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.
