Giving the Covenant back their bomb. Sticking a plasma grenade on a swooping banshee, throwing my hands up in the air and screaming. That soundtrack—my god, that soundtrack. Defending Reach, to the end. Reuniting with Cortana in the ruins of High Charity. Making it out at the last second on a Warthog, with a friend riding shotgun in the back.

Finishing the fight, only to start another.

It’s difficult to pick a favorite. But if you had to settle on a single, striking, unforgettable experience throughout this twenty-year-old video game series, perhaps the best choice can be found in the 2001 original, when you stepped out of that crashed escape pod. At first, there’s grass, rushing water, cliffs, and clouds—and then you looked up to see the unyielding curvature of one of those eponymous ringworlds, a view soon interrupted by a hostile alien dropship.

That initial experience of seeing a Halo ring in person, so to speak, was thrilling to millions of gamers across the globe. It was hardly surprising, then, that at the end of the first teaser trailer for the series’ latest release, this December’s Halo: Infinite, the camera pans up from the beauty of the landscape to reveal another of those oh-so-familiar arches encircling the sky in a star-flecked arm of the Milky Way. It’s what we all wanted to see. Souls were stirred.

It’s impossible to dissociate the ringworlds from the exhilarating games themselves, so seeing one appear in a teaser for a future game is a thrill. But there is something about the Halo constructs themselves, about their enchanting design, that light a special kind of spark.

As it happens, I’m an Earth and space science journalist, one who primarily writes about the way worlds work. Learning about strange worlds drifting about in the endless dark, and writing stories about them for the lay reader gives me a profound feeling of awe. And recently, while zipping about on a Halo ring, it occurred to me this game was almost eliciting a similar feeling.

I wanted to know why. So who better to ask than planetary scientists, researchers who deal with interplanetary marvels on a daily basis?

For the uninitiated: the Halo Array is a network of ring-shaped worlds scattered throughout our galaxy. Built by an über-powerful but long-extinct species called the Forerunners, they are habitable research facilities designed to study a parasitic life form referred to as the Flood. Just in case these parasites ever began to proliferate through the galaxy, the Halo rings would be ignited, each unleashing an expanse burst of energy to wipe out all sentient life within a 25,000-lightyear radius—and thereby robbing the Flood of its, um, food. The Flood, with their biblical namesake, is the Halo mythology answer to zombies...in space.

The Covenant, a theocratic hegemony comprised of multiple alien species, sees these superweapons as powerful religious tools. Humanity sees them as an existential threat. And Craig Hardgrove, a planetary scientist at Arizona State University, thinks they are just goddamn cool.

When the original Halo came out, Hardgrove was an undergraduate student. But he’s been a fan of Bungie—the studio behind the original trilogy of Halo games and several spinoffs—since the early 1990s when they made another science fiction first-person shooter series named Marathon. Throughout, “there was this spirit at Bungie of making these epic sci-fi games,” he says. And the studio’s keenness on crafting gigantic, visually stunning, and fully realized worlds never lost its appeal to this future planetary scientist. “For the background of my phone, I had a picture of Reach for a little while,” he says, referring to a planet in the series that was successfully invaded and decimated by the Covenant.

Caitlin Ahrens, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, MD., is also a fan of Halo. She is a more recent convert, becoming a gamer in her mid-twenties. “I still suck at video games,” she says. “But I’m so drawn into stories.” And from the moment she dove into Halo, playing with a friend in tow, she was hooked. “Halo is a beautiful game. The whole world of it is just so cool.”

But Hardgrove and Ahrens know that if they were to ever set foot on one of the Array’s rings, their astonishment would be eclipsed—by bafflement. Each installation is a largely metallic ring, a subterranean honeycomb network filled with, among other things, research facilities containing Flood specimens. But each ringworld’s inner surface is also decorated with copious geological features: there are mountains, valleys, lakes, and rivers. “Wait a minute. There’s elevation, there’s topography,” Ahrens says. “What’s going on?”

Those mountains would be especially problematic. If you make them too high, they will sink into the underlying metal ring. “How high could your mountain be before that honeycomb structure collapses?” says Ahrens, frowning as she ponders on it for a moment.

That parts of the rings appear to be in the middle of summer, while others are buried in several feet of snow, also doesn’t make much sense. “I could never figure that out. Why would there be different climates?” Hardgrove says. Having seasons doesn’t really work when your world is ring-shaped. But even if it’s a function of the ring’s engineering, why do those seasons, or geographically distinct climate zones, exist? Where does all that winter snowmelt go when it melts? It would have to go somewhere, or all that accumulated mass would, once again, just depress that section of the ring and break it.”

