What is high gravity in space?

Gravity is the pull of a mass; g-load is the acceleration a vehicle imposes on the people inside it, expressed in multiples of the 9.80 m/s² we feel at sea level. The two produce the same sensation, weight, which is why the words get mixed up. A crew strapped in during ascent, or decelerating in the upper atmosphere on the way home, feels several times its normal weight for minutes at a time. That is high g-load, and it comes from the engines and from the air, not from any gravitational field out there. Space itself offers no such thing: between the planets, gravity is weak rather than strong.

High gravity needs mass, and the Solar System keeps most of it in the giant planets. At Jupiter's cloud tops the pull is 24.79 m/s², two and a half times Earth's 9.80 m/s². Further out the numbers fall back: Saturn, Uranus and Neptune all sit close to the terrestrial value despite their bulk, because their radii are so large and gravity weakens with the square of the distance from the centre. Genuinely extreme gravity belongs to collapsed objects, where a stellar mass is folded into a sphere a few kilometres across and nothing material survives at the surface.

The only way to experience sustained high gravity is to manufacture it. Large centrifuges spin samples, cell cultures and occasionally people at several times Earth gravity, to study how organisms respond when weight increases instead of disappearing; pilots train on the same principle. Those machines also hint at the eventual answer to weightlessness in space, since a rotating habitat would press its occupants outwards against a floor. Nothing flown so far does it, so crews still swing between minutes of high g-load and months of almost none.