Weightlessness: why space stations fall without crashing

Gravity is present in orbit too

At the typical altitude of a space station in low Earth orbit, a large proportion of the gravity we experience at Earth's surface is still acting. The fact that people float there therefore cannot be due to Earth's gravitational pull having disappeared. What matters is the absence of a sustained supporting force. On the ground, the surface beneath us prevents our free fall and pushes against our feet. We perceive this opposing push as weight. In a space station, by contrast, the crew, walls and loose objects all fall together. There is therefore no floor that has to support people continuously. Their mass remains unchanged: even when floating, a heavy piece of equipment cannot be accelerated or slowed down with arbitrarily little effort. Weightlessness therefore does not mean masslessness; rather, it describes, in the first instance, a particular mechanical situation in the reference frame of the falling laboratory.

An orbit is a continuous fall past Earth

If an object were released with no sideways velocity, it would fall towards Earth. Give it enough horizontal velocity, and its falling trajectory curves in such a way that Earth's surface effectively slopes away beneath it. It is then in orbit. Gravity continuously provides the acceleration that changes its direction of motion. A satellite therefore does not, in principle, need an engine running all the time to stay up. In low orbits, however, the thin residual atmosphere slows it down, so occasional orbit-raising manoeuvres may be necessary. Nor is weightlessness perfect: atmospheric drag, vibrations, rotation and small differences in the gravitational field produce residual accelerations. For scientific experiments, the more precise term microgravity is therefore often used. This expression describes the actual conditions better than the idea of an entirely force-free space.

What research without a supporting floor reveals

Weightlessness can also be created for limited periods. Drop towers use seconds of free fall, parabolic flights use corresponding sections of a flight path, and sounding rockets use several minutes of their ballistic flight. DLR's MAPHEUS programme uses such rockets for materials science experiments, among other purposes. Without the usual effects of buoyancy and sedimentation, researchers can investigate, for example, how constituents mix or how materials solidify. This does not mean that all disruptive influences disappear. Temperatures, electromagnetic forces and surface tension remain active and may even become more prominent. This is precisely why comparative measurements from ground-based experiments are important. The benefit lies in selectively reducing one dominant influence and seeing other processes more clearly. Here, spaceflight becomes a laboratory method, not merely transport to an unusual location high above Earth.

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