Stargazing Notes — Where does space begin?
Ken Tapping is an astronomer with the National Research Council's Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory in Penticton.
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Updated: September 02, 2009 10:12 AM
Astronomer Fred Hoyle wrote, "space isn't remote at all. It's only an hour's drive away, if your car can drive straight up." If we could drive straight up at 100 kilometres an hour, that would take us to a height of 100 kilometres. Would that be high enough to take us into space? The answer's not that easy.
It depends upon how you define space. If space starts at the minimum height at which we can have a satellite orbit the Earth, then no. The Earth's atmosphere at that height is still dense enough to drag down satellites and make meteors burn up.
As a thought experiment, imagine us taking that drive that Fred Hoyle mentioned. We start at the ground and proceed vertically upwards. On the ground the air pressure is about 15 pounds per square inch, 100 kiloPascals or 1,000 millibars. As we get higher the pressure drops. This is because air pressure is due to the weight of the overlying atmosphere; as we ascend, there is less atmosphere above us, and the pressure needed to support is less. That means that each breath takes in less oxygen.
By the time we are at between 4,000 and 5,000 metres, about the height of our telescopes on top of Mauna Kea in Hawaii, the shortage of oxygen becomes noticeable. For astronomers and engineers visiting the Canada France Hawaii Telescope, the Gemini North Telescope or the James Clerk Maxwell Telescope, this manifests itself as a growing inability to concentrate, clumsiness and persistent headaches.
When we get to about nine kilometres' altitude, roughly the height of Mount Everest, we would lose consciousness very quickly without an oxygen mask. At about 20 km altitude, the air is so rarefied we will need pressure suits to stay alive. We would also notice that the sun's ultraviolet radiation is getting stronger. Starting at about 10 km above sea level we enter the ozone layer, which extends upwards to about 50 km altitude. This layer protects us from solar ultraviolet radiation. As we move upwards, the radiation gets stronger.
Around 90 km up we start to detect electrons. The solar ultraviolet radiation is energetic enough to break up the atoms of the gases making up the atmosphere. We are entering the ionosphere. These electrons reflect radio waves, making long-distance radio communication possible. Although there is some atmosphere at those heights, compared with the density at sea level it is a pretty good vacuum, and we need space suits to survive. At several hundred kilometres the density is low enough for spacecraft to orbit the Earth. However, we are still inside the Earth's magnetic field, and screened by the magnetopause from the space outside. At 200,000 km we are about halfway to the moon and definitely outside the magnetopause. However, we are still not in deep space; we are in the outer parts of the sun's atmosphere, experiencing the solar wind. Somewhere out 'way beyond Pluto, about 15 billion kilometres away, we cross the edge of the solar wind and enter interstellar space. However, we need to travel many thousands of light years to get out into intergalactic space, the nearest thing to empty space there is.
Jupiter is visible in the southern sky all night. Mars rises just after midnight, and Venus rises in the early hours. The moon will be full on Sept. 4.
Ken Tapping is an astronomer with the National Research Council's Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory in Penticton.
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