Space Propulsion: Separating Fact From Science Fiction

An unfortunate property of science-fiction is that it is, tragically, fiction. Instead of soaring between the stars and countless galaxies out there, we find ourselves hitherto confined to this planet we call Earth. Only a handful of human beings have ever made it as far as the Earth’s solitary moon, and just two of our unmanned probes have made it out of the Earth’s solar system after many decades of travel. It’s enough to make one despair that we’ll never get anywhere near the fantastic future that was seemingly promised to us by science-fiction.

Yet perhaps not all hope is lost. Over the past decades, we have improved our chemical rockets, are experimenting with various types of nuclear rockets, and ion thrusters are a common feature on modern satellites as well as for missions within the solar system. And even if the hype around the EMDrive vanished as quickly as it had appeared, the Alcubierre faster-than-light drive is still a tantalizing possibility after many years of refinements.

Even as physics conspires against our desire for a life among the stars, what do our current chances look like? Let’s have a look at the propulsion methods which we have today, and what we can look forward to with varying degrees of certainty.

Rulers of Low Earth Orbit

RS-68 rocket engine under test at NASA Stennis Space Center in 2000.

When it comes to getting things into orbit around Earth and keeping them there, we are doing pretty well. Since the early days of the rocket engine in the first half of the 20th century, we have come up with numerous improvements and new technologies. We have developed new solid and liquid fuels and learned to use hypergolic as well as cryogenic fuels. This has made the process of launching new satellites and new probes into orbit around the Earth and on inter-planetary transfer orbits practically a matter of routine.

Once out of Earth’s gravity well, or safely in orbit around the planet, a propulsion method capable of less brute force suffices as the gravitational pull of the Earth is no longer a concern. This is where ion thrusters shine: using relatively small amounts of propellant and the electricity from solar panels or other sources like RTGs, they manage to generate significant amounts of thrust in the form of ion beams. Because ion thrusters have very high specific impulse, they are very efficient in their fuel usage, yet they come with the disadvantage of having very little thrust.

This gets us to the core of the issue with rockets and space-based propulsion: balancing performance between energy required and fuel spent. Whereas a chemical rocket can be easily scaled up to use more fuel for more thrust, its specific impulse is pretty atrocious, meaning that for every unit of fuel burned, most of the energy contained in the fuel is wasted, i.e. not used for the purpose of propulsion.

Specific impulse (Isp) is defined in seconds, where the indicated value specifies for how long the rocket engine or equivalent device can provide thrust to the rocket using the available propellant. This determines the duration of thrust and thus the total acceleration. In addition, chemical rockets get lighter as they use up their propellant, causing the acceleration for the same thrust increases over time. The thrust-to-weight ratio thus determines how well a rocket performs.

As a direct comparison, a chemical rocket such as SpaceX’s Falcon 9 with Merlin 1D (full thrust) engines has an Isp of 311 seconds in a vacuum, and 282 seconds at sea level. Meanwhile the Isp of an ion thruster is measured not in seconds, but in weeks or even months to years. This despite the ion thruster in a satellite or probe having only a fraction of the propulsion that a chemical rocket has. Meanwhile, the ion thruster has a very low thrust-to-weight ratio which prevents it from lifting as much as a sheet of paper out of Earth’s gravity well.

Beyond Earth orbit

Exhaust side of NASA’s Evolutionary Xenon ion Thruster (NEXT).

Using these chemical rockets and ion thrusters we can get and keep satellites as well as the International Space Station in orbit, even when they are in lower orbits where atmospheric drag is an issue. And as recently demonstrated by the US, China and UAE by getting the Hope (Misabar Al Amal) & Tianwen-1 (Questions to Heaven) orbiters around Mars, as well as the Perseverance rover on Martian soil, we’re getting pretty good at traveling to at least one of our nearest neighbors in the solar system.

Most of travel within the solar system makes use of orbital mechanics, with the current ESA BepiColombo mission a prime example of this. Instead of traveling in a straight line from Earth to Mercury, this mission spans seven years, during which BepiColombo will use gravity assist: essentially using the gravity of various planets and the Sun of our solar system in order to both gain and lose speed, as well as changing its orbit around the Sun so that it can ultimately align itself with Mercury and park itself in its orbit.

This also shows that another important factor here is one of time. Without the consideration of how long traveling within the solar system or beyond may take, using gravity assist from the Earth and other planets is a valid and very efficient way to travel around in space. The Voyager probes have made it successfully out of the solar system this way, taking only around forty years for this. Of course, outside of scientific missions, discarding the time factor is only an option when one begins to consider something like generation ships.

Much like on Earth, we prefer to travel faster and waste less time. After all, who wants to be stuck in a sailing ship at the Cape on a months-long sailing trip when one can just take an airplane to the East, for example? Similarly, we are looking for ways to travel faster in space.

