On August 2, 1971, at the end of the last EVA of the Apollo 15 mission, Commander David Scott took a few minutes to conduct a classic science experiment in front of the TV camera that had been set up just outside the LM Falcon at the Hadley Rille landing site. Scott, a former Air Force pilot, recreated a famous demonstration often attributed to Galileo (which may or may not have actually been performed by the astronomer in Pisa in 1586) that shows how objects of different masses react the same way to gravity when dropped – that is, they fall at the same rate.*
By performing the “acceleration test” in the vacuum environment of space (but where there is still an observable downward pull of gravity) the Earthly factor of air resistance is negated – especially on such a low-mass and low-density object as a falcon feather – thereby creating a more “pristine” setting for the centuries-old experiment than could ever be achieved here.
According to a report on the mission’s science objectives: “During the final minutes of the third extravehicular activity, a short demonstration experiment was conducted. A heavy object (a 1.32-kg aluminum geological hammer) and a light object (a 0.03-kg falcon feather) were released simultaneously from approximately the same height (approximately 1.6 m) and were allowed to fall to the surface. Within the accuracy of the simultaneous release, the objects were observed to undergo the same acceleration and strike the lunar surface simultaneously, which was a result predicted by well-established theory, but a result nonetheless reassuring considering both the number of viewers that witnessed the experiment and the fact that the homeward journey was based critically on the validity of the particular theory being tested. ” (Joe Allen, NASA SP-289, Apollo 15 Preliminary Science Report, Summary of Scientific Results, p. 2-11. Source.)
And for those who have posed the question: yes, the Moon does have gravity. One-sixth of Earth’s in fact, which means a 170-lb astronaut wearing a 180-lb Apollo space suit would weigh 350 lbs on Earth but only weighed 58 lbs on the Moon. (Which means Dave Scott’s 3-lb hammer weighed less than half a pound when he dropped it.)
Launched on July 26, 1971, Apollo 15 was the first of the “J” missions capable of a longer stay time on the moon and greater surface mobility, thanks to the first use of the Lunar Roving Vehicle (LRV).
This post was originally published on Aug. 1, 2015.
(*And yes, they did in fact hit the lunar surface at the same time!)