[6] The Mariner 10 spacecraft was launched on the first day of the scheduled launch period, November 3, 1973, at 0045 Eastern Standard Time (0545 Greenwich Mean Time) from Cape Canaveral, Florida, using an Atlas/Centaur D1-A launch vehicle.12 The spacecraft received a gravity assist from Venus on February 5, 1974 and encountered Mercury on March 29, 1974, 146 days after launch ( Figure 4). The exploration of Mercury was the primary objective of the mission and the basis for the selection of the Mariner 10 experiment complement. Experimenters wished to determine, at least in general terms, several of the important properties of this littleknown planet. In particular, it was desired to ascertain the nature of Mercury's surface morphology; whether an atmosphere is present, and, if so, the constituents; the planet's interaction with the solar wind; and a refinement of its mass and radius. Because solar wind data can provide important information on a planet's bulk properties, the study of the interaction between Mercury and the solar wind was given a high scientific priority, and a darkside passage at 705 km altitude was selected for the flyby.
An aim point within the solar occultation zone made possible a sensitive search for a tenuous neutral atmosphere by observation of the extinction of solar extreme ultraviolet radiation and by a favorable ground-track for studying the infrared thermal emission of the surface from mid-afternoon to midmorning, local time. Mariner 10 passed through the region in which Earth is occulted by Mercury (as viewed from the spacecraft) to permit a dual-frequency (X- and S-band) radio occultation probe in search of an ionosphere and to measure the radius of the planet.
After completing a 176-day solar orbit following its first Mercury flyby, the Mariner 10 spacecraft successfully encountered Mercury for a second time on September 21, 1974 (Figure 4). The reencounter was at the same position in the solar system, 0.46 AU from the Sun. The spacecraft passed by the sunlit side of Mercury at an altitude of 48,069 km. The main objective of this second flyby was to extend the photographic coverage of Mercury. The new photographs obtained were used to tie together the incoming and outgoing portions of Mercury photographed during the first encounter and provided new views of the south polar area.
Mariner 10 passed Mercury for the third time on March 16, 1975, at 327 km altitude. This encounter yielded the most accurate celestial mechanics data of the mission because of the close passage and the absence of an Earth occultation. The main objective of the third encounter was to define the source of the weak magnetic field discovered on the first encounter. Like the first encounter, it was a dark-side pass. Photographs at a resolution of about 100 m were obtained during the third encounter. Partial-frame pictures were acquired in areas not previously photographed at this resolution.
THE SPACECRAFT
Figure 5 is a schematic of the Mariner 10 spacecraft. The weight of the spacecraft was 504 kg, which included 20 kg of hydrazine fuel and 79.4 kg of scientific experiments. When fully deployed' the spacecraft measured 3.7 m from the top of the low-gain antenna to the bottom of the heat shield of the thrust vector control assembly of the propulsion subsystem. Its total span was 8.0 m with the two solar panels extended. Each panel measured 2.69 m long and 0.97 m wide and was attached to outriggers on the octagonal bus. The high-gain antenna. magnetometer boom, and the plasma science experiment boom also were attached to the bus. The two-degrees-of-freedom scan platform contained the two television cameras and the ultraviolet airglow spectrometer.
The high-gain antenna was an aluminum, honey-combed parabolic dish reflector antenna 1.37m in diameter [7] with a focal distance of 0.55 m. Right-handed, circularly polarized radiating feeds were attached to the antenna to allow transmission at both S-band (2295 MHz) and X-band (8415 MHz) frequencies. Transmissions from Earth were received at an S-band frequency of 2113 MHz. 'The antenna was attached to a deployable support boom and was driven by two-degrees-of-freedom actuators to obtain optimum pointing toward Earth.
SCIENTIFIC EXPERIMENTS
The scientific experiments ( Table 2 ) were selected to take advantage of the opportunity to encounter Mercury and to approach the Sun more closely than ever before. The television science and infrared radiometry experiments provided measurements of the surface of the planet. The plasma science, charged particles, and magnetic field experiments supplied measurements of the environment around the planet and the interplanetary medium. The dual-frequency radio science and ultraviolet spectroscopy experiments were designed for detection and measurement of the characteristics of Mercury's neutral atmosphere and ionosphere. The celestial mechanics experiment provided measurements of planetary mass characteristics and tests of the theory of relativity. Although all experiments were designed and selected to achieve the scientific objectives at Mercury, important data were obtained during the Venus encounter and during the cruise phase. The arrangement of these experiments on the spacecraft is shown in Figure 5.
Television Science. Because Mariner 10's trajectory at Mercury passed through the solar occultation regions ( Figure 6 ), the closest approach to the planet occurred....
....when the cameras could not see the sunlit portion of Mercury. Consequently, the cameras were equipped with 1500-mm focal length lenses so that high-resolution pictures could be taken during the approach and post-.....
Table 2. Mariner 10 Scientific Experiments.
.
