There is considerable scientific interest in designing an interstellar spacecraft, and the technologies required to build one are theoretically feasible. If we ever venture to explore a distant stellar system, one of these technologies might get us there.
Proxima Centauri b: A Destination Case Study
It might go without saying, that we want to look for life elsewhere in the universe. We all look to the heavens and wonder: are we alone? To answer this question, we are going to need to develop new technologies. We now know that the majority of stars in our universe have a complement of orbiting planets. There are several known possibly Earth-like planets we could explore. The problem is that they are so far away. Our nearest star, Proxima Centauri, is 4.25 light years away, shown as a yellow dot below. This might be the first exoplanet system we actually get to.
|NASA/Penn State University; Wikipedia|
Proxima Centauri is a small low-mass main sequence red dwarf star. It is seen as fairly white through the Hubble lens because even though it is technically a “red” dwarf, its surface temperature is 3040 K (as hot as a light bulb burning warm white).
Proxima Centauri has at least one confirmed planet orbiting it, called Proxima Centauri b (or Proxima b for short). Below is an artist’s impression of the planet orbiting its star. Based on one of several possible models of formation, it could be an arid, but not water-free, super-Earth (1.3 times Earth’s mass).
|Artist’s impression; ESO (European Southern Observatory)/M. Kommesser; Wikipedia|
A significant challenge faced by any life on Proxima Centauri b is the radiation it would endure from its star. Red dwarfs are more violent and less stable than larger stars such as ours. They are covered in starspots that can dim their output light up to 40% for months on end. At other times the planet’s atmosphere, assuming it has one, would face erosion from frequent and powerful radiation flares. Does it have a magnetosphere that would protect an atmosphere?
Shielded from the direct impact of its star’s unpredictable radiation, the “in between” zone might stand a chance to support life. It’s a bit of a slim shot at alien life, but fortunately it isn’t our only shot. There are numerous other exoplanets, which might be more suitable for life to evolve, and they might all be within our eventual physical reach. The first question, however, is how do we find them?
The Search For Exoplanet Candidates
It is startlingly amazing what we can glean through indirect observation. Proxima Centauri b, our case study planet, was discovered by ESO’s HARPS Spectrometer, located at La Silla observatory in the Chilean desert. Like other planets, it is illuminated by reflected light only and it is far too faint to be been by the naked eye. High-precision HARPS detects minute Doppler shifts in the radiation from its parent star, Proxima Centauri. These tiny shifts in the star’s radial velocity are in reality the tiny regular wobbles it makes as its planet orbits around it. Each time it orbits, the planet shifts the center of gravity of this simple two-body system back and forth. Even though this detection method detects only a fraction of possible planets - those planets whose orbital plane lines up with Earth - in just two decades, more than 4000 exoplanets have been confirmed. We are in the golden age of exoplanet discovery.
HARPS isn’t alone. Sophisticated ground-based telescopes of various kinds (listed here in Wikipedia), have detected exoplanets. New ones are detected as our ability to look improves. HARPS, so far, has discovered over 130 exoplanets. Many additional observatories are located in orbit around Earth, such as the Kepler Space Telescope, for example, with 2421 exoplanets detected and counting. And now TESS has detected 9 exoplanets during its first operational year. The tantalizing question is what kinds of alien life could some of these exoplanets harbour?
These sophisticated observations can answer questions about planetary mass, orbit, the atmosphere, and even possible surface composition regarding planets that are far too distant, too small and too dim for us to observe directly in the night sky. This information is tantalizing. Yet there is no substitute for being there. It is a human need to see with ones own eyes, feel with ones own fingers. Perhaps someday a manned interstellar mission to a planet harbouring alien life will happen. It’s perhaps more likely that we might never surmount the daunting technical obstacles faced with such a long manned spaceflight, one that out of necessity would take many years, perhaps even longer than a single human lifetime to accomplish. Wikipedia lists our manned travel options, and obstacles, in this regard.
There is hope. A future robotic mission equipped with suites of sensory instruments that could see, hear, physically feel, chemically taste and smell for us might be feasible in the near future. Besides crafting a vehicle that can house these instruments AND withstand years of interstellar travel, the principle challenge facing us is the time it will take to get to any distant solar system. Even traveling at near light speed to our closest exoplanet system, Proxima Centauri, would take at least four years. Assuming we have the propulsion technology to make this happen, imagine how difficult it would be to get funding for a certainly expensive exploratory mission that would take over eight years to receive any data back IF nothing goes wrong.
Spacecraft Propulsion: A Very Brief History
Perhaps the most daunting question is how to propel the spacecraft, and that is really what I’d like to explore in this article. To put this problem in perspective, consider the chemical propulsion technology we currently use. In 2013, Voyager 1 (shown below) entered interstellar space at a velocity of 62,000 km/h (about 17 km/s).
|Artist’s rendition; NASA/JPL- Caltech|
|Artist’s impression; NASA|
|Artist’s rendition; NASA/John Hopkins APL/Steve Gribben|
Current Technique: Gravity Assist
Let’s start at the beginning. In order to successfully launch, a rocket must overcome Earth’s gravitational well. A gravity well? We can think of a mass “denting” the space around it, as depicted below right, with a mass such as a planet, at the bottom of the well. The gravity well is a conceptual model arising from Einstein’s theory of general relativity, which predicts that gravity arises from the curvature of space-time. Technically it is four-dimensional space-time, not three-dimensional physical space, that curves .
|An image of Voyager 2 aboard Titan III-Centaur. Both Voyager probes were launched by Titan III rockets, NASA/MSFC|
|Delta IV Heavy Rocket launch in 2013; U.S. Air Force/Joe Davila. This rocket is currently in use with several launches expected in 2020-2023.|
One key problem with chemical rocket propulsion is fuel weight. Voyager 1 was launched on the expendable 4-stage rocket, Titan 3E, which used solid and liquid chemical fuels in its engines, providing enough thrust to lift the fairly heavy (823 kg) Voyager probe. At lift-off, the fully fueled rocket weighed about 633,000 kg, almost all of which was fuel. It needed most of it just to escape Earth’s gravity, with its fuel weight working against it. Even starting out with a respectable launch velocity of 130,000 km/h, Voyager 1 would have quickly petered out to become a dead weight in space were it not for the fortuitous and anticipated line-up of massive planets at that time.
