telescopes were used to collect highly energetic electrons and atomic nuclei, specifically oxygen nuclei or less massive. These particles then passed through a set of detectors and were counted. Magnetometers Two fluxgate magnetometers were entrusted with discerning whether Mercury produced a magnetic field, and studying the interplanetary magnetic field between flybys. In designing this experiment, scientists had to account for interference from the magnetic field generated by Mariner 10's many electronic parts. For this reason, the magnetometers had to be situated on a long boom, one closer to the octagonal hub, the other one further away. Data from the two magnetometers would be cross-referenced to filter out the spacecraft's own magnetic field. Drastically weakening the probe's magnetic field would have increased costs. Celestial Mechanics and Radio Science experiment This experiment investigated the mass and gravitational characteristics of Mercury. It was of particular interest because of the planet's closeness to the Sun, large orbital eccentricity, and unusual spin-orbit resonance. As the spacecraft passed behind Mercury on the first encounter there was an opportunity to probe the atmosphere and to measure the radius of the planet. By observing phase changes in the S-band radio signal, measurements of the atmosphere could be made. The atmosphere was assessed as having a density of about . Departing Earth Boeing finished building the spacecraft at the end of June 1973, and Mariner 10 was delivered from Seattle to JPL's headquarters in California, where JPL comprehensively tested the integrity of the spacecraft and its instruments. After the tests were finished, the probe was transported to the Eastern Test Range in Florida, the launch site. Technicians filled a tank on the spacecraft with of hydrazine fuel so that the probe could make course corrections, and attached squibs, whose detonation would signal Mariner 10 to exit the launch rocket and deploy its instruments. The planned gravity assist at Venus made it feasible to use an Atlas-Centaur rocket instead of a more powerful but more expensive Titan IIIC. The probe and the Atlas-Centaur were attached together ten days prior to liftoff. Launch posed one of the largest risks of failure for the Mariner 10 mission; Mariner 1, Mariner 3, and Mariner 8 all failed minutes after lift-off due to either engineering failures or Atlas rocket malfunctions. The mission had a launch period of about a month in length, from 16 October 1973, to 21 November 1973. NASA chose 3 November