We’ve seen footage of astronauts floating around the International Space Station playing ping pong with water balls and Pac-Man with strings of M&Ms Watching these astronauts flourish in an environment that is completely foreign to us, we can imagine for a moment that we are floating around with them
The weight of our buttocks pressed firmly into our seats brings us back to planet Earth, back to reality
Is the magical experience of weightlessness really limited to the small portion of people who are allowed to call themselves something-nauts (you know, astronauts, cosmonauts, taikonauts, spacenauts)?
Weightlessness may be just 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, which is how much matter we are made of, as well as the attraction between our mass and the mass of planet Earth
This attraction, known as gravity, is a contactless force that acts on us from a distance
As the name implies, a non-contact force is one that acts between two objects that have no physical contact with each other, meaning we don’t have to touch the Earth for gravity to act on us
In fact, we don’t feel gravity unless there is an opposing contact force to counteract it
This opposing force is called 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 Earth’s gravity pulls our body toward the ground
However, because our feet are in physical contact with the ground, there is also a normal force that pushes our feet up (Figure 1A)
This contact force (or normal force) on our feet allows us to perceive gravity as weight
If the ground disappeared from under our feet, gravity would still act on us, but we wouldn’t be able to feel it
This inability to sense gravity would make us feel weightless (at least for a moment; Box 1)
Astronauts feel weightless when there is nothing to oppose gravity
(A) An astronaut standing on Earth does not feel weightless because the ground creates a normal force that opposes gravity
(B) An astronaut orbiting Earth feels weightless because there is no ground or normal force to counteract gravity
However, because the astronaut is also moving super fast, he/she is constantly falling 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 affected by the Earth’s gravity
In that sense, they still have weight, even though Earth’s gravity is less in orbit than on the Earth’s surface (Box 1)
However, they don’t feel their weight because nothing pushes them back
Essentially, the ground has disappeared from under them and both the astronauts and the spaceship fall down (Figure 1B)
Wait, so weightlessness is just free fall?
Yes Free fall is defined as “any movement of a body where gravity is the only force acting on it” In the vacuum of space, where there are no air molecules or supporting surfaces, astronauts are only affected by gravity
So they fall to earth with the acceleration of gravity
This begs the question: How can starships stay in orbit instead of falling back to Earth’s surface?
Although gravity pulls astronauts toward Earth, the spaceship travels so fast in the forward direction that it eventually spins in a circle around Earth, much like a ball swinging at the end of a string
For example, the International Space Station travels at about 17,150 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 arguably cheapest way to experience weightlessness is to take advantage of parabolic flight (aka a journey aboard the Vomit Comet)
To understand how flying in parabolic arcs creates the feeling of weightlessness, we first need to review the four basic forces acting on an aircraft (Figure 2A)
The first force is drag, which is caused by air molecules hindering the aircraft’s forward motion
The third force is weight
To create the feeling of weightlessness, the pilot equals thrust 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 hitting the ground
Then the plane experiences 20-30 seconds of free fall as it completes the climb and begins to fall back to Earth
Finally, as the aircraft returns to the same altitude it started at on the leading half of the arc, the pilot reengages the 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 flights allow passengers to experience weightlessness without actually going into space
(A) The four forces acting on an aircraft are weight, lift, thrust and drag
Because acceleration occurs toward an unbalanced force, aircraft accelerate in a forward direction when thrust exceeds drag and increase in altitude when lift exceeds weight
(B) When the pilot equates thrust to drag and eliminates lift, weight is the only unbalanced force acting on the aircraft
Accordingly, the plane falls, and the passengers feel weightless for about 20-30 seconds
To prevent the aircraft from hitting the ground, this weightless maneuver is preceded by a controlled ascent and followed by a controlled descent
This cycle of controlled ascent, weightlessness, and controlled descent creates the parabolic flight path that characterizes zero-g experiences
In general, parabolic flight is very similar to a hypothetical elevator ride
As the elevator accelerates to floor 10, the passengers feel heavier than usual (plane climbs to 30,000 feet)
As the elevator approaches floor 10 and immediately changes direction to return to floor 1, the passengers feel weightless (free-fall maneuver)
Finally, if the elevator slows down when returning to floor 1, the passengers will feel heavier than usual (plane descending to 20,000 feet)
Such a flight with the Zero G Corporation starts at $4,950 per person and includes 15 parabolic maneuvers
You just won a free second of weightlessness
Although a journey on the Vomit Comet gives the feeling of weightlessness, you don’t get the name of an astronaut
Thankfully, SpaceX, Blue Origin, and Virgin Galactic are all working to make that happen
While SpaceX is poised to become the first private company to send humans into space, its clients 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 adapted their weightless experiences to those with slightly smaller checkbooks and slightly less ambitious space travel plans
While Blue Origin’s New Shepard and Virgin Galactic’s SpaceShipTwo are very different in vehicle design, both private individuals promise the ability to travel to space
Paying customers leave the Earth’s atmosphere, see the curvature of the Earth and experience weightlessness for a few minutes before returning safely to the ground
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 Applied Physics at Harvard University’s School of Engineering and Applied Sciences
Read this article from Spacecom to learn more about the effects of weightlessness on astronauts
Visit this page to learn more about experiments conducted aboard the International Space Station, including experiments that investigated the effects of long-term weightlessness on human health
Visit this site to learn how NASA studies the effects of weightlessness on nonliving things
Visit their websites and follow them on social media to track the progress of SpaceX, Blue Origin and Virgin Galactic