Once boosted to a certain velocity, any body will maintain that velocity in a vacuum, and space is an almost perfect vacuum. Nonetheless, gravity acts on it. In Voyager 1’s case, gravity was working against it and for it. It had to climb up the Sun’s gravity well but it had several gravity assists to help it along. It reached its current velocity of 62,000 km/h, by making use of how the planets lined up in 1977. It passed very near Jupiter and Saturn, and then Uranus and Neptune, using each of their gravity wells to slingshot around them. It went with the planet’s spin to gain momentum at the expense of the planet’s momentum in order to boost its velocity. The decrease in spin of a planet would be immeasurably miniscule because its momentum is so much greater than a relatively infinitesimal probe. Voyager 1 was able to enter interstellar space by escaping the Sun’s gravitational well, which we can think of as that same inverted cone with a gradually widening, flattening base. An object traveling near the edge of that cone base, far from the Sun, experiences a much weaker gravitational pull. The solar gravity well isn’t so steep near the edge. Very close to the Sun, where the well is steepest, an object needs to travel at 525 km/s, or almost 2 million km/h, (away from it) to escape its gravity. Escape velocity then drops exponentially with distance. To help visualize how this works, see the graph below. Using data extrapolated from Voyager 2’s similar statistics, Voyager 1’s launch speed was about 36 km/s (about 130,000 km/h) (red line), very close to solar system escape velocity (blue line) at Earth’s orbital distance from the Sun. Its initial thrust had to work against the Earth’s gravity and then against the Sun’s enormous gravity, rapidly decelerating it to about 10 km/s by the time it approached Jupiter. If it kept experiencing that rate of deceleration it would never escape the Sun’s pull. Jupiter’s gravity boosted it back up to about 28km/s, well above solar escape velocity at that distant orbit. Shortly afterward, and with the help of subsequent planetary boosts, Voyager 1 kept getting boosted to keep it well above solar escape velocity. And that’s how Voyager was able to leave our solar system behind for the adventure of interstellar space beyond.
Velocity and Travel Time
We have solved the issue of leaving our solar system but the issue of achieving a reasonable journey time to an extremely distant solar system remains. There is a limit to what kind of velocity a craft can achieve using chemical fuel for an initial thrust followed by gravity assists. Even if a spacecraft uses the Sun as its gravity assist, like the Parker probe will, reaching a velocity of 692,000 km/h, it would still take about 7000 years to reach Proxima Centauri, 4.25 light years away. The only way to shorten that travel time is to go faster, much faster.
There is a built-in limit to the usefulness of any chemical rocket fuel, and that is its thrust-to-weight (T/W) ratio. This is a very useful dimensionless ratio calculated by dividing engine thrust (in units of force called newtons (N) by the weight of the fully fueled rocket at sea level (again in newtons), This is an example of weight treated as a force. It is generated by Earth’s gravitational field. Thrust is directly proportional to the acceleration of the rocket (force = mass x acceleration).
A high T/W ratio means high acceleration. Three force vectors work on a rocket as it lifts off: thrust (upward), weight (downward) and drag (downward). Drag is a mechanical force. It is the friction created as a solid object (the rocket) moves through a fluid (air in this case). Drag diminishes as the rocket exits Earth’s atmosphere. Thrust is the force generated by the rocket’s propulsion system.
If the ratio is greater than 1 and the drag is minimal, the rocket can lift off and accelerate upward. The T/W ratio is used as a static measure for a rocket at sea level. In operation, this ratio changes constantly as the rocket uses up fuel, reducing weight, and as drag decreases up through the atmosphere. It also changes as engine efficiency changes. To get an idea of the values involved, we can compare an Airbus A380 fully fueled and loaded at takeoff with a T/W ration of 0.227. It can take off and get airborne but with a T/W less than 1 but it can’t accelerate straight upward. To get a little more nuanced, its weight overcomes its thrust so airspeed always decays during a climb, whether it’s vertical or not. Fighter jets, in contrast usually achieve a T/W of close to 1, which affords them much greater maneuverability and speed. You might find it interesting to compare the W/T ratios of various aircraft here. The Space Shuttle fully fueled at takeoff had a T/W of 1.5 (using three main engines and two solid rocket boosters), plenty of vertical thrust to escape Earth’s gravity and reach orbital speed. To achieve this, the shuttle would launch vertically for a few kilometres while performing a gravity turn. That means the shuttle allows gravity to gradually bend its trajectory from straight up into horizontal to the ground as it accelerates up to the speed and altitude where it can maintain its orbit around Earth once its rockets shut off.
You won’t find a simple T/W ratio for modern rockets at takeoff because this ratio isn’t all that useful. They are all over 1. More important are the individual thrusts of the engines and the overall weight of the fuel, which is constantly changing, and the payload weight. You might guess that a higher T/W ratio is always better for a rocket launch but it isn’t. Instead there is an optimal acceleration rate. A higher T/W ratio means you build up great velocity in the lower atmosphere where the air is thickest. This means the rocket is subject to a lot of drag and therefore heating. These two factors increase wear and tear and engine inefficiency. A lower T/W ratio, on the other hand, means it will take longer to reach orbital speed so more fuel is used up fighting against gravitational forces. There is a sweet spot. Most rockets are designed for a T/W ratio of slightly higher than 1 with a maximum acceleration of about 4 g for a few seconds at the end of the first stage rocket burn, when most of the rocket fuel is burned and the weight is lowest compared to the upward thrust. 4 g means that our bodies feel 4 times heavier than they normally do (at 1 g). That is about the maximum acceleration that is “comfortable” for astronauts. To achieve the very high velocity required for a feasible interstellar mission, such a rocket will need to continue to accelerate after it has achieved orbital speed, and it will also need more boost than a gravity assist from our Sun. Considering that the nearest system is over 4 light years away, it will need to be boosted to a significant fraction of the speed of light. It will also need enough reserve thrust to maneuver into orbit around its target planet, but I am getting ahead of myself.
