Weightlessness

One of the most famous sights in space exploration is an astronaut floating inside the International Space Station, gently drifting between handholds with no apparent weight at all. This state is called weightlessness, or microgravity. It seems magical, but the explanation is surprising: astronauts in orbit are not weightless because there is no gravity. They are weightless because they are in free fall: constantly falling around the Earth.

  • What it isNo felt weightEven though gravity still acts
  • Real causeFree fallFalling alongside everything else
  • Gravity in ISS orbit90% of surfacePlenty of gravity, just falling
  • ISS altitude400 km upLow Earth orbit
  • ISS speed28,000 km/h17,500 mph
  • Time per orbit90 minutes16 sunrises a day

Free fall, not zero gravity

If you are in a lift and the cable suddenly snaps, you would briefly feel weightless as the lift and you fall together. Gravity is still pulling you. You just cannot feel your weight because there is nothing solid pressing back on you. The lift floor falls at the same rate you do, so your feet do not push on it.

That is exactly what is happening to astronauts in the International Space Station. Both the station and the astronauts are falling towards Earth. They are also moving sideways so fast (28,000 km/h) that as they fall, the curved surface of Earth keeps falling away beneath them at the same rate. They keep falling without ever hitting the ground. This is called orbit.

The astronauts have weight in the strict scientific sense (gravity is pulling on them), but they are weightless in the sense that they cannot feel that weight. There is nothing pressing back on them.

Fact At the altitude of the International Space Station, Earths gravity is still about 90 per cent as strong as on the surface. If you could somehow stand still on a giant tower 400 km tall, you would feel almost as heavy as on the ground. The astronauts feel weightless only because they are falling sideways fast enough to keep missing the ground.

What weightlessness feels like

Astronauts describe weightlessness as both wonderful and disorienting:

  • You can push gently off one wall and float to the other.
  • Tools, water and food all float if not strapped down.
  • Your body fluids redistribute. Faces puff up; legs become thinner ("chicken legs").
  • There is no "up" or "down". You can sleep against any surface.
  • Some new astronauts feel space sickness for a few days, similar to severe motion sickness.
  • Drinking is done from a closed bag with a straw; food is often sticky or in pouches.

How astronauts get weightless

Astronauts experience weightlessness whenever they are in free fall. This happens:

  • In orbit around Earth (ISS, China Space Station, brief missions).
  • On the way to the Moon, Mars or further (between Earths gravity and the destinations gravity).
  • For about 25 seconds at a time on parabolic flights, where a special aircraft dives downwards along a precise arc, putting everything inside into free fall. NASA uses these for training and is nicknamed the "Vomit Comet".
  • Briefly inside any falling object on Earth: a roller coaster drop, a free-fall ride at a theme park, or even when you jump off a diving board (although the time is very short).

Effects on the body

Weightlessness changes the body in interesting and sometimes worrying ways:

  • Bones lose density: without weight to push against, bones lose about 1-2 per cent of their density per month, similar to osteoporosis.
  • Muscles get smaller: especially leg and back muscles that normally hold you up against gravity.
  • Heart shrinks slightly: it does not have to pump as hard against gravity, so it weakens.
  • Vision can blur: shifting fluid can press on the eyes.
  • Astronauts grow taller: without gravity squashing their spine, they gain 3-5 cm in height. They shrink back when they return.
  • Radiation exposure increases: without Earths atmosphere protecting them, astronauts get more cosmic radiation.

To counter these effects, astronauts spend 2 hours every day exercising, using treadmills, exercise bikes and resistance equipment. Even so, returning astronauts often need months of physical therapy to readjust to gravity on Earth.

Did you know? Drinking water in space is tricky. You cannot pour it (it just floats away in a globe), and you cannot drink from a cup (the water will not flow out). Astronauts drink from sealed bags through long straws, sucking the water out. Spilled water has to be chased around with a vacuum or sucked into a towel, or it will float into the equipment and cause electrical problems.

Weightlessness in research

Microgravity is useful for science because it removes the effects of gravity on experiments. On Earth, gravity causes things to sink, mix or settle. In space, fluids form perfect spheres, crystals grow more evenly, and cells behave differently. Research on the ISS has led to better medicines, stronger metals and a better understanding of how the human body works. Some industrial materials may be best produced in space, where gravity does not get in the way.

Try this The next time you are on a swing, push off hard and time the moment at the very top of the arc when you feel briefly "light". For a fraction of a second, you and the swing are at the peak of free fall and you feel almost weightless. The same feeling, much exaggerated, is what astronauts experience all the time. Roller coaster drops are designed to extend this feeling for several seconds at a time.
Deeper dive: how do astronauts go to the toilet in zero gravity?

Going to the toilet in microgravity is one of the trickier engineering challenges of space travel. On Earth, gravity does most of the work, pulling waste downwards. In space, with no gravity, special equipment is needed.

The toilet on the International Space Station uses gentle suction instead of gravity. The user secures themselves to the seat with foot restraints and thigh bars. A small fan creates an airflow that pulls liquid and solid waste away from the body. The waste is collected in containers; liquids are recycled into fresh drinking water (because every drop of water has to be precious in space). Solid waste is dried out, stored, then ejected from the station to burn up in the atmosphere.

The whole process takes more time than on Earth. New astronauts practise on Earth-based mockups before they go up. Even so, mistakes happen. NASA archives include several stories of escaped waste floating around the cabin.

For the upcoming Moon and Mars missions, NASA and SpaceX are designing new toilets that work for both men and women, that can recycle even more water, and that take up less precious space. The competition for the best space toilet design has even been the subject of NASA-sponsored prizes. Behind every weightless astronaut photo is a lot of clever plumbing.

For more, see what is gravity and gravity in space.