Ralph McNutt

Member of the Plasma Science team from before launch

Co-Investigator on both the Voyager Low-Energy Charged Particle and Plasma Science teams

Ft. Worth, Texas

One of my more memorable moments, after watching Voyager 1 launch from the Cape in September of 1977, was focused on whether Neptune had a magnetosphere, a region of space surrounding the planet in which a planetary magnetic field dictates the interaction of the planet with the solar wind and the subsequent motion of plasma and charged particles near the planet. To the more visually aligned members of the Voyager science team -- and to members of the press -- this was often referred to as “squiggly-line” science, because particle counts, plasma fluxes, plasma wave intensities and magnetic field components all typically were (and are) plotted as graphs of numbers versus time. To the uninitiated, this all just looked like a bunch of squiggly lines. (To be fair, even those of us chasing such data -- and what they could and did reveal about planets and other phenomena in the solar system -- liked pictures as well, but dealing with those was someone else’s job at the time.)

Part of the challenge was that certain conclusions could only be drawn from actually “being there,” and in mid-August 1989, Voyager was bearing down on its rendezvous with Neptune. The various “fields and particles” instruments were measuring the conditions in the solar wind at the spacecraft. All we needed to know was exactly where the wind stopped, and from that, we could deduce the strength of Neptune’s planetary magnetic field.

So, we waited, and then we set up a prediction pool -- when we would know we were at the boundary of Neptune’s influence in the solar wind. And, we made a list on a chalkboard in one of the rooms at the Jet Propulsion Laboratory assigned to the science teams (as shown in this picture). The prediction was for when Voyager 2 would fly into the “bow shock” of the magnetosphere of Neptune in PDT-ERT (Pacific Daylight Time-Earth Received Time, which was the time the data arrived back at the computers at JPL in California). So, there were only five entries -- beginning at Day of Year (DOY) 235, also known as August 23rd (in non-leap years). At the flyby, the one-way light time was 4 hours and 6 minutes. The spacecraft event time (SCET) is typically given in Greenwich Mean Time (GMT), so to go from PDT-ERT to GMT-SCET, add 7 hours to get to GMT, but the event occurred on the spacecraft 4 hours and 6 minutes earlier, so that has to be subtracted off, a difference of 2 hours and 54 minutes. My guess was a lot later than the others, and in favor of a smaller magnetic field, hence, first seen closer to the planet. I picked 12 noon on DOY 236 -- that is, August 24th at 1454 GMT-SCET. The Plasma Science experiment detected the bow shock on the 24th of August at 1438. Not too bad! But, the truly amazing things are that this was documented and the photo still exists.