We’ve seen footage of astronauts floating around the International Space Station playing ping pong with water balloons and Pac-Man with strings of M&Ms For a moment, as we look at these astronauts thriving in an environment completely unfamiliar to us, we can imagine ourselves floating along with them
The weight of the back pressed hard against the 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 can call themselves nauts (you know, astronauts, cosmonauts, taikonauts, 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 so far-fetched
Our weight on Earth depends on our mass, which is how much matter we are made of, as well as the gravitational force between our mass and the mass of planet Earth
This force of attraction, better known as gravity, is a non-contact force that acts on us from a distance
As the name suggests, a non-contact force is one that acts between two objects that are not in physical contact with each other, meaning that we don’t have to touch the Earth for gravity to work
In fact, we don’t feel the force of gravity unless there is an opposing contact force to oppose it
This opposing force is called the normal force, which, unlike gravity, is a contact force that acts on objects that are physically connected to each other
For example, when we stand on the ground, the gravitational force of the Earth pulls our body towards the ground
However, since our feet are in physical contact with the ground, a normal upward force also acts on our feet (Figure 1A)
It is through this contact (or normal) force on the legs that we are able to perceive the force of gravity as weight
If the ground disappeared beneath our feet, gravity would still act on us, but we wouldn’t be able to perceive it
This inability to feel gravity would make us feel weightless (at least momentarily; Box 1)
Astronauts feel weightless when there is no opposition to the force of gravity
(A) An astronaut standing on the ground does not feel weighted because the ground 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 gravity
However, since the astronaut is also moving forward at extremely high speed, he is constantly falling around the Earth, rather than hitting the Earth
Why do astronauts feel weighed down?
In space, astronauts and their spacecraft still have mass and are still affected 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 own weight, because nothing pushes them back
In essence, the ground has disappeared from under them, and both the astronauts and the spacecraft fall (Figure 1B)
Wait, so weightlessness is just freefall?
Yes Free fall 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, only gravity acts on astronauts
Thus, they fall towards the Earth with the acceleration of gravity
This begs the question: How can spacecraft stay in orbit and not fall back to Earth’s surface?
Although gravity pulls the astronauts toward Earth, the spacecraft moves forward so fast that it orbits Earth in a circular motion, much like a ball swinging on a string
For example, the International Space Station travels at about 17,150 miles per hour, and this propulsion keeps astronauts in orbit despite being pulled toward Earth
Is weightlessness only possible in space?
So how can we experience weightlessness?
Well, the easiest and possibly cheapest way to experience weightlessness is to take a parabolic flight (aka a trip aboard the Vomit Comet)
To understand how flying in a parabolic arc creates the feeling of weightlessness, we must first review the four basic forces acting on an airplane (Figure 2A)
The first force is the drag caused by air molecules that prevent the aircraft from moving forward
The third force is weight
To create a sense 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 as far as they hit the ground
The plane then experiences 20-30 seconds of free fall as it finishes climbing and begins to fall back towards Earth
Finally, when the aircraft returns to the same altitude as it started at the front of the arc, the pilot applies lift again to bring 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 crash safely (Figure 2B)
Parabolic flights allow passengers to experience weightlessness without going into space
(A) The four forces acting on an airplane are weight, lift, thrust, and drag
Because acceleration occurs in the direction of the unbalanced force, airplanes accelerate forward when thrust is greater than drag and increase altitude when lift is greater than weight
(B) If the pilot sets thrust equal to drag and eliminates lift, the only unbalanced force acting on the plane is weight
Accordingly, the plane falls and the passengers feel weightless for about 20-30 seconds
To prevent the aircraft from falling to 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 experiences
In general, a parabolic flight is very similar to a hypothetical elevator ride
As the elevator accelerates toward the 10th floor, passengers feel heavier than usual (the plane climbs to 30,000 feet)
When the elevator approaches the 10th floor and immediately changes direction to travel back to the 1st floor, passengers feel weighed down (free fall maneuver)
Finally, as the elevator slows to return to the 1st floor, passengers feel heavier than usual (the plane descends to 20,000 feet)
Such a flight with Zero G Corporation starts at $4,950 per person and includes 15 parabolic maneuvers
You just won a free second of weightlessness
While a trip on Comet Vomit creates weightlessness, it doesn’t earn you the name of an astronaut
Fortunately, SpaceX, Blue Origin and Virgin Galactic are working on it
Although SpaceX is poised to be the first private company to send humans into space, its customers are currently limited to NASA astronauts, wealthy individual Yusaku Maezawa, and 6-8 of Maezawa’s art friends
Fortunately, Blue Origin and Virgin Galactic have made their weightless experience 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 terms of vehicle design, both promise private space travel
Paying customers leave the Earth’s atmosphere, see the curvature of the Earth and experience weightlessness for a few minutes before safely returning to Earth
Lisa Heppler is a fifth-year PhD student in the Biological and Biomedical Sciences Program at Harvard
Jovana Andrejevic is a third-year applied physics PhD student at Harvard University’s School of Engineering and Applied Sciences
To learn about the effects of weightlessness on astronauts, read this article on Spacecom
For information on experiments conducted on the International Space Station, including those on the effects of long-term weightlessness on human health, visit this page
If you want to know how NASA studies the effects of weightlessness on inanimate objects, visit this site
To follow the progress of SpaceX, Blue Origin and Virgin Galactic, visit their websites and follow them on social media