Editing Mariner 10 (section)

== Design and trajectory ==
[[File:Mariner 10 gravitational slingshot.jpg|thumb|left|An artists' impression of the ''Mariner 10'' mission. It used a flyby of the planet [[Venus]] to decrease its perihelion. This would allow the spacecraft to meet Mercury on three occasions in 1974 and 1975.]]

''Mariner 10'' was the first mission to use a [[gravity assist]] from one planet (in this case, Venus) to reach another planet.{{efn| [[Luna-3]] was the first spacecraft to use [[Gravity assist]].}} It used Venus to bend its flight path and bring its [[perihelion]] down to the level of Mercury's orbit.<ref name=nasa1/><ref name=beyond/> 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 [[Photovoltaic module|solar panel]]s and its high-gain antenna as a means of [[Spacecraft attitude control|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 10''{{'}}s 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.<ref name=nssdc3/>{{sfn|Clark|2007|pp=22-23}}{{sfn|Strom|Sprague|2003|p=16}} The Mariner 10 spacecraft was manufactured by Boeing.<ref name=nasa3/> 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.{{sfn|Reeves|1994|p=222}} 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.<ref name=Biggs/> The mission ended up about US$1 million under budget.{{sfn|Murray|Burgess|1977|p=142}}

[[Spacecraft attitude control|Attitude control]] is needed to keep a spacecraft's instruments and antennas aimed in the correct direction.<ref name=Doody/> 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 10''{{'}}s orientation along three axes.{{sfn|Dunne|Burgess|1978|p=58}}{{sfn|Murray|Burgess|1977|p=50}}<ref name=ezell/> 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.<ref name=Paul/> Commands for the instruments could be stored on ''Mariner 10''{{'}}s computer, but were limited to 512 words. The rest had to be broadcast by the Mission Sequence Working Group from Earth.<ref name=Shirley/> 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' [[direct current|DC]] output to [[alternating current|AC]] and alter the voltage to the necessary level.<ref name=nasa2/> The subsystem could store up to 20 [[ampere hour]]s of electricity on a 39-volt [[nickel–cadmium battery]].<ref name=wilson/>

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]].{{sfn|Dunne|Burgess|1978|pp=32-33}} However, solar shielding was unfeasible for some of ''Mariner 10''{{'}}s other components. ''Mariner 10''{{'}}s two solar panels needed to be kept under {{cvt|115|°C|°F}}. 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°.{{sfn|Strom|Sprague|2003|p=16}}{{sfn|Murray|Burgess|1977|p=21}} Additionally, ''Mariner 10''{{'}}s 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.{{sfn|Dunne|Burgess|1978|pp=30-32}}

Accurately performing the gravity assist at Venus posed another hurdle.{{sfn|Reeves|1994|p=242}} If ''Mariner 10'' was to maintain a course to Mercury, its trajectory could deviate no more than {{convert|200|km|sp=us}} from a critical point in the vicinity of Venus.{{sfn|Dunne|Burgess|1978|p=56}} 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.{{sfn|Murray|Burgess|1977|pp=25-26}}

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.{{sfn|Strom|Sprague|2003|p=14}} 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.{{sfn|Murray|Burgess|1977|pp=25-26}}{{sfn|Murray|Burgess|1977|p=38}} The unused backup was sent to the Smithsonian museum for display.<ref name=smithsonian2/>