Where Does Space Begin?
Eighty kilometers above the Earth's surface? 100 km? 600 km?
On Wednesday morning, the privately financed American manned rocket SpaceShipOne successfully flew into space and returned to Earth as part of an attempt to win the $10 million Ansari X Prize, which will go to the first team that creates a reusable, reliable spacecraft without government funding. The AP reports that organizers claimed "the ship crossed the official 62-mile-high border of space." Why does space officially begin 62 miles above the Earth?
Because 62 miles is about 100 kilometers, and 100 is a nice round number. The creators of the prize settled on 100 kilometers because it's the figure used by the World Air Sports Federation (FAI), the organization that maintains the aeronautical record book and keeps track of achievements in flying. But the FAI's secretary general, Max Bishop, admitted that the choice was "fairly arbitrary." In the mid-1950s the federation, knowing many aeronautic records would be blown away by the coming space age, wanted to declare a point at which aeronautics ended and astronautics began. An informal group of aeronautics researchers led by Hungarian Theodore Von Karman tried to predict the altitude below which significant lateral thrust would be required to keep a craft flying level. The group speculated that this would happen somewhere around 100 kilometers, so Von Karman suggested the federation just use that nice round number everyone could agree upon. The 100-kilometer standard, sometimes called the Karman Line, has since been adopted by many agencies and organizations worldwide.
The National Aeronautics and Space Act, which created NASA in 1958, simply defines space as "outside Earth's atmosphere." But it's tricky to pinpoint where the Earth's atmosphere ends. NASA could use a figure as lofty as 600 kilometers, the outer limit of the upper atmosphere, or thermosphere—high above the International Space Station, which generally orbits about 354 kilometers above sea level. Or it could say space begins 50 kilometers up, at the top of the stratosphere, below which one finds 99 percent of the air in the atmosphere. But when determining who is an astronaut, NASA uses the FAI's 100-kilometer figure. The U.S. Air Force, however, awards astronaut wings to rated officers who fly higher than 50 miles (or about 80 kilometers) above sea level.
So what makes a particular altitude "space"? The ability of a satellite to maintain an orbit around the Earth? While SpaceShipOne achieved a brief orbital trajectory, satellites must stay at least 200 miles (about 320 kilometers) above sea level to maintain a long-term orbit. The existence of a vacuum? While on average, all space is a vacuum, every cubic meter of space contains a few hydrogen atoms—the number of which begin to gradually increase more than 625 miles (1,000 kilometers) from Earth.
The only organization with international jurisdiction over this knotty question, the United Nations' Office for Outer Space Affairs in Vienna, has not taken a stand on the issue. "There is no agreement on the limit of outer space," said Hans Haubold, the OOSA's senior program officer. Haubold added that the distinction between space and the atmosphere is too fuzzy for a physics-based definition ever to be established. Asked if, in his opinion, SpaceShipOne did indeed reach space when it reached 100 kilometers, Haubold replied in Continental fashion: "Basically yes, of course, but what is space? What is time? What is physics?"
Explainer thanks X Prize Executive Director Gregg Maryniak, Maj. Karen Finn of the U.S. Air Force, and Elvia Thompson at NASA.
Dan Kois is a senior editor at Slate and a contributing writer to the New York Times Magazine.
Photograph of SpaceShipOne by Jim Campbell/Getty Images.