Lonne Lane

Team Chief, Science Integration Team; Assistant Project Scientist for Jupiter; Photopolarimeter Principal Investigator

Retired

Arcadia, California

In July 1979, Voyager 2 flew past Jupiter and Europa. The Photopolarimeter instrument (PPS), which measures polarized light, came out of the Jovian radiation field with damaged command shift registers and loss of control over aperture, color filters and polarization measurements. But, the basic instrument was still functional. Over the next 18 months, the Voyager mission operations team at the Jet Propulsion Laboratory worked with the PPS scientists and engineers to develop operational codes to enable measurements at Saturn and to perform a long-duration stellar-ring occultation sequence, where scientists can learn more about the rings by watching how they block starlight. But, the aperture wheel remained “stubborn” in its performance. Sometimes the aperture wheel was fully open (a position of 161 or 162-163) and sometimes it only moved on command to a half-open position, which would not work properly for the ring occultation. In spite of special “exercise” routines to clean the aperture wheel’s electrical contacts, the commanded positions were only proper about half of the time.

During the course of the approach to a near encounter with Saturn, the PPS team commanded the aperture wheel more than a dozen times to see if a pattern could be discovered that would provide higher confidence. The proper position would occur when the ring occultation measurement was to begin. We thought we found a weakly correlated pattern that might give a better than 50 percent chance of the proper setting. Coming into the near encounter, the instrument was given one last wheel contact cleaning operation and then everyone just had to wait to see what would occur. The team and a number of spacecraft engineering personnel gathered at JPL where the telemetry line printers were spewing out pages of engineering status reports. The operations team transferred the PPS telemetry channels to the overhead video monitors. We all waited for the critical command event; everyone who knew what was at stake was nervous with anticipation and some dread.

The appointed time arrived. Everyone was watching the monitors. Aperture wheel position number 161 suddenly appeared and everyone was cheering. The occultation measurement was going to be a success! Two hours later the measurement ended and the recorded PPS intensity data were transferred to magnetic tape and given to us to format and graphically print at the PPS data computer room in building 264. The PPS data computer drove a high-speed analog strip chart recorder that began to pour out squiggly lines representing time of measurement and star intensity. A single paper plot over a quarter of a mile long showed the attenuation of the starlight by the ring material, at a resolution of greater than 20 meters, for the entire 70 million-meter length of the cut through the rings’ radius.

What a spectacular event that unfolded after so much hard work by so many people!