Physics, Earth & Space Icon Physics, Earth & Space

Is Space Travel Our Destiny?

A few days ago I published the paper “The Solar System: Favored for Space Travel” in the journal BIO-Complexity. I thought it would be helpful for me to give a short summary of the paper to Evolution News readers.

I was motivated to do the study after two papers were published in 2018 on the difficulty of launching rockets from super-earths. Super-earths are the most common type of planet that are being discovered around exoplanets. They are somewhat loosely defined as being larger and more massive than Earth but smaller and less massive than Uranus or Neptune. From observations, super-earths seem to transition from rocky to gas-dominated composition above 1.5 times the size of Earth. 

Two Studies

Together, the two studies not only showed that it is more difficult to launch rockets from super-earths, but it is also more difficult to launch interstellar missions from stars less massive than ours. One of the authors, astronomer Mike Hippke, wrote:

I am surprised to see how close we as humans are to end up on a planet which is still reasonably lightweight to conduct space flight. Other civilizations, if they exist, might not be as lucky. On more massive planets, space flight would be exponentially more expensive. Such civilizations would not have satellite TV, a moon mission, or a Hubble Space Telescope.

Another author, Harvard astronomer Abraham Loeb, noted:

Chemical propulsion requires a fuel mass that grows exponentially with terminal speed. By a fortunate coincidence the escape speed from the orbit of the Earth around the Sun is at the limit of attainable speed by chemical rockets. But the habitable zone around fainter stars is closer in, making it much more challenging for chemical rockets to escape from the deeper gravitational pit there.

A Choice of Words

Notice their choice of words: “surprised,” “lucky,” and “fortunate coincidence.” The authors of the papers made use of simple equations relating rocket propulsion and gravitational forces from planets and their host stars to arrive at their conclusions. The starting point is the Tsiolkovsky equation. The inescapable conclusion is that as you increase a planet’s surface gravity, an increasingly larger fraction of the rocket’s mass is propellant. In fact, the propellant mass fraction rises exponentially with the planet’s mass. 

But, it’s even worse than Hippke and Loeb concluded. The atmosphere makes it harder to launch rockets in two ways. First, it creates a drag force on the rocket. Second, the higher pressure at the surface of a super-earth reduces the effective thrust of a rocket. It gets still worse, since super-earths should have thicker atmospheres than Earth. And, if you expect to return astronauts to the planet, you’ll have to deal with the greater heat generated on reentry. 

Those super-earths with substantial hydrogen in their atmospheres (probably a substantial fraction) will make it even harder on our wannabe space explorers. Since hydrogen is a lightweight molecule, the atmosphere will be much more puffy (technically, it has a larger scale-height). This would require a rocket to travel much farther from the planet before the air drag became negligible; the overall air drag would be greater.

Basic Raw Materials

We shouldn’t overlook the basic raw materials for building rockets. Earth is well-endowed with abundant minable metal ores and fossil fuels. What’s more one of the best rocket propellants, hydrogen, is easily extracted from water, as is one of the best oxidizers, oxygen (which can also be obtained from the atmosphere). Think of that next time you take a drink of water.

Once you leave a planet, you can begin to consider further adventures. As Loeb noted, launching an interstellar probe from the habitable zone of a low mass star, a red dwarf, is more difficult. “Gravity assists” from massive planets in a system help, but red dwarfs rarely have giant planets like Jupiter. What’s more, if you’re going to build a large interstellar ship, the raw materials available in an asteroid belt are an essential ingredient. An asteroid belt is not a given in a planetary system. 

Distance and Dust

Once you depart your home system, there are two main challenges to interstellar travel: distance and dust. The typical spacing between stars in the solar neighborhood is several light-years. The nearest star other than the Sun is Proxima Centauri, at 4.22 light-years. This means it would take us 4.22 years to travel to Proxima Centauri if we were traveling at the speed of light.

But, when you are traveling to other stars you invariably run into solid material. Most of it is in the form of tiny dust grains smaller than 0.1 micron. Dust impacting a spaceship traveling at high speed can cause a lot of damage. You can compensate for it by taking into account some amount of eroded spaceship mass in its design, but that increases the required initial mass. Best to minimize the interstellar dust encountered on the voyage.

The density of stars and the abundance of interstellar dust vary greatly depending on your home address in the Milky Way Galaxy. We not only reside in a low dust region of the Milky Way, but we are in a “dust hole.” As the Solar System bobs up and down relative to the flat disk as it orbits about the center of the Milky Way, sometimes we are a little closer to the center. Right now we are closest to the center in our ~220 million year orbit, and we are also close to the mid-plane of the disk. This means the Sun is currently passing through the densest region of stars in its entire orbit through the Milky Way. There is no better time to set off on our first interstellar mission!

What should we make of this? Should we be surprised that the Earth and the Solar System seem to be better than most other locations for space travel? We certainly don’t need to do space travel to survive. Our ancestors weren’t space-faring folk. Why didn’t we find ourselves living on one of the much more common planetary systems where space exploration is more difficult? If you subscribe to The Privileged Planet thesis, then it makes perfect sense. It follows the same pattern — the cosmos is designed for discovery. We were meant to do science and explore the cosmos.