I’ve seen images of astronauts floating around the International Space Station playing ping pong with water balls and Pac-Man with rows of M&Ms For a moment, as we watch these astronauts thrive in an environment completely foreign to us, we can imagine ourselves floating along with them
The weight of our backsides pressed firmly into our seats brings us crashing back down to planet Earth, back to reality
Is the magical experience of weightlessness really limited to the tiny proportion of human beings who get to call themselves something-nauts (you know, astronauts, cosmonauts, taikonauts, spacemen)?
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 hopeless
Our weight on Earth depends on our mass, which is how much matter we are made of, and the force of attraction between our mass and the mass of planet Earth
This force of attraction, more commonly 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 do not have to touch the Earth for gravity to act on us
In fact, we do not feel the force of gravity unless there is an 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 sit 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 force pushing up on our feet (Figure 1A)
It is through this contact (or normal) force on our feet that we are able to perceive the force of gravity as weight
If the ground beneath our feet disappeared, gravity would still act on us, but we would be unable to feel it
This inability to feel gravity would make us feel weightless (at least momentarily; Box 1)
Astronauts feel weightless when there is nothing to oppose the force of gravity
(A) An astronaut standing on Earth does not feel weightless 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 the force of gravity
However, because the astronaut is moving forward very fast, he/she continuously falls around the Earth instead of crashing into the Earth
Why do astronauts feel weightless?
In space, astronauts and their 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 pull is less in orbit than it is on Earth’s surface (box 1)
However, they don’t feel their weight because nothing is pushing them back
Essentially, the ground has disappeared from under 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 in which gravity is the only force acting on it” In the vacuum of space, where there are no air molecules or supporting surfaces, astronauts are acted upon only by gravity
Thus, they fall towards the Earth under the acceleration of gravity
This begs the question: How are spacecraft able to stay in orbit instead of falling back toward the Earth’s surface?
Although gravity pulls the astronauts toward Earth, the spacecraft travels so fast in the forward direction that it ends up orbiting Earth in a circular pattern, much like a ball dangling on the end of a string
For example, the International Space Station travels at about 27,150 miles per hour, and this forward 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 parabolic flight (aka a trip aboard the Comet Vomit)
To understand how flying in parabolic arcs creates the sensation of weightlessness, we must first 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 airplane 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 plane then experiences 20-30 seconds of free fall as it completes its climb and begins to fall back toward Earth
Finally, once the airplane returns to the same altitude it started from in the forward half of the arc, the pilot reactivates lift to bring the aircraft back 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 flights allow passengers to experience weightlessness without actually going into space
(A) The four forces acting on an airplane are weight, lift, force, and thrust
Because acceleration occurs in the direction of an unbalanced force, airplanes accelerate forward when the force is greater than drag and 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
As a result, 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 is preceded by a controlled climb and followed by a controlled descent
This cycle of controlled ascent, weightlessness, and controlled descent creates the parabolic flight path characteristic of zero-g experiences
In general, parabolic flight is very similar to a hypothetical elevator ride
As the elevator accelerates to the 10th floor, the passengers feel heavier than normal (as the plane climbs to 30,000 feet)
As the elevator approaches the 10th floor and immediately reverses direction to return to the 1st floor, passengers feel weightless (free fall maneuver)
Finally, as the elevator decelerates on the return to floor 1, the passengers feel heavier than normal (the plane descends to 20,000 feet)
One such flight with Zero G Corporation starts at $4,950 per person and includes 15 parabolic maneuvers
You just won a free second of weightlessness
Although a trip to Comet Vomit offers the sensation of weightlessness, it won’t earn you the name of an astronaut
Fortunately, SpaceX, Blue Origin and Virgin Galactic are 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 wealthy person named Yusaku Maezawa, and 6-8 of Maezawa’s artistic friends
Fortunately, Blue Origin and Virgin Galactic have offered their weightless experiences 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 individuals the opportunity to travel into space
Paying customers will leave the Earth’s atmosphere, see the curvature of the Earth and experience several minutes of weightlessness before returning safely to the ground
Lisa Heppler is a fifth-year doctoral student in the Biological and Biomedical Sciences Program at Harvard
Jovana Andrejevic is a third-year PhD student in applied physics at Harvard University’s School of Engineering and Applied Sciences
To learn about the effects of weightlessness on astronauts, check out this article on Spacecom
To learn about experiments aboard the International Space Station, including those that look at the effects of long-term weightlessness 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