Los Angeles---Changes in the performance of two scientific instruments aboard NASA's Mars-bound Mariner IV spacecraft were reported here today by a Jet Propulsion Laboratory engineer.
Dan Schneiderman, JPL's Mariner Project manager, said data from the solar plasma probe, which ceased normal function 10 days after Mariner was launched last November 28, now has become understandable to scientists through analysis of the component failure in the probe.
However, telemetry from a second instrument, Schneiderman reported, indicates that a portion of the ionization chamber experiment, which measures radiation in space, is not operating properly. The new failure, he said, is in the GeigerMueller tube which counts the number of charged particles in the energy region measured by the ion chamber.
The chamber itself, which measures the total effect of radiation in the region of energy of electrons greater than 0.5 million electron volts and protons greater than 10 million electron volts, continues to function normally. The malfunction of the Geiger-Mueller tube portion of the experiment was ascertained after several days of analyzing data that did not fall within predicted levels and could not be correlated with other measurements.
Schneiderman related the latest events in the Mariner IV mission in an address before the Unmanned Spacecraft Meeting, sponsored by the American Institute of Aeronautics and Astronautics, at the International Hotel.
He said the malfunction in the solar plasma probe was found to be the failure of a resistor in the instrument's high voltage circuitry. By taking this failure into account, experimenters were able to recalibrate the instrument and interpret data which otherwise would be unintelligible.
Schneiderman pointed out to the AIAA delegates that this is the 95th day of Mariner IV's flight to Mars and that the spacecraft is now 26 million miles from Earth and travelling at a speed of 22,700 miles per hour relative to the Earth.
Mariner IV has received and acted upon 42 commands from Earth, he said, and a number of others issued by the on-board central computer and sequencer.
The latest CC&S-commanded event occurred last Saturday morning when the spacecraft's Canopus tracker was updated to compensate for the changing relationship between the spacecraft, the sun and the star Canopus. This was accomplished without changing the attitude of the spacecraft, but by electronically changing the "look angle" of the tracker. Three more Canopus updates are scheduled to occur before Mariner IV flys by Mars on July 14.
Next Friday, Schneiderman said, Mariner's transmitter will be switched automatically from the omni-directional antenna, in use since launch, to the high-gain fixed directional antenna which is needed for the spacecraft-to-Earth radio link as the communications distance increases. Antenna-switching also is an event controlled by the CC&S and backed up by ground command capability.
Performance of the spacecraft's attitude control, thermal control and power subsystems has been excellent, Schneiderman said.
Based on nitrogen gas consumption to date, he estimated there is enough gas available to keep Mariner IV stabilized for about six years. Since last December 17, when a special command was transmitted to the spacecraft, there has been no reoccurrence of loss of lock with Canopus.
Telemetry readings, which report measurements of voltages, currents and temperatures of various Mariner IV subsystems, have been close to nominal throughout the mission, Schneiderman said.
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