We see images and videos from the International Space Station (ISS) where astronauts floating in the space freely. That’s because they’re in the space, so there is no gravitational force of Earth there, right?

Wrong.

The International Space Station is in Low Earth Orbit see notes 1 with an altitude of between 330 and 435 km (205 and 270 mi). It is so close to the Earth that on a clear day easily visible to the naked eye from the ground as it is the third brightest object in the sky (NASA has actually launched a new interactive map at its Spot the Station web site).

At that altitude, the Earth’s gravity is about 90 percent of what it is on the planet’s surface – still pretty strong, right? To reduce the gravity of the Earth by a factor of one million, one needs to be at a distance of 6 million kilometers (around 3,728,227 miles) from the Earth – more than fifteen times the distance between the Earth and 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 of a spacecraft and reach the conclusion that there is no gravity.

Anywhere mass and space exist, gravity exists. Gravity is the curvature of spacetime due to the presence of mass. The gravitational influence from the Sun that keeps the Earth in orbit around the Sun is felt equally by the astronauts in space. The gravitational influence from the Earth that keeps the Moon and the ISS in Earth orbit is also felt by the astronauts floating inside and outside the spacecraft. If these influences were not felt, the astronauts would not stay in orbit.

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?

The answer is: astronauts float in space because they are free-falling see notes 2 towards the Earth (see the “orbital speed” title below). They experiencing the same thing experienced by a roller coaster rider on an amusement park ride. In fact, every object orbiting another object in the space is actually free-falling towards the object which they are orbiting.

And, keep in mind that the ISS is in a vacuum – where gravity causes all objects to fall at the same rate regardless of the mass of the object. If a person drops a bowling ball and a feather on the Earth’s surface, air will make the feather fall more slowly. But if there were no air, they would fall at the same acceleration.

So, the ISS (or any spacecraft that orbiting Earth or any other celestial body), its crew, and any objects aboard are all falling toward but around Earth. 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. In other words, 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.

Here is a beautiful video explaining what is microgravity and how does it help science research. Narrated by David Mitchell, the fifth “60 Second Adventures” series explores how and why we recreate microgravity conditions on Earth using “drop towers” and “parabolic flights” as well as asking people to spend a lot of time in bed; that way we can understand processes from bacterial resistance to human aging or even planet formation. Funded by the UK Space Agency.

## Orbital Speed

The International Space Station has an orbital speed of 7.67 km/s (27,600 km/h; 17,200 mph). Its orbital period is 92.65 minutes. In other words, the ISS orbits the Earth 15.54 times per day! Why is it moving so fast?

A beautiful explanation can be found here. It is really hard to put an object into Earth’s orbit – but it’s not because space is high up. It’s hard because you have to go so fast. Getting to space is relatively easy. The problem is staying there.

As explained above, the gravity in low Earth orbit is almost as strong as gravity on the surface. To avoid falling back into the atmosphere, you have to go sideways really, really fast.

The speed you need to stay in low Earth orbit is about 8 kilometers per second. Only a fraction of a rocket’s energy is used to lift up out of the atmosphere; the vast majority of it is used to gain orbital (sideways) speed. So, reaching orbital speed takes much more fuel than reaching orbital height.

## Notes

1. Low Earth orbit (LEO) is an orbit around Earth with an altitude between 160 kilometers (99 mi) (orbital period of about 88 minutes), and 2,000 kilometers (1,200 mi) (orbital period of about 127 minutes).
2. Freefall is any motion of a body where gravity is the only force acting upon it. An object moving upwards would not normally be considered to be falling, but if it is subject to the force of gravity only, it is said to be in free fall. The International Space Station is thus in free fall.

## Sources

1. Low Earth orbit on Wikipedia
2. What Is Microgravity? on NASA.gov
3. Free fall on Wikipedia
4. Micro-g environment on Wikipedia
5. Orbital Speed on what-if.xkcd.com