Alan Stahler: Understanding spacecrafts |

Alan Stahler: Understanding spacecrafts

Alan Stahler

Launch a spacecraft into orbit and it becomes part of the "clockwork of the universe" — you can calculate where it will be a day, a week, a month from now.

But such precise calculations were not possible as China's first space station, Tiangong-1, spiraled downward over the past few weeks, finally splashing down in the Pacific Sunday evening.

Satellites pass over our heads all the time. Unlike aircraft, satellites carry no red or green lights, nor flashing white lights. We can only see satellites because they reflect sunlight. Most satellites reflect a steady light, because they maintain a constant orientation toward Earth — they're "station-keeping," so as to keep their cameras trained, say, on the weather below.

Occasionally, though, a satellite will dim and brighten, brighten and dim. That satellite is no longer station-keeping — it's tumbling, out of control, reflecting sunlight off one face, then another.

Back in 2016, Chinese ground control lost communication with the uninhabited Tiangong-1 … and thus lost control of the spacecraft. They could no longer command it to fire its jets, slow itself down, fall into the atmosphere and splash down in a selected part of the world ocean.

Our atmosphere is an ocean of air. It keeps us alive: We breathe in, breathe out.

Recommended Stories For You

The atmosphere, too, breathes in, breathes out — slowly and unpredictably, which made Tiangong-1's fate unpredictable.

The particles — molecules — in a glass of water are always moving, slamming into each other, pushing each other away.

Warm the water and the particles move faster and crash harder, and thus push each other away harder, causing warmed water to expand. (Were the water in a narrow tube, rather than a glass, we could use it as a thermometer.)

Molecules in a gas are also in constant motion. Warm the air, and, like water, it expands (the earliest thermometers used air, rather than water).

Light & heat

Raw sunlight, coming in from space, contains wavelengths — colors of radiation — beyond the visible light we see with our eyes. Raw sunlight contains extreme ultraviolet — just down the energy spectrum from x-rays. And raw sunlight contains x-rays, too.

Extreme UV and x-rays carry so much energy, they can damage biology. Fortunately, both are absorbed by atoms of air, high over our heads. Absorbing this energy heats the upper atmosphere, big time: Surrounding our planet, hundreds of miles up, is a layer of very thin, very hot, air.

Sometimes the sun gets frisky, and lets loose a magnetic explosion on its surface. The explosion shoots a burst of x-rays and extreme ultraviolet into space. Reaching Earth, the radiation heats the upper atmosphere more than usual, making it swell outward.

If a spacecraft collides with anything — even molecules of air — the collisions will slow the craft. As the upper atmosphere swells, spacecraft in low earth orbit hit more and more air molecules, which brake the craft.

When an orbiting spacecraft slows, it drops … strikes more air … slows yet more … and falls out of orbit.

We cannot predict just when the sun will suffer a magnetic explosion, and thus cannot predict when the sun will inflate the upper atmosphere. Even if we witness such an explosion, we cannot very well predict how much (or how little) the atmosphere will inflate, nor how long it will take to deflate again. Thus, no one knew precisely when and where Tiangong-1 would come down … until it did.

With rain forecast for this weekend, local astronomers will wait 'til next — 8 p.m. Saturday, April 14 — to set up scopes and welcome the public to view Venus (the "evening star,") and the very young stars in Orion.

Alan Stahler enjoys sharing his love of nature with students of all ages. His science stories can be heard on KVMR-FM (89.5 MHz), and he may be reached at