We’ve seen footage of astronauts running around the International Space Station playing Ping-Pong with water cannons and Pac-Man with strings of M&Ms For a moment, as we watch these astronauts thrive in an environment that is completely foreign to us, we can imagine ourselves with them
The weight of our backs, held firmly in our seats, brings us back to planet Earth, back to reality
Is the magical experience of weightlessness really limited to a small number of people who call themselves something-nauts (you know, astronauts, cosmonauts, taikonauts, spationauts)?
Weightlessness might just be for astronauts, but with the help of private companies like SpaceX, Blue Origin, and Virgin Galactic, becoming an astronaut might not be so far-fetched
Our weight on Earth depends on our mass, how much matter we are made of, as well as the force of attraction between our mass and the mass of planet Earth
This attractive force, commonly known as gravity, is a non-relational force that acts on us at a distance
As the name suggests, a non-contact force is one that acts between two objects that are in physical contact with each other, meaning that we do not need to touch them for gravity to act on us the ground
In fact, we don’t feel the force of gravity unless there is an opposing force to counteract it
This opposing force is called the normal force, which, unlike gravity, is a relativistic force that acts on objects that are physically connected to each other
For example, when we stand on the ground, the force of gravity pulls our body towards the ground
However, because our feet are in physical contact with the ground, a normal force also acts upward on our feet (Figure 1A)
It is through this relation (or normal) force on our feet that we can perceive the force of gravity as weight
If the ground were to disappear beneath our feet, gravity would still act on us, but we would not be able to feel it
This inability to feel gravity will make us feel weightless (at least for a moment; Box 1)
Astronauts feel weightless when there is nothing to oppose the force of gravity
(A) An astronaut standing on the surface of the earth does not feel weightless because the earth creates a normal force that opposes the force of gravity
(B) An astronaut orbiting the Earth feels weightless because there is no earth or normal force to counteract the force of gravity
However, because the astronaut is also moving forward very quickly, he is constantly orbiting the Earth rather than hitting the Earth
Why do astronauts feel weightless?
In space, astronauts and their spacecraft still have mass and are still acted upon by the earth’s gravity
In this sense, they still have weight, even though the Earth’s gravitational pull in orbit is smaller than it is on Earth’s surface (Box 1)
However, they do not feel the weight because nothing is holding them back
In essence, the ground has disappeared from beneath them, and both the astronauts and the spacecraft fall (Figure 1B)
Wait, so weightlessness is just free fall?
Yes Free fall is defined as “any motion of a body on which gravity is the only force acting” In the vacuum of space, where there are no air molecules or support surfaces, astronauts are acted upon only by gravity
Therefore, they fall towards the ground with the speed of gravity
This begs the question: how can spacecraft stay in orbit, instead of returning to Earth’s surface?
Although gravity pulls the astronauts toward Earth, the spacecraft moves forward so fast that it orbits the Earth in a circular motion, much like a ball swinging on the end of a string
For example, the International Space Station travels at about 17,150 miles per hour, and this increased speed keeps astronauts in orbit despite being pulled toward Earth
Is weightlessness only possible in space?
So how can we really experience weightlessness?
Well, the easiest and probably cheapest way to experience weightlessness is to take advantage of parabolic flight (aka travel on the Vomit Comet)
To understand how flying in parabolic arcs creates the feeling of weightlessness, we first need to consider the four basic forces that act on an airplane (Figure 2A)
The first force is drag, which is created by air molecules that hinder the forward motion of the aircraft
The third force is gravity
To create the sensation of weightlessness, the pilot sets the throttle equal to drag and eliminates lift
At this point, the only unbalanced force acting on the plane is weight, so the plane and its passengers are in free fall
However, planes can only go so far before they hit the ground
Then, the aircraft undergoes a 20-30 second free fall as it completes its ascent and begins its descent toward Earth
Finally, when the aircraft returns to the same altitude as it started from the front half of the arc, the pilot resumes the lift to bring the aircraft back to a stable altitude and prepare for the next takeoff
The resulting parabolic flight path gives the pilot enough time and distance to land safely (Figure 2B)
Parabolic flights allow travelers to experience weightlessness without actually going into space
(A) The four forces acting on an airplane are weight, lift, thrust, and drag
Because acceleration occurs in the direction of an unbalanced force, airplanes accelerate forward when drag is greater than drag, and altitude increases when lift is greater than gravity
(B) When the pilot applies thrust to drag and eliminates lift, the only unbalanced force acting on the airplane is weight
Accordingly, the plane falls and the passengers feel weightless for about 20-30 seconds
To prevent the plane from crashing into the ground, this weightless maneuver involves a controlled takeoff followed by a controlled descent
This cycle of controlled, weightless ascent and controlled descent creates the parabolic flight path characteristic of zero-g experiments
In general, parabolic flight is very similar to a hypothetical elevator ride
As the elevator accelerates to the 10th floor, passengers feel heavier than usual (planes climb to 30,000 feet)
As the elevator approaches the 10th floor and immediately changes direction to return to the 1st floor, passengers feel weightless (free-fall maneuver)
Finally, as the elevator decelerates as it returns to floor 1, passengers feel heavier than usual (a plane descending to 20,000 feet)
One such flight with Zero G Corporation starts at $4,950 per person and includes 15 parabolic maneuvers
You’ve just earned a free second of no weight
Although a trip on the Vomit Comet offers the feeling of weightlessness, it won’t earn you the name of an astronaut
Fortunately, SpaceX, Blue Origin, and Virgin Galactic are all working to make that possible
While SpaceX is poised to become the first private company to send humans into space, its customers are currently limited to NASA astronauts, a wealthy individual named Yusaku Maezawa, and 6-8 of Maezawa’s artistic friends
Fortunately, Blue Origin and Virgin Galactic have made their weightless experiences available to those with slightly smaller checkbooks and slightly less ambitious space travel plans
Although Blue Origin’s New Shepard and Virgin Galactic’s SpaceShipTwo are very different in vehicle design, both promise private individuals the opportunity to travel to space
The spacecraft will leave Earth’s atmosphere, experience Earth’s gravity, and experience weightlessness for several minutes before safely returning to Earth
Lisa Heppler is a fifth-year doctoral candidate in the Biological and Medical Sciences Program at Harvard
Jovana Andrejevic is a third-year Applied Physics PhD student at the School of Engineering and Applied Sciences at Harvard University
To learn about the effects of weightlessness on astronauts, check out this article from Spacecom
Visit this page to learn about experiments being conducted on the International Space Station, including those looking at the effects of long-term inertia on human health
To learn how NASA studies the effects of gravity on non-living things, visit this website
To follow the progress of SpaceX, Blue Origin, and Virgin Galactic, visit their websites and follow them on social media