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Moon Tennis

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August 30, 2005: They call him the "Rocket Man." Tennis pro Andy Roddick holds the world's record for fast serves: 155 mph. By the time opponents realize where the ball is going, very often, it's already gone. Ace! His groundstrokes are like rockets, too.

Believe it or not, this can be a problem. Balls hit so hard want to go long, flying straight out the back of the court. Out! It's hard to win a game that way.

Right: Andy Roddick serves--a 100+ mph blur. [More]

So what do you do when you're so powerful? Roddick has a trick up his sleeve: topspin. By hitting the ball obliquely (at a slant), he causes it to spin. Topspin makes the ball curve downward. Instead of going out, it drops neatly into play on the other side of the net.

Now the Rocket Man is wondering, "What if the US Open were played on the Moon? Would topspin do me any good?" [video]

It's a perfectly reasonable question. In 2004 when President Bush set out the nation's Vision for Space Exploration, he declared "human beings are headed into the cosmos." He never said, except for the tennis players. Where people go, sport follows. Decades from now, tennis might be a popular pastime on the Moon.

Picture this: Two space-suited athletes stare at each other across the net of a tennis court drawn in moondust. Their own reflections stare back from the faceplate of the opponent. Overhead, the sun is bright, the sky is black and Earth looks absolutely beautiful; it's a crescent today.

Above: An artist's concept of Moon tennis. Credit: Paula Vargas and Terry Longbottom of NASA/JSC. [video]

One player tosses the ball up … up … up. Smash! Soundlessly, a 100+ mph serve glides across the net, long. Another serve, long. And another, long. Groundstrokes go long, too.

Something's wrong. On the Moon, topspin doesn't work.

Actually, the ball spins just fine, but spinning doesn't make it curve downward. The Moon has no air, and spinning balls only curve when they're flying through an atmosphere. (Note: Gravity causes balls to curve downward, too, but not enough to rescue every shot.)

Physicists call this the Bernoulli Effect: Air pressure on one side of a spinning ball is higher than it is on the other side. High pressure pushes the ball toward low pressure--hence the curve. Swiss physicist Daniel Bernoulli wrote down the equations describing this curious phenomenon in the 18th century, setting the stage for Andy Roddick's devastating power game almost 300 years later.

The Bernoulli Effect is very important in sports. In baseball, it lets pitchers throw curveballs. In tennis and ping pong, it helps players ace their serves. In golf, it's responsible for the dreaded slice.

Right: The Bernoulli Effect. [More]

Aside: Apollo 14 astronaut Al Shepard became the first extraterrestrial golfer on February 6, 1971, when he tried hitting some balls on the Moon. His club was homemade, consisting of a 6-iron attached to the end of a geology tool handle. "Unfortunately, the suit is so stiff, I can't do this with two hands, but I'm going to try a little sand-trap shot here," said Shepard. One shot, remarked mission control, "looked like a slice." Shepard countered "Straight as a die!" In fact, the ball rolled just 2 or 3 feet. Later, Shepard did get off a good shot which famously went "for miles and miles and miles." Practice makes perfect, especially on the Moon.

Moon tennis is going to differ from Earth tennis in many ways. For example: In the Moon's lower gravity, only 1/6th Earth's, lobs will fly six times higher and hang six times longer. Thirsty? Hit a lob. While it's in flight you can go get a drink of water.

Here on Earth we have clay courts and grass courts. On the Moon there will be dust courts. The surface affects the style of play: clay is a slow surface, while grass is faster and prone to crazy bounces. A dusty surface will probably be slow, until it packs down a bit.

Moondust adds an interesting twist to tennis. For one thing, moondust is extremely dry and insulating, and thus prone to static cling (like a crackling sock pulled out of the dryer). Balls hit into this material over and over will soon gather a layer of electrified dust. Want to shock your opponent? Smash the ball straight at him!

There's more: When a ball bounces off a mooncourt, it's going to kick up a little dusty plume. Spin the ball just right and you could send that dust jetting toward your opponent, craftily blinding him. Topspin, or in this case, backspin, might come in handy after all.

And don't forget the spacesuits. On Earth, players wear shorts and light shirts. On the Moon, they'll be climbing into a bulky full-body pressure suit. As Apollo astronauts discovered, you don't really "run" in a 1/6-g moonsuit, you "bound," hopping around like Tigger of the Hundred Acre Wood. A player might spend more time off the ground than on it. ("Footwork!" cries your tennis coach. Footwork? How can you have footwork when your feet don't touch the ground? Old-fashioned strategies might not work on the Moon.)

Right: Andy Roddick ponders tennis on other worlds. [video]

Furthermore, joints at the elbows, knees and especially shoulders of spacesuits are less flexible than real human joints. Overhead serves are going to be mighty difficult. Sorry Andy! Forehand or underhand serves, legal in Earth tennis, might be required on the Moon. Meanwhile, the backhand stroke could become extinct: it's tough to reach across the body of a turgid spacesuit.

Eventually, special suits might be manufactured for moon tennis, designed to allow overhand serves, backhand shots, and normal running. The dimensions of courts could change, too, probably made bigger to accommodate no-Bernoulli serves and low-gravity volleys.

Low gravity. No topspin. Bulky spacesuits. Moon tennis is going to be different. Says Roddick, "…that might be kind of cool!"

Authors: Dr. Tony Phillips & Phil West | Production Editor: Dr. Tony Phillips | Credit: Science@NASA

More Information

Hey, wait a minute! While this story is grounded in serious physics, it shouldn't be taken too seriously. We won't know what it's like to play tennis on the Moon until someone tries it. Will tennis balls really accumulate electrical charge as they bounce back and forth across the court? It depends on what the ball is made of and how it interacts with moondust. Scientists do know that the Moon "crackles" with static electricity--Moon tennis might crackle, too. Also, by the time people are actually living on the Moon, advanced space suits will surely be lighter and more agile than Apollo suits. This will help astronauts go about their duties: exploring, building the colony, and acting as field geologists. Improved suits will help them play tennis, too, making backhand strokes and over-the-shoulder serves easier to execute. And, of course, athletes playing inside a pressurized lunar sports dome could shed their spacesuits altogether. (Imagine the "bounding" you could do in there!) There would be no moondust, plenty of air and plenty of Bernoulli. How different will Moon tennis be? Time will tell.

All Things Bernoulli -- a review of the Bernoulli Effect (and related phenomena) suitable for high school and undergraduate physics students

The Magnus Effect -- When applied to tennis and other ball sports, the Bernoulli Effect is often called "the Magnus Effect," after 19th-century German physicist H. G. Magnus who studied the forces on spinning balls and cylinders.

Bernoulli vs. Newton -- Topspinning balls sink because they have "negative lift," the opposite of "positive lift" generated by an airplane's wing or a backspinning ball. How is lift created? Scientists are still debating the underlying reasons.

Follow the bouncing ball -- a look at the physics of tennis bounces or, in physics-speak, the "ball-court interaction"

More physics links:

Alan Shepard, Out of this World Golfer -- from pasturegolf.com

A Nice Day for a Game of Golf -- NASA's Apollo 14 Lunar Surface Journal recounts the first-ever round of extraterrestrial golf.

Daniel Bernoulli -- a short biography

NASA's Vision for Space Exploration