We’ve seen the feet of astronauts floating around the International Space Station playing Ping-Pong with water balls 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 floating with them
The weight on our rear ends pressed firmly into our seats bringing us crashing back to planet Earth, back to reality
Is the magical experience of weightlessness really limited to the small proportion of human beings who have come to call themselves something-nauts (you know, astronauts, cosmonauts, taikonouts, spationauts)?
Weightlessness may only be for astronauts, but with the help of private companies like SpaceX, Blue Origin, and Virgin Galactic, becoming an astronaut may not be too far away
Our weight on Earth depends on our mass, which is the amount of matter we are made of and the force of attraction between our mass and the mass of the planet Earth
This attractive force, more commonly known as gravity, is a non-contact force that acts on us at a distance
As the name implies, a non-contact force is one that acts between two objects that are not in physical contact with each other, meaning that we do not need to touch the Earth for gravity to act on us
In fact, we do not feel the force of gravity unless there is some opposing contact force to counteract it
This opposing force is called the normal force, which unlike gravity, is a contact force that acts on objects that are physically associated with each other
For example, when we stand on the ground, the force of the Earth’s gravity pulls our body towards the ground
However, because our feet are in physical contact with the ground, there is also a normal upward force on our feet (Figure 1A)
It is in this contact (or normal) force on our feet that we can see the force of gravity as weight
If the ground beneath our feet were to disappear, gravity would act on us, but we would not be able to feel it
This inability to feel gravity would make us feel weightless (at least momentarily; Box 1)
Astronauts feel weightless when nothing opposes the force of gravity
(A) An astronaut standing on 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 ground or normal force to counteract the force of gravity
However, since the astronaut is also moving very fast, he is continuously falling around the Earth rather than falling back to the Earth
Why do astronauts feel weightless?
In space, astronauts and spacecraft still have mass and are still acted upon by Earth’s gravity
In this sense, they still have weight, even though Earth’s gravitational force is less in orbit than it is on Earth’s surface (Box 1)
However, they do not feel their weight because there is nothing pushing back on them
In essence, the ground has disappeared beneath them, and both the astronauts and the spaceship are falling (Figure 1B)
Wait, so weightlessness is just free fall?
Yes Free motion is defined as “any motion of a body in which gravity is the only force acting on it” In the vacuum of space, where there are no air molecules or support surfaces, the astronauts only act on gravity
Thus, they fall towards the Earth under the acceleration of gravity
This begs the question: how can spaceships stay in orbit, instead of falling back towards the Earth’s surface?
Despite gravity pulling the astronaut toward Earth, the spacecraft travels so fast forward that it ends up orbiting the earth in a circular pattern, much like a ball swinging on the end of a string
For example, the International Space Station is traveling at about 17,150 miles per hour, and this momentum keeps the astronauts in orbit despite being pulled toward Earth
Is weightlessness only possible in space?
So how can we actually experience weightlessness?
Well, the easiest and perhaps cheapest way to experience weightlessness is to take advantage of parabolic flight (aka a trip aboard the Vomit Comet)
To understand how flying in parabolic arcs creates the sensation of weightlessness, we first need to review the four basic forces that act on an airplane (Figure 2A)
The first force is drag, which is caused by air molecules that prevent the aircraft from moving forward
The third force is weight
To create the sensation of weightlessness, the pilot sets thrust 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 fall so far before they hit the ground
The aircraft then experiences 20-30 seconds of free fall as it completes the climb and begins to fall back towards Earth
Finally, once the aircraft returns to the same altitude it started at in the forward half of the arc, the pilot re-engages elevator to return the aircraft to a stable altitude and prepare for the next climb
The resulting parabolic flight path gives the pilot enough time and distance to fall safely (Figure 2B)
Parabolic flight allows passengers to experience weightlessness without actually going into space
(A) The four forces acting on an airplane are weight, lift, thrust, and drag
Since acceleration occurs in the direction of an unbalanced force, aircraft accelerate in the forward direction when thrust is greater than drag and the increase in altitude when lift is greater than weight
(B) When the pilot sets thrust equal to drag and eliminates lift, the only unbalanced force acting on the airplane is weight
Consequently, the plane falls and the passengers feel pain for about 20-30 seconds
To prevent the aircraft from crashing into the ground, this weightless maneuver is preceded by a controlled climb and followed by a controlled descent
This cycle of controlled climb, weightlessness, 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 toward floor 10, the passengers feel heavier than usual (plane rising to 30,000 feet)
As the elevator approaches floor 10 and immediately changes direction to travel back toward floor 1, passengers feel weightless (free fall maneuver)
Finally, as the elevator slows down on the return to floor 1, passengers feel heavier than usual (plane descends at 20,000 feet)
Such a flight with Zero G Corporation starts at $4,950 per person and includes 15 parabolic maneuvers
You just have a free second of weightlessness
Although a trip on the Vomit Comet gives the feeling of weightlessness, it will not give you the name of astronaut
Fortunately, SpaceX, Blue Origin, and Virgin Galactic are all working to make this possible
While SpaceX is poised to be the first private company to send humans into space, its customers are currently limited to NASA astronauts, a rich man named Yusaku Maezawa, and 6-8 of Maezawa’s artistic friends
Fortunately, Blue Origin and Virgin Galactic have provided weightless experiences to those with slightly smaller paychecks 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 in space
Paying customers will leave the Earth’s atmosphere, see the curvature of the Earth, and experience a few minutes of weightlessness before returning safely to Earth
Lisa Heppler is a fifth-year PhD candidate in the Biological and Biomedical Sciences Program at Harvard
Jovana Andrejevic is a third-year PhD student in 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
To learn about experiments conducted aboard the International Space Station, including those looking at the effects of long-term subgravitation on human health, visit this page
To learn how NASA studies the effects of weightlessness on non-living things, visit this site
To follow the progress of SpaceX, Blue Origin, and Virgin Galactic, visit their websites and follow them on social media