From Point A to Point B: How Do We Achieve Interstellar Flight?

The Universe is a pretty big place, and getting from one star system to the next presents some seriously astronomical challenges!

From Point A to Point B: How Do We Achieve Interstellar Flight?
Credit: Adrian Mann

To travel to another star in our galaxy, finding another system of planets, and maybe even finding life there. Such has been the dream for scientists, futurists, and dreamers for centuries. But it has only been within the past century or so that human beings have been able to contemplate this as a serious possibility.

RELATED: NASA IS PLANNING AN INTERSTELLAR MISSION TO EXPLORE POTENTIALLY HABITABLE PLANETS

And while we are still a long way from being able to venture to other star systems, we are getting closer. In recent years, not one but two robotic spacecraft have managed to venture beyond our Solar System and make it into interstellar space.

These are the Voyager 1 and 2 probes, which explored the outer Solar System during the 1970s and 1980s and entered the interstellar medium in 2012 and 2018, respectively. In the not-too-distant future, they will be joined by two more spacecraft (Pioneer 10 and 11).

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Artist's impression of the Voyager Golden Record, which accompanied both probes into space. Credit: NASA

However, it will be many thousands of years before these spacecraft reach any of the closest stars to our Solar System. The same goes for any spacecraft that have launched from Earth in more recent years. For example, consider the New Horizons mission, which launched from Earth in 2006 and achieved its historic flyby with Pluto in 2015.

Since then, it became the first spacecraft to rendezvous with a Kuiper Belt Object (KBO) - known as Ultima Thule (2014 MU69) - and the fifth to achieve the necessary escape velocity to leave the Solar System. At its current velocity and heading, it is estimated that the spacecraft will reach interstellar space by 2040.

But like its interstellar peers, it will not reach another star system for thousands of years. In fact, even if we launched a spacecraft tomorrow using our most cutting-edge propulsion technology, it would still be many centuries or millennia before it ever made it to the closest star system.

Our Nearest Cosmic Neighbors:

The closest star to our Solar System is Proxima Centauri, a main sequence M-type (red dwarf) star roughly 4.24 light years away. This star is part of a triple star system that also consists of the Alpha Centauri system, which consists of a main sequence G-type star (yellow dwarf) similar to our Sun and a main sequence K-type (orange dwarf) star.

In addition to being the closest star to our own, the Alpha Centauri system also contains the nearest Sun-like star to our own. The next closest, Tau Ceti, is just under 12 light years away, though it is the closest single Sun-like star to the Solar System. This system also has the honor of being where the closest confirmed exoplanet is known to exist.

This is none other than Proxima b, a rocky planet whose discovery was announced in 2016 by the European Southern Observatory. This planet is at least 1.3 times as massive as Earth and orbits at a distance of about 0.05 AU from its star - which places it within the star's habitable zone - and has an orbital period of approximately 11.2 days.

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Since it's discovery, researchers have conducted multiple studies to determine whether or not it could support life. What they found was that depending on the composition of the planet's atmosphere and whether or not it had a magnetic field, life could exist there in the form of basic organisms, most likely in a day-side facing ocean.

For this reason, multiple proposals have been made to send a robotic spacecraft there that would be capable of studying the planet, its atmosphere, and assessing what conditions are like on the surface. One of the top contenders is Breakthrough Starshot, which hopes to send a nanocraft using directed energy and a lightsail within our lifetimes.

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Artist's impression of Proxima b. Credit: ESO/M. Kornmesser

"Within our lifetimes" is the key phrase here. In order for us to send missions that can make it to the nearest stars in a short amount of time, we will likely have to restrict our efforts to sending robotic spacecraft, or we'll need to come up with some very sophisticated methods (or both!)

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To put it mildly, space is really, REALLY big! Hence why we use units like light-years (the distance that light particles travel in a single year) to measure the distance. And just to put that in perspective, the speed of light is 299,792,458 m/s (1,079 million km/h; 670.6 million mph).

And since this whole Faster-Than-Light (FTL) thing is still entirely speculative (or just impossible) we are stuck with a smaller range of options in the meantime. So what are our options? Is there a way to make spacecraft fast enough to get the nearest stars in a respectable amount of time, or will we have to settle for sending tiny robotic probes if we want to hear from them "in our lifetimes"?

Conventional Methods:

When we talk of "conventional methods", we mean those that rely on technology that currently exists and/or has been proven to be effective. The most obvious of these is chemical rockets, where solid or liquid fuels are ignited and the resulting combustion is channeled through nozzles to generate thrust.

