From the Project Manager

Calendar year 1997 is the final year of Galileo's primary mission. The Project will celebrate the 20th anniversary of its start this October and complete primary mission data return in December--2 years after arriving at Jupiter on December 7, 1995.

Remarkably, 1997 is to be the most productive year ever for Galileo Orbiter science, which says a lot considering all the impressive science already returned. We're off to a great start. The Europa-4 recorded science data were returned as planned before and after solar conjunction. Europa-6 was another grand success--we're five for five! (Remember no Orbit-5 encounter because of conjunction.) Six of the ten primary mission encounters occur this year, so five remain ahead: two with Ganymede, two with Callisto, and the final with Europa. And, between the Callisto encounters, we will fly the dramatic magnetotail orbit to measure the magnetosphere 10 million km away from Jupiter.

A very important, but subtle, science observation was made a week after Europa-6. The orbital tour was designed so that Galileo would pass directly behind Io as seen from Earth on February 26th. Even though the distance from Galileo to Io was over 3 million km, the change to Galileo's radio signal frequency caused by its passing through the Io ionosphere allows the Radio Science Propagation Team to measure the ionosphere and estimate characteristics of Io's extremely thin atmosphere. This was the first Io radio occultation experiment since Pioneer 10 over 20 years ago and only the second in history. It is the first of six Galileo Io occultations, which together will provide the first global picture of Io's complex ionosphere.

The Europa-6 gravity assist had to very precisely change Galileo's orbital inclination to achieve the Io occultation. Subsequently, on March 14th, orbit trim maneuver (OTM)-22, the apojove OTM be-tween Europa-6 and Ganymede-7, was the biggest OTM ever to be performed with Galileo's 10-N thrusters. Requiring over 12 hours, in aggregate the thrusters were pulsed nearly 2800 times to impart 15.8 m/s to "tip" Galileo's orbit plane opposite to what Europa-6 had done. This got Galileo headed to Ganymede-7. Our celestial "mechanics" are still having a great time and doing superbly.

Happily, due to the continuing outstanding performance of the Galileo Orbiter and the "can do" spirit and performance of the Project Team, NASA has enthusiastically endorsed our 2-year follow-on mission--the Galileo Europa Mission (GEM) briefly described at the close of my last column. Galileo is now slated to continue returning rich science data from the Jupiter system until the end of the century, with the grand finale being one or two breathtakingly close encounters of Io! One of the greatest challenges of the GEM is that we are committed to perform it with an annual budget of one-fifth the amount we received annually for our primary mission. Although we cannot do as much, the most important unique remaining science Galileo can do in the Jupiter system is our clear goal, and we have every intention of achieving it.

--Bill O'Neil

• It's Time for Europa!
• Educational Outreach Corner