February’s talk: ‘The Grand Tour – Mission to the Giants’

Given by Rob Slack of Swindon Stargazers.

His talk ‘The Grand Tour – Mission to the Giants’, namely the Voyager missions took us back to the sixties and the space race and the two people who made the Grand Tour possible.  I will introduce the second person first: Gary Flandro, who worked out that we could use gravity assist on spacecraft to propel them from one gas giant to the other.   Also that an amazing planetary alignment would take place in September 1977 and that there would not be another chance for 175 years.

These gravity assists were a recent big thing and could be used to speed up or slow a vehicle to help it cross the interplanetary vastness of space.  This was actually part of Flandro’s postgraduate studies, so he wasn’t high on the academic ladder at that point.  Rob then introduced another person (person number one!) whose computations enabled Flandro to make his own amazing proposition: Michael Minovitch.  He was a mathematician who was set a challenge to work on the ‘three body problem’, in other words you have scenarios of the Sun, a planet, and a third object such as a comet, asteroid or spacecraft acting on each other.  He was given time on the massive IBM 7090 computer at UCLA in order to work out different trajectories and came up with gravity assists.  This was in 1961 and sadly the space race meant it got buried.

Luckily Flandro was around to build on it and in 1969 NASA wanted a ten year mission to the outer planets.

At the time it was not known how populated the asteroid belt was, and how strong was Jupiter’s radiation.  (James Alfred) Van Allen had done lots of work on the radiation belts around the Earth, but it was not known yet that Jupiter’s Van Allen belts are millions of times stronger.  (If you have a short wave radio you can hear Jupiter, mainly as Io moves through its magnetic field.)

Those with longer memories may already know that the two Pioneer craft (1972/73) did not even have proper cameras; they had photopolarimeters, which were brightness sensors and those wonderful Jupiter and Saturn pictures they returned were created by the spacecraft spinning and scanning in the brightness changes.  The energy to run everything was created by electricity coming from thermocouples wrapped around plutonium which was so radioactive that it was kept on booms away from the main craft.  Pioneer 11 gave us views of Saturn and Titan but its route via Jupiter had to be altered in order to miss the worst of Jupiter’s radiation belts.

By the time the Voyagers came along the use of gyros meant that the craft did not need to spin in order to take pictures.  There was a ten day launch window for the grand tour originally calculated by Flandro but now they were able to launch the two craft on 20th August and 5th September 1977 respectively.  (Voyager 2 was the first one to leave.)  Energy came from a radioisotope thermoelectric generator.  Fortran was the computer language.  The Jupiter closest approaches were V1 5/3/79 and V2 9/7/79. 

Their cameras were a kind of cathode ray colour tv tube.  Voyager 2 had a slightly better camera, and it was able to detect aurorae and lightning on Jupiter and lightning discharges from Io.  It photographed Jupiter’s ring.  Amazingly there had been no plans to photograph the Galilean satellites but sense prevailed and we now have visible records of the extreme volcanic activity on Io.  We now know Io’s surface changes day by day as it gets mangled by Jupiter’s effects.  Europa’s frozen watery landscape was seen to be riddled with lines.

Voyager 1’s arrival at Saturn was hastened in order to allow it to view Titan.  It then swung up out of the Solar System.  On 14/2/90 it looked down on the Solar System and took a ‘family portrait’ series of shots of the Sun and planets.  Remember the ‘pale blue dot’ (the Earth, less than a pixel size) and the poignant words of Carl Sagan, reminding us how anyone who’d ever lived had been on that tiny dot.  Voyager 1’s cameras were turned off after that.  Voyager 2 stayed on the ecliptic to continue to the other planets.

In 1980 the spokes on Saturn’s rings were detected, as were the ‘shepherd satellites’ that meandered along the ring edges.  Uranus and Neptune….Rob didn’t really talk about them but the pictures are well remembered: greenish tilted Uranus and the storms on blue Neptune.

Voyager 2 continues to send back data, although the signal is very weak.  In December 2018 the number of solar particles dropped and cosmic rays increased, indicating it has reached the heliopause and is entering interstellar space.  It is still generating enough heat to keep equipment going, although it is now functioning at much lower temperatures than expected.

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