One of the trailblazers was Pioneer 11, a robotic forerunner of such exploration and the first spacecraft to reach Saturn, the ringed and most iconic planet in the solar system.
Launched 50 years ago -- on April 5, 1973 -- Pioneer 11 is still going. Next destination: one of the stars in the Aquila (Eagle) constellation. Estimated time of arrival: 4 million years.
But Pioneer 11 already achieved its fame, with fly-bys of Jupiter and Saturn, sending back volumes of data about the distant planets and even discovering a couple more moons at the latter planet, as well as a new ring.
In comments on Pioneer 11's "spectacular" night launch, the late physicist James Van Allen, of the University of Iowa, said he expected the spacecraft to provide "a wealth of information even before it gets close to the approach with Saturn."
"This explorer will teach us a great deal about the solar system, never mind our own innate drive to learn," Van Allen said.
Plans for exploration
Approved in February 1969, the Pioneer missions were part of NASAs' unprecedented plans to explore the outer solar system. They involved twin spacecraft soaring past the orbit of Mars and the asteroid belt, to survey Jupiter and Saturn and to learn the physics of their surroundings.
They would do all that during amazingly high-speed flybys. The total budget: $350 million -- $2.4 billion in 2023 dollars.
Aerospace manufacturer TRW built Pioneer 10 and 11. While Pioneer 10 would fly to meet and survey Jupiter in early December 1973, Pioneer 11 used all the acceleration the giant planet would provide -- in a so-called gravity assist (a slingshot effect) -- yet it took 6 1/2 more years to reach Saturn in September 1979.
Both Pioneers crossed the space medium beyond the orbit of Mars and checked the asteroid belt so that scientists could evaluate possible hazards to missions much farther out in cosmic space.
And the twin spacecraft facilitated complex missions to Uranus and Neptune by their Voyager successors to the edge of the solar system and beyond.
During the spacecraft design phase performed in the 1960s, technology wasn't sophisticated enough to allow Pioneers 10 and 11 to enter orbit around the solar system's gas giants.
Pioneer 11, however, made the most of the boost from Jupiter, which hurled it up to a speed of 106,000 mph. Still, the spacecraft took until Sept. 1, 1979, to reach its closest approach to Saturn.
Its trajectory took it to thread the gap between the planet's top clouds and the edge of the innermost rings 13,000 miles overhead. The onboard instruments detected a thin, extra ring, as well as two extra moons.
The closest encounter lasted only a few hours and coincided with one of the biggest discoveries of the mission.
When the onboard instruments observed Saturn's largest moon, Titan, signals from the spacecraft showed a puzzling aberration.
It didn't take long for the science team to interpret that measurement as a possibility that that moon had an atmosphere. They had suspected it for some time, and at that point they had reached a final conclusion.
"It wasn't just any atmosphere, it was a massive one," said Andrew Ingersoll, one of the mission lead scientists.
A professor emeritus of planetary science at the California Institute of Technology in Pasadena, Calif., Ingersoll explained that the atmospheric pressure, measured at that moon's surface, was surprisingly 1 1/2 times that of Earth.
"Finding out that it really had such a thick atmosphere was pretty interesting," he said. "It didn't tell us everything about Titan, but science continuously advances, if you have good instruments and good data."
From that wealth of information, scientists established that Titan was similar to primordial Earth. There were some differences, however, as that atmosphere was made of methane.
At temperatures of minus-290 degrees F, it could mean something amazing: rain, rivers and lakes were all liquid methane, Ingersoll said.
That set the stage for a future mission, the Cassini-Huygens, in 2005. As the descent probe parachuted toward Titan's surface, cameras showed mountains and liquid basins, which confirmed that predictions from a quarter-century earlier were indeed correct.
Besides hauling panoply of scientific instruments, and just like its twin, Pioneer 11 carried a gold-plated aluminum plaque with greetings from Earth, just in case some extraterrestrial civilization ever found it.
Public relations stunt
Given the scientific prominence of the mission and the assured media interest, late astronomer and author Carl Sagan had a genius idea -- and a public relations stunt that NASA gladly accepted.
Finding room to improve, Sagan devised something more: including identical gold-plated LP records holding two hours of Earth sounds, including salutes in 58 languages.
"Hello from the children of planet Earth," Sagan's first son says in the LP's very first audio bite.
There was a lot of printed information on the metallic plaque, actual instructions --graphics and binary code -- to figure out where in the galaxy the Pioneers came from, Ingersoll said.
"Putting our location on a spacecraft that might be detected by aliens, was incredibly naïve," he said. "And this was something Sagan was perfectly aware of."
"It was amusing, and I think Sagan was having fun, also. In interstellar space, the chance of finding that little spacecraft is zero."
After 50 years, Pioneer 11 is an estimated 10.2 billion miles from Earth.
Last data in 1995
Whatever data Pioneer 11 still was sending suddenly stopped in late 1995. NASA officially announced that there was no way to confirm whether the spacecraft still was transmitting. And even NASA's original website has been archived.
As a reminder that Pioneer 11 is venturing out on its own, its NASA Solar System Exploration page's elapsed time is frozen at 22 years, 7 months, 17 days, 21 hours, 49 minutes -- the last time scientists received engineering data from the spacecraft, Nov. 24, 1995.
The final decision about Pioneer 11's ultimate path came straight from celestial mechanics, Ingersoll said.
"Mission planners had to make sure Pioneer 11 was on track to Jupiter, making sure it'd pick up its own speed, enough speed to make it to Saturn," he said.
And that decided the only trajectory geometry would allow.
The million outer planets of a star called Sol
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