Portland, Ore. The National Aeronautics and Space Administration's "weightless wonder" aircraft will be the testbed this week for a robot that exhibits catlike motion in free fall. The work could one day lead to designs that eliminate the need for retrorockets, gas jets and gyroscopes in satellites and other spacecraft.
The robot, thus far tested only under less-than-ideal conditions in a university lab, rotates without angular momentum by changing the length of parts of its body while rotating them in opposite directions, much as a falling cat does. A successful NASA test of the concept could transform all space gear now based on gyroscopes including satellites, the space station and the space shuttle by enabling precise, vernier-like orientation control with zero angular momentum.
Managing that momentum is the bane of space navigation today. The law of conservation of angular momentum dictates that when an object rotates in one direction, there must be an equal and opposite rotation in the other direction. The law complicates almost every space-based maneuver, introducing unwanted spin that must be compensated for with retrorockets or gas jets.
In devising the concept, project leader Gregory Ojakangas, a physics professor at Drury University (Springfield, Miss.), borrowed from a species that flouts the rules of angular momentum. "When you drop a cat upside down, it always lands on its feet, with no spin. It doesn't paddle the air that would impart spin it just expands its upper body and rotates it, then contracts its upper body and rotates in the opposite direction," said Ojakangas. "In that sense, ours is not a new idea; nature has been doing it for I don't know how many millions of years."
This week, the catlike robot will be put to the test over the Gulf of Mexico in a Boeing KC-135a Stratotanker aircraft the plane NASA calls the weightless wonder which simulates the effect of weightlessness in space.
When an astronaut friend of Ojakangas' described to the physicist how his catlike motions while weightless enabled him to ratchet around without spinning, seemingly violating the law of conservation of angular momentum, the professor hit on an idea. Ojakangas researched the physical laws governing the physiological traits that let cats eliminate spin, then proposed the robot idea as a class project to a group of students.
The resultant robot has a bottom part that always counterrotates with the top part, thereby canceling angular momentum, and yet the different weights extended from the top and bottom allow for a net rotation without spin. The students have tested the robot in the lab by hanging it from a long wire, but twists in the wire have introduced unwanted angular momentum that have foiled the team's ability to take precise measurements. Ojakangas' team will test its theory more reliably with the robot aboard the NASA flight.
"It's a curious little device, and potentially a way of turning satellites without any net spin, without gyroscopes and without gas jets or any kind of propulsion," said Ojakangas.