Going Nuclear

There are a few possible destinations which we would like to travel to faster: one is of course Mars, but other planets in our solar system are also of interest, such as Jupiter’s moon Europa. Here we run into a big issue with both our chemical rockets and our ion thrusters: one cannot provide thrust for long enough, and the other doesn’t provide enough thrust. A possible solution here dates back to the 1950s, in the form of nuclear propulsion.

Many are probably aware of DARPA’s Project Orion, which saw the use of Nuclear Pulse Propulsion as a way to fly to Mars and back in the span of four weeks. While that project never got off the ground, new NPP-based concepts have been worked on. Here much of the most recent research focuses on the use of nuclear fusion in some way to create high-speed exhaust. We see something similar in the general scope of nuclear thermal rockets of which NPPs are a part, where the focus has shifted away from fission and towards fusion. Some, like the Direct Fusion Drive, can be thought of essentially an improved ion thruster.

PFRC-2 device during a magnetic field pulse at Princeton in 2016.

The DFD along with others are some of the concepts which NASA is currently looking at to slash travel times to Mars and other destinations, including for Orion (the spacecraft). The DFD uses findings from the Princeton field-reversed configuration (PFRC) experiments to provide continuous thrust at significantly higher levels than today’s ion thrusters. This would make it suitable for interplanetary travel, with a travel time of four years predicted to travel to Pluto at the edge of our solar system.

Of course, none of this would make interstellar travel outside of our solar system significantly easier.

Making Light of Things

In this universe there are only a few things which are certain. One of which is that space is very big, not to mention very empty. Another is that objects have mass, and yet another being the speed of light (c) exists. The latter two combined dictate a very real limit to how much an object can accelerate. This is problematic in light of challenges such as getting a human being to the nearest star system (Alpha Centauri, 4.37 light years) within that person’s lifespan.

As the most distant human-made object, Voyager 1 is traveling at 1/18,000 of the speed of light, which would mean that it would be capable of reaching Alpha Centauri in approximately 80,000 years. Yet as we’ll see, the solution here is not simply to accelerate more, as this creates two new problems. The first is one of sheer kinetic energy, as the energy required to accelerate to an appreciable fraction of light speed is larger than one could hope to produce with any kind of current or future propulsion method.

Astronauts on the International Space Station experience time 0.01 seconds slower per Earth year than people on Earth’s surface.

The second problem is defined by general relativity (GR).  Simply put, if an object experiences acceleration, then the reference frame of the object and that of any outside observers begin to drift apart. This gravitational time dilation effect in a visual representation means that to an outside observer, a clock held by an accelerating object slows down, while vice versa an outside observer’s clock will seem to move faster than the clock which they are holding.

Although the effect of this time dilation are relatively minor around Earth (e.g. astronauts in the ISS versus people on Earth), the brutal truth here is that we do not want to accelerate significantly at all. That is, unless we wish to deal with situations where the people onboard a spaceship traveling at 0.6c will themselves experience weeks passing during a mission, while back on Earth decades will have passed. This renders even fraction-of-c space unmanned probes relatively pointless.

A potential solution here lies in the concept of a warp drive, also known as the Alcubierre drive and its derivatives. This method essentially allows someone to travel effectively faster than light (FTL), without changing their effective gravity and thus their reference frame. This also avoids the need for enormous amounts of energy.

Physical Warp Drives

The ‘Phoenix’ warp-capable ship from Star Trek, built around a converted ICBM.

FTL drives are pretty much a staple of science fiction, and take on many forms. Of these, the warp drive is one of the rare few which is both based on scientific theory and which has seen a few decades of study and refinement. At its core the principle is simple enough: the ‘warp drive’ establishes (warp) a shell of space time around the object (‘warp field’), which can then move without having to increase its kinetic energy. Its effective speed would be limited by how rapidly it can warp space time.

A recent addition to the literature on this topic is Introducing Physical Warp Drives by Bobrick et al., which works through the past decades of literature, while creating a classification system for the different warp drive types imaginable.

Most importantly, it covers how an assumption made with the Alcubierre drive — in that it requires a large amount of negative mass — was merely based on a lack of understanding of the underlying theory. Effectively, this means that the negative mass requirement can be reduced or even fully eliminated, and that within the realm of physics there is so far nothing to stop humanity from constructing actual, physical warp drives and embark on FTL trips through the galaxy and beyond.

Space: The Final Frontier?

It would appear then that at least some part of science fiction could within the near future become science fact, with the starships portrayed in the original Star Trek series (TOS, TNG and VOY) on the side of the Federation providing a tantalizing template for what humanity’s future could be like. Interestingly, in the Star Trek universe it’d take until 2063 for an inventor to test the first warp drive.

What our own timeline will look like is still up for grabs, fortunately. Whether we truly will be able to build warp drives in another forty years from now or not, and what we will find out there if we do are all still open questions. As we find ourselves reminiscing about Yuri Gagarin’s historic flight into space sixty years ago, it’s exciting to look ahead, to what the next decades may bring.

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