KininarunoteA notebook of curious things

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Everything here is gathered from things other people actually researched, thought through, and built: books, papers, historical records, and the people behind them. If something here catches your interest, go look at the original sources listed at the bottom of the page. They carry more depth than a page like this ever can.

Why Do Astronauts Float If Gravity Is Still Pulling on Them?

Category: Space & Astronomy

Here is a number that spoils the most popular answer. Up where the International Space Station flies, the Earth's gravity is still about 90 percent as strong as it is where you are sitting right now [1]. Gravity has not let go of anyone up there. What disappears is the feeling of weight.

So why do the astronauts drift around the cabin as if nothing were holding them? The answer is a little odd: they are falling, all day long, and so is everything around them. This note follows that idea from a thrown baseball to a space station, and ends with a bottle you can drop at home.

Gravity Is Still Strong Where the Space Station Flies

NASA says the station orbits at an altitude between 200 and 250 miles (roughly 320 to 400 kilometers), and that at that height Earth's gravity is about 90 percent of what it is at the surface [1]. That sounds high, but the Earth is very large, so climbing a couple of hundred miles only weakens its pull a little.

That rules out the simplest explanation, “there is no gravity in space.” If gravity were gone, the station would not circle the Earth at all. It would fly off in a straight line. Gravity is exactly what keeps it in orbit [3].

Floating Means the Astronauts and the Station Are Falling Together

NASA’s explanation is short: the spacecraft, its crew, and everything aboard are all falling toward Earth, and they are falling together, so the crew and the objects appear to float [1]. NASA’s Glenn Research Center puts it as a rule: the condition called microgravity comes about whenever an object is in free fall [3].

Think about what that means. If you and the floor beneath you drop at exactly the same rate, the floor never rushes up to meet your feet, and you never press on it. Let go of a pen in that cabin and it stays beside your hand, because the pen and the cabin fall at the same rate. This is why the feeling of weight, which is really the floor pushing back on you, disappears. The pull of gravity has not.

The Station Falls Around the Earth Instead of Into It

If everything is falling, why does the station not crash? Because it is also moving sideways, very fast. NASA gives the speed as about five miles per second (around 17,500 miles per hour, or roughly 28,000 kilometers per hour) [1][2].

NASA’s explanation starts with a thrown baseball. Throw it, and gravity curves its path down so it lands fairly quickly [1]. A famous way to picture what happens next is a thought experiment credited to Isaac Newton, the English scientist of the 1600s: put a cannon on a very tall mountain and fire it level with the ground. A slow ball lands nearby. Pack in more gunpowder and it lands farther away. Pack in enough, and the ball falls toward the ground just as before, but the ground curves away beneath it, so it never lands. In NASA’s words, it falls completely around Earth, and that is what being in orbit means [4].

The station is like that cannonball with no ground to hit. According to NASA it circles the Earth about every 90 minutes, which adds up to 16 orbits, with 16 sunrises and 16 sunsets, in 24 hours [2].

Why Scientists Say “Microgravity” Instead of “Zero Gravity”

You will often hear “zero gravity” or “zero-g.” NASA’s Glenn Research Center says that objects in orbit appear to float in a state called zero gravity, or, more accurately, microgravity, which it gives as about one millionth of the gravity we feel at the surface (1 × 10-6 g). The reason is that gravity is not absent. It is present, and it is what holds the spacecraft in orbit [3]. NASA’s space-flight textbook adds that words like “weightless” and “microgravity” have to be taken with a grain of salt, because gravity is still dominant [4].

So the cabin is not a place with no gravity. It is a place where falling hides gravity’s usual effects.

You Can Feel a Few Seconds of Free Fall on Earth

Free fall does not need a rocket. NASA lists a roller coaster going over a big hill and amusement-park drop rides as everyday ways to feel it [5], and a car going over rolling hills or jumping on a trampoline gives short moments of weightlessness too [3].

Scientists make longer stretches on purpose. Airplanes flying a curved, hill-shaped path give people and equipment about 20 to 30 seconds of free fall at the top of each arc [5]. NASA Glenn has a drop shaft about 500 feet deep where test packages fall for just over 5 seconds in a vacuum [3]. The station is the same idea with one big difference: it never reaches the bottom.

Try It at Home: Does Water Keep Pouring From a Falling Bottle?

Here is a small test you can do with ordinary things. This is a classic classroom demonstration, not something I found in the sources above, so treat it as an experiment where the result is yours to report.

  1. Take an empty plastic drink bottle. Ask an adult to poke two or three small holes in the side near the bottom with a nail or a push pin.
  2. Go outside, or stand over a bathtub or a large basin. Put a finger over each hole and fill the bottle with water.
  3. Take your fingers away and watch the streams. Then cover the holes again, hold the bottle at about waist height over the basin, uncover the holes and let go at the same moment, and watch the streams during the fall. A phone video played back slowly can help.

If free fall matters the way the space-station explanation says it does, the water inside the bottle and the bottle itself are falling together, so the water may stop pushing out of the holes while the bottle drops. What did you see? Try again from a different height, and decide whether a bigger drop changes what you see. Keep the drop low, away from people, and wipe up any spills afterward.

If you want to read more, the NASA pages in the sources below are written for school-age readers, and you can compare how they explain the same idea for different ages.

Sources

I read the full text of each NASA page listed here and checked the numbers against each other. The station’s speed and orbit time come from the NASA pages; the 90 percent figure and the 200 to 250 mile altitude come from the NASA grades 5–8 page. Speeds given in miles per hour are converted to kilometers for convenience, and different NASA pages round the figures differently. The Newton’s cannon passage is NASA’s retelling of the thought experiment; I did not check Newton’s own writing. The bottle experiment is not from these sources.

  1. NASA, “What Is Microgravity? (Grades 5-8).” https://www.nasa.gov/learning-resources/for-kids-and-students/what-is-microgravity-grades-5-8/ (altitude, 90 percent gravity, falling together, 17,500 mph, baseball)
  2. NASA, “Space Station Facts and Figures.” https://www.nasa.gov/international-space-station/space-station-facts-and-figures/ (five miles per second, about 90 minutes per orbit, 16 orbits and sunrises per day)
  3. NASA Glenn Research Center, “What Is Microgravity?” https://www.nasa.gov/centers-and-facilities/glenn/what-is-microgravity/ (free fall, microgravity wording and 1 × 10-6 g, drop shaft, everyday examples)
  4. NASA Science, “Basics of Space Flight, Chapter 3: Gravity & Mechanics.” https://science.nasa.gov/learn/basics-of-space-flight/chapter3-4/ (cannon on a mountain, falling completely around Earth, “grain of salt”)
  5. NASA, “What Is Microgravity? (Grades K-4).” https://www.nasa.gov/learning-resources/for-kids-and-students/what-is-microgravity-grades-k-4/ (roller coaster, drop rides, 20 to 30 seconds of free fall in an airplane)

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