Ultimately no chemical rocket fuel can provide the kind of long-term thrust required to reach a significant fraction of the speed of light, often loosely called relativistic velocity. Why? To answer this question we can start by exploring the ideal rocket equation. This equation describes how a rocket accelerates itself using thrust created by shooting out part of its mass at high velocity in the opposite direction. That’s the Sun-bright blast from the rocket’s engines as it lifts off its pad. The rocket moves according to the common-sense principles of the conservation of momentum, Newton’s second law. But there is a bit more to it than what at first seems quite simple.
The rocket equation steps in where Newton’s second law cannot. Put formally, Newton’s second law states that the acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object. Or, simply put, it is force = mass x acceleration. In a rocket system, mass is always changing, so Russian scientist Konstantin Tsiolkovsky, in 1903, derived a formula using straightforward calculus, that describes its unique motion. It’s a lovely example of why calculus is good, and it’s a good equation on which to practice your calculus skills. Below is a screenshot of the equation taken from the Wikipedia page to show you what it looks like:
The rocket equation describes the motion of a rocket system. If you’d like to see how it is derived (in its non-relativistic form), this NASA site goes through it well. For our needs, it’s good enough to simply know the rocket equation relates a vehicle’s maximum possible change in velocity (delta-v) to the engine exhaust velocity, as the vehicle’s mass changes while fuel is consumed. It is from this equation that we get such terms as delta-v and impulse, which we will get into. Using this equation we can explore how the momentum of a rocket system changes as its fuel is consumed. We can compare the efficiencies of various rocket fuels in a rocket system. We can calculate how much fuel is required to change the velocity of a system over time, and what the maximum change in velocity is. Ultimately this equation can tell us which kinds of fuel will get us the acceleration and maximum velocity we need to achieve interstellar travel.
If you are sci-fi fan like me, you’ve probably heard or read, “Go to full impulse!” In physics, impulse is another outgrowth of Newton’s second law. It is the integral of a force over a time interval. In other words, we use calculus to describe a force changing over time. As a result, impulse creates a change in momentum of a system. To see how these two concepts are related to each other, check out this physicsclassroom.com link.
Impulse can also be used to figure out the efficiency of the fuel used in the rocket system if the mass of the fuel (also called propellant) is taken into account. It is total impulse per unit of fuel used up. More specifically, we can measure how many seconds a fuel can accelerate its own initial mass at 1 g. The resulting value, in seconds, is called the specific impulse. If we turn this relationship around slightly, we also determine how much of a particular fuel is needed for a given delta-v. There are several different ways to calculate specific impulse. Another method brings us back to the rocket equation. It can be calculated as the generated thrust of a system per unit mass fuel flow rate. This gives us the effective fuel exhaust velocity relative to the rocket. Effective exhaust velocity is proportional to the specific impulse, and both can be used to measure the efficiency of a rocket fuel. To get a feel for how these two values relate to each other, consider a fighter jet’s rocket-like engine running within Earth’s atmosphere. We can calculate the jet’s effective exhaust velocity from the rocket equation. Its actual exhaust velocity, however, will be reduced by atmospheric pressure. This pressure works against the exhaust, and therefore the momentum, and the delta-v. In return, the specific impulse of the engine’s fuel, its efficiency in other words, is also reduced under these real conditions. In jet engines, there is a big difference between effective and actual exhaust velocity. In rockets operating in the vacuum of space, there is none. If you go back to the Delta IV Heavy rocket Wikipedia page, on the right you can compare the specific impulses of its engine stages at sea level (in air) and in vacuum.
Specific impulse can be derived from yet another rocket science term called thrust. Thrust is a mechanical force. It is equal and opposite to the force generated by the mass of gas accelerated and shot out as the rocket’s fuel exhaust. This force, an example of Newton’s third law of motion, is in the forward direction of the rocket. Newton’s third law states that for every action (or force), there is an equal and opposite reaction (force). Specific impulse can be calculated by dividing the thrust by the rate at which the fuel is used up.
We can compare the efficiencies of various rocket fuels by comparing their specific impulse values. Most rocket fuels range from about 175 s to 300 s. “Seconds” might seem like a weird unit to measure how good a fuel is, but it is quite practical, and it makes sense now that you know how it is derived. In practice, thrust and efficiency (which usually increase with the price) tend to trade off one another. Solid rocket fuels tend to have high thrust and relatively low cost. They are usually used in the first stage engines on modern rockets because you need a lot of force to achieve lift off and its okay, even advantageous, to burn off a lot of heavy fuel right away. The more expensive higher impulse fuels (many of these being liquid hydrogen-based) are saved for higher stages. These fuels have a higher specific impulse (more seconds) than solid rocket fuels. If you are curious to compare efficiencies and other features among an extensive list of rocket fuels, see NASA’s list here,
No chemical rocket fuel is efficient enough to accelerate to a relativistic velocity. It is tempting to think that, because there is no air resistance in space and, away from stars and planets, there is no significant gravity to work against, it would be easy to eventually accelerate a spacecraft to near light speed. The problem is that in space, there is also nothing to push against to make the craft go faster except the fuel it brings along with it. The mass of the fuel resists being accelerated. It has its own inertia to overcome. If the fuel is too inefficient, you can’t pack in enough fuel to accelerate the spacecraft long enough to reach relativistic speed and overcome the inertia of the fuel mass itself. (We’ve now covered all three of Newton’s laws.)
