Orbital mechanics · CSF physiology

Falling Around the Earth

By Dr Ganesalingam Narenthiran BSc(MedSci) MB ChB MRCS(Ed.) FEBNS FRCS(SN)

Astronauts on the International Space Station float, yet gravity at their height is still almost as strong as on the ground. This is apparent weightlessness: the station and everyone in it are falling together. Three working models show how a rocket turns its climb into sideways speed, why that speed keeps the station from hitting the ground, and what constant free fall does to the fluid around the brain.

1

From launch pad to orbit

The rocket flies straight up, then gradually tilts over. Watch the speed arrow: at lift-off it points up; by engine cut-off almost all of it points sideways, parallel to the ground, at about 7.7 km/s. That sideways speed is what the module carries into orbit.

T+ 00:00
Velocity (speed and direction) Sideways part Upward part Pull of gravity
2

Why sideways speed keeps it up

The module starts 400 km up, moving parallel to the ground. Gravity pulls it toward the centre of the Earth the whole time; only its sideways speed changes. Too slow and it falls into the Earth. At 7.67 km/s it falls around the Earth in a circle. Faster and it swings out into a higher, stretched orbit. Fast enough and it escapes. The Earth and the 400 km height are drawn to scale.

7.67 km/s
Now 7.7 km sideways if there were no gravity falls ≈ 4.3 m surface curves away ≈ 4.3 m 400 km 400 km 1 second later Not to scale: the 4.3 m drops are hugely exaggerated

One second of orbit

At 400 km, gravity is 8.7 m/s², about 89% of its pull at the ground. In one second the station falls about 4.3 m toward the Earth.

In that same second it travels 7.7 km sideways. Over 7.7 km the curved surface of the Earth drops away by the same 4.3 m. So the station keeps falling but never gets closer to the ground.

Think of throwing a ball. The harder you throw it, the further away it lands. Throw it hard enough and the ground curves away before the ball can reach it.

3

What happens to CSF in the head

On Earth, gravity pulls cerebrospinal fluid (CSF) and blood toward the feet when we stand, like water in a tall bottle that presses hardest at the bottom. In free fall that top-to-bottom difference disappears in every posture. Step through from standing on Earth to months in orbit.

Whole body: posture, felt weight, fluid
Head and eye, magnified
CSF pressure along the neuraxis
Pressure in the head over 24 hours

Standing on Earth

  • Ventricles enlarge
  • Optic nerve sheath distends
  • Back of globe flattens
  • Optic disc swells
  • Choroidal folds
  • Hyperopic shift
Pressure in head
4cmH₂O
Lumbosacral
45cmH₂O
Top-to-bottom
41cmH₂O
Headward shift
0%
Weight felt
1 g, head → feet

Pressures are illustrative, chosen to match the description: lying flat ≈ 14–18 cmH₂O, near-uniform along the neuraxis in orbit, low in the head and high at the lumbosacral spine when upright. Eye and ventricle changes are exaggerated so they can be seen. The leading theory is that spaceflight-associated neuro-ocular syndrome (SANS) comes mainly from this headward fluid shift; the exact mechanism is still debated. Weightlessness, cosmic radiation and mildly raised CO₂ on the station also cause other neurological and systemic problems.