We can leave the Solar System, but arriving anywhere is not happening soon

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.

In that sense, interstellar space doesn’t seem that far away. That’s just one generation, a fraction of a human lifetime, to make it to the edges of our Solar System. But Voyager’s journey has only just begun. Beyond the heliopause sits the Oort cloud, a loose sphere of small, icy comets lazily orbiting the Sun, leftovers scattered from the formation of the Solar System billions of years ago. Here in the Oort cloud, those bodies spend lonely lives, knowing the Sun as only a point of light slightly brighter than all the rest.

The inner edge of the Oort cloud is somewhere between 2,000 and 5,000 thousand Astronomical Units (AU, the length of the typical Earth-Sun distance) away. At its current speed, Voyager will reach that shell of material between 600 to 1,500 years from now, long after it runs out of power.

Put another way, if the Byzantine emperor Justinian had launched the Taxidiótis probe from his capitol in Constantinople in the year 540 AD, it would just now, in the modern era, be reaching the Oort cloud. (Assuming Taxidiótis could hit similar speeds).

The outer edge of the Oort cloud is even more uncertain. At its maximum possible extent, it’s spread across nearly a light-year. Voyager 1 will have to silently cruise for nearly 20,000 years to make it through to the other side. Not that there will be any risk of an encounter; the five Earth masses' worth of material that inhabit the Oort cloud are spread so thinly that collision is nearly impossible.

If Voyager were headed toward Proxima Centauri—and it’s not—it would take roughly 80,000 years to complete its journey.

Eighty thousand years. Modern humans were all still in Africa 80,000 years ago, while Europe and Asia were the domains of Neanderthals and Denisovans. That encompasses the appearance of abstract language, the development of art and music, the beginnings of monumental construction, the spread of agriculture, the birth of cities, and, in the last few thousand years, the awakening of written history. Everything that we can associate with modern humans—our culture, our ways of living, even our ways of thinking—can be contained in the length of a single voyage to our nearest star.

The fast way

If we want to travel among the stars, we have to figure out a way to conquer the unfathomable gulfs of nothingness that separate us. Let’s leave aside the idea of sending humans on that kind of expedition anytime soon, if ever. We can’t even send people to Mars, the next nearest planet, without intense investment to solve the engineering challenges associated with such a journey.

So let’s focus on robotic probes for now, as that will have to do for several generations to come. If we want to get to Proxima Centauri quickly, we have to accelerate our craft to a decent fraction of the speed of light (and even that’s not that fast; a probe capable of near-light speed would still take nearly half a decade).

But we can’t achieve these kinds of speeds with conventional means—rockets powered by chemical reactions. One of the nastiest aspects of space travel is known as the "tyranny of the rocket equation." Essentially, to go faster or go farther, you need more fuel. But the more fuel you have, the heavier you become. So you need yet more fuel to overcome that weight, which makes you heavier. Soon enough, certain objectives—like, say, traveling to the nearest stars in less than a generation—become impossible. Not "really hard." Impossible.

One answer to the rocket equation is known as the Breakthrough Starshot Project. Funded to the tune of $100 million by Soviet-Israeli venture capitalist Yuri Milner, the project aims to develop the benchmark concepts and technologies needed to launch a probe to the nearest star that could reach it in less than a human lifetime. While that’s not nearly enough funding to actually build anything, the hope is that it will be enough to lay the groundwork, which will attract more serious funding (something surely in the tens of billions of dollars) to actually do the thing.

To achieve this extremely ambitious goal, the current Starshot thinking is to use a lightsail, which is a spacecraft attached to a large reflective membrane. The lightsail itself doesn’t carry any fuel onboard, freeing itself from the tyranny of the rocket equation. Instead, a gigantic laser on the ground will be used to accelerate the spacecraft. The radiation from the laser doesn’t have mass, but it does have momentum, so by bouncing off the sail, the photons can impart a small, but crucially not-zero, amount of thrust to the spacecraft.

As long as the laser fires, the spacecraft can continue accelerating right up to the edge of the speed of light. The only limiting factor is the power of the laser. And the ability to maintain its focus over large distances. And the reflectivity of the lightsail membrane.