In order to achieve a reasonable interstellar velocity, we need a fuel with a much higher efficiency, or specific impulse, than any chemical combustion reaction can provide. I’d like to caution us here because I mentioned exhaust velocity earlier. It would be easy to assume that the final velocity of a rocket is limited by its exhaust velocity. This calls for a close examination of Newton’s third law. The forces must balance out, but not the velocities. The fuel in the engine accelerates from rest to whatever the engine exhaust velocity happens to be. This accelerating mass of fuel creates the equal and opposite force of thrust. The force of thrust will continue to accelerate the rocket as long as the engine keeps working – in a vacuum. A jet flying through the atmosphere, on the other hand, will continue to accelerate until air resistance balances out the thrust of the aircraft. As a little test of this, consider a jet with enough thrust to go supersonic. This doesn’t mean its engine exhaust (which unlike a rocket is almost entirely accelerated air) is supersonic.
Fuel Options for Interstellar Flight
Most of these options are based on the likelihood that our first interstellar exploratory mission to another planetary system will be an unmanned probe. In theory, interstellar (unmanned) exploration is now possible, but much needs to be worked technically out before we can send off our first interstellar probe. We need a promising target planet, one that we will be reasonably certain has liquid water and an atmosphere containing oxygen, the bare essentials for life as we know it. New exoplanets are discovered almost every day, and new details about them are being discovered. We don’t know enough about any one of them to know if it could support life. To get there, a daunting number of technical problems need to be solved, such as propulsion and communications, but also protection from the brutal micro-impacts and radiation of interstellar space. Today’s new projects are at the brain-storming-for-ideas stage. Breakthroughs scientists are making right now are built on from the successes of previous projects, and it’s a very exciting time to follow this progress.
How It All Started: Orion
A silver lining emerged from the aftermath of WWII as some scientists began to wonder if they could harness the awesome power of a nuclear explosion to explore the depths of space. Since then, a number of additional novel propulsion concepts for such as mission have been proposed. To get an idea of them all, see this list in the Wikipedia article linked in the paragraph above. One of the most intriguing possibilities uses either a fission or fusion chain reaction. This concept is a powerful one. It offers a perfect scenario by combining two essential qualities into one: high thrust AND high specific impulse. It can be very efficient and it can also perform the high delta-v maneuvers required to insert itself into orbit or to land onto an exoplanet.
This dream took root in the late 1950’s, at the peak of the atomic age epitomized by Disney’s (original) Tomorrowland and later on by The Jetsons, well before news of the Three Mile Island accident, the Chernobyl meltdown and the Fukushima disaster soured public opinion in the decades to come. The dream was Project Orion, developed by theoretical physicist, Freeman Dyson.
|An artist’s conception of an interplanetary Project Orion spacecraft; NASA|
Originally cloaked in government top-secrecy, the Orion concept nonetheless eventually emerged to capture the imagination of the world. Stanley Kubrick’s 1968 movie, “2001: A Space Odyssey,” utilized a fictitious nuclear-powered high-performance Orion III to shuttle passengers from Earth to (also fictitious) Space Station V.
The actual Orion spacecraft would have been propelled by a series of nuclear fission explosions (nuclear pulse propulsion). Of course, one chief challenge was how not to blow the whole ship apart (how to harness and dampen almost instantaneous surges in momentum). This challenge would be revisited and refined decades later. The Partial Test Ban Treaty of 1963 as well as concerns about the radioactive fallout from its propulsion system, really a series of full-scale nuclear bombs, led to its fall from favour in the 1970’s.
But the idea didn’t fall away completely. This concept has been refined over the decades since into smaller better-controlled fission propulsion systems that use tiny pellets of fuel to generate chains of micro-explosions. Advances in techniques to confine and direct the explosions would further increase the efficiency of the system. The size of the original Orion interplanetary prototype vehicle was a huge full-scale manned 90 metre high 4000 tonne rocket. Requiring 800 0.14-kilotonne nuclear bombs detonating in rapid succession just to lift off, the original idea was to use this behemoth to get to Mars. While Orion scientists were focused on Mars, a final nail in the Orion coffin came from NASA’s decision to focus instead on (chemical propulsion) missions to the Moon.
From the grand-plan-anything-goes mindset of the 50’s, a once crazy-sounding idea is gathering new public interest. Building from the Orion days, a possible future micro-fission, or even micro-fusion, version of this system will be far smaller, lighter, and less dangerous, thanks to advances in technology. A nuclear pulse engine could consist of a series of anti-hydrogen pellets as small as 0.1 mm in diameter suspended within 100-gram hollow shells of nuclear fuel. A series of explosive lenses (shaped focusing charges) could break a Penning trap used to suspend antimatter in a vacuum, triggering an annihilation explosion with enough energy to trigger nuclear fission or even fusion, which in the latter case would create a shower of fast neutrons and very little radioactive fallout. Project AIMStar and Project ICAN, both proposed in the 1990’s, depend on versions of this much more compact antimatter-triggered nuclear pulse system. Meanwhile, Project Daedalus and Project Longshot, projects in the1970’s and 1980’s respectively, refined the concept of a new type of fusion energy called inertial confinement fusion for use in a nuclear pulse propulsion system.
1) Nuclear Photonic Propulsion
This theoretical system introduces an interesting twist to nuclear propulsion. Like other nuclear systems, it offers very high efficiency. The maximum specific impulse of any chemical propellant is only about 400 seconds. A variety of theoretical propulsion systems can achieve specific impulses magnitudes higher, with some also potentially achieving relativistic velocities. To compare, an ion propulsion system, which I will describe, can theoretically achieve a specific impulse of about 10,000 s. With this efficiency such a craft could achieve much faster speeds, but probably not relativistic, velocity. A photon rocket, however, could top this, potentially achieving an impulse of 30,000,000 s. This type of propulsion could propel a spacecraft up to 1/10 the speed of light.
