Some are baffled and others saddened by the fact that humans put footprints on the moon more than 40 years ago and have not ventured a fraction of that distance from home since. Have we lost our spirit of exploration?

Not at all, said Arizona State University historian Stephen Pyne, but we're seeing the end of one great era of exploration and the start of a new one. In a talk May 15 at Drexel University, Pyne said we are just entering a third great era of exploration kicked off by the Voyager spacecraft, which explored thousands of times farther than any human-led expedition could go.

The twin spacecraft Voyagers 1 and 2 were launched in 1977 and since then have brought us spectacular pictures not only of the planets but their bizarre and diverse moons. Today, Voyager 1 is on the verge of crossing through a theoretical boundary called the heliopause, which marks the end of the solar wind's reach and the beginning of interstellar space.

Pyne's talk, part of a series sponsored by the Philadelphia Area Center for the History of Science, was titled Voyager and the Third Great Age of Discovery. So what were the first two?

The three great ages

To Pyne, the history of exploration divides not by ocean, land and space, but by cultural shifts that have propelled mankind outward with different motivations. The first age peaked in the 16th and 17th centuries, he said, with the voyages of the great sailing ships. Before that time, Europeans didn't even know the Pacific Ocean existed and by the end they had mapped out many of its islands. The goals then all came back to god, gold and glory. The crowning achievement or "grand gesture" he said, was Magellan's circumnavigation of the globe.

But exploration declined in the early 1700s, before a new motivation took hold in the second half of that century – the era of Benjamin Franklin, Lewis and Clark and Charles Darwin. Gold was still important but the new era was less about God and more about science, said Pyne. "Naturalists replaced missionaries."

People used the oceans now as a way to reach new islands and continents and many of the great expeditions focused on the land. Scientists and artists could both find fame and fortune on underexplored regions of the globe.

By the 20th century, however, we were left with the poles, and the great explorers focused on the formidable and still uninhabited continent of Antarctica. The great scientists stayed home to study the atom and the gene, he said.

Though many of us associate Apollo's moon shot with the space age, Pyne fits into this same second age. It marks the end of an era. But Voyager was something new entirely. There were no human crew members and the two spacecraft are never coming home.

The Voyagers' 36-year adventure

Pyne said he doesn't mind that the Voyagers and Mars Rovers and other expeditions of this third era are crewed by robots and hot humans. The robots are our proxies, our avatars. We made them to be our ears and eyes and hands.

Voyager 2 was actually launched first, he said, but Voyager 1's trajectory would reach Jupiter first. The launch was planned to take advantage of a rare configuration of the planets that would allow both Voyagers to use gravity to "slingshot" from one world to the next. And so without having to come back, they sent us our first close-ups of Jupiter, Saturn, Uranus and Neptune, and dozens of exotic moons.

In their journeys they discovered geysers on frigid Triton, and volcanoes on desolate Io. In a geological sense, the moons of Jupiter and Saturn were alive. The single mission has lasted 35 years. "There is nothing else comparable," he said.

The mission pushed humans to think of a way to represent ourselves to whomever or whatever might find these craft. Each carried a gold-plated record with a representation of 55 languages, and songs by Mozart, Chuck Barry, some birds and a whale. "It spoke to what we thought was best about ourselves," he said. "The odds of an alien decoding the records are astronomically tiny, but the message was noted by millions of Earthlings."

And there was something else that marked Voyager as the harbinger of a new era. "One of its alluring peculiarities is that it has looked back as much as it has looked forward." When Voyager 1 aimed its cameras toward home on Valentine's Day of 1990, it took a family portrait of the inner planets, he said, with Earth appearing as nothing but the famous "pale blue dot".

This image and those of Voyager tell us something about our future, if we are willing to see what's there. As much as we may need a new planet to inhabit after overflowing our ecological niche and changing our atmosphere, there is no replacement Earth offering us for another chance. Pyne questions whether we would behave any less destructively if we were given a new planet. "I think the only planet we're going to have is Earth."