We see images and videos from the International Space Station (ISS) where astronauts float in space freely. That’s because they’re in space, so there is no gravitational force of Earth there, right?
Wrong.
The International Space Station (ISS) orbits in Low Earth Orbit at an altitude between 330 and 435 km (205 and 270 miles). Due to its proximity to Earth, it is easily visible to the naked eye on a clear day, being the third brightest object in the sky. NASA provides an interactive map on its Spot the Station website to help you locate it.
At this altitude, Earth’s gravity is still about 90% as strong as it is on the surface. To reduce Earth’s gravitational pull by a factor of one million, you would need to be 6 million kilometers (about 3,728,227 miles) away from Earth, which is more than fifteen times the distance to the Moon.
Robert Frost, Instructor and Flight Controller at NASA, says:
“People do not have a good understanding of what weightlessness is. They see astronauts floating around inside and outside a spacecraft and conclude that there is no gravity.”
Gravity exists wherever there is mass and space. It is the curvature of spacetime caused by the presence of mass. The gravitational pull from the Sun that keeps the Earth in orbit also affects astronauts in space. Similarly, the gravitational pull from the Earth, which keeps the Moon and the ISS in orbit, is felt by astronauts both inside and outside the spacecraft. Without these gravitational forces, the astronauts would not stay in orbit.
Frost explains:
“At the altitude the astronauts in the ISS inhabit, the gravitational influence from the Earth is 8.75 m/s². That is only about 11% less than the 9.81 m/s² felt by you and me on the Earth’s surface.”
So, why do astronauts float in space? Here’s the explanation
Astronauts float in space because they are in a state of free fall, creating the sensation of weightlessness. The International Space Station (ISS) and other spacecraft orbit the Earth at high speeds, essentially falling towards the Earth while moving forward fast enough to keep missing it.
Freefall is any motion of a body where gravity is the only force acting upon it. The International Space Station, or any object in orbit, is thus in free fall.
This continuous free-fall state creates a microgravity environment, where the effects of gravity are still present but not noticeable. In this condition, everything inside the spacecraft, including the astronauts, falls at the same rate, eliminating any relative force between them and causing the sensation of floating.
In other words, since they are all falling together, the crew and objects appear to float when compared with the spacecraft. This is called microgravity, the condition in which people or objects appear to be weightless. So that’s why astronauts and other objects float in space.
A zero-g flight, also known as a parabolic flight, uses the same freefall concept to achieve weightlessness. During the flight, the airplane follows a series of parabolic arcs. At the peak of each arc, the plane and everyone inside it enter a brief state of freefall, creating a microgravity environment. This causes passengers to experience weightlessness, similar to the conditions astronauts feel in space. The sensation lasts for about 20-30 seconds per arc, allowing passengers to float and move freely as if they were in space. This method is used for astronaut training and scientific research.
The orbit itself is a balance between the forward motion of the spacecraft and the pull of Earth’s gravity, resulting in a constant state of free fall around the Earth. This state of free fall and microgravity allows astronauts to experience weightlessness, making them appear to float inside the spacecraft.
The microgravity or Micro-g environment is more or less a synonym of weightlessness and zero-g but indicates that g-forces are not quite zero, just very small. The symbol for microgravity is µg.
Eternal Free Fall: The Role of Orbital Velocity in Why Astronauts Experience Weightlessness in Space
Getting to space is relatively straightforward, but staying there requires achieving and maintaining a specific speed and trajectory, which is much more challenging. Gravity in low Earth orbit (LEO) is nearly as strong as gravity on Earth’s surface, with the International Space Station (ISS) experiencing about 90% of the gravitational pull we feel on the ground.
To stay in orbit, a spacecraft must move sideways at a very high speed to counteract the pull of gravity. This is known as orbital speed. For the ISS, this speed is about 8 kilometers per second (approximately 17,500 miles per hour). The concept here is that while the spacecraft is constantly falling towards Earth due to gravity, its high sideways velocity ensures that it keeps missing the Earth, effectively putting it in a continuous state of free fall around the planet.
Achieving this orbital speed requires a significant amount of energy. Most of a rocket’s fuel is expended not just in lifting the spacecraft out of the atmosphere but primarily in accelerating it to the required sideways speed. This is why launching into orbit involves such large booster rockets and why the journey to space requires such careful planning and immense energy.
In practical terms, reaching orbital speed is much harder than just reaching orbital height. It takes a tremendous amount of fuel to accelerate a spacecraft to 8 kilometers per second. This requirement makes it impractical to carry enough fuel to slow down upon re-entry into the Earth’s atmosphere. Instead, spacecraft rely on aerodynamic braking, using heat shields to dissipate the enormous kinetic energy built up during re-entry by converting it into heat, which the atmosphere can absorb.
To give a sense of how fast 8 kilometers per second is, consider this: when the ISS passes overhead, it circles the entire Earth in about 90 minutes. This means that it travels around the planet roughly 16 times a day. Despite appearing to drift slowly across the sky, the ISS is moving at blistering speeds.
This high-speed, sideways motion is the essence of why an orbit is an “eternal free fall.” The spacecraft is continuously falling towards Earth, but because it is moving so quickly sideways, it keeps missing the Earth, thus remaining in orbit. This balance of gravitational pull and orbital speed keeps the ISS and other satellites perpetually falling around the planet rather than crashing back to the surface.
Here’s How Curved Spacetime (Einstein’s General Relativity) Explains Floating in Space
A cornerstone of Einstein’s General Relativity is the equivalence principle, which states that local observations in a freely falling reference frame are indistinguishable from those in an inertial frame in the absence of gravity. This principle leads to the understanding that free-falling objects follow geodesics in curved spacetime.
A geodesic is the shortest path between two points in a curved space or spacetime, analogous to a straight line in flat space. In the context of General Relativity, a geodesic represents the trajectory that a free-falling object follows under the influence of gravity, without any other forces acting on it.
This path is determined by the curvature of spacetime, which is influenced by the presence of mass and energy. Essentially, geodesics describe how objects move naturally in a curved spacetime, and their paths are dictated by the geometric properties of that spacetime.
Space Oddity by Commander Chris Hadfield
Chris Hadfield, a former commander of the International Space Station (ISS), created a unique cover of David Bowie’s iconic song “Space Oddity” while aboard the ISS. Released in 2013, this rendition is remarkable for being the first music video filmed in space. Hadfield’s performance, recorded in a microgravity environment, showcases him floating effortlessly as he sings and plays the guitar.
The video features stunning visuals of Earth from the ISS, adding a breathtaking backdrop to the music. Hadfield adapted the lyrics to reflect his experiences in space, making it a heartfelt tribute to both Bowie and the wonders of space exploration.
The video quickly became a viral sensation, captivating audiences worldwide and highlighting the human aspect of living and working in space. Through this creative project, Hadfield demonstrated the unique intersection of art and science, bringing the beauty of space closer to people on Earth.
Here’s the amazing clip of Commander Chris Hadfield’s cover of David Bovie’s Space Oddity.