A robotic probe trip to Alpha Centauri, for example, could become feasible, at a little over 40 years one-way with an additional 40 years to begin receiving information back on Earth. A nuclear photonic rocket would have an onboard nuclear (fission) reactor, which would create enough heat to emit a broad band of very intense blackbody radiation. Analogous to the radiant heat you feel near red-hot glowing embers, the thermal radiation from nuclear fission works exactly the same but consists of highly energetic gamma and X-ray photons rather than infrared photons. If all the nuclear fuel on board a spacecraft could be converted into photon energy and it was all directed in a perfect beam out the back of the craft, it could provide an enormous impulse. However, because thrust is derived from massless photons, their overall momentum is limited. Therefore, this seemingly ideal scenario, although it is nuclear, has the problem of low thrust.
Low thrust is a problem built into photonic systems. This is actually a classic Newton’s Third Law problem. Chemically fueled rockets shoot out literally tons of mass to achieve high thrust (high acceleration), even though the spent fuel is exhausted at a comparatively low – chemical explosion-level – velocity. A rocket driven by a flow of very low-mass ions or, going further, massless photons, has an extremely low propellant mass flow (low acceleration) but very high exhaust velocity. That means that these systems can eventually achieve very high rocket velocity, but it would take a long time to reach it. A question inherent with such low-thrust systems is how to change velocity quickly enough to fall into orbit around a planet and also possibly land on a planet surface, all things that a probe mission to an exoplanet would likely have to do. It would seem a massive waste to send a craft all that way only to do a fly-by. These space flight orbital maneuvers require steep changes in velocity, which in turn require massive thrust.
On looking up “Energy requirements and comparisons” on the Nuclear Photonic Rocket Wikipedia page linked above, it paints a rather bleak picture for our hopes of a photon propulsion system for interstellar space flight, at least for a manned flight. An example 300,000 kg spacecraft set up with reasonable perimeters, will indeed reach 1/10th speed of light. That’s feasible. However, nuclear energy is not nearly as efficient as we might assume it is. Using even the highest grade of nuclear fuel possible, the fission process itself never converts all of its fuel mass into photons. It converts only about 0.10 % of it. Reaching final velocity would require a conversion of 240 grams of mass into photon energy, which would require an enormous 240,000 kg of nuclear fuel, almost all of which would have to be carried the whole journey. We can assume the full-size reactor itself would contribute significant additional mass. Even with such a high specific impulse, and here the term is estimated because fuel is not consumed in the same way as it is in a chemical reaction, it would take a year to escape Earth’s gravity from a starting position in low orbit and it would then take 80 years to reach its final speed, accelerating at a very anemic rate of 10-5 g.
2) Micro-Probe With a Laser-Driven Sail
We could totally switch up the game by going small and moving away from nuclear energy to a highly focused laser for the photon drive. It would be much easier to focus the photon beam, perhaps into a sail to push the spacecraft and it would obviously do away with the need for a nuclear reactor and all that fissionable material. However, a limitation of lasers is that they are much less efficient at converting energy into light than blackbody radiation emitters. The photon energy will also be many scales smaller than any nuclear system, even with the most powerful laser that is currently technically possible.
Still, it is a tantalizing possibility. There is no need to carry any fuel, so the idea here is to go as small as possible. The 2016 Breakthrough Starshot privately funded initiative describes a series of ultra-light probes (on the scale of a gram) accelerated toward Alpha Centauri by beaming laser photons into their relatively large (a few meters wide) sails from Earth. The mission plans to send back flyby photographs of Proxima Centauri b.
|An artists conception of the Starshot solar sail deploying near Earth; Kevin Gill; Wikipedia|
3) Ion Propulsion
An ion thruster ionizes propellant to produce a beam of ions. The thrust from an ion propulsion engine is almost imperceptible. It is equivalent to the weight of a piece of copy machine paper on your hand (about 90 mN), and this means that even a small light spacecraft would take a very, very long time to accelerate to near light speed. On the upside it is extremely efficient, achieving a specific impulse of about 10,000 seconds.
3a) Electrostatic Ion Thruster
One way to do this is to use a gridded ion thruster design, schematically shown below.
The exhaust velocity of such an engine system can approach about 160,000 km/h. Still, the thrust of such a system would be small. Even using heavy xenon ions, the ion beam would not be able to overcome ordinary air resistance. Such a craft would have to be launched into orbit, and even once in the near vacuum of interstellar space, acceleration would be vey low. On the upside, this technology is very doable, based on test data from a variety of ion thruster engine designs.
In fact, xenon ion thrusters, as part of an NSTAR engine system, were already used very successfully to propel the recent Dawn probe to Vesta and Ceres in the asteroid belt. Arriving at Ceres in 2015, it is currently in an uncontrolled orbit around Ceres, having exhausted all of its fuel as of late 2018.
|Artist’s concept of NASA’s Dawn spacecraft above Ceres; NASA/JPL-Caltech. The blue glow comes from excited ions in the engine outflow.|
3b) Electromagnetic Ion Thruster
While an electrostatic thruster uses the Coulomb force from an electrical field to accelerate ions, an electromagnetic thruster uses the Lorentz force in which both electric and magnetic fields exert forces on the ions. An electric field can be produced by a stationary charge, whereas a magnetic field is produced by a moving charge. The electric and magnetic fields, in either of these technologies, are generated using a power source, which can be electric solar panels if used near the Sun. Far from the Sun, another source such as nuclear power could be used instead.
In an electromagnetic thruster, an ion (+q, shown below left) in motion (vector v) is acted upon by two force fields. The magnetic field q(v x B) exerts a perpendicular force (vector B) while an electric field (q E) exerts a force in the same direction as the field (vector E). The Lorentz force is the force responsible for the circular trace patterns you see after particles are bombarded in accelerators.
