Not Lost in Space

By Eric M. Jones

GOOD SCIENCE FICTION WRITERS are not necessarily good scientists; nor do they have to be. But if one wants more-or-less realistic writing, some understanding of the how the universe works could be helpful. "How the Universe Works” was once called “Natural Philosophy” but is now called “Physics,” and it needn’t be as scary as some would have you believe.

Now I don’t insist for a moment that the strict constraints of reality need to be imposed on an otherwise good story. For one thing we don’t know what future reality will look like any more than those old pulp era artists did. We have a hunch that spaceships won’t be made of wood, balloons, and have sails. But we are otherwise pretty clueless.

Let’s start with some basics:

There’s no air in space?

Well, there’s just not much when you get away from planets and stars, but gas molecules (and dust) are all over the place. Most of what one could collect in deep space would be hydrogen and helium, with traces of everything else.

There is a good explanation of the fact that there can be no absolute vacuum. All materials, even solid metal alloys, have a temperature dependent “vapor pressure” below which it will evaporate (or sublimate). At absolute zero there is no evaporation or sublimation, but for several reasons, absolute zero does not exist in deep space. So absolute vacuum doesn’t either. Interstellar space has a few atoms per cubic centimeter and a few degrees above absolute zero is the best you can get. Cosmic rays and electromagnetic emissions of all sorts stoke the cosmos.

Spacecraft and high-vacuum equipment designers are well aware of this. After reading a list of what materials can and cannot be used in vacuum, one appreciates how difficult designing spacecraft really is. And of course, if something warms up, like maybe your engine ... it can sublimate and disappear.

Some of the most common materials that outgas in a vacuum include lead, zinc, PVC, plastics, and paints. Materials that offer the best choice for use in space include stainless steel, aluminum, gold, Teflon, borosilicate glass, and porcelain.

A brief look at orbital mechanics (you probably missed that class) ...

Any relatively tiny object in free orbit around a large celestial body travels in a flat plane passing through the center of the body. This makes a polar orbit possible, and an equatorial possible and all other orbits are slanted at various angles ... But one cannot orbit, let’s say, at any other latitude or on any path that is not on a plane passing through the center of the larger body. One cannot “orbit” at the magnetic pole as was described once on “Star Trek.”

The higher an altitude a body orbits, the slower it goes. When an Earth satellite is high enough (an altitude of approximately 36,000 km), it orbits in one sidereal day, thus apparently remaining over the same spot on the planet. We call these orbits Geosynchronous or Clarke Orbits after the science fiction writer Arthur C. Clarke who popularized communication satellites in geostationary orbits in a 1945 magazine article. There are now approximately six hundred geosynchronous satellites, many no longer operational. The whole business of satellites, orbital locations, and their communication frequencies is decided by international agreements.

Occasionally you will see a writer refer to a geostationary orbit around the Moon. There is one unstable Lagrangian location (L1) where the gravitational attraction of the Moon and that of the Earth cancel, but the “lunar”-synchronous orbital position (analogous to geosynchronous orbit) is the Earth itself. So an Earth-synchronous-type orbit is not possible for the Moon.

A special note on orbits—objects that approach the Earth, Sun, or other body from a great distance can be determined to be on an elliptical, parabolic or hyperbolic path. This is an important distinction because an object on an elliptical path will return someday, an object on a hyperbolic path will never return, and an object on a parabolic path is on the cusp and is indeterminate ... it might or might not return. This fact was known as early as Newton.

There are special names for many orbits, and the fact that A orbits B means that B orbits A. Orbits can be quite complicated. For example, there is the amazing horseshoe orbit of asteroid 3753 Cruithne, or 2010 SO16. From the Earth’s point of view, Cruithne actually follows a kidney-bean-shaped horseshoe orbit ahead of the Earth, taking slightly less than one year to complete a circuit of the “bean.” [Diagram of a horseshoe orbit, above left, courtesy of NASA. Image has been enhanced to highlight the orbital path.]

Remember that there are an infinite number of paths that can be travelled by one unpowered body around another, and an infinite number that can’t exist.

There’s no gravity in space?

If we constructed a tower a hundred kilometers tall above the Earth’s surface, by definition we would be in “space.” So what would the gravitational force sitting at the top of such a tower be? It would be reasonably close to that felt on the surface of the Earth. You would not feel much difference.

How about a million kilometers tall? At that distance you would feel a difference based on the fact that you are very far from the Earth’s center of mass.

So why do astronauts float around in zero-g while in orbit? It’s because they are in orbit. Which means they “fall freely” while moving forward to miss the curve of the Earth. Being in free fall anywhere will cause the inhabitants (or loose parts) to experience zero-g. If a ship or anything else is not moving relative to some point in space, it will experience some g-force. That’s what astronauts in space stations refer to as “microgravity.” Even something floating in space is drawn to the local center of gravity. So everything in the International Space Station feels some tiny g-force, while the ISS itself does not—relative to the Earth.

Many science fiction outer space dramas refer to ships or space stations with “artificial gravity.” Of course we know about centripetal forces, but this is acceleration not artificial gravity. Can some as yet undiscovered artificial gravity ever exist? Probably not. Gravity—it’s a safe bet—is a consequence of the curvature of space-time caused by the uneven distribution of mass-energy. There doesn’t seem to be much speculation that things will ever change. Still, it’s a great savings of production money not to have everyone floating around, so the “future invention” of artificial gravity makes for easier TV and movie production.

