By Harry Keller
Science Editor
[Related: Beyond Earth: The Dynamics of Human Expansion Across the Stars, Outlook for Interstellar Travel Is Improving, Trump Releases Unresolved UAP Files: Credible Acknowledgments, Are Interstellar Visitors Really Alien Ships?, see Harry’s list of ETC publications for numerous space-related articles.]
When asked about interstellar travel, people raise concerns about cosmic rays, interstellar dust, how to survive a centuries-long trip, and how to obtain the necessary energy, among other issues. These are valid concerns. Interstellar cosmic rays are about ten times stronger than those inside our solar system. At 10% of the speed of light, an average dust particle would carry the energy of tons of TNT upon impact with the starship. A habitable planet could be as close as 40 light-years away. If you could travel that fast, the trip would take 400 years. That speed is too low to produce any time dilation.
Consider the usual suspects creating obstacles, hurdles, and roadblocks to interstellar travel.
ENERGY:
As those of you who have taken courses that include energy calculations know, the kinetic energy of an object is one-half of the object’s mass times the square of its velocity. The energy must come from somewhere, and the ship’s power plant has to supply it. The numbers are, well, astronomical. Given the length of the trip, the power demands, even for such enormous amounts of energy, are modest. The sum of all the power used to propel the ship is the energy.
In simpler terms, long-term power adds up to a lot of energy. A steady, low power can produce plenty of energy over centuries. The power plant must keep running, even intermittently, for that long.
I suspect that the means of supplying energy for an interstellar trip will be one of the earliest obstacles to fall to our clever engineers. The outcry over energy is misplaced.
COSMIC RAYS:
Cosmic rays consist primarily of protons, hydrogen atoms stripped of their sole electrons, traveling at close to the speed of light. Their energy far exceeds a simple kinetic-energy calculation because of Einstein’s Special Theory of Relativity.
Here on Earth, some cosmic rays pass through Earth’s magnetic field and its atmosphere to reach us on Earth’s surface. Most are removed by these filters. A scintillation detector can measure these rays. You can even turn your cellphone into a scintillator detector. At high altitudes, with less atmospheric attenuation, the detector will detect more radiation than at the beach. They can damage your DNA and lead to cancer. In space, it’s much worse, and in interstellar space, it’s far worse. Your spaceship’s hull doesn’t help much.
In fact, the metal hull of your starship will make cosmic rays even more damaging. It’s a cascading effect. A high-speed proton hits a large nucleus. The impact blasts that nucleus to bits, and the bits have very high speeds. They hit other nuclei, breaking them up. Furthermore, the collision creates new particles that act like the original cosmic ray. The one cosmic ray that might have passed through a body without interacting has created dozens of energetic subatomic particles with low enough velocities to leave a trail of destruction in a human body. The metal hull of a starship will not act as a shield to cosmic rays.
INTERSTELLAR DUST:
Space has dust, not much, but over a long trip at high speeds, a ship will encounter plenty of dust. Space has no oxygen to support combustion. TNT does not rely on oxygen, and there’s no TNT in space either. If a bit of dust hits a starship head-on, it dumps all of its kinetic energy into the ship’s hull. The dust may be just sitting there in space with no velocity with respect to the stars around it. (I simplified, but the principle remains the same if you consider the complications.) The ship, at 10% of the speed of light, can also be considered as standing still, and the dust relative to the ship moves at that speed to hit the ship. Depending on the size of the dust mote, it hits the ship with the power of tons of TNT. In other words, it can blow a hole in the ship.
No one has ever had to find a shield for this sort of impact. Before we whisk off into interstellar space, we must. The first line of defense will be hull shape. All of the interstellar ships you see in science fiction movies are wrong. The ship must be sleek, with a pointed nose, so the dust glances off the hull without dumping all of its relative kinetic energy into it. As the ship approaches dust particles, it should have a system that pushes the dust to the side. The only mechanism I’ve come up with shoots a beam of electrons ahead of the ship to ionize the dust particles. A magnetic field can then push them to the side. I can think of reasons this won’t work. Some clever engineer might fix that.