Like the electrostatic ion thruster, a gas is ionized. It could be xenon once again but lithium so far has been a better performer in tests on this technology. Ionized gas particles accelerated by an electromagnetic field could in theory generate an extremely high specific impulse, achieve an exhaust velocity about three times higher a xenon ion thruster and produce a respectable thrust of 20 N and possibly even up to 200 N, which is far higher than electrostatic ion propulsion. Like an electrostatic ion thruster, an electric field is generated by a power source. The magnetic field can be externally applied to the particles through magnetic rings around the exhaust chamber. Or, the field can be induced by the ion’s electrical current while the ions are accelerated. In this case, a cathode extends down through the middle of the ion chamber. At lower power levels, the magnetic field must be externally applied because the self-induced field is too weak. A CGI rendering of Princeton University’s lithium self-field MPD thruster is shown below.
A problem with this propulsion system is that it would require far more power to operate than electrostatic ion propulsion, on the order of hundreds of kilowatts (kW) compared to 1-7 kW. As with all ion thrusters, thrust increases with power input. There are no current spacecraft power systems that can provide hundreds of kilowatts of power. For a manned trip to Mars, a Earth-based laser or even a very large solar panel array might do the job. For a much longer interstellar flight, however, nuclear power would be a reasonable choice. NASA’s Project Prometheus’ reactor, dropped in 2005, would have been a small nuclear reactor that could generate electrical energy in this power range to run such an ion engine. (These nuclear power generators should not to be confused with nuclear thermal propulsion, described earlier). This technology would also have been useful for deep space probes in our system that are too far from the Sun to use photovoltaic panels.
Several countries around the world have experimented with MPD thruster technologies. Russia, then the Soviet Union, flew some experimental prototypes on their spacecraft and more recently Japan successfully operated an MPD thruster in space as part of an experiment. Research on this kind of ion thruster is still being carried out today at the Electric Propulsion and Plasma Dynamics Lab at Princeton University as well as at NASA’s Jet Propulsion Laboratory and Glenn Research Center.
5) Antimatter Propulsion
An antimatter rocket might sound a bit like science fiction but this technology could be one of the most promising for interstellar travel, provided a few practical matters are sorted out. Antimatter has the highest energy density of any proposed rocket fuel. In fact, up to 75% of the mass of the matter/antimatter fuel mixture is directly converted into energy. This is an incredibly high figure but it is not 100% as one might at first guess. Only the annihilation of electrons with positrons yields 100% energy. In practice, other particles and their antimatter twins annihilate each other as well, yielding both energy and a variety of yet more particles, representing mass that is not converted to energy. Condensed antihydrogen fuel might someday be feasible. However, at least with current technology it would be extremely difficult to create any sizeable quantity of antihydrogen. First, antiprotons and positrons (anti-electrons) need to be created. Positrons are relatively very easy to make. Physicists at Lawrence Livermore National Laboratory in California use a special laser to irradiate a plate of gold to create billions of positrons per pulse.
Antiprotons are not easy to make. A few antiprotons are created for each one million particle collisions in a particle collider. These rare particles are separated out from other particle products using a magnetic field. Then, these very fast antiprotons need to be slowed down using electric and magnetic fields so that they can capture positrons in order to make antihydrogen atoms. Then the atoms need to be trapped in an ultracold magnetic “bottle.” So far, a bottle of a few (less than 100) antihydrogen atoms have been stored for almost 17 minutes before they annhiliated. If CERN used its colliders only to make antimatter, it could only make 1 billionth of a gram per year! It takes about a billion times more energy to make antimatter than what is stored in its mass. One (likely very generous) estimate puts the price of 1 gram at about a trillion dollars US. NASA estimates that a trip to Mars would take around 20 milligrams of antimatter. The xenon or lithium required for ion thruster engines, described above, is quite expensive too but nowhere near this scale. This being said, the tiny amounts of antihydrogen mankind might someday feasibly manufacture would provide one hell of a wallop, on the scale of several billion times more than the most efficient chemical rocket fuel.
An antimatter rocket could use the products of annihilation for direct propulsion or it could be used indirectly to power a different drive, for example, to heat a fluid such as liquid hydrogen, which would be expelled, and which would potentially supply more thrust.
The theoretical antimatter AIMStar (Antimatter-Initiated Microfusion) rocket, developed in the 1990’s, would use gamma rays created from matter/antimatter annihilation to trigger a fission nuclear reaction in a deuterium/tritium mixture that would in turn start a fusion burn within that mixture. That superheated plasma would be ejected to propel the rocket using a series of fusion pulses. It would have a specific impulse of about 61,000 s and achieve an exhaust velocity of about 1/3 the speed of light. AIMStar propulsion technology might be feasible in a few decades. Developed by Penn State University, it could be used to visit the distant Oort Cloud, 10,000 A(astronomical units) away. The mission itself would take several decades, accelerating constantly for 22 years to achieve 3/1000th light speed, and require 28.5 micrograms of antimatter, more than we can create with current technology.
I briefly mentioned fusion as part of a theoretical antimatter engine earlier, but I think the fascinating possibility of nuclear fusion as a practical energy source for propulsion deserves a closer look. One reason that fusion propulsion theory is attractive is it is fuel-efficient. A typical chemical rocket engine has a specific impulse of about 450 s. A fission-based design, such as Orion, would have maximum specific impulse of about 10,000 s. By contrast, a fusion rocket would have a specific impulse over ten times higher, at about 130,000 s.
This option comes with serious challenges, however. One of them is low thrust. The first fission drive concept, Project Orion, demonstrated the potential power of nuclear energy for space travel. It promised high specific impulse AND thrust. Expelled from a series of fission detonations, debris particles, radiation and heavy radioactive nuclei could theoretically achieve an engine output average velocity of around 100,000 km/h and achieve an eventual maximum velocity in interstellar space of an impressive 36 million km/h, just over 3% light speed.. Because the debris would contain lots of large atomic nuclei (lots of mass), it would have high thrust as well. It is one of the few theoretical propulsion systems that could achieve significant thrust and impulse at the same time, a chief advantage of a fission system.