By the Principle of Equivalence, gravity and acceleration are in a sense, indistinguishable. But there’s always a catch. A person accelerating in a space ship at 1g will simply run out of acceleration in less than a year. (Non-relativistically: 350 days × 1g=c), and feeling the ersatz gravity of centripetal force in a wheel-shaped space station or a carnival rotor requires that the wheel keep spinning, and your local universe be cylindrical.

The speed of light.

At the top of the wish list of every science fiction author is that some form of Faster Than Light travel, or “FTL” will be discovered ... Einstein be damned. There was a time in physics when the universe was viewed as Cartesian, the speed of light was viewed as infinite, and there didn’t seem to be speed limits on spaceships.

Physics, especially Einstein’s Special Theory of Relativity and quantum mechanics conspired to scramble the earlier understanding into the form we see today: As objects move faster, they gain mass and their own internal clocks—relative to the clock they left back at the origin—tick more slowly. At the speed of light, the ultimate velocity possible in the universe, time stands still. A photon traveling across the universe experiences no time. From the photon’s point of view, its speed is infinite because it goes from one end of the universe to the other, seemingly instantaneously. This appears to be a sort of FTL. (A photon travels at Warp 10 by “Star Trek: TNG” definitions.)

Unfortunately, there are very few zero-rest-mass particles: gluons (which never go anywhere) and photons, and just maybe one of the three neutrino flavors. Any particle or material with a non-zero rest mass becomes infinitely heavy at the speed of light. The photon is the carrier for the electromagnetic force, so when it stands still, it does not really exist. Despite headlines that say “Scientists Make Light Stand Still,” they really don’t, except in a very special way in very special circumstances. Light, if it stops, leaves a “virtual photon” ... or a “hole” in reality ready to be refilled when you want the photon to speed up again. Light travels slower than “c” in any optical medium, but in any vacuum anywhere, measured by any observer, photons always travel at “c.”

But FTL is a convenient plot device because the galaxy is so large. Interesting factoid: “Star Trek’s” action takes place basically in only one or two quadrants of the Milky Way. “Star Trek: Voyager” had the Voyager flung into the opposite side of the Milky Way (the Delta Quadrant), where it would take 70,000 years to return to Earth at Warp 9.8 (TNG scale). Traveling to other galaxies, with perhaps one small exception, was never used.

Which way is up?

People are used to visualizing the solar system as ringed orbits laid out horizontally on a flat table. The orientation of everything else in space is so complicated and so non-intuitive that we ignore it. Nobody (except for this writer apparently) seems to object to a cannon ball on a rubber sheet with a bocce ball traveling around it representing the Earth traveling around the Sun. This is obviously a two-dimensional representation of a three- or four-dimensional phenomenon. The simplicity of the demonstration destroys any possible extension of the idea presented.

One of the most surprising discoveries first-time astronomy buffs discover is that images may appear in any sort of strange orientation depending on the type of telescope. Your first look at Mars might show it upside-down. Telescope images may appear upside-down, rotated, or inversed from left-to-right. That’s why telescopes are classed as “terrestrial use” if the image orientation is correct. Newtonian-reflector telescopes always have inverted images.

For looking at objects in space, orientation is not important at all. For example, looking at Jupiter from the northern hemisphere puts the Great Red Spot in Jupiter’s lower Hemisphere. Looking at it from Australia puts it in the upper hemisphere [below right]. But both images have been inverted by the telescope optics. The “Man in the Moon” is also upside down in Australia relative to the northern hemisphere. In space there is no up or down.

We are used to seeing Earth as a globe with the north pole pointed up, usually tilted at about 23½° (ε=23° 26' 21.406") and the south pole pointing opposite. Astronomers call this planetary tilt the obliquity of the ecliptic (which rolls off the tongue). The ecliptic is the orbital plane of the planet around its star. This changes over time.

The tilt, spin direction, magnetic pole axis, and orientation of the plane of the celestial body is generally a result of the formation of the planetary system around its star. So almost all of it goes in the same direction unless a body is smacked by something else.

Most celestial bodies have geographical poles but some don’t. Saturn’s moon Hyperion and many asteroids have no stable geographical pole. Uranus’ geographical and magnetic poles are sixty degrees apart. The Earth’s poles flip every once in a while. A flip might happen soon. The magnetic flips seem random and complicated and leave their history in the magnetization of magma in sea floor spreading.

It seems obvious somehow that the magnetic poles and the geographic poles of celestial bodies must be more-or-less aligned, but they’re frequently not. In general, the geographic and magnetic poles of known neutron stars are not
aligned. If they were aligned they would not be pulsars and we could not see
them. Accreting black holes can apparently sweep their own accretion disks into alignment with their magnetic field. And, of course, the geographical orientation of a black hole cannot be observed. Astronomers are searching for a pulsar orbiting a black hole to confirm Einstein’s theories of how clocks behave in extreme gravitational fields.

Studies indicate that there is no correlation between galactic spin direction and orientation. Recently it was discovered that black hole jets do seem to show some alignment with their neighbors, especially in the early universe. Why? Currently, this is a puzzle. 

Further Reading

Materials for Use in a Vacuum.
Satellite Around the Moon.
LIGO Vacuum Compatible Materials List.
List of Orbits.
Horseshoe Orbit.
Artificial Gravity.

Eric M. Jones is the Associate Editor of “Perihelion.” He is an engineer, designer, consultant, and entrepreneur, working in the experimental aircraft community, NASA, space transportation companies, and the International Space Station.