Telemetry from the Mariner IV spacecraft, now en route across interplanetary space to the planet Mars, indicates that one of its eight experiments has ceased to function.
Engineers at the Jet Propulsion Laboratory said the instrument -- the solar plasma experiment -- began operating only intermittently yesterday. No useful data was received from the instrument since early today. Project officials analyzing the problem have not discounted the possibility of recovery of the instrument.
JPL also reported that the Mariner IV temporarily lost its lock on the star Canopus at 4:30 a.m. PST today and went into its programmed roll search for the star. Loss of lock on Canopus does not alter Mariner's course.
When the spacecraft's Canopus tracker lost lock with the Star, Mariner rolled 342 degrees in 44 minutes, stopping at another bright star, Gamma-Vela. Project officials plan to command the spacecraft to reacquire Canopus sometime in the near future.
Current analysis supports the theory that a dust particle, reflecting the light of the sun, passed through the field of view of the Canopus tracker. According to project officials, a dust particle no larger than .005-inch, passing within 2 feet of the sensor, could have triggered the loss of lock on Canopus.
For 8 days, the solar plasma probe functioned perfectly, measuring and transmitting to Earth the density, velocities, temperatures and direction of low energy prot outward from the Sun at supersonic speeds to form what has termed the "solar wind."
This is the first time that such measurements have been made in interplanetary space farther from the Sun than the orbit of Earth.
Breakdown of the plasma probe has not affected other instruments or spacecraft subsystems.
Five other scientific instruments aboard the Mariner IV are functioning normally. Two additional experiments will be conducted in the vicinity of Mars next July.
Mariner IV performed a mid-course maneuver Saturday, seven days after launch from Cape Kennedy. The post-maneuver trajectory data are now being analyzed to determine precisely the new path of the spacecraft.
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