The bulk of the blast from a fusion reaction, in contrast, is made up of massless gamma rays and (low mass) protons. It’s very low in mass but it’s going very fast (on average close to slight speed), delivering an enormous specific impulse but low thrust. There are two general options for nuclear fusion propulsion. Direct propulsion means that the plasma itself is directed out the back of the craft: low thrust. Indirect propulsion means that fusion energy is used to generate electricity, perhaps to power an MPD ion drive. This is a propulsion system that is very efficient but requires at least a few hundred input watts of electricity. Fusion power could supply that, and the efficient MPD drive could achieve enough thrust to push a heavy cargo rocket or even a manned spaceflight over very long distances at significant velocity.
Besides low thrust, the biggest challenge of fusion is achieving ignition temperature. A fusion rocket will, out of necessity, use fusion plasma because that is where a fusion reaction takes place, in extremely hot pressurized plasma (millions of degrees C). In fact, in nature one must look into the interior of a star to find matter in this extreme state. In order to get plasma energetic enough to initiate a fusion reaction, it must be confined under extreme pressure – pressure equivalent to the interior of a star as massive as our Sun – and kept stable. The problems with fusion, in a nutshell, are how to heat gas into a fusion state, how to confine it and how to keep it stable. You must keep nuclei energetic and in close enough proximity to fuse together in an ongoing stable reaction.
Nuclear plasma cannot be confined physically. It is far too hot and under too much pressure for any material in contact to withstand, even momentarily. However, there are other ingenious ways to tame it, and these methods come with the added advantage that they can be very lightweight and compact. Weight is an overarching concern with long-term spaceflight. Everything is designed to minimize the mass that must be hauled across space, so that energy expenditure can be minimized. Fuels can be heavy, adding their own load, and their supply is limited. The main advantage of a fusion propulsion system is that it can be very efficient, requiring a smaller and much lighter fuel supply than a fission system. The fuel would consist of hydrogen isotopes such as deuterium or tritium rather than a heavy fissile isotope such as uranium-235 or plutonium-239. In addition to this advantage, more fuel mass would be turned into energy in a fusion drive than in a fission drive. In a fusion drive, 676 units of energy would be converted from 1 kg fusion fuel. A fission reaction would yield 176 units of energy/kilogram of fuel. See also that hyperphysics website link to see how these values are calculated. Furthermore, because fusion produces less radiation than fission does, less, necessarily heavy, shielding would be required to protect sensitive hardware. A fusion drive will still require a reactor, but it could be built much lighter and smaller than a fission reactor if technologies such as magnetic or inertial plasma confinement are used. Not only can these methods heat the plasma but they also keep it under pressure.
Magnetic confinement works by inducing a circular electrical current in plasma. The current creates a magnetic field that squeezes the plasma into a thin ring. This results into two kinds of heating: Joule heating and adiabatic heating. Joule heating is microscopic friction caused when a current passes through a conductor. During adiabatic heating, the thermal energy (temperature) of a gas or plasma increases as it is compressed, according to the first law of thermodynamics. A technology that currently uses magnetic fusion confinement is the tokamak, a Russian-designed device that confines and stabilizes plasma by winding magnetic field lines in a helix around a toroid (doughnut) shape. Several tokamaks are now working around the globe. To make such a device feasible as a generator, it must produce more energy than is required to maintain the fuel in a fusion state, a tall order. The first fusion tokamak-type full-size reactor, called ITER, just achieves this and is currently under construction in France. The problem with current tokamak technology is that the toroidal reactor is very heavy, severely reducing a potential craft’s weight/thrust ratio.
A different and more feasible approach for a fusion drive would be to go small and introduce a different confinement method. One could focus intense energy onto a very small target, using a powerful laser to heat and compress a tiny fuel pellet of the hydrogen isotopes tritium and deuterium until the pellet ignites a fusion reaction. The fuel is relatively cheap and plentiful, not just on Earth but elsewhere in the universe as well, and the system could be highly efficient, perhaps used for a power source for a secondary thrust technology. An intense focused blast of laser photons heats the outer layer of the fuel pellet exploding it outward in all directions, producing an equal reaction force inward in the form of shockwaves. The shockwaves compress and heat the interior of the pellet enough to ignite fusion. This is an example of Newton’s third law in action.
This type of fusion requires a very powerful and focused laser in the vicinity of megajoules (MJ), which is doable. The National Ignition Facility (NIF), which researches inertial confinement fusion, has such a laser in use. This system also requires a perfect sphere of fuel in order to create a shockwave that is properly focused inward. A way around this issue has been to surround the fuel in a tiny metal cylinder. The laser focuses on the inner surface of the metal, heating it into plasma, which then radiates X-rays. The thermal energy from the X-rays is absorbed by the fuel sphere (more efficiently than by the photons). It then perfectly implodes into fusion plasma.
To directly produce an impulse from this reaction, a magnetic field could confine and direct the intense blast energy of the fusion explosion. The field could also be set up to form a pusher plate. A tremendous amount of energy, in the form of gamma rays, released from a series of fusion pulses could push against a pusher plate, in this case a magnetic field, in order to translate force into acceleration. The plate could also be designed to absorb the shock of the individual pulses to allow for smooth acceleration of the craft. The drawback of this system, as mentioned, is its very low thrust.
Relativistic Travel And Einstein
It would seem that once we work out extremely efficient and continuous fuel consumption, our future interstellar spacecraft, perhaps even manned, should be able to accelerate across the light years of space, perhaps at a comfortable rate of 1 g, the same as Earth’s gravity, to eventually reach light speed. After all, can’t we simply use the formula f = ma? Newton’s second law states that force equals mass times acceleration. Imagine the scenario. We can use the rocket equation to refine our calculations. The rocket equation deals with the fact that we must accelerate both ship and fuel mass and the fuel mass is decreasing as we go. Then imagine that we can reverse the engine thrust and decelerate at 1 g, over several years, to reach planet X. We can assume there is no friction in this space vacuum to work against us. We will assume there are no particles in interstellar space (which is not the case and would cause a serious problem at relativistic velocity). Here, then, is our system in effect: the propellant removes momentum in one direction so the ship can gain momentum in the other direction.
