In 1961, UCLA grad student Michael Minovitch (image here) figured out the gravity assist maneuver for space travel. The young mathematician, working for the Jet Propulsion Laboratory (JPL), crunched some numbers. And the numbers showed that if a rocket was aimed carefully at a planet at just the right time, its course could be bent to arrive at a second planet with a savings in both fuel and time.
A few years later, another grad student spending his summer at JPL, Gary Flandro (image here), was tasked to design some exploratory missions to the outer planets: Jupiter, Saturn, Uranus, and Neptune. He noticed that all four of them would align in the late 70’s to permit one space probe to visit two, three, or even all four of them. And better yet, it could get to Neptune in something like twelve years, as opposed to forty or more if it flew there unassisted. It seemed too good to be true. But NASA had to get hopping. These alignments only occurred every two centuries. Could they put a mission together in a decade?
In those days, NASA had just gotten its feet wet with single-planet missions. Mariner 2 flew by Venus in 1962 and Mariner 4 visited Mars in passing three years later. But eventually the muckety mucks came around and the Grand Tour was rolled out for congressional approval in 1971. The following year, its budget was sliced, and scientists cut back from four to two planets, and from four to two probes.
On the bright side, Dr Flandro’s idea was successfully employed as the 10th Mariner, aimed first at Venus, and later gave us our first close-up views of the planet Mercury.
So the JPL scientists poured their efforts into Voyager space probes to each visit Jupiter and Saturn. They secretly kept their options open for Uranus, Neptune, and Pluto. But they didn’t tell Congress.
To guide this “stealth” mission to the way-out planets, Voyager would have to survive a fairly close pass to Saturn’s rings. Potential problem: Saturn is almost a billion miles away. And what if Saturn’s neighborhood were messy with ring debris and other gunk? The JPL Voyager folks needed a sacrificial lamb, a test project to clear the road, as it were. They found one in Pioneer 11. (Artist’s depiction below.)
I recommend Mark Wolverton’s The Depths of Space: The Story of the Pioneer Planetary Probes. Excellent book. Tells a great story. The eleventh Pioneer was the crowning achievement of that series. And its Silicon Valley handlers weren’t excited about sacrificing their probe for the sexy, souped-up Voyagers.
The Pioneer project began as inner solar system explorers–built with economy, and aimed at studying the sun and the space between the inner planets. They scored big time to put together NASA’s first mission beyond the asteroid belt. Unlike what you see in Star Wars and Star Trek, navigating the sun’s asteroid belt isn’t evasive maneuvers. But still, people weren’t sure about things like small particles, meteors, and such. Maybe the intrepid Pioneers couldn’t make it to Jupiter.
But they did.
After a successful Pioneer 10, Ames Research Center decided to pull a Gary Flandro. They stole a page from JPL and targeted their Pioneer 11 mission 43,000 miles above the cloud tops of Jupiter, bending its trajectory over the orbit plane of the planets (right) to aim at Saturn which then, was on the opposite side of the sun from target number one.
I can imagine the JPL crew was a little pee-oh’ed at getting beat to a second planet. They had designed the Grand Tour mission, and the resulting Voyagers which weren’t due to launch until 1977. Here this little upstart 600-lb weakling was going to get a first look at Saturn. And it relied on the JPL imagination of Gary Flandro, the guy who originally imagined multiple-planet missions.
But Pioneer 11 has to survive an extra five year jaunt. And whatever mess is to be found near Saturn’s rings.
If you’ve been following this series, by now you’re probably wondering where the satellites enter the show. Well, look at the pair there on your left.
In the 1960’s these two satellites were imaged from earthbound telescopes. But astronomers lacked the imagination to think of two bodies sharing nearly the same orbit. None of the observations worked out, so that expected one moon inside the orbit of Mimas was a big question mark.
One year before Pioneer 11 hit up the neighborhood, astronomers Stephen M. Larson and John W. Fountain suggested that two satellites were a better fit for the observation profile. Maybe Pioneer would settle it. Little did they know …
The danger 20,000 miles above Saturn’s clouds turned out not to be ring particles and small debris. Much closer to the intrepid space probe was Epimetheus, one of those two Larson-Fountain satellites. Closer than California and Cape Canaveral. Twenty-five hundred miles close, which, on Earth doesn’t seem all that close. But remember that Pioneer 11 traveled over a billion miles to miss a moon by so little. It’s like nailing a golf hole-in-one … from two or three miles away with a bank shot. But a fifty-mile wide satellite nearly wiped out the little Pioneer–they didn’t see it coming.
Voyager nailed down the two moons–Epimetheus indeed has a partner, Janus (imaged left, from Cassini). And they have an interesting relationship, despite not quite sharing the same orbit. They almost share an orbit. The inner satellite slowly gains on the outer, taking four years to catch up. Then something cool happens. The two moons switch orbits–as the inner one closes in on the outer, it moves to the outside, and the outer gets bumped closer to Saturn, thus starting another four-year chase. Lather, rinse, repeat.
Pioneer and Voyager opened our eyes to new satellites, close in to their father planets. Shepherd moons. Orbit-sharing. Crazy misses by a cosmic fraction. 1979 ushered in a new era of satellite imagination.