Why Antarctica?

Good question. To answer that, consider that I am sending a huge instrument essentially to outer space, hanging from a monstrous helium-filled balloon. CREST weighs about 7000 lbs. What would happen if that came crashing into your living room? Nothing good, I think.  To avoid dropping our instrument Wizard-of-Oz-like uninvited into something man-made we go where the likelihood of finding human structures is very small. This includes places like northern Canada, Australia, the oceans, New Mexico, and Antarctica.

A balloon and instrument just after launch.  The pre-deployed parachute is visible.

Still, with all those choices, the question remains: why Antarctica?  Consider what happens after our instrument achieves float altitude at 120,000 feet. (Challenge: How many miles is that? How many kilometers?) It begins to drift with the wind, just like a hot air balloon would.  But at 120,000 ft, winds blow either east to west or west to east, depending on the time of year.  And the wind speed is usually quite high: many tens of mph. This means that the balloon and instrument cover a lot of ground very quickly. What may have started as empty country (eg New Mexico) quickly ends up being populated (eg Los Angeles!). 

A balloon and instrument at float altitude, fully expanded (seen through a telescope).

Antarctica has the advantage that east and west directions form tight circles about the south pole. So a balloon sent up from McMurdo Station will take about 10 or 15 days to go around the pole and end up back at McMurdo. Very convenient, since then we can decide whether to take one more time around or bring it down. And all along we never have to worry about running into someplace like Los Angeles or Buenos Aires.  Any other launch site mentioned above runs that risk.  Plus, there is no chance that our instrument will get tangled up in a tree in Antarctica.