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For example, the New Horizons mission was the fastest object launched from Earth, reaching a launch velocity of 58,500 km/h (36,400 mph). At this speed, it was able to make it to the Moon - which lies at an average distance of about 384,400 km (238,850 mi) from Earth - in just 8 hours and 35 minutes.

However, it was the Helios 2 mission – launched in 1976 to study solar processes - that established the record for highest speed achieved by a spacecraft - 240,000 km/hr (150,000 mph). This was done with the help of a gravity assist, where a spacecraft uses the gravitational force of a large object (like a planet or star) to slingshot around it and pick up a boost in velocity.

But even at this speed, it would still take a whopping 19,000 years to reach Proxima Centauri. Another problem is the fact that the spacecraft that rely on chemical propellants exhaust their fuel very quickly in order to achieve top velocity. Ionic propulsion (aka. the Hall-Effect Thruster) are much more fuel-efficient and achieve maximum velocity more slowly.

One of the first missions to rely on an ion drive was NASA's Deep Space 1 mission, a technology demonstrator that rendezvoused with the asteroid 9969 Braille and comet Borrelly in 1998. DS1 relied on a xenon-powered ion drive that over the course of 20 months managed to reach a velocity of 56,000 km/hr (35,000 miles/hr).

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Prototype Hall-effect thruster being tested at NASA's Glenn Research Center. Credit: NASA

Ion thrusters are therefore more economical than rocket technology, as the thrust per unit mass of propellant (a.k.a. specific impulse) is far higher. But it takes a long time for ion thrusters to accelerate spacecraft to any great speeds, and the maximum velocity it can achieve is dependent on its fuel supply and how much electrical energy it can generate.

At this velocity, it would take a spacecraft over 81,000 years to travel from Earth to Proxima Centauri. Once again, that's a very long time. To put it in perspective, a spacecraft that relied on conventional engines would take over 750 generations to reach Proxima Centauri, while one that used ion engines would take over 3,200 generations.

Now compare that to crewed missions. The Apollo 10 spacecraft, which flew to the Moon without landing in 1969, holds the record for the highest speed attained by a crewed vehicle with 11.08 km/s (39,888 km/h; 24,791 mph). At this speed, it was able to make it to the Moon in just under 2 days and 4 hours.

But to get to the nearest star, it would take roughly 114,800 years (or about 4600 generations). That kind of defeats the purpose of sending a mission to an extrasolar system, doesn't it? By the time the spacecraft reached it and was in a position to send back information, anyone who witnessed the launch would be long dead.

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The Apollo 10 Command Module in orbit of the Moon. Credit: NASA

Moreover, technology would have advanced so much in that intervening period that chances are, subsequent missions using more advanced technology would have made it there first. So what options does that leave us? How do we bridge the immense gulf between our planet and the nearest extrasolar one in a reasonable time frame?

Possible Methods (Existing Technology):

Currently, there are many options for building a spacecraft that would utilize proven technology to cut the amount of time it takes to reach a destination. These technologies are favorable because they offer a high amount of specific impulse, in that they extract the maximum amount of energy from their fuel source at a minimal cost.

With enough time and energy, a spacecraft might be able to reach "relativistic speeds" (a fraction of the speed of light). As expected, the main barrier is cost, since none of these concepts have been built before and would require some serious engineering to get them ready.

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Nuclear-Thermal/Nuclear Electric Propulsion (NTP/NEC):
Nuclear propulsion has been explored extensively by NASA and other space agencies for the sake of conducting long-duration missions to Mars and other locations far from Earth. In addition to offering a high level of energy, thrust, and fuel-efficiency, nuclear reactors are a proven technology.

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Artistic impression of a "bimodal" nuclear spacecraft. Credit: NASA

Applications that call for the use of nuclear reactors can be divided into two basic categories: nuclear-thermal propulsion (NTP) and nuclear-electric propulsion (NEP). In the case of the former, the decay of radioactive elements (uranium or deuterium) is used to heat liquid hydrogen, which turns it into ionized gas (plasma) that is then channeled through nozzles to generate thrust.

In the case of the latter, a nuclear reactor converts heat into electrical energy to power an electrical engine. In both cases, the rocket would rely on nuclear fission or fusion to generates propulsion rather than chemical propellants. These methods offer a number of advantages over conventional methods, the first and most obvious being fuel-efficiency.

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Compared to chemical rockets and ion engines, