Where does space begin?

Answers to your questions about the news.
Sept. 30 2004 1:31 PM

Where Does Space Begin?

Eighty kilometers above the Earth's surface? 100 km? 600 km?

Maybe they should have called it ThermosphereShipOne
Maybe they should have called it ThermosphereShipOne

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?

Dan Kois Dan Kois

Dan Kois is Slate's culture editor and a contributing writer to the New York Times Magazine.

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.



Crying Rape

False rape accusations exist, and they are a serious problem.

Scotland Is Just the Beginning. Expect More Political Earthquakes in Europe.

I Bought the Huge iPhone. I’m Already Thinking of Returning It.

The Music Industry Is Ignoring Some of the Best Black Women Singing R&B

How Will You Carry Around Your Huge New iPhone? Apple Pants!

Medical Examiner

The Most Terrifying Thing About Ebola 

The disease threatens humanity by preying on humanity.


The Other Huxtable Effect

Thirty years ago, The Cosby Show gave us one of TV’s great feminists.

Lifetime Didn’t Find the Steubenville Rape Case Dramatic Enough. So They Added a Little Self-Immolation.

No, New York Times, Shonda Rhimes Is Not an “Angry Black Woman” 

Brow Beat
Sept. 19 2014 1:39 PM Shonda Rhimes Is Not an “Angry Black Woman,” New York Times. Neither Are Her Characters.
Sept. 19 2014 1:11 PM An Up-Close Look at the U.S.–Mexico Border
  News & Politics
Sept. 19 2014 6:22 PM Blacks Don’t Have a Corporal Punishment Problem Americans do. But when blacks exhibit the same behaviors as others, it becomes part of a greater black pathology. 
Sept. 19 2014 6:35 PM Pabst Blue Ribbon is Being Sold to the Russians, Was So Over Anyway
Inside Higher Ed
Sept. 19 2014 1:34 PM Empty Seats, Fewer Donors? College football isn’t attracting the audience it used to.
  Double X
The XX Factor
Sept. 19 2014 3:07 PM Everything Is a "Women's Issue"
  Slate Plus
Slate Picks
Sept. 19 2014 12:00 PM What Happened at Slate This Week? The Slatest editor tells us to read well-informed skepticism, media criticism, and more.
Brow Beat
Sept. 19 2014 4:48 PM You Should Be Listening to Sbtrkt
Future Tense
Sept. 19 2014 5:03 PM White House Chief Information Officer Will Run U.S. Ebola Response
  Health & Science
Medical Examiner
Sept. 19 2014 5:09 PM Did America Get Fat by Drinking Diet Soda?   A high-profile study points the finger at artificial sweeteners.
Sports Nut
Sept. 18 2014 11:42 AM Grandmaster Clash One of the most amazing feats in chess history just happened, and no one noticed.