by Lisa Heppler

figures by Jovana Andrejevic

Weightlessness is something many of us have dreamed about since we were kids. We have seen footage of astronauts floating around the International Space Station playing Ping-Pong with balls of water and Pac-Man with strings of M&Ms.

For a moment, as we watch these astronauts thriving in an environment completely alien to us, we are able to imagine ourselves floating around with them. Unfortunately, the magic is short-lived. The weight of our rear ends pressed firmly into our seats brings us crashing back to planet Earth, back to reality.

So, is dreaming really as close as we’ll ever get to floating in space? 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, spationauts)? Not so fast.

Weightlessness may only be for astronauts, but with the help of private companies like SpaceX, Blue Origin, and Virgin Galactic, becoming astronauts may not be so far-fetched. Our dreams of floating in space are closer to becoming reality than ever before.

To prepare for our journey, we must first understand what the heck weightlessness actually is.

What is weight?

Our weight on Earth depends on our mass, which is 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, more commonly known as gravity, is a non-contact force that acts on us from a distance. As the name implies, a non-contact force is one that acts between two objects that are not in physical contact with one another, meaning that we need not be touching Earth for gravity to be acting upon 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 termed normal force, which in contrast to gravity, is a contact force that acts upon objects that are physically associated with one another.

For example, when we are standing on the ground, the force of 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 upwards 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 were to disappear, gravity would nonetheless be acting upon us, but we would be unable to sense it. This inability to feel gravity would make us feel weightless (at least for a moment; Box 1).

Why do astronauts feel weightless?

So what does this mean for orbiting astronauts? In space, astronauts and their spaceship 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 smaller in orbit than it is on Earth’s surface (Box 1). However, they do not feel their weight because nothing is pushing back on them. In essence, the ground has disappeared from beneath them, and both the astronauts and spaceship are falling (Figure 1B).

Wait, so weightlessness is just free fall? Yes. Free fall is defined as “any motion of a body where gravity is the only force acting upon it.” In the vacuum of space, where there are no air molecules or supportive surfaces, astronauts are only acted upon by gravity. Thus, they are falling towards Earth at the acceleration of gravity.

This begs the question: how are spaceships able to stay in orbit, rather than falling back towards Earth’s surface? Although gravity pulls astronauts towards Earth, the spaceship is traveling so quickly in the forward direction that it ends up orbiting around the earth in a circular pattern, much like a ball swinging at the end of a string. For example, the International Space Station is traveling at about 17,150 miles per hour, and this forward momentum keeps the astronauts in orbit despite being pulled towards 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 obstruct forward movement of the airplane. The second force is thrust, which is a propulsive force supplied by the engine. The third force is weight. The final force is lift, which results primarily from interactions between the airplane wings and air molecules, and depends on the density of air, the shape of the wings, and the orientation of the airplane in the air. The combination of these four forces determines the speed and direction of the airplane.

Let’s return to the concept of parabolic flight. 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. This is what creates the zero-g experience. However, airplanes can only fall so far before they hit the ground. So, prior to this maneuver, the pilot aims the plane upward and applies a burst of thrust. Then, the plane experiences 20-30 seconds of free fall as it completes the climb and starts to fall back toward Earth. Finally, once the plane returns to the same altitude it started from on the front half of the arc, the pilot re-engages lift 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).

In general, parabolic flight is very similar to a hypothetical elevator ride. Imagine that an elevator travels from floor 1 (20,000 feet) to floor 10 (30,000 feet) and back to floor 1 (20,000 feet) without a noticeable stop at floor 10. As the elevator accelerates towards floor 10, the passengers feel heavier than normal (airplane climbing to 30,000 feet). As the elevator approaches floor 10 and immediately changes direction to travel back towards floor 1, the passengers feel weightless (free fall maneuver). Finally, as the elevator decelerates upon returning to floor 1, the passengers feel heavier than normal (airplane descending to 20,000 feet).

Such a flight with the Zero G Corporation starts at $4,950 per person and includes 15 parabolic maneuvers. That comes to about $14 per second of weightlessness. So, the next time you feel your stomach drop on a Delta flight, smile and enjoy the ride! You just won a free second of weightlessness.

How to book a trip to space?

Although a trip on the Vomit Comet does provide the sensation of weightlessness, it will not give you the name of astronaut. For that, you have to go to space! Luckily, SpaceX, Blue Origin, and Virgin Galactic are all working to make that possible.

While SpaceX is poised to be the first private company to send people 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 catered their weightless experiences to those with slightly smaller checkbooks and slightly less ambitious space traveling plans. Although Blue Origin’s New Shepard and Virgin Galactic’s SpaceShipTwo are very different in vehicular design, both promise private individuals the opportunity to travel to space. Paying customers will leave Earth’s atmosphere, see the curvature of the Earth, and experience a few minutes of weightlessness before returning safely to the ground. Although pricing information and launch dates have yet to be released, several news outlets have reported that tickets will cost $200,000 to $300,000 a piece, and trips will begin as soon as 2019.

Thus, the countdown to becoming something-nauts has officially begun!

Lisa Heppler is a fifth-year PhD candidate in the Biological and Biomedical Sciences Program at Harvard. She studies the role of STAT transcription factors in cancer.

Jovana Andrejevic is a third-year Applied Physics PhD student in the School of Engineering and Applied Sciences at Harvard University.

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