trajectory) to show different aiming points at the target,
such that if you pass through one of these, then you will
later fly through any chosen point beyond the planet.
“In the delivery of Voyager 2 to this targeting plane
relative to Neptune, we only missed our two coordinates
by about 6.5 kilometres. And the timing accuracy
was better than one second. As you passed over the
pole of Neptune, there were some amplifications of
those errors, so the difference was greater when we
got to Triton, but the accuracy was still phenomenal.
“It’s the equivalent of sinking a golf putt from a
distance of over 30,000 miles! And the target is moving.”
RS: You talked earlier about having a few
sleepless nights. Was the need for this degree
of accuracy a cause for concern?
planned JPL programme to explore the outer Solar
System between 1990 and 2010, designed to be more
affordable than the so-called multi-billion dollar ‘flagship’
missions of the 1970s. Mariner Mark II was replaced by
the very successful lower-cost Discovery Programme)
I had people working on the comet mission who were
heartbroken and had to leave and find other jobs.
“In fact, NASA Administrator Dan Goldin threatened
to also cancel Cassini in ’94. He was big on ‘better,
faster, cheaper’ and Cassini as a Flagship Mission would
cost NASA over $2 billion. But when word got out about
this, the Director General of ESA, Jean-Marie Luton,
wrote a special letter of appeal that went to VicePresident Gore. It more or less said ‘We are partners with
NASA on this project. We want you to know that if you
cancel Cassini, don’t look for any further international
cooperation.’ Our relationship with the Europeans would
have been damaged. In my mind that letter saved
Cassini and I stayed with the program from that point.”
KOHLHASE: “One thing worried me on Voyager which
would awake me in the middle of the night. I was
responsible for the
navigation and would
RS: And since then,
calculate analytically,
Cassini, like Voyager,
when we flew through
has been a major
the Jupiter system
achievement in solar
and experienced
system exploration.
the gravity assist and
KOHLHASE: “It has been
deflection of the
an enormous success,
trajectory by roughly
orbiting Saturn for the
90 degrees, how much
last ten years, studying
it would cost us on the
the planet and its rings,
outbound leg leaving
finding the hydrocarbon
Jupiter to correct
lakes on Titan, the
any navigation errors
geysers on Enceladus,
made when inbound.
the intricate detail in
These errors would be
the Saturn rings, and
amplified by Jupiter
Charley’s analogy of the gravity assist flyby of Voyager as it passed Jupiter.
variety among the
and the large Galilean
Credit: Charles Kohlhase with artwork by Gary Hovland.
smaller icy satellites. A
satellites, and we
remarkable success.”
only had a limited amount of propellant onboard to
make corrections. Well, I did the analysis and the results
RS: You were Science and Mission Design
showed that it wasn’t going to be that bad, only 10 or
Manager for this very complex mission. How
15 m/s. I thought ‘That can’t be right’ and kept worrying
did you and your team plan for the orbital part
about it, but it turned out OK, and the rest is history.”
of the mission after it arrived at Saturn?
RS: And then you moved onto another
historic mission, Cassini.
KOHLHASE: “Yes, then I went to a large Saturn orbiter,
and an international cooperative venture with the
Europeans carrying their Huygens probe to Titan.
I had the best job anybody could ever have.”
RS: What was the situation at that early
stage in planning Cassini?
KOHLHASE: “When I first joined Cassini, it was called
Mariner Mark II. It was initially designed as two missions,
one an orbiter to Saturn with a Titan probe, the other
CRAF – Comet Rendezvous Asteroid Flyby. The comet
mission was subsequently cancelled in 1992 because
of reduced funding. (Note: These were part of a
18
18
KOHLHASE: “At the time of launch (October 15th 1997)
the way Cassini worked was that, after gravity-assist
flybys of Venus, Earth and Jupiter, we planned to go
into orbit around Saturn then make an initial tour based
on 45 flybys of Titan. Titan is big enough that, if you
fly by at close range, it can reshape the trajectory of
the spacecraft using the laws of gravity assist. Every
flyby designed before launch was done for a purpose.
We’d be raising the inclination of the orbit, or dropping
down, altering the orbit to look at Saturn, its rings or icy
satellites. And of course we looked at Titan itself every
time we passed by. So, my team designed the complete
first tour, which ran from July 2004 until July 2008.”
RS: Cassini had a much bigger payload of experiments
and instruments than the earlier missions you worked
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