Where, exactly, is the edge of space? It depends on who you ask.

The moon seems to hang in the balance between Earth's atmosphere and the blackness of space in a picture taken from the International Space Station.

Ask someone where outer space is, and they’ll probably point at the sky. It’s up, right? Simple.

Except, no one really knows where “air space” ends and “outer space” begins. That might sound trivial, but defining that boundary could matter for a variety of reasons—including, but not limited to, which high-flying humans get to be designated as astronauts.

Now, with Virgin Galactic seemingly on the cusp of launching paying passengers onto suborbital trajectories, many people are wondering whether those lucky space tourists will earn their astronaut wings. As of right now, they will, according to U.S. practices.

Is that a problem? “No, I think it’s great!” says NASA astronaut Mike Massimino, who helped repair the Hubble Space Telescope.

Here, we take a look at the ways space is currently defined, the confusion surrounding the demarcation, and what the future might bring.

 After three failed launches, SpaceX CEO Elon Musk scraped together enough funding to launch a fourth version of SpaceX's Falcon 1 rocket. On September 28, 2008, Musk's gamble paid off when the Falcon 1 became the first privately developed liquid-fuel rocket to orbit Earth. The rocket is seen here lifting off from the Reagan Test Site in the Marshall Islands.

 In late 2010, the second flight of SpaceX's next-generation rocket, the Falcon 9, was literally capped with a milestone: a fully functioning version of SpaceX's Dragon capsule. For the first time, a privately funded company successfully launched, orbited, and recovered a spacecraft. While this demonstration mission wasn't carrying anything for science, in an elaborate nod to the British comedic troupe Monty Python, the historic launch did ferry a wheel of cheese into space.

 With Earth looming behind it as an epic backdrop, a SpaceX Dragon craft docked with the International Space Station on May 25, 2012. The Dragon, laden with supplies for the ISS and its crew, became the first commercially developed space vehicle to be launched to the station. On May 31, Dragon safely left the station and returned to Earth.

 Getting a payload into geosynchronous orbit isn't easy. To achieve GEO, a rocket must travel so high, and so fast, that any satellite it drops there will orbit Earth above a fixed point on the planet's surface. On December 3, 2013, SpaceX's Falcon 9 Heavy pulled it off, making SpaceX the first private company to send a satellite into geosynchronous orbit.

 Ever since Musk founded SpaceX in 2002, his mantra has been reusability. On December 21, 2015, that vision became reality when the booster stage of a Falcon 9 rocket successfully returned to Earth and landed back on its launchpad at Florida's Cape Canaveral Air Force Station. It was the company's third attempt to stick the landing, and it paved the way for future successes.

 Not every SpaceX rocket can feasibly work its way back to land, so the company built an autonomous drone ship, named Of Course I Still Love You, to give its rockets a place to set down at sea. On April 8, 2016, a Falcon 9 flight ended with a successful ocean landing, the first ever pulled off in spaceflight history.

 Once SpaceX demonstrated a knack for recovering used rockets, they took things to the next level: actually launching a previously flown spacecraft. On March 30, 2017, a Falcon 9 rocket powerful enough to send satellites into orbit became the first successfully reused launch vehicle.

In its latest milestone moment, SpaceX successfully launched its Falcon Heavy rocket for the first time on February 6, 2018. The rocket, which lifted off from the historic launchpad 39A at NASA’s Kennedy Space Center, is now the most powerful launch vehicle in operation anywhere in the world.

Does it really matter where space starts?

International treaties define “space” as being free for exploration and use by all, but the same is not true of the sovereign airspace above nations. The laws governing air space and outer space are different; flying a satellite 55 miles above China is just fine if space begins at 50 miles up, but define the edge at 60 miles, and you might find your satellite being treated as an act of military aggression.

“Where does a country’s air space stop and space begin?” asks Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics. “Once you agree on a boundary of space, you agree on a boundary where space law applies.”

However, the United States and some other countries have resisted a formal, international delimitation of space, stating that it’s not necessary and that “no legal or practical problems have arisen in the absence of such a definition.” Others argue that maintaining a distinct boundary will be crucial, given an increase in the number of national space programs and in private spaceflight endeavors that are boosting the amount of suborbital traffic.

So, how is “space” currently defined?

Broadly, most experts say that space starts at the point where orbital dynamic forces become more important than aerodynamic forces, or where the atmosphere alone is not enough to support a flying vessel at suborbital speeds.

