Taylor Dinerman has a lengthy piece in the Journal that’s a worthwhile read. There is one nit to pick, though (very few people get this right):
Being in microgravity (since there is always some gravitational effect, this is the correct term) for any length of time changes one’s metabolism. The human heart, for example, becomes like a ball rather than the “heart”-shaped organ it is on Earth. Blood flows closer to the outer layers of the skin, giving astronauts a characteristic puffy face. It is estimated that a six-month stay on the International Space Station (ISS) causes an average 11% loss of bone density. NASA is working hard to find a way to keep its personnel healthy during long-duration space operations, either on the moon base planned for sometime in the 2020s or on a later trip to Mars.
No, “microgravity” is not the correct term. Microgravity means literally a millionth of a gee of acceleration. Using it in any other way is very confusing.
A quick tutorial.
First, there is no such thing as zero gravity, anywhere in the universe. Gravity, which is the force that one mass exerts on another, as a function of the product of the two masses and the inverse of the square of the distance between them, is ubiquitous, because the universe is filled with masses of various sizes and shapes. And in a so-called “Zero-G” flight, the gravity level is in fact almost exactly the same as it is on the ground, since the aircraft isn’t flying all that high, relative to the distance from the center of the earth. Even in low earth orbit, gravity is still about 90% of what it is on the surface. We have to be very careful with the word gravity. In this context, we are using it as a unit measure of an acceleration, not the amount of pull that is exerted on us by the earth (or other objects).
What happens in both parabolic flight (which is technically a portion of an ellipse, rather than a parabola, but if one makes a flat-earth assumption for the gravity model, a parabola is close enough) and in orbit is that the craft is in weightlessness, or free fall, which are the proper terms.
When one is falling (and in a circular orbit, one is continually falling, with the rate of fall toward the earth the same as the rate of the horizon dropping in front of you, so you never get any closer), one doesn’t sense gravity. Gravity is only sensed when one resists it, by standing on the ground, or sitting in a chair, or having air drag slow you down when you sky dive. Also, there is only a single point of your body that is in true free fall; most of it is experiencing various (tiny) levels of gravitational force. Because the trajectory is a line in space, only the portion of an object through which that line passes is in true weightlessness. But for practical (in this case, visceral entertainment) purposes, your whole body will seem to be floating. And the easiest way to describe it, if not the accurate one, is “zero gravity.” Hence the company’s name.
One other note, which is a pet peeve of mine. In Hawking’s note to Taylor, he uses the phrase “risk-adverse” to describe NASA. The correct phrase is “risk averse.” That is, one has an aversion to risk. Adverse has a different meaning entirely, but many people get this wrong.