Mariner 10
Mariner 10 was an American robotic space probe launched by NASA on 3 November 1973, to fly by the planets Mercury and Venus. It was the first spacecraft to visit Mercury and the first to perform a flyby of multiple planets.
Mariner 10 was launched approximately two years after Mariner 9 and was the last spacecraft in the Mariner program.
The mission objectives were to measure Mercury's environment, atmosphere, surface, and body characteristics and to make similar investigations of Venus. Secondary objectives were to perform experiments in the interplanetary medium and to obtain experience with a dual-planet gravity assist mission. Mariner 10s science team was led by Bruce C. Murray at the Jet Propulsion Laboratory.
Design and trajectory
Mariner 10 was the first mission to use a gravity assist from one planet to reach another planet. It used Venus to bend its flight path and bring its perihelion down to the level of Mercury's orbit. This maneuver, inspired by the orbital mechanics calculations of the Italian scientist Giuseppe Colombo, put the spacecraft into an orbit that repeatedly brought it back to Mercury. Mariner 10 used the solar radiation pressure on its solar panels and its high-gain antenna as a means of attitude control during flight, the first spacecraft to use active solar pressure control.The components on Mariner 10 can be categorized into four groups based on their common function. The solar panels, power subsystem, attitude control subsystem, and the computer kept the spacecraft operating properly during the flight. The navigational system, including the hydrazine rocket, would keep Mariner 10 on track to Venus and Mercury. Several scientific instruments would collect data at the two planets. Finally, the antennas would transmit this data to the Deep Space Network back on Earth, as well as receive commands from Mission Control. Mariner 10s various components and scientific instruments were attached to a central hub, which was roughly the shape of an octagonal prism. The hub stored the spacecraft's internal electronics. The Mariner 10 spacecraft was manufactured by Boeing. NASA set a strict limit of US$98 million for Mariner 10's total cost, which marked the first time the agency subjected a mission to an inflexible budget constraint. No overruns would be tolerated, so mission planners carefully considered cost efficiency when designing the spacecraft's instruments. Cost control was primarily accomplished by executing contract work closer to the launch date than was recommended by normal mission schedules, as reducing the length of available work time increased cost efficiency. Despite the rushed schedule, very few deadlines were missed. The mission ended up about US$1 million under budget.
Attitude control is needed to keep a spacecraft's instruments and antennas aimed in the correct direction. During course correction maneuvers, the spacecraft may need to rotate so that its rocket engine faces the proper direction before being fired. Mariner 10 determined its attitude using two optical sensors, one pointed at the Sun, and the other at a bright star, usually Canopus; additionally, the probe's three gyroscopes provided a second option for calculating the attitude. Nitrogen gas thrusters were used to adjust Mariner 10s orientation along three axes. The spacecraft's electronics were intricate and complex: it contained over 32,000 pieces of circuitry, of which resistors, capacitors, diodes, microcircuits, and transistors were the most common devices. Commands for the instruments could be stored on Mariner 10s computer, but were limited to 512 words. The rest had to be broadcast by the Mission Sequence Working Group from Earth. Supplying the spacecraft components with power required modifying the electrical output of the solar panels. The power subsystem used two redundant sets of circuitry, each containing a booster regulator and an inverter, to convert the panels' DC output to AC and alter the voltage to the necessary level. The subsystem could store up to 20 ampere hours of electricity on a 39-volt nickel–cadmium battery.
The flyby past Mercury posed major technical challenges for scientists to overcome. Due to Mercury's proximity to the Sun, Mariner 10 would have to endure 4.5 times more solar radiation than when it departed Earth; compared to previous Mariner missions, spacecraft parts needed extra shielding against the heat. Thermal blankets and a sunshade were installed on the main body. After evaluating different choices for the sunshade cloth material, mission planners chose beta cloth, a combination of aluminized Kapton and glass-fiber sheets treated with Teflon. However, solar shielding was unfeasible for some of Mariner 10s other components. Mariner 10s two solar panels needed to be kept under. Covering the panels would defeat their purpose of producing electricity. The solution was to add an adjustable tilt to the panels, so the angle at which they faced the sun could be changed. Engineers considered folding the panels toward each other, making a V-shape with the main body, but tests found this approach had the potential to overheat the rest of the spacecraft. The alternative chosen was to mount the solar panels in a line and tilt them along that axis, which had the added benefit of increasing the efficiency of the spacecraft's nitrogen jet thrusters, which could now be placed on the panel tips. The panels could be rotated a maximum of 76°. Additionally, Mariner 10s hydrazine rocket nozzle had to face the Sun to function properly, but scientists rejected covering the nozzle with a thermal door as an undependable solution. Instead, a special paint was applied to exposed parts on the rocket so as to reduce heat flow from the nozzle to the delicate instruments on the spacecraft.
Accurately performing the gravity assist at Venus posed another hurdle. If Mariner 10 was to maintain a course to Mercury, its trajectory could deviate no more than from a critical point in the vicinity of Venus. To ensure that the necessary course corrections could be made, mission planners tripled the amount of hydrazine fuel Mariner 10 would carry, and also equipped the spacecraft with more nitrogen gas for the thrusters than the previous Mariner mission had held. These upgrades proved crucial in enabling the second and third Mercury flybys.
The mission still lacked the ultimate safeguard: a sister spacecraft. It was common for probes to be launched in pairs, with complete redundancy to guard against the failure of one or the other. The budget constraint ruled this option out. Even though mission planners stayed sufficiently under budget to divert some funding for constructing a backup spacecraft, the budget did not permit both to be launched at the same time. In the event that Mariner 10 failed, NASA would only allow the backup to be launched if the fatal error was diagnosed and fixed; this would have to be completed in the two and a half weeks between the scheduled launch on 3 November 1973 and the last possible launch date of 21 November 1973. The unused backup was sent to the Smithsonian museum for display.
Instruments
Mariner 10 conducted seven experiments at Venus and Mercury. Six of these experiments had a dedicated scientific instrument to collect data. The experiments and instruments were designed by research laboratories and educational institutions from across the United States. Out of forty-six submissions, JPL selected seven experiments on the basis of maximizing science return without exceeding cost guidelines: together, the seven scientific experiments cost US12.6 million dollars, about one-eighth of the total mission budget.Television photography
The imaging system, the Television Photography Experiment, consisted of two Cassegrain telescopes feeding vidicon tubes. The main telescope could be bypassed to a smaller wide angle optic, but using the same tube. It had an 8-position filter wheel, with one position occupied by a mirror for the wide-angle bypass.TV camera exposures ranged from 3 ms to 12 s with each camera being able to take a picture every 42 s. The picture resolution was 832 x 700 pixels, 8-bit coded.
The entire imaging system was imperiled when electric heaters attached to the cameras failed to turn on immediately after launch. To avoid the Sun's damaging heat, the cameras were deliberately placed on the spacecraft side facing away from the Sun. Consequently, the heaters were needed to prevent the cameras from losing heat and become so cold that they would become damaged. JPL engineers found that the vidicons could generate enough heat through normal operation to stay just above the critical temperature of ; therefore they advised against turning off the cameras during the flight. Test photos of the Earth and Moon showed that image quality had not been significantly affected. The mission team was pleasantly surprised when the camera heaters started working on 17 January 1974, two months after launch. Later investigation concluded that a short circuit in a different location on the probe had prevented the heater from turning on. This allowed the vidicons to be turned off as needed.
Of the six main scientific instruments, the cameras were by far the most massive device. Requiring 67 watts of electricity, the cameras consumed more power than the other five instruments combined. The system returned about 7,000 photographs of Mercury and Venus during Mariner 10's flybys.