When a feather and a hammer are dropped at the same time why does the hammer hit the ground first?

AUSTIN, Texas — If you get a chance to step outside on one of these warm summer nights, you might glance up at the moon. It's hard to imagine that 50 years ago this week, three Americans had just landed their spacecraft on the moon. 

It was known as the Apollo 15 Mission – the fourth time the U.S. sent astronauts to the moon.

But this visit, this week in 1971, was different. It was the first time astronauts traveled on the surface of the moon in a battery-operated, 400-pound, four-wheel lunar roving vehicle. They drove along the moon’s rugged terrain picking up rock samples, at times venturing nearly 20 miles from their spacecraft.

This was also the mission that Astronaut David Scott got to test a scientific theory. The question was: If you drop a hammer and a feather from the same height, will the hammer fall to the ground faster than the feather since it’s heavier?  

On Earth, the feather would likely float on air currents while the hammer would hit the ground first. But what about in the vacuum of space?

The answer? Based on Scott’s experiment, on the moon, both hammer and feather reached the ground at the same time, proving Galileo’s 400-year-old theory that objects dropped from the same height reach the surface at the same time regardless of their weight.

The astronauts returned to Earth safely, but they left their lunar rover on the moon. They also left behind a small memorial dedicated to those Americans and Russians who had died during early space exploration.  

It all happened this week in 1971.

Taking air resistance out of the picture doesn't mean that the feather and the hammer have the same mass. If the feather is much less massive, then how can it fall at the same rate?

Say you have two objects: a billiard ball and a feather. You drop both from the same height at the same time. You lay odds on the ball hitting the ground first -- and you're probably right, even if it's just by a split-second. However, as demonstrated by Galileo in 1589, mass does not affect gravitational pull; theoretically, all things should fall at the same rate, regardless of how heavy they are.

Back in 1971, on his last day on the moon, Apollo 15 Commander David Scott tested this theory. In one hand, he took a 1.32kg aluminium geological hammer. In his other, a 30g falcon feather, 44 times lighter than the hammer. Sure enough, when he dropped them both from the same height at the same time, they hit the ground simultaneously -- thus demonstrating Galileo's theory.

On Earth, it doesn't necessarily work this way. This is because the planet is enclosed in a bubble of gas -- the atmosphere -- which causes an effect called aerodynamic drag, particularly on objects that have a comparatively large surface area -- such as feathers or fabric. It is caused by the pressure of a medium, such as air, on a solid object. Every time you move, you are pushing against air. The denser the medium the object is moving through, the stronger the drag pressure -- which is why water is more difficult to move through than air. Aerodynamic drag is also what allows parachutes to work. The drag pressure across the surface area of the fabric is enough to slow descent to a safe speed. On the moon, there is no atmosphere -- and therefore no aerodynamic drag to slow the fall of high surface area objects. If you were to use a parachute on the moon, you'd end up looking pretty silly and possibly broken.

So why did Curiosity have a parachute? Mars, in fact, does have an atmosphere -- albeit a very thin one, made up mostly of carbon dioxide. Curiosity's parachute, about 51 feet (15.5 metres), is twice the size of a parachute that can safely drop a human on Earth -- and isn't considered safe for human missions, since a human-carrying spaceship will be a lot heavier than the Curiosity lander. To that end, NASA is currently developing what it is calling the Low-Density Supersonic Decelerator to be used in concert with a 110-foot-diameter (33.5 metres) parachute. The LDSD is a saucer-shaped inflatable designed to slow a craft while travelling at supersonic speeds through a low-density atmosphere.

And that's why Galileo was boss.

It may seem obvious at first – but is it?

Let's find out!

Footage courtesy of NASA

Spaceman:
Well, in my left hand, I have a feather. In my right hand, a hammer. And I guess one of the reasons that we got here today was because of the gentleman named, Galileo a long time ago, and made a rather significant discovery about falling objects and gravity fields. And we thought that, where would be a better place to confirm his findings than on the moon? And so, we thought we would try it here for you, as the feather happens to be appropriately a falcon feather for our Falcon. And I'll drop the two of them here, and hopefully they'll hit the ground at the same time. How about that? Mr Galileo was correct in his findings.

Because the Apollo crew were essentially in a vacuum, there was no air resistance and the feather fell at the same rate as the hammer.

This is exactly what Galileo had concluded hundreds of years before: all objects released together fall at the same rate regardless of mass. The result was predicted by well-established theory, but was reassuring for the Apollo crew as their homeward journey from the Moon depended on this theory being correct!