Mar 16 1975
From The Space Library
Mariner 10 made its last and closest flyby of Mercury at a distance of 327 km at 6:39 pin EDT. To prepare for the flyby, the trajectory had been corrected on 30 Oct. 1974 and 13 Feb. 1975. An additional correction 7 March was to reduce the probability of Mariner 10's impacting the surface of Mercury from 30% to 1%, targeting Mariner 10 to approach the planet from the sunlit side, then loop around behind it.
One last-minute problem was navigational difficulty in locking onto the star Canopus so that the spacecraft would point in the proper direction for data collection. Because a commanded roll search for Canopus would expend all remaining attitude-control gas, Mariner 10 had to be maneuvered using solar-radiation pressure on the solar panels and high-gain antenna. The problem was intensified because of the tandem demands of Helios 1 and Mariner 10 on the antennas of NASA's deep-space tracking network. However, the primary users West German scientists tracking Helios 1-gave receiving time to the Mariner 10 flight controllers, who obtained Canopus lock in time for the encounter.
Major objective of the third encounter was to determine whether Mercury had an intrinsic or induced magnetic field. In a press briefing 17 March, Dr. Norman F. Ness, principal investigator of the magnetic fields experiment, said that Mercury did have an intrinsic magnetic field, encountered on the incoming trajectory at precisely the time predicted from the first Mercury pass. Mariner 10 had measured the field to be a maximum of 400 gamma. Plasma science experiment data supported the presence of the field; Dr. Ness observed that the interface between the solar wind and Mercury's magnetic field appeared to be a scaled-down version of the interaction between the solar wind and the magnetic field of earth.
The spacecraft returned about 300 TV pictures of the planet's surface between 12 and 17 March, with best resolution at 100 m obtained from quarter-frame near-encounter pictures. All photos were transmitted in real time and at 22 kilobits per sec, instead of the planned 117, because of overheating of the spacecraft. (NASA MORs, 10 March 75, 7 April 75; NASA Releases 75-58, 75-59; NASA Activities, April 75, 12-13; Av Wk, 24, March 75, 24-25)
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