There’s also plenty of hydrogen out there, about one atom per cubic centimeter. The hydrogen will abrade the hull. After centuries of abrasion, the hull could fail.
DECELERATION:
A starship must slow down as it approaches its destination. Designers could use the engines after turning the ship around to slow it down. That would mean carrying much more reaction mass over many light-years to the chosen exoplanet. Having taken centuries to reach this destination, the ship could use much less reaction mass to enter a highly elliptical orbit around a gas giant. At perihelion, it could skim the outer reaches of the atmosphere to brake its speed. Using its engines just to make course adjustments shouldn’t be a problem.
It all depends on the choices made in the planning phases.
TRAVEL TIME:
At 10% of the speed of light, a speed that may not be attainable in practice, humans would be looking at centuries of travel in space. Will the machinery function for that long? Can humans survive that long and still be ready to do the work of setting up a settlement? Robots will be advanced by the time an interstellar flight launches.
Much can go wrong in such a long time. How can testing be thorough enough? The readers here can use their imaginations and come up with scenarios. There’s no simple answer here. Perhaps, two centuries from now, solutions will be available.
THE REAL PROBLEM:
Every preceding issue is real and will require excellent, even breakthrough, engineering to solve. In each instance, scientists and engineers can imagine a theoretical solution. Whether they can achieve it is another issue.
An ordinary nuclear power plant could provide the necessary energy, but building one for a starship and having it produce power for over 400 years is a serious engineering challenge. Fusion power remains a dream of the future. If a fusion power plant can be made compact enough, it would serve.
A strong magnetic field around the starship would deflect most of the cosmic rays. Water is good at absorbing that radiation. Engineers can, in theory, figure out how to reduce radiation to acceptable levels.
The problem of interstellar dust at very high velocities is not trivial, but we have clever engineers to work on it.
Deceleration (aka negative acceleration) merely requires more of what enabled the ship to fly to the distant star in the first place.
Travel time might be less of a problem if the ship could fly faster, but that’s not possible, as you will see. Instead, human lifespans must increase, and some form of human pseudo-hibernation must be developed. Scientists are working on both and might have a solution before the end of this century.
I won’t walk you through the calculations to prove that the real problem does not lie among those just mentioned. Without solving this problem, the others don’t matter. You’ve probably heard this word in a different context. It is momentum. In physics, momentum is mass times velocity, and it must be conserved. To gain momentum for one object, another must balance it precisely by gaining momentum in the opposite direction. That’s the simplest case. With many objects, the entire system’s momentum must remain the same. This concept is so fundamental that it is called the Law of Conservation of Momentum.
Applying this law to a starship means considering how the ship can increase its speed. There’s nothing to push against in space. It’s a vacuum out there. The only way the starship can move forward is for something else to move backward. In simple terms, something must be expelled out the back of the ship. This is the task of rocket engines.
The more stuff we toss out, the more momentum (and velocity) the ship will achieve. Tossing it out faster achieves the same result. This problem becomes serious because the ship must carry all this reaction mass for the entire trip, until it’s tossed out. Energy and momentum must be expended on the stuff you are throwing away. It’s not enough to produce the energy and momentum for the starship; you must do the same for its reaction mass up to the point where it’s tossed out.
Ignoring this new problem, it’s easy to determine how much reaction mass to load on a starship to achieve a given velocity. The ship will require as much reaction mass to decelerate. You can use some tricks to reduce this problem. This analysis raises a crucial question. What exhaust speed must the starship’s engines have to reduce the amount of reaction mass to a small fraction of the starship’s mass? I show the calculations in my new book, Aliens and Humans. The engine exhaust velocity must be greater than 99% of the speed of light to reduce reaction mass to 10% of the ship’s mass when the ship’s speed is 10% of the speed of light. Lower ship speed would reduce the required reaction mass but would make the trip, already too long, even longer.
Until someone invents a spaceship engine with relativistic exhaust speeds, the stars remain out of reach. Given the temperatures and pressures involved, this invention may never come to fruition. If so, we may never reach the stars anytime soon. By soon, I mean within three centuries.
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