Reaching the stars just got a bit easier. A new study argues that the boundary between Earth’s atmosphere and outer space—known as the Kármán line—is 20 kilometers, or about 20%, closer than scientists thought. Though the new definition won’t make a difference for launching rockets and spacecraft, it could help clarify a legal debate that will set the rules for space policy—and commercial spaceflight—for years to come.
Until now, most scientists have said that outer space is 100 kilometers away. At that point, it’s been thought, the speed needed to achieve lift in the superthin atmosphere is equal to the speed needed to simply orbit the planet; once there, a spacecraft’s horizontal pace would counteract the tug of Earth’s gravity. It’s an insight that has long been credited to aerodynamics pioneer Theodore von Kármán, though he credited the term to the world’s first “space lawyer,” Andrew Haley, in 1963.
A close look shows that the traditional definition flies in the face of evidence, says Jonathan McDowell, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. As a hobby, McDowell compiles an influential, detailed record of rocket launches online. “I’ve been making lists of rockets since I was 13,” he says. He often has to decide which launches qualify as reaching outer space, and which do not. Given how low many orbiting satellites fly, the 100-kilometer limit never seemed right to McDowell. He preferred the mesopause, the coldest point in Earth’s atmosphere, located roughly 85 kilometers up. (Recent estimates have bumped it somewhat higher.)
Most people continued to use 100 kilometers as a boundary, including the World Air Sports Federation (FAI) in Lausanne, Switzerland, the keeper of outer space records. Although definitions are always points of contention in science, it seemed worthwhile to McDowell to dig deeper, knowing such companies as Virgin Galactic and Blue Origin could soon be offering tourism flights to this boundary realm.
He started with data: namely, public records of satellite telemetry he had downloaded from the North American Aerospace Defense Command about the orbits of 43,000 satellites. Most didn’t matter for his project—they orbited far too high above the edge of outer space. But at least 50 had orbits that occasionally operated below 100 kilometers, such as the Soviet Elektron-4 satellite, which made 10 spins at 85 kilometers or below before disintegrating into the atmosphere in 1997. “Are you going to say [these satellites are] in space and then not in space every 2 hours?” he asked. “That doesn't seem very helpful.” Below 80 kilometers, the story changes: It is highly unlikely a satellite will make another orbit, as thickening atmosphere sends it to a fiery end.
The empirical evidence seemed clear, so McDowell turned to the math of the Kármán line. For his calculations, he looked at a satellite’s return to Earth, rather than its exit. That meant comparing orbital velocity, which is derived from gravity, against the drag of the atmosphere. McDowell used a standard atmospheric model to simulate the past 50 years, calculating how the line behaved at different latitudes and longitudes. He found that the atmosphere’s tug turns negligible between 66 kilometers and 88 kilometers, he will report in an upcoming issue of Acta Astronautica.
This lines up with work published last year by Thomas Gangale, a former U.S. Air Force officer and a legal scholar, in a paper titled “The Non Kármán Line: An Urban Legend of the Space Age,” in the Journal of Space Law. In an exhaustive survey, Gangale suggests that Von Kármán never proposed 100 kilometers as a boundary in his own work. Rather, Gangale proposes, “His engineering work was misinterpreted by lawyers” seeking to define where national airspace ended—and international space began. The “delimitation” debate, as it is known, continues to this day.
It's time for the Kármán line to face some scrutiny, says Alan Stern, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, who also serves as chief scientist at World View Enterprises, a high-altitude-balloon venture. “I do think that McDowell makes a cogent argument for something near the 80-kilometer altitude,” he says.
George Whitesides, CEO of Virgin Galactic, agrees. “[McDowell] lays out a solid case that … a reasonable position for ‘where space begins’ is around 80 kilometers,” he says. That’s consistent with the precedent set by NASA and the Air Force, which award astronaut wings to X-15 pilots who fly above 80 kilometers, he adds, “a recognition we have always respected.”
Like many definitional debates, the question of where space begins can feel pedantic. But if scientists can settle on a rigorous answer to where outer space begins, McDowell says, that could eventually filter out to the world of international treaties and space law, where commercial interest is intense. The United States has long resisted any legal definition of outer space to avoid restrictions on high-altitude military activities.
Although McDowell makes an excellent technical case, however, it’s unlikely that science will solve something like the delimitation debate, adds Brian Weeden, director of program planning at a space policy think tank in Washington, D.C., called the Secure World Foundation. “Unfortunately, that’s a legal and political debate,” he says. “Technical [solutions] rarely tend to be the answer in those.”