The problem is in the assumption of f = ma. This Newtonian equation assumes that the relationship between mass and acceleration always holds up. And it does, but only until we get to relativistic velocities, a scenario Newton, I suspect, never thought of. At these velocities, the relationship starts to break apart. We must translate our motion equations from Galilean transformations into Lorentz transformations. This sounds complicated, but it means that at relativistic velocities, space-time itself starts to makes itself known within our accelerating spaceship scenario. For Newton, space and time are invariant. You can always count on them to be the same. A ruler is always one foot long and time always flows at the same rate. A second lasts a second. For Einstein, space and time are no longer invariant and we can no longer think of them as separate from each other either. Now they must be treated as part of a 4-dimensional single continuum, space-time. Two things to consider: space-time can stretch and warp and, second, we now must think of any situation in terms of frame of reference. The ONLY constant that doesn’t change with frame of reference is the speed of light.
As our ship revs up to, say 30% light speed, we would begin to notice changes. Time dilates – clocks on the ship slow down compared to those on Earth. Conversely, measured from the ship, Earth’s clocks speed up. Length contracts in the direction of motion. This means that even though length contracts, from Earth the ship would look fine. From a dock in which the ship whizzed past, however, you would see the ship look shorter or squished. If the ship could travel exactly at light speed, it would have no length at all in this reference point. Relative mass increases. The ship, as measured from Earth, would be growing more massive.
What does all this mean for Newtonian dynamics? At lower velocities, Newtonian physics holds up. As velocity becomes relativistic, the ship’s momentum/velocity/acceleration begins to lose what looked like a linear relationship to one another. This happens, according to Einstein, because the time coordinates between the ship and the observer (such as us on Earth) are no longer the same. Time, distance, and mass are now relative, that is, their values depend on your frame of reference. Only the speed of light and the basic laws of physics remain constant from every reference point in the universe.
This isn’t intuitive. The ship’s changes we would measure from a different reference point such as Earth - its mass, its physical dimensions and its time dilation - are not optical illusions. They are real valid measurements. Yet, their values depend entirely upon where we measure them. So, if you were inside the ship, your clock would be running normally, your mass would be normal and the ship would be of the same dimensions as always. Looking out the front window, however, you would notice that your frame of reference is now drastically different from the relatively stationary space around you. The stars whizzing past the ship would begin to bunch up closer, the distances between them shrinking, while the starlight reaching the front window of the ship shifts to blue (Doppler effect). In your reference, they are approaching relativistic velocity toward you.
The spaceship can never exceed light speed. Witnessed from Earth, the reason is clear. As the ship approaches light speed, more and more of the force of 1 g acceleration is converted into RELATIVISTIC mass rather than velocity, until, as the theory goes, the relativistic mass approaches infinity, an impossibility, at light speed. This doesn’t mean that, at any and all reference points, your ship is now so massive it would take infinite force to continue to accelerate it (although that’s what we would correctly observe from Earth). At the same time, all processes on the ship would appear from Earth to slow down to a stop. Frozen there, there would be no way to advance further.
Time, mass and distance in space-time change as your reference changes. Inside the ship, now approaching light speed, you would see your star field bunching up, turning blue and then falling entirely beyond your visual range into an almost perfectly black invisible surface in front of your window. Unfortunately you would soon need to install a powerful blast shield because the photons of starlight will eventually transform into X-rays and then deadly gamma rays as the Doppler shift continues to shorten the wavelength of this electromagnetic radiation. Is there a limit to the Doppler shift of a photon? The wavelength of a photon should theoretically approach zero as the ship approaches light speed but there is no theoretical description that makes sense for a shorter-than-zero photon wavelength. The photon energy would approach infinity. The speed of the photons, however, will always be the same, because light speed is invariant.
Time would seem to speed up for all the processes going in space in front of your ship, say an exploding supernova somewhere. At even 90% light speed, you might still be able to perceive the explosion by instruments. Once your ship passes the supernova you might see it – a Doppler-shifted slow motion explosion – in you rearview mirror. You will have to trust Einstein to continue accelerating to a final velocity of, say 99% light speed, and then decelerate when you approach your destination. As you do so, the star field will deepen, come back into view, as the reel of time once again becomes your time. Remember, everything inside your ship is normal for the whole journey. It’s the objects and their motions outside it, moving near light speed relative to your ship that will seem weird.
A tantalizing feature of near light-speed travel comes from time dilation. As time relatively slows down inside the spacecraft, including for any occupants, time relatively speeds up outside of it, including on Earth. A trip to a star system 100 light years away could be shortened into, perhaps a few years FOR the occupants. This seems to break the rules of faster-than-light travel, but in the reference frame of the ship and its occupants, space contracts and stars appear much closer to each other. What, for Earth would be at least a 100-year trip, would be experienced as much shorter by the occupants because their clocks are running very slowly compared to the universe (and normally for them). To them, the distance to the star system would be just a few light years away, perhaps less, depending on how close to light speed the ship traveled and how long it would take to accelerate and decelerate. Consider that if a ship could travel at exactly light speed (which it cannot), the universe would be completely length contracted (there would be no distance between stars or galaxies at all in the direction that it is moving) and time would not exist, as time in the universe surrounding the ship would run infinitely fast, an infinite nonsense answer to the equation that describes relativistic motion. To a photon, then, there is no time or distance between its emission and its eventual absorption. That is its universe, a mind-bending thought for the next time you are in the shower. Sadly for any ultrafast interstellar travellers, Earth will have run like a movie on extreme fast forward, a minimum of over 200 years would have passed by the time the travellers returned, a few years later.
Unfortunately, this journey is unfeasible. It is impossibly perilous. If your ship strikes even a cold gas cloud in space, it will blast into smithereens. Every gas atom will be a significantly massive relativistic missile striking your ship hull. Unless the ship has a powerful magnetic field to deflect particles away perhaps . . .