The Rigors of Space Travel

This paragraph appeared in an article on CNN's web site, but just for variety, let's blame the Associated Press who wrote it, instead of CNN, who printed it without, apparently, a moment's ratiocination.

The return to Earth, though quick, can be physically taxing; the heavy deceleration once in the Earth's atmosphere -- from about 500 mph to 180 mph (800 kilometers/hour to 288 kilomters[sic] per hour) -- inflicts severe G-forces on space explorers who have spent the previous weeks or months weightless. As it nears the ground, the Soyuz fires its engines to slow the descent again to about 3 mph.

Yes, Anousheh Ansari, apparently in fine physical condition, managed to land without damage to herself after experiencing just about the same deceleration you do when descending from cruising altitude (~500 mph) to the vicinity of an airport (~180 mph) in preparation for landing.

Anousheh Ansari is a woman who seems to have taken unto herself the advice that I offered Warren Buffett.  You might argue that she took that advice long before I offered it, to which I'd reply "picky, picky."  We owe a lot to her:  She rescued the "X Prize," which became the Ansari X Prize and which served as a major impetus to individual space travel.  She just spent a reported $20 million for a "tourist" trip to the space station.  And without overanalyzing this, the fact that she is an American who was born in Iran might offer a smidgen of encouragement to denizens of certain nations. 

Returning to the return to earth, let's see what really happened.  Initially, her spacecraft was in orbit.  Disregarding the drag of the almost nonexistent atmosphere at that altitude, that means that there was a balance between the gravitational force pulling it toward the earth, and the inertia of the craft keeping it moving forward, resulting in an essentially circular motion.  The orbital velocity necessary to result in this balance is about 17,000 miles an hour, which so much above the 500 mph listed in the news story that even the Associated Press might have taken notice.   In order for the spaceship to land, that velocity must be reduced to zero, or at least zero relative to the place of landing.  The way this is accomplished is by firing "retro" rockets, i.e., rockets in a direction opposite the motion of the space ship.  Doing this disrupts the balance of forces that keep the ship in orbit to start it going down faster than it does forward.  As it gets closer to the earth, it reaches thicker atmosphere, which then drags the ship more strongly, which decreases its speed, which brings it lower and thus into thicker atmosphere, and so on. 

This is a delicately balanced process.  If you enter the atmosphere too fast the atmospheric drag slows you down at an excessive rate, and, notwithstanding the mission of CATWOKE, the conversion of kinetic energy melts the skin of the spacecraft and crushes the occupants.  Getting it right, as almost always occurs, results in the occupants experiencing several times the pull of gravity during the descent, but not so many times as to be damaging.  That is the "physically taxing" part referred to in the article.  The mystery is where those two mph numbers, 500 and 180, came from.

The last time I found similarly preposterous numbers in one of these AP/CNN articles, they were silently corrected, one on each of two subsequent days.  That will probably happen here, leaving us ignorant as to where the writer obtained the original.  That may be for the best.
 

Follow-up 28 July 2006

I found a nice, more nearly complete, and presumably accurate description of both a normal and abnormal Soyuz landing procedure on the web.  It explains in more detail how the spacecraft enters the atmosphere and where the occupants suffer high G forces.

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