By Jackie Wattles, CNN
The flight will last about 10 minutes. Photo: AFP or licensors
Blue Origin is taking a star-studded crew of six female passengers to the edge of space on Monday in one of the most closely watched suborbital space tourism missions in years.
The flight will last about 10 minutes - carrying the group more than 100 kilometres into the sky and offering a few minutes of weightlessness before they descend.
But at what point during the flight will singer Katy Perry, journalist Gayle King and their fellow passengers reach "space"?
Is it when they look outside their window and the blue glow of the sky fades to black? Is it when they reach an altitude at which satellites can orbit? Or is it when the atmosphere grows so thin that it no longer plays a defining role in the flight physics?
In the spaceflight community, there is no hard-and-fast definition.
Space can be defined in several ways, and the usefulness of the criteria for determining where it starts can depend on the scenario. That's why various organisations around the world use different altitudes to mark that invisible threshold for recordkeeping purposes.
And for suborbital space tourism, quibbling over definitions can take on a life of its own.
Blue Origin and Virgin Galactic, for example, have been known to spar publicly over the matter - mostly because of one, specific means of defining space: the Kármán line.
As part of a six-member, all-female crew, Katy Perry is set to "shoot across the sky" on Blue Origin's next space flight, the company announced on February 27, 2025. Photo: MAURO PIMENTEL
What makes an astronaut?
Perhaps the most well-known and controversial demarcation of space, the Kármán line lies at 100 kilometres (62 miles) above sea level.
Blue Origin's suborbital New Shepard flights travel just past that altitude.
But the company's chief competitor Virgin Galactic does not.
Its flights so far have reached up to about 88.5 kilometres which is still beyond the 81-kilometre mark that the US government has long used to define space.
Still, Blue Origin has pointed to the Kármán line to assert its rides as a more legitimate path to earning "astronaut" status, saying in one 2021 social media post that "none of our astronauts have an asterisk next to their name" - a subtle dig at Virgin Galactic.
Defining an astronaut, however, is almost a different matter entirely.
In the early days of spaceflight, the US government established the 81-kilometre definition as a basis for awarding astronaut badges to military and NASA pilots.
The Federal Aviation Administration has also given out commercial astronaut wings to private-sector adventurers who reached higher than 81 kilometres.
But the agency mostly did away with that programme in 2021, opting to list participants on its website rather than award physical badges to private-sector fliers.
SpaceX has also awarded its own set of silver wings to nongovernment passengers who fly on its orbital Crew Dragon capsule.
So, in practice, deciding who is and isn't an astronaut is up to the recordkeeper.
Former NASA astronaut Terry Virts told National Geographic in 2018 that he was not too concerned with gatekeeping the "astronaut" designation.
"If you're strapping your butt to a rocket, I think that's worth something," Virts told National Geographic in 2018 when asked about the issue.
"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."
Where weightlessness happens
In the public imagination, however, the idea of an astronaut often conjures images of a person floating in weightlessness, surrounded by the black cosmic expanse.
But experiencing weightlessness has little to do with altitude - at least, not at the relatively low ones that Blue Origin and Virgin Galactic fly.
Earth's gravitational pull will still be tugging on the Blue Origin capsule when it reaches weightlessness at apogee, the term in spaceflight for very top of a flight path.
But the astronauts will be weightless because the energy the rocket and capsule drummed up after liftoff will be cancelled out by Earth's gravity, giving them a minutes-long version of the sensation people experience when they reach the peak of a big roller-coaster hill.
In contrast, astronauts on the International Space Station remain weightless for months at a time because they are in orbit around Earth - which requires far faster speeds than New Shepard's suborbital flights travel.
Recordkeepers, however, have not stipulated that a person must travel to orbit to be deemed an astronaut.
Civil and military pilots who commanded X-15 aircraft to altitudes above 81 kilometres during a test campaign in the 1960s, for example, were given astronaut wings.
The science of defining space
The US government uses the 81-kilometre mark to define space for many of the same reasons that other organisations use the Kármán line. The latter may just calculate the phenomenon differently.
Theodore von Kármán, a Hungarian American engineer and physicist and cofounder of NASA's Jet Propulsion Laboratory, was among the first to attempt to describe the altitude at which aeronautics becomes astronautics.
But even Kármán's initial attempts in the 1950s were inexact.
"The atmosphere is indeed dynamic and fluctuates in density which makes any delimitation imprecise," according to one 2014 study on the definition of space.
So, "the Kármán line fluctuates between" 84 kilometres and 100 kilometres.
Notably, 100 kilometres is how the Swiss-based Fédération Aéronautique Internationale, or the World Air Sports Federation, defines the Kármán line.
"It's been that way since the '60s," president of the federation's Astronautic Records Commission (ICARE) Scott Neumann told CNN.
The federation suggested in 2018 that it might lower its definitional altitude in response to new research about the Kármán line - but the organisation ultimately didn't take such a step after crunching the numbers.
"You have this nice maximum point, which we call the 'maximum equilibrium velocity altitude', and you can point to that on a graph and say, 'This is where you're starting to become more like a spacecraft and less like an aircraft'," Neumann said.
The "maximum equilibrium velocity altitude" refers to the altitude at which the atmosphere becomes so thin that a vessel can no longer rely on the flight dynamics of airplanes.
Instead, the vehicle will need the type of speed that rocket engines provide.
Another perspective
While the Fédération Aéronautique Internationale decided to stick with its 100-kilometre definition, other researchers and institutions in the space industry don't share the same view.
Senior vice president of space missions at aerospace manufacturing company Redwire Space Spence Wise told CNN he believed a more accurate altitude is closer to 88 kilometres.
Wise said that definition derives from the behaviour of vehicles - mostly discarded rocket boosters - that have returned from space after being dragged out of orbit by atmospheric friction.
For context, the "vacuum" of space isn't always a vacuum. Earth's atmosphere actually thins out over thousands of miles.
"But what you can consistently see is that somewhere between 90 kilometres and 88 kilometres, these rocket bodies, as they're re-entering (the atmosphere), stop acting like they're mostly drifting in space - being driven by Keplerian orbit kind of behaviour," Wise said, referring to the principles of planetary motion described by Johannes Kepler in the 17th century.
"And they start just falling out of the sky."
Satellites and spacecraft can also briefly travel even closer to Earth during their orbits - as near as 80 to 90 kilometres high - without being immediately dragged down.
In aerospace parlance, such low-flying spacecraft are referred to as Very-low Earth Orbit, or VLEO, satellites.
Atmospheric drag grows more prominent at lower orbital altitudes, and that's a key consideration when designing a vehicle.
"You can really drive orbits down, maybe down as far as 150 kilometres - but there's a cost you pay for that. …You're probably designing something that spends all of its time trying to stay in orbit," Wise said.
Put simply: When it comes to defining space, calculations and considerations change depending on the spaceborne object in question.
"Some experts have noted that function and purpose might lead to more appropriate distinctions between aircraft and spacecraft than altitude," the 2014 paper reads.
And the distinctions can grow ever blurrier as technology evolves and new types of vehicles can achieve different types of flight.
So, do these definitions really matter at all?
"It's funny to think of it," Wise said.
"Is it still worthwhile to climb Everest? We need these agreed-to norms or definitions of this hard and epic thing to do. So particularly in the context of commercial travel to space, I think the Kármán line is a totally fine thing."
- CNN
