image credit: STS-41B, NASA; image source; larger image
Astronomy Picture of the Day: To Fly Free in Space
This is astronaut Bruce McCandless, orbiting along with the Space Shuttle in 1984 as he tests his rocket pack. When he stepped outside to begin his spacewalk, why didn't he fall back to Earth? He stayed in orbit because before and after he stepped outside, McCandless and the Shuttle had the same velocity. The force of Earth's gravity bent his path into the same orbit as the shuttle; that's because the acceleration of gravity does not depend on the mass of the object being accelerated. To learn more about McCandless' spacewalk, see Astronomy Picture of the Day: To Fly Free in Space and Footloose.
Now look at the image at the top right of Felix Baumgartner beginning his supersonic skydive. When he stepped out, his balloon had been lifting him slowly in the thin stratospheric air, so his initial velocity was quite small. Gravity then pulled him back to Earth.
To learn more about gravitational orbits, visit Satellite Motion.
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Felix Baumgartner was nearly in free fall as he stepped out of his capsule at an altitude of 39 kilometers (24 miles). At this altitude, the atmosphere is so thin that a falling object experiences hardly any air resistance, so it accelerates rapidly to very high speed (Baumgartner's skydive exceeded the speed of sound).
Watch the video at Feather Drop to see how the air resistance on a feather depends on the density of the air it is falling through. First you'll see the feather fall through air at atmospheric pressure, and then through air at a very low pressure (a very good vacuum).