Weightlessness

Are astronauts, in Earth orbit, without weight?

First, I should mention that there are two types of weight: actual weight and apparent weight. An Earth-bound object's actual weight is the downward force exerted upon it by the Earth's gravity. The object's apparent weight is the upward force, typically transmitted through the ground, that opposes gravity and prevents the object from falling through the floor or ground (towards the center of the Earth). When you stand on your bathroom scale, it's measuring your apparent weight (i.e. how hard it's having to push up on you to prevent you from accelerating downwards through the scale, crushing it). This doesn't necessarily have to be equal to your actual weight. In your bathroom, your apparent weight is equal to your actual weight, but if you carry the scale into an elevator and weigh yourself while accelerating upwards, the scale will register an apparent weight that is greater than your actual weight. Since a cable is pulling the elevator up rapidly, the elevator's floor is pushing up on the scale, and the scale is, in turn, pushing up on your feet. In order to force you upwards, against gravity, the scale is having to push on you harder than if it (and you) were stationary. Your actual weight doesn't change here, because it's dependent on your mass (which isn't changing) and your distance from the center of the Earth (which is changing only a negligible amount). But to the scale, it feels like you're growing heavier, because it's having to not only support you but push you (accelerate you) upwards. So it registers a heavier "weight", which we now know to be your apparent weight. What about when the elevator accelerates downwards? Your apparent weight becomes less than your actual weight. And now, what if the elevator's cable were to break and the elevator, scale, and you were all to freefall towards the ground below? The scale would be falling at the same rate you were falling, and so it wouldn't be supporting any of your weight. It would indicate a weight of zero. That is, your apparent weight would be zero. This is the definition of weightlessness. Weightlessness means without apparent weight; it has nothing to do with your actual weight. So ... an astronaut in orbit, in a constant state of freefall, kept to a near circular path around the Earth by gravity, is weightless, but only in the sense that he or she has no apparent weight. Astronauts most definitely do have actual weight!

Apparent weight can change fairly easily, we see. All you have to do is take a ride on an elevator, or rollercoaster, or some other device that accelerates you in a vertical direction. Does actual weight ever change (ignoring the effects of food)? Yes, it does. The force of gravity on you (which determines your actual weight) is dependent on your mass, the mass of the Earth, the gravitational constant G, and the distance between you and the center of the Earth. Assuming your mass is held constant, you can reduce your actual weight by increasing your distance from the center of the Earth. So astronauts have actual weight, but an actual weight that is slightly less than their actual weight back on Earth. How much less? In orbit around 300 km (185 miles) above the surface of the Earth, astronauts' actual weight will be about 8.8% less than back on Earth.

What makes you feel weightless when you're falling, even though you still have an actual weight? Or one could ask, what makes you feel heavy (or with weight) when you're standing on the ground? It's not gravity. It's the force of the surface you're standing on, pushing against you. If you're standing on a sidewalk, the concrete is pressing against your feet, which are in turn pressing against your ankles, which are pressing against your lower legs, which are pressing against your upper legs, and on and on. The feet are supporting your entire mass. Your chest, for example, only supports the mass of the body above the chest. You don't feel pressure evenly distributed throughout your body. (Well, you're used to the feeling of standing on a surface, and so you may have a hard time sensing this uneven pressure distribution, but it's there.) Your sense of weight also comes from your arms pulling down on your shoulders. When in freefall, this pressure gradient (or change over space) disappears. Each section of your body, each cell, is falling at the same rate. Therefore, your upper body isn't pushing on your lower body. Your ankles aren't pushing on your feet. There is no pushing at all. Neither are your arms pulling down on your shoulders. The absence of these sensations is what one equates to feeling weightless.

How does NASA simulate a weightless environment for astronaut training? They could put their astronauts in an elevator, take it to the top of a tall building, cut the supporting cable, and allow the elevator and its inhabitants to freefall for several seconds. But the impact upon hitting the ground would be extreme and most unpleasant. Instead, NASA sends its astronauts up in an airplane, and the airplane flies in the parabolic trajectories of freely falling objects. Soaring over the Gulf of Mexico, pilots level off at about 26,000 feet. They then shoot the plane upward at about a 45-degree angle. At this point, the apparent weight of the people inside the nearly empty, padded fuselage increases to about 1.8 times their actual weight. Half a minute later, pilots push the aircraft's nose over the top of this "parabola", and the plane falls some 8,000 feet or so until its pointing downward at about 30 degrees. During this freefall, the aircraft's acceleration matches Earth's acceleration of gravity, making everything inside weightless for 17 to 25 seconds. (Parts of the movie Apollo 13 were filmed on this aircraft.) Over a two-hour flight, the aircraft may fly through some 40 of these parabolas. NASA used two KC-135 Stratotanker aircraft for these sessions from 1973 until 2005, when they were retired and replaced with a McDonnell Douglas C-9. The plane, not too surprisingly, earned the nickname "Vomit Comet."