Fusion technology accidentally revolutionized astronomy
Pity the poor astronomer. Biologists can hold examples of life in their hands. Geologists can fill test cabinets with rocks. Even physicists can examine subatomic particles in laboratories built here on Earth. But throughout its millennia-long history, astronomy has always been a science of separation. No astronomer has stood on the shores of an alien exoplanet orbiting a distant star or seen an interstellar nebula up close. Except for a few trapped light waves traversing the great void, astronomers have never had intimate access to the environments that fuel their passion.
Until recently, that is. At the beginning of the 21st century, astrophysicists opened a new and unexpected era for themselves: large-scale laboratory experiments. Powerful machines, especially some very large lasers, have provided ways to recreate the cosmos, allowing scientists like myself to explore some of the universe’s most dramatic environments in contained, controlled settings. Scientists have learned to explode mini-supernovae in their laboratories, reproduce environments around newborn stars and even probe the hearts of massive and potentially habitable exoplanets.
How we got here is one of the great stories of science and synergy. The emergence of this new, large-scale, laboratory-based astrophysics was an unexpected side effect of a much broader, more fraught, and now quite newsworthy scientific journey: the quest for nuclear fusion. While humanity has worked to capture the energy of the stars, we have also found a way to bring the stars down to Earth.
Last month, researchers at the Lawrence Livermore National Laboratory announced that they had crossed a fusion milestone. For the first time, more energy came out of a fusion experiment than was put in. Although the world is still likely decades away from any kind of working fusion power generator, the experiment was a scientific breakthrough, moving us one step closer to purity and essentially limitless energy through self-sustaining fusion reactions. To achieve this, the researchers relied on lasers to recreate a place where thermonuclear fusion reactions already occur: the Sun’s core. They focused the lasers on small pellets of hydrogen, mimicking the Sun’s extraordinarily high temperatures and densities to squeeze the hydrogen nuclei into helium and initiate fusion reactions.
Stars don’t give up their secrets so easily. The lasers used are factory-sized affairs that require enormous power to do their job. It was in the process of building these multi-story light machines that the researchers realized that they also happened to be building a unique tool for studying the sky. Called High Energy Density Laboratory Astrophysics, or HEDLA, the field that has emerged around these lasers has given astronomers entirely new ways to practice their craft.
Work began in earnest in the early 2000s with the investigation of one of the most energetic events in the cosmos: supernovae, the titanic explosions that end the lives of massive stars. Supernovae are powered by powerful shock waves that develop in a star’s core and then propagate outward, blowing the star’s outer layers into space. The heavy elements found deep inside a star are the key to life eventually forming somewhere, so a long-standing question for astronomers has been whether the explosion from a supernova mixes a star’s core elements with its lighter surface, and through that mixing spreads essential-to-life heavier elements throughout the cosmos. Working together, astronomers and fusion plasma physicists recreated the layers of a star in miniature with thin strips of plastic and less dense, foam-like material. Then they zapped the mini star sandwiches with the big fusion lasers. Powerful shock waves formed that tore through the targets, snapping them like wet cardboard. Mixing between the layers, it turned out, was real. The experiments confirmed a large part of the astronomers’ map of how elements cycle around the galaxy.
This was an exciting direction for astronomy. Not only could astronomers now tinker with star stuff in a laboratory; they could do it over and over again. By tweaking one variable after another, they were able to run real ground-based experiments, test hypotheses, and watch the results play out before their eyes. Soon they were developing experimental platforms to study a wide range of astronomical environments, including the swirling disks of gas that accompany star formation and the collision of giant interstellar clouds. HEDLA still has limits; not all astrophysical phenomena can be studied in the laboratory. Strong gravitational effects, for example, cannot be captured, because they need a star’s mass, and no funding agency pays for it. The trick for astrophysicists has been to find an overlap between the questions they want to answer and the extreme conditions that giant fusion machines can create.
A sweet spot in the HEDLA Venn diagram lies in the search for distant worlds where alien life can form. In recent decades, an “exoplanet revolution” has revealed that nearly every star in the sky hosts its own family of worlds. Because life almost certainly needs a planet to originate, understanding the different conditions on all these alien worlds has risen to the highest priority on astronomers’ to-do list. So far, many of the exoplanets we’ve discovered are strange beasts that look very different from the eight worlds orbiting our Sun. The most important among these are the super-earths, planets weighing from 2 to 10 times the mass of our world. We do not have these types of planets in our solar system, and yet they turn out to be the most common world in the universe. So what kind of planet is a super-Earth? Are this mass of generic worlds worth probing for alien life?
The conditions on the surface of a planet, where life will form, depend heavily on what happens deep inside. Thousands of miles down, the pressure is so high that rocks are squeezed until they ooze like asphalt on a scorching day and iron liquefies. Under certain circumstances, the swirling motions of this molten soup drive planet-wide protective magnetic fields that support life. This is where HEDLA’s high-power lasers come in: They prove to be a uniquely perfect tool for probing pressure deep inside planets. By using lasers to push samples of rocks and metals to the deep planetary pressures, scientists can see how the samples behave, discover their resistance to flow (important for plate tectonics) or their ability to conduct electricity (important for magnetic field generation).
This is also where I come in. The research my colleagues and I are conducting is part of a multi-year, multi-institution push funded by the National Science Foundation to make HEDLA a major tool for understanding planetary conditions, including those of super-Earths. A recent experiment in this initiative actually used the same huge 192-laser beam facility at California’s Lawrence Livermore National Laboratory where the recent fusion breakthrough occurred – the big daddy of all big lasers. Scientists wanted to understand how iron would react to super-Earth pressure, because swirling liquid iron in planetary cores is key to creating planetary magnetic fields. Does iron remain liquid inside a super-Earth, or does it “freeze” over time, solidifying into a crystal lattice that will kill any chance of a magnetic field? By driving the iron to pressures 10 million times Earth’s surface pressure, the study tracked exactly when iron fell from liquid to solid state. From this data, the team found that super-Earths can keep their cores afloat long enough for magnetic fields to offer a billion years or more of planetary shielding. If these results are correct, these large planets may have the right conditions not only to allow life to form, but also for it to develop and thrive.
Experiments like this show how far the new field of laboratory astrophysics has come in just a couple of decades. It’s a story of convergence and even coming of age. Almost a century ago, astrophysicists discovered the physics of thermonuclear reactions in stars. Their efforts were not aimed at one day running the cities of mankind, but at answering an ancient cosmic question: What makes the stars shine? Only after the nuclear weapons of the Cold War did some scientists begin to explore the possibilities of peaceful fusion power. Now, moving a little closer to abundant, clean energy, we have limited our own separation from the power of the stars and the cosmos as a whole. The universe is more in our hands than ever before. And by capturing even a fraction of its capacity in our laboratories, we are reminded of how vast and magnificent it has always been.