Historically, it’s been difficult to pin that point at a particular altitude. In the 1900s, Hungarian physicist Theodore von Kármán determined the boundary to be around 50 miles up, or roughly 80 kilometers above sea level. Today, though, the Kármán line is set at what NOAA calls “an imaginary boundary” that’s 62 miles up, or roughly a hundred kilometers above sea level.

The Federation Aeronautique Internationale (FAI), which keeps track of standards and records in astronautics and aeronautics, also defines space as beginning a hundred kilometers up. It is, after all, a nice round number.

But the Federal Aviation Administration, the U.S. Air Force, NOAA, and NASA generally use 50 miles (80 kilometers) as the boundary, with the Air Force granting astronaut wings to flyers who go higher than this mark. At the same time, NASA Mission Control places the line at 76 miles (122 kilometers), because that is “the point at which atmospheric drag becomes noticeable,” Bhavya Lal and Emily Nightingale of the Science and Technology Policy Institute write in a 2014 review article.

How come people can’t agree?

“It’s very political, it turns out,” McDowell says.

There is no easy distinction between “space” and “not space,” in part because Earth’s atmosphere doesn’t simply vanish; rather, it gradually becomes thinner and thinner over about 600 miles. Technically, the International Space Station—which orbits at an average height of 240 miles—would not be in space if we defined “space” as the absence of an atmosphere.

Furthermore, there’s no single altitude above which a satellite can stably remain in orbit; that depends on the type of satellite and its orbital trajectory, McDowell says.

A prodigious maker of lists, McDowell was compiling records for rockets, astronauts, and other space objects, and he went looking for an accepted international boundary that would help him decide which records to include. When he realized none existed, he decided to find one by revisiting the types of calculations von Kármán did.

He pulled publicly available orbital paths for 43,000 satellites and sorted them based on the lowest points in their orbits (called perigee) during decommissioning and atmospheric re-entry. From there, he realized that satellites could orbit the planet numerous times below an altitude of 62 miles, but those dipping beneath 50 miles met a quick and flaming end more often than not.

After that, he redid the von Kármán math and found that atmospheric contributions on orbiting spacecraft become negligible at around 50 miles up.

“What you don’t see is satellites dipping down to 70 and coming back out,” he says. “There is a fairly sharp boundary, a decently sharp boundary, between how low the perigee can be and where you just won’t make it back out again.”

With suborbital spaceflight companies edging closer to the edge of space, could 2019 be the year we formally define it?

McDowell thinks that’s unlikely, although he’s sure the conversation will pick up speed as commercial spaceflight ventures begin spending more time in the region between 50 miles and 200 miles up, where the space station orbits.

“I think that as space activity moves more into this regime, the pressure to agree on a boundary will be greater,” he says.

In fact, the FAI says that because of “compelling” recent analyses suggesting that space ought to begin around 50 miles up, it will propose a meeting this coming year to evaluate the idea.

Will paying suborbital spaceflight passengers be called “astronauts”?

As of now, yes, at least if they make the trip from a U.S. launch site. The FAA and the U.S. Air Force both agree that flying higher than 50 miles above our planet qualifies a person for the title. (See where active spaceports exist around the world.)

What do NASA astronauts think about that?

Some people might argue that getting into orbit is what defines an astronaut. However, “I think Alan Shepard and Gus Grissom would disagree,” says Terry Virts, a former commander of the International Space Station who has spent more than 213 days in orbit. “They’re the first two U.S. astronauts who didn’t get into orbit.”

Virts says there’s a big difference between riding along on a five-minute suborbital flight and performing a six-month orbital mission, but when it comes down to it, folks on both types of trips have earned the “astronaut” title.

“If you’re strapping your b**t to a rocket, I think that’s worth something,” Virts says. “When I was an F-16 pilot, I didn’t feel jealous about Cessna pilots being called pilots. I think everybody’s going to know if you paid to be a passenger on a five-minute suborbital flight or if you’re the commander of an interplanetary space vehicle. Those are two different things.”

Massimino agrees that there’s an important distinction between being selected as a NASA astronaut—“the training, the struggle, the rejections, all of that”—and being a paying customer. But he’s also completely on board with space tourists earning the title.

“I think if you get above that line, you certainly qualify as an astronaut, absolutely,” he says. “The more the merrier!”