Ad Astra: The Future of Propulsion Technology (Part II)

In the long term, humanity may realize some truly exotic forms of propulsion that leverage new advances in technology or physics that go beyond the Standard Model. These proposed systems could enable rapid transit to neighboring star systems and exoplanets, combining immense fuel efficiency and unparalleled energy density with high acceleration rates (aka. delta-v).

If nuclear propulsion opens the Solar System to further exploration and settlement, these more exotic ideas could open our cosmic neighborhood and even the entire galaxy!

Directed Energy 

Solar sails have been the subject of research and development for decades and are considered a cost-effective way of exploring the Solar System. Compared to conventional spacecraft, they are relatively cheap to manufacture, being made of large sheets of reflective material and lightweight electrical components. More importantly, they rely on solar pressure for propulsion, requiring no fuel (or bulky fuel tanks).

Several technology demonstrators have been built and tested in space. This includes the IKAROS space probe by the Japanese Aerospace Exploration Agency (JAXA) that operated in space from 2010 to 2015. Between 2015 and 2019, the Planetary Society launched two solar sails to orbit, LightSail-1 and LightSail-2. 

The same technology has been considered for interstellar exploration applications in recent years. This is known as Directed Energy Propulsion (DEP), where “lightsails” are accelerated to a fraction of the speed of light (relativistic speeds) using arrays of focused lasers. The technology combines the benefits of low mass, no propellant, and extremely high delta-v.

The concept was proposed by physicist and science fiction author Robert Forward, which he described in a 1984 study titled “Roundtrip Interstellar TravelUsing Laser-pushed Lightsails”:

“It is a form of beamed-power propulsion in that the ‘engines’ of the vehicle are left behind in the Solar System and the power and reaction mass are transmitted out to the rest of the vehicle that carries the payload… these systems can be designed so that the outward thrust of the Solar System-based lasers not only an push the lightsails up to relativistic velocities but also can be used to bring the lightsails to a stop in the target system.”

According to a 2000 study produced by Robert Frisbee, a Senior Member of the Technical Staff in the Advanced Propulsion Technology Group at NASA JPL, a laser sail could be accelerated to half the speed of light. However, this would require a steady flow of 17,000 terawatts (TW) of power (close to what the entire world consumes in a single day) for nearly a decade. 

Frisbee also calculated that a sail composed of composite materials and measuring about 200 mi (320 km) in diameter could reach Proxima Centauri in a little more than 12 years, while a larger sail (600 mi; 965 km) could make the transit in just under nine years. 

Efforts to realize directed energy propulsion began in 2014 with Project Dragonfly, a feasibility study for an interstellar mission of small sailcraft capable of reaching a target star system within a century. The top two concepts have since matured into Project Lyra and  Breakthrough Starshot, currently being developed by the Initiative for Interstellar Studies (i4is) and Breakthrough Initiatives (respectively). 

The former envisions lightsails that could achieve velocities of up to 16 mi/s (26 km/s) and rendezvous with interstellar objects like ‘Oumuamua and 2I/Borisov. The latter calls for a 100 Gigawatt (GW) laser to accelerate lightsails to 37,256.5 mi/s (59,958.5 km/s), or 20% the speed of light, allowing it to reach Alpha Centauri within 20 years of launch.

Antimatter 

Another truly exotic idea is the use of matter-antimatter annihilations, a concept that is commonly featured in science fiction. To break it down, antimatter is matter composed of antiparticles with the same mass but opposite charge as regular particles. In an antimatter propulsion system, particles of hydrogen and antihydrogen collide in a reaction chamber.

This reaction would unleash as much energy as a thermonuclear bomb and showers of subatomic particles called pions and muons that travel at relativistic speeds. These are channeled by a magnetic nozzle to generate thrust, gradually pushing a spacecraft up to half the speed of light. An immediate advantage of this propulsion system is the way it combines incredible energy density and high delta-v. 

These benefits make antimatter propulsion the most fuel-efficient and powerful concept ever conceived. Because of its potential, the NASA Institute for Advanced Concepts (NIAC) has investigated the technology to realize future missions to deep space. For such a mission, a few grams of antimatter would be enough to provide rapid transit to Mars and back.

Multiplied exponentially, antimatter propulsion could enable missions to nearby stars in a few years or decades. According to a 2003 report prepared by Frisbee for the 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, a two-stage antimatter rocket would need more than 900,000 tons (815,000 metric tons) of fuel to make the journey to Proxima Centauri in roughly 40 years. 

A 2001 study by Dr. Darrel Smith & Jonathan Webby of the Embry-Riddle Aeronautical University in Arizona was more optimistic. According to their estimates, an interstellar spacecraft weighing 441 tons (400 metric tons) with 187 tons (170 metric tons) of antimatter could reach 0.5 the speed of light and make it to Proxima Centauri in just over eight years. 

The downside is the cost of producing even the most modest amounts of antimatter. Only a handful of nanograms have been created by particle accelerators to date. According to CERN, producing about one billionth of a gram costs a few hundred million Swiss francs (equivalent to a few hundred million US dollars).