The current reference design for Starshot is to run a 100 Gigawatt laser for roughly ten minutes, which will accelerate a spacecraft to roughly 10 percent of the speed of light. At that speed, a Starshot-style probe could buzz by Proxima Centauri in less than half a century. While that would make it longest spaceflight ever achieved in human history, it’s at least shorter than the typical human lifetime, so the same people who launched the probe could conceivably see it to its successful completion—a completion that would entail a high-speed zoom through the Proxima system (don’t even think about slowing down, let alone returning to Earth). Its data describing what it encountered there would take another four years to make it back to Earth.

While this design is not outright impossible, the difficulties in actually achieving it highlight just how far humanity has to go before interstellar is as banal as intercontinental.

The energy required for the laser, for starters, is 100 gigawatts. That’s roughly the energy output of all of the nuclear power stations in the entire United States combined. There are more powerful lasers out there, reaching up to a petawatts and even zettawatts, but those lasers get away with less total energy expenditure by firing for only a brief amount of time—usually around a femtosecond, which is 10-15 seconds.

The Starshot laser will have to take the entire nuclear energy output of the United States and run continuously for around 10 minutes, which is about a billion billion times longer than we can currently sustain such intense laser pulses.

The lightsail material will have to be the most reflective substance ever created. If even a tiny percentage of the laser light is absorbed, it will impart enough energy that the lightsail will simply melt away.

This is all assuming that we can accurately point the laser through our ever-shifting atmosphere at the increasingly distant, and thereby smaller, lightsail. (We’ll also assume that once the world’s most powerful laser is completed, it won’t be used to take over the world, à la Dr. Evil).

To make the math work—the power of the laser, the size of the lightsail, the maximum speed—there’s an upper limit to the size of the Starshot spacecraft. That limit is around one gram.

For reference, that’s about the mass of a paperclip.

The “no” way

Assuming that the collective dream for the legacy of our species isn’t to flood the Milky Way with paperclips, those tiny spacecraft will have to pack a lot of punch (sensors, communications, controlling electronics, shielding, and a power source for all of it) in its small package, which presents its own set of tremendous challenges.

Still, nothing about Starshot is impossible. It’s just really hard and incredibly expensive, and it will take an unknown amount of time to achieve. Humanity has solved hard, possibility-pushing challenges before.

But Starshot also underscores the inescapable realities of interstellar travel. We can send capable, advanced, large spacecraft pretty much anywhere in the Solar System. Sending humans is mostly an extrapolation of existing technologies. And yet it will take unprecedented leaps in technological sophistication to send a smattering of ultra-tiny probes to the nearest few stars in our local neighborhood, leaving the other hundreds of billions in the galaxy unexplored.

That’s not necessarily a bad thing, though. Surely those distant stars host untold riches in knowledge and would help us understand the evolution and fate of stars, the potential for life on other worlds, and more. But that knowledge comes at an enormous cost. And that gives us an intriguing alternative.

What if we spent even a small fraction of the time, effort, and money that it would take to achieve a rapid interstellar mission and deployed it instead to understanding the Solar System?

We are already well on our way to mastering the Solar System, as we can send spacecraft pretty much anywhere we want… as long as we have the budget for it. And what we’ve learned already in a mere half-century of discovery has no precedent in human history—a fact that we can easily forget in our pursuit of interstellar dreams. An acceleration to this new age of solar discovery will help us answer many questions that scientists are aching to answer.

The surface of Venus is hot enough to melt lead, yet the upper atmosphere has a pleasant temperature. What will we find if we floated through its clouds? Mars once hosted raging rivers and sparkling seas. What chemicals, what critters once swam through its oceans? The inner depths of the giant planets remain unmapped and unknown, with familiar elements behaving very differently as they’re crushed by pressure. What strange physics powers their interiors? The outer moons of those giants, like Europa and Enceladus, host more liquid water than the Earth does. What secrets are buried under their globe-spanning sheets of ice? The asteroid belt, the Kuiper belt, and the Oort cloud retain memories of the infant Solar System. What can they teach us about our home’s ancient history?

Our Solar System is beautiful, mysterious, largely unknown… and accessible. In the next hundred years, we can send robotic craft, and even human hands, to many of those worlds. We can ride along with comets and dig into cores of asteroids. We can send submersibles into oceans that have never known the light of the Sun. We can fly craft through atmospheres that would be lethal to us yet might give rise to their own kinds of life.

We don’t have to travel to the distant stars to explore the grand mysteries of the Universe. The Universe has delivered many of them to our very doorstep.

Correction: Unit errors in mass and power have been fixed.