On August 25th, 2012, humanity became an interstellar species. There was no fanfare or galactic welcome party as a humble robotic probe, the Voyager 1 spacecraft, crossed an invisible threshold. It slipped between the region dominated by the physics of the Sun and into the thin milieu of plasma between the stars.

Whatever fate befalls us now, whatever future civilizations rise and fall, whether we heal the Earth or continue our self-destructive path, we will still, and always, have this. A monument, a marker, a testament to the existence of our species and the ingenuity of our minds. It’s unlikely that any alien civilization will encounter our spacecraft, yet it will still exist, circling the center of the Milky Way for eons to come.

In the coming decades, Voyager 1 will be joined by other craft sent along solar-escape trajectories: the Pioneer probes, New Horizons, and more. And now that we’ve crossed this astrophysical threshold, we are forced to ask a difficult question: Is this it? Is this all we’ll ever accomplish beyond the Solar System, a scattering of wayward probes sent out into the infinite night?

For decades, scientists, engineers, and dreamers have worked to develop technologies that can radically expand our presence outside the Solar System. But they all face one enormous challenge: the brain-breaking enormity of the cosmos. Sustained interstellar travel is simply beyond the means of our technology, and any reasonable projection of anything we’ll develop over the next few generations.

Thankfully, that doesn’t mean our space dreams are dead. We’ll have to learn to love the one we’re with and stop looking beyond to impossible frontiers and instead turn our curious eyes and minds to the wonders and mysteries of our own Solar System.

The slow way

To put the challenge of interstellar travel in perspective, let’s build a scale model of our local neighborhood. Go find a couple willing volunteers (friends, relatives, sleeping cats, whatever). Place one in the center of the room—that’s the Sun. Place yourself, playing the part of the Earth, and stand three feet away from them. That’s our Solar System, with those three feet representing the 93 million miles between our planet and our star.

Your second volunteer will be Proxima Centauri, the nearest star to the Sun, which sits 4.246 light-years away. Our choice of light-year as a unit here masks the true monstrosity of the distance, tucking it away inside a term that’s really beyond comprehension instead of letting us appreciate what the numbers mean. To get that appreciation, play a game with your second volunteer: Ask them to position themselves where they think Proxima should be in the scale model.

Then pack them a lunch and shove them in the car because they have to travel over 200 miles away.

The Voyager 1 probe has been playing the real-life version of this. Launched on September 5th, 1977, the spacecraft reached Neptune, the outer sentinel of the Solar System, over a decade later.

And since then, it has not encountered another world.

Voyager 1 has enough speed to overcome the gravitational pull of the Sun. Save for a freak cosmic accident, it’s never coming back. Cruising at a speed of over 56,000 kilometers per hour (35,000 mph)—it could circumnavigate the Earth in less than 45 minutes—it managed to cross the heliopause after roughly 35 years of travel time. The heliopause is widely considered to the boundary of the Solar System and is a region marked by a sharp change in the density of charged particles that constantly float through space. The interior of the heliopause is dominated by the particles emanating from the Sun itself; beyond it, you’ll find yourself in a mixture generated by billions of distant stars.