This sort of counterproductive chaos only exists at the surface. Geologically active worlds, like Earth, have a crust sitting atop the mantle, a superhot, solid-but-squishy section of the planet that shifts about and carves up the crust through volcanic eruptions, earthquakes, and the like. “With a ring system, you don’t have to worry about a mantle,” says Ahrens — and, by extension, you wouldn’t have to worry about those geologic hazards either.

But without a beating geologic heart, the ringworlds’ weather and water cycles would present a landscaping conundrum. By having constantly flowing water and patches of precipitation, the land’s features would be inexorably eroded over time. And without any major tectonic forces available to sculpt the surface, no new mountains or basins would arise to replace them. Eventually, all that beautiful topography would be sanded away, turning the surface into a grim slurry of sediment.

With these problems in mind, it’s reasonable to wonder why the engineers would have bothered to make ringworlds anything other than giant space stations. Why would they willingly make their jobs more difficult?

Then again, if you’re technologically advanced enough to engineer giant galaxy-sterilizing ringworlds, you might as well make them look nice. “The rocks are just like window dressing, or something,” says Hardgrove — something more aesthetically pleasing than a geologically barren mechanical megastructure. At least this way, you can go on a pleasant hike when you’re on a break from your subterranean scientific research. And as the rings spin around, they generate gravity, allowing an atmosphere, and anyone gallivanting about, to stay stuck to the surface.

The notion of a spinning circular space station has been around for over a century, and the concept of a habitable ringworld was popularized by the 1970 novel Ringworld by Larry Niven. They have popped up in works of popular culture both before and after. Clearly, mapping the features of a familiar world on a distinctly alien canvas has a strong appeal. “Because it’s such a weird shape, it gives you such a different sense of world exploration,” says Ahrens. “It kind of twists your reality a little bit.”

You don’t have to care about the world-building of the Halo universe to enjoy the series. They are kickass action games, whether you’re playing them solo, or you’re teaming up or fighting against your friends. But the Halo rings themselves, and the lore and mystery enveloping them, are potent draws for many.

Hardgrove read the stories of Arthur C. Clarke and Isaac Asimov growing up, many that featured that same sort of world-building and immense scale. “As a planetary scientist, I feel like those inspirational stories are what you need as a kid, early on, to propel you to be interested in these things,” he says. “You know, maybe one day we’ll find these crazy things out there.” And maybe soon—NASA just this year officially supported the search for alien megastructures.

Halo and planetary science offer more than flights of fancy. They give something else that other research fields in astronomy don’t: tangibility.

On July 4, 1997, NASA’s Pathfinder mission landed on Mars — and, soon after, the lander’s rover, Sojourner, popped out and scooted across the rust-hued world. Watching proceedings unfold back on Earth, Hardgrove thought: “Man, I wanna do that.” He wanted to work on missions to the Moon and Mars. “Astronomy and astrophysics are super cool, but for me, I want to send a robot,” he says. “They’re close. You can go and touch the rocks.”

And that’s the thing about video games: they offer an experience like none other. Reading about a character exploring a ringworld can be exciting, as can seeing a similar tale unfold on film. But video games give you the chance to do something no other media can: you can use an avatar to make the journey yourself, in the first person. You are the one making the choices to react to things — to fight, to explore, to look around at this remarkable alien landscape. You aren’t watching from afar. You are, essentially, there, taking a walk in the woods of a Halo ring.

With all these parallels in mind, it was only a matter of time before Halo and planetary science found a way to cross-pollinate.

Bungie, who bequeathed the Halo series to another studio named 343 Industries a few years ago, today makes another futuristic first-person shooter: Destiny. Hardgrove has been helping them out with their literal world-building, making sure that familiar realms, including the Moon and Mars, look realistic.

Hardgrove also happens to be the principal investigator on the Lunar Polar Hydrogen Mapper, or LunaH-Map for short, a shoebox-sized spacecraft that will search the permanently shadowed parts of the lunar south pole for water-ice. And it seems that the game developers love planetary science as much as planetary scientists love Bungie. After LunaH-Map is launched, and once its mission is complete, it will crash into the lunar south pole.

“They actually put our spacecraft in Destiny 2,” Hardgrove says. If you visit the Moon in Destiny, you might come across the wreckage of this real-life probe.

Like the ringworlds themselves, these easter eggs come full circle. The Marathon video game logo found its way into the first Halo game: it was used as the symbol for humanity’s spaceship the Pillar of Autumn—the very first environment you explore in the series.

Hardgrove simply couldn’t resist. “I put the symbol that was on the Pillar of Autumn…onto the nameplate for our spacecraft that's going to the Moon,” he says.

Planetary science makes us wonder. It lets us dream of alien worlds—what they are like, were like, will be like. It fills in the gaps in the vast shadows of the cosmos. Halo is no different in this respect. It doesn’t matter that this is all a work of fiction. What matters, says Hardgrove, is that “they play a role in inspiring people.”

Long may it continue.