passed behind the planets as viewed from Earth.
Gases in an atmosphere refract and scatter a radio signal, and by measuring these effects scientists can calculate the pressure and temperature of the atmosphere. The presence of an ionosphere is revealed by its special effects upon the characteristics of the radio signal. The cutoff of the radio signal as it grazes the surface of the planet provides a measurement for accurately determining the radius of the planet. Because the thick atmosphere of Venus bends the radio signal and traps it in a path around the planet, the high-gain antenna of Mariner was steered along the limb to compensate for the expected bending so as to allow deeper penetration of the radio waves through the atmosphere. The experiment used two frequencies to provide more accurate information about Venus's atmosphere and the inter...
....planetary medium than is obtainable with a single frequency.
The television system centered around two vidicon cameras, each equipped with an eight-position filter wheel. The vidicons were attached to telescopes mounted on a scan platform that allowed movement in vertical and horizontal directions for precise targeting on the planetary surfaces. These folded optics (Cassegrain) telescopes were required to provide narrow-angle, high-resolution photography ( Fig. 3 -10 ). They were powerful enough for newspaper classified ads to be read from a distance of 400 meters (a  quarter of a mile). An auxiliary optical system mounted on each camera allowed the acquisition of a wide-angle, lower-quality image. Changing to the wide-angle photography was done by moving a mirror on the filter wheel to a position in the optical path of the auxiliary system.
In addition to wide-angle capability, the filter wheels included blue bandpass filters, ultraviolet polarizing filters, minus ultraviolet high-pass filters, clear apertures, ultraviolet bandpass filters, defocussing lenses for calibration, and yellow bandpass filters.
A shutter blade controlled the exposure of the 9.8- by 12.3-mm image face of the vidicon for an interval that could be varied from 3 msec to 12 sec. The light image formed on the photosensitive surface of the vidicon produced an electrostatic charge proportional to the relative brightness of points within the image. During vidicon readout, an electron beam scanned the back side of the vidicon and neutralized part of the charge so as to produce electric current variations proportional to the point charge being scanned at the time.
These analog signals produced from the vidicon readout process were electronically digitized as 832 discrete dots or picture elements (pixels) per scan line, and presented to the flight data system in the form of 8-bit elements for transmission. Each TV frame-one picture-consisted of 700 of these vidicon scan lines. All timing and control signals, such as frame start, line start/stop, frame erase, shutter open/close, and filter wheel step, were provided by the systems on board the spacecraft.
The television experiment had the objectives of providing data to permit the following scientific studies of Mercury: gross physiography, radius and shape of the planet, morphology of local features, rotation and cartography, photometric properties, and regional color differences. For Venus the experiment aimed at obtaining data on the visual cloud structure, scale and stratification, and the ultraviolet markings and their structure and motions. The television experiment also searched for satellites of Mercury and Venus and was used for targets of opportunity such as Comet Kohoutek.