The answer to the question of exactly where Earth's atmosphere gives way to the bleakness of beyond is blowing in the winds of space, suggests research from the University of Calgary.
New data confirms that, by one definition, the boundary is 118 kilometres above the Earth. That's about the same distance as the drive between Calgary and Banff, or Toronto and Waterloo, Ont.
Astronomy Prof. David Knudsen says a specialized instrument designed by the school was attached to a NASA rocket shot through the windy area where Earth's atmosphere meets space.
The university's specialized Supra-Thermal Ion Imager recorded detailed information about the extreme heat produced by the friction between Earth's gentle winds and the dizzying blasts of space.
Solar wind, made up of charged particles blowing away from the sun, can be thousands of kilometres an hour, he said.
"If you think of these two regions — atmospheric winds and the space winds — being two different flows, then right at the boundary there's friction where they slip across each other, and it actually heats up that region," Knudsen said.
"It's an important source of heat and energy for the upper atmosphere.
"The boundary that we were reporting is the halfway point, really, between very fast winds in space and slower winds in the upper atmosphere."
Not easy to tell where outer space begins: researcher
It's been hard to research that boundary because it's lower than the altitude at which satellites orbit, but higher than research balloons can fly.
The limit of 118 kilometres had been suggested by computer models and research done from the ground, but little detailed information had been gleaned from the region itself.
Knudsen stressed that it's not easy to put an exact figure on where outer space begins.
While they came up with 118 kilometres, those working with space shuttles consider it the place where the shuttles switch from thrusters to aerodynamic flaps — about 122 kilometres above sea level.
Other people have put it closer to 110 kilometres, said Jaymie Matthews, an astrophysics professor at the University of British Columbia. He was not involved in the University of Calgary research.
"It all depends on what physical characteristics of the atmosphere are important to you," said Matthews, adding most people think of space as a vacuum, but it's hard to say at what pressure that technically begins.
"It's all very gradual."
He said that if you want to do a "high jump" into space, 118 kilometres will probably do the trick. But if you're looking for a "marathon," where you've obtained enough height to actually stay up in space, you've got to go a lot higher.
Knudsen said he's not that interested in putting an exact figure on where space begins, but rather pulling apart the little-studied secrets of the boundary region.
He said the ion imager is being constantly refined and readied for other space missions, including two with the European Space Agency.