The Voyager 2 spacecraft has just passed through the heliopause and into interstellar space, forty years after it was launched.

On the one hand that’s a stunning technological achievement and a reminder of the wonderful universe we live in. On the other, it’s a reminder that humans will never go out to explore this universe, or even leave Earth in significant numbers.

Although Voyager 2 has passed the heliopause it is still within the gravitational field of the sun. It would take another 30,000 years to fly beyond the Oort cloud which marks the boundary.

These facts could have been computed when Voyager was launched though at the time its mission was limited to five years. But if they had been pointed out as an argument for the impossibility of interstellar travel, the response would surely have been that the problem would be solved by technological progress. Forty years before Voyager was launched, flying across the Atlantic ocean was a major feat. Forty years or so before that, the first heavier-than-air flight was undertaken by the Wright brothers.

Extrapolating one could reasonably expect that forty years more progress would produce massive advances in space travel including human space travel. In fact, though no one knew it at the time, the heroic age of space travel (indeed, of nearly all kinds of travel) had already passed. No one has travelled to the moon since Voyager 2 was launched and, quite possibly, no one ever will. The promise of easy access to space through the space shuttle has been abandoned in favor of the 1950s technology of the Atlas rocket. Meanwhile physicists have closed off just about every possible loophole that might allow us to evade Einstein’s conclusion that the speed of light is an absolute limit.

The other achievement of the Voyagers and their successors has been a comprehensive exploration of the planets and moons of the solar system. They have revealed many marvels, but nowhere remotely habitable compared to, say, Antarctica or the Atacama desert.

The biggest lesson of our decades of space exploration is that Earth is the only planet we have.