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) 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