Television science
B. C. Murray
California Institute of
Technology
Twin 1.5 m telescopes,
vidicon cameras
Infrared radiometry
C. S. Chase
Santa Barbara Research
Corporation
Infrared radiometer
Ultraviolet spectoscopy
A. L. Broadfoot
Kitt Peak National
Observatory
Airglow spectrometer and
occulation spectrometer
Celestial mechanics and
radion science
H. T. Howard
Stanford University
X-band transmitter
Magnetic field
N. F. Ness
Goddard Space Flight
Center
Two triaxal fluxgate
magnetometers
Plasma science
H. S. Bridge
Massachussetts Institute of
Technology
Scanning electrostatic
analyzer and electron spectrometer
Charged particles
J. A. Simpson
University of
Chicago
Charges particle
telescope
[8] ....encounter phases. The schematic view of the television camera is shown in Figure 7, and the camera characteristics are given in Table 3.
The imaging sequence was initiated 7 days before the encounter with Mercury when about half of the illuminated disk was visible and the resolution was better than that achievable with Earth-based telescopes. Photography of the planet continued until some 30 min before closest approach, providing a smoothly varying sequence of pictures of increasing resolution and decreasing areal coverage. Pictures with resolutions on the order of 2 to 4 km were obtained for both quadratures on the first encounter (Figures 18 and 19). Variation in resolution, ranging between several hundred kilometers to approximately 100 m, assisted in the extrapolation of large-scale features observed at high resolution over broad areas photographed at lower resolution. The highest resolution photographs were obtained approximately 30 min prior to and following closest approach on the first and third encounters. Pictures taken in a number of spectral bands enabled the determination of regional color differences.
The second Mercury encounter (Figure 6) provided a unique opportunity to observe regions of Mercury with more favorable viewing geometry than was possible during the first encounter. In order to permit a third encounter, it was necessary to target the bright-side encounter for a south polar pass. This trajectory allowed unforeshortened views of the south polar region, the exploration of areas not previously accessible for study, a geologic and cartographic tie in the southern hemisphere between the two sides of Mercury photographed on the first encounter, and the acquisition of stereoscopic coverage of the southern hemisphere. Because of the small field of view resulting from the long focal length optics, it was necessary to increase the periapsis altitude to about 48,000 km to ensure sufficient overlapping coverage to make a reliable geologic and cartographic tie. 'The resolution of the photographs taken during closest approach ranged from 1 to 3 km (Figure 20 ).
Table 3. Television Camera Characteristics
Focal length
1500 mm (62 mm) a
Focal number,
f/8.4
Shutter speed range
33.3 ms to 11.7 s
Angular field of view
0.38° X 0.47° (9° X
11°) a
Vidicon target image area
9.6 X 12.35 mm
Scan lines per frame
700
Image elements per line
832
Bits per image element
8
Frame time
42 s
Spectral filters
Blue, ultraviolet, ultraviolet
polarizing, orange, minus ultraviolet, and clear
a Wide-angle optics.
The third Mercury encounter was targeted to optimize the acquisition of magnetic and solar wind data. Therefore, the viewing geometry on the third encounter was very similar to that on the first encounter. However, the third encounter presented the opportunity to target high-resolution pictures to areas of geologic interest seen previously at lower resolution. Because of ground communication problems, these pictures were acquired as quarter frames.
Infrared Radiometry. The primary goal of the infrared radiometry experiment was to measure infrared thermal radiation emanating from the surface of Mercury between late afternoon and early morning. These temperature measurements taken on the first encounter provided much more accurate values for the average thermal properties of the planet than can be obtained from ground-based studies. An important secondary objective was to search for possible correlations between thermal anomalies and topographic features.
Ultraviolet Spectroscopy. The occultation spectrometer provided a sensitive detection of any atmosphere present, and of its composition, with a detection threshold improved by a factor of about 107 over current ground-based studies. The airglow spectrometer provided quantitative information on the abundance of H, He, He+, C, [9] O, Ne, and A in the atmosphere of Mercury by measuring the intensity and spatial distribution of their ultraviolet emission lines. Data were taken on the first and third encounters.
Celestial Mechanics and Radio Science. The celestial mechanics experiment provided improved measurements of the mass and gravitational characteristics of Mercury. The planet's close proximity to the Sun, large orbital eccentricity. and unusual spin-orbit resonance made this experiment of primary interest.
The occultation of the spacecraft by Mercury on the first encounter afforded an opportunity to probe the atmosphere and to measure the radius of the planet. Phase changes in the S-band radio signal allowed measurement of an atmosphere with about 1016 molecules per cm3. A more sensitive but less direct measurement of atmospheric gas density was provided by the ionospheric refractivity measurements.
Magnetic Field and Plasma Science. Vector magnetic field and plasma measurements were made to study the interaction of Mercury with the solar wind. Because of the nature of the solar wind and the physical processes under investigation, these phenomena are strongly interrelated and mutually supporting. Data were taken on the first and third encounters.
Charged Particles. The charged particle telescope was designed to detect high-energy particles at Mercury. This experiment complemented and extended the magnetic field and plasma science measurements of the interaction of Mercury with the solar wind.
