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....