A 1999 NASA report estimated that producing a single gram of antimatter would cost $62.5 trillion, while a 2006 report by Dr. Gerald Smith of Positronics Research LLC placed the estimated cost at  $25 billion per gram. This essentially means that antimatter is the most expensive material in the world to manufacture right now.

A possible solution was recommended by astrophysicist Richard Obousy (co-founder of Icarus Interstellar) in a 2011 paper, where he described a Vacuum to Antimatter Rocket Interstellar Explorer System (VARIES). Similar to the Bussard Ramjet discussed in the previous installment, the VARIES spacecraft would harvest its own propellant directly from space. This would be accomplished by firing lasers (powered by massive solar arrays) into the vacuum of space and scooping up the resulting antiparticles.

However, the high cost and energy requirements of such a concept would still be prohibitive and very challenging using today's technology.

Give me Warp Speed!

If humanity ever wants to realize the dream of becoming an interstellar or intergalactic species, we’re going to need to go much faster than any of these concepts allow. Unless we’re willing to wait for centuries (or longer) to travel between star systems, we will have to develop faster-than-light (FTL) propulsion. Unfortunately, the very idea of exceeding the speed of light violates Einstein’s Theory of General Relativity. 

As it stands, there’s one possible candidate for FTL, known as the Alcubierre Warp Drive. This concept was proposed by Mexican theoretical physicist Miguel Alcubierre in 1994 as a way of reconciling faster-than-light space travel with General Relativity. As Alcubierre described it in his seminal paper, “The warp drive: hyper-fast travel within General Relativity”:

“[I]t is possible to modify spacetime in a way that allows a spaceship to travel with an arbitrarily large speed. By a purely local expansion of spacetime behind the spaceship and an opposite contraction in front of it, motion faster than the speed of light as seen by observers outside the disturbed region is possible. The resulting distortion is reminiscent of the “warp drive” of science fiction."

According to Alcubierre, quantum field theory allows for the existence of spacetime regions with negative energy densities. This is known as the Casimir Effect, which describes the attractive force between two surfaces in a vacuum. If a “ring” of negative mass could be created around a spacecraft, spacetime could theoretically be contracted in front of the ship and expanded behind, allowing the spacecraft to effectively travel faster than the speed of light.

The spacecraft would not be violating Relativity since it is merely riding a wave generated by the expansion and contraction of local spacetime. It would also allow the spacecraft to avoid the relativistic effects of rapid travel through space, such as time dilation (time slows down as the speed of light approaches), the massive increase in inertial mass, and the extreme energy required to keep accelerating.

According to Alcubierre’s original paper, the amount of negative mass required to achieve a warp field was beyond anything humanity could achieve. However, multiple studies have presented revised estimates of the amount of exotic mass required that place it within the realm of possibility. The most notable of which was the work of Dr. Harold “Sonny” White, the former head of the Advanced Propulsion Physics Research Laboratory (NASA Eagleworks) at NASA’s Johnson Space Center.

Dr. White’s work in warp fields began in 2011 while preparing to deliver a speech for the first 100-Year Starship symposium. After performing a sensitivity analysis with Alcubierre’s field equations, he realized that negative vacuum energy density could be significantly reduced. Dr. White published these findings a year later in a paper titled “Warp Field Mechanics 101.”

This paper is built on previous research by one of Dr. White’s colleagues, astrophysicist Richard Obousy. In 2009, Obousy co-authored a paper titled “Casimir energy and the possibility of higher dimensional manipulation,” where he argued that next-generation particle accelerators could produce Standard Model fields that could adjust the density of dark energy locally and change the expansion of spacetime. 

Between 2012 and 2019, Dr. White and his colleagues at NASA Eagleworks investigated the possibility of an Alcubierre warp drive. Since then, he has continued to pursue these efforts through the non-profit Limitless Space Institute, along with other advanced propulsion concepts. However, the concept is still very much in the realm of theory and nowhere near to being realized.

Which will it be?

As noted, exotic methods of propulsion are needed if humanity ever hopes to break free of the Solar System and become an interplanetary species. However, most of the methods proposed to date are either too much of an engineering challenge, too expensive, require future advancements and breakthroughs, or any combination thereof.

Right now, the only concept that appears to be feasible and within the realm of possibility is that of directed energy propulsion. In the coming decades, it may become a regular feature of space exploration: lightsails with tiny smartphone-sized spacecraft being sent throughout the Solar System and beyond to explore distant planets, moons, and other celestial objects.

For those working to realize this propulsion technology, these missions would serve as pathfinder missions, paving the way for larger spacecraft that would follow. If there's enough time between these missions, other propulsion concepts might be able to mature to the point where they are possible. Step by step, we could find ourselves sending more sophisticated robotic explorers to space, culminating in the first interstellar crewed missions.

Perhaps this will be how future generations will explore and ultimately settle beyond the Solar System. Given enough time, perhaps they will put down roots across a significant part of the galaxy. Only time, and ongoing research and development, will tell...