Crossing the Interplanetary Divide

May 2021

True or false and why: Here in the Americas, most of us have heard of the Continental Divide, the invisible line demarcating where watersheds either flow west toward the Pacific Ocean or east toward the Atlantic Ocean. Interplanetary travel will be similar: On your way to Mars, at some point you are no longer in free fall toward Earth. You are in free fall toward Mars. It’s that simple. Draft a response of no more than 250 words and email it by noon Eastern May 13 to aeropuzzler@aiaa.org for a chance to have it published in the June issue.

FROM THE APRIL ISSUE

WINNER: The baseball player’s comparison of swimming and the flight of a baseball is inappropriate. It is true that saltwater is denser than fresh water. This causes a floating object (person) to displace less volume of water and have more volume immersed in the air above. The person swimming is operating on the interface between two fluids. The water is 800 times as dense and is much more viscous than the air so it is beneficial to have a smaller proportion of the volume in the water and the remaining, larger portion in the air. This is what makes swimming easier. The baseball is immersed only in air, not at the interface with another fluid, and the buoyancy is extremely small. There is no “cushion of cold air.” For the baseball flying through the air, the denser, cold air creates more drag and reduces the flight distance. The player is only half right about the heat and humidity. Humid air is less dense than dry air and hot air is less dense than cold air, so the ball will fly farther on a hot, humid day. They both help, not just the humidity. The humidity is not really providing lift, just less drag.

Douglas Dobbin, an AIAA senior member, of El Paso, Texas, performs trajectory and risk analysis at White Sands Missile Range in
New Mexico and is in a graduate airplane design program at Georgia Tech. douglas.dobbin@gmail.com

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