The lack of a uniform definition of where space begins has scientific and engineering implications that extend well beyond which billionaire gets there first.
Shortly after Richard Branson’s VSS Unity spaceship successfully touched down in New Mexico last Sunday, the billionaire and his crewmates were awarded shiny astronaut wings to pin onto their cobalt-colored flight suits.
Astronaut wings are an honor given to people who have completed a flight in space, which NASA, the U.S. Air Force, and the Federal Aviation Administration consider to begin 50 miles (80 kilometers) above the Earth’s surface. Yet the international governing body for aeronautic and astronautic records defines the boundary as 62 miles (100 kilometers), suggesting that Branson’s flight, which reached 53.5 miles, didn’t quite make it into outer space.
Because Jeff Bezos is scheduled to fly above 62 miles—a theoretical boundary known as the Kármán Line—on July 20, he’ll also be able to claim that he’s the first billionaire to fly his own craft into outer space.
Should both billionaires be considered astronauts? The answer to this question depends upon your definition of outer space—and that can vary widely, say scientists and engineers from the Laboratory for Atmospheric and Space Physics(LASP) at the University of Colorado Boulder.
“I’ve always thought of the edge of space to be where, when looking at the Earth edge on, things go black,” says Lynn Harvey, who studies the dynamics of the middle atmosphere. The change from the yellow, light blue, and dark blue regions of Earth’s atmosphere to the black backdrop of outer space occurs at an altitude of about 62 miles, she says. This definition suggests that Branson didn’t quite reach the final frontier.
Other experts define the edge of space differently. LASP atmospheric chemist Cora Randall considers the edge of space to be the lower boundary of the thermosphere—an upper layer of Earth’s atmosphere where there are too few molecules to transfer radiation efficiently. This boundary is just above where Earth’s highest clouds form, and its location varies by season, ranging from about 55 miles above the Earth in summer to 60-62 miles in the winter. Since Branson’s and Bezos’ flights are both occurring in the summer, she considers 55 miles to be the altitude they need to achieve to earn their astronaut wings—a threshold that Branson’s VSS Unity did not quite reach.
Rick Kohnert, the program manager for LASP’s small satellite initiatives, considers the altitude at which space begins to be where the human eye can no longer discern airglow—an eerie, often emerald-green light visible from high above the Earth. Airglow occurs when molecules and atoms energized by sunlight cast off their extra energy as light.
The altitude beyond which airglow is no longer visible can shift with solar activity, Kohnert says, but is typically at an altitude of about 56 miles, which means from his perspective, Branson was “right there on the edge” of outer space. LASP’s Richard Eastes, who leads NASA’s GOLD mission, says this is also about the altitude where extreme ultraviolet sunlight is stopped by the atmosphere, protecting humans from its damaging effects.
A different approach is to ask the question: at what altitude would a small satellite survive in orbit for more than a day before friction would bring it down? LASP’s David Malaspina, who submitted a recent satellite proposal including this calculation, says that this important threshold is about 110-120 kilometers (68-75 miles).
Like many things in life, something that at first glance seems quite simple, such as where outer space begins, can in reality vary widely, depending on your perspective. Researchers from LASP and other institutions around the globe take advantage of these nuances to better understand the full spectrum of space and develop innovative ways to explore and study it. Regardless of whether color, eerie airglow, the highest clouds, or atmospheric drag is used to determine the boundary of space, the fuzziness of this boundary has scientific and engineering implications that extend well beyond which billionaire gets there first.
This story was written by Terri Cook – Head of LASP’s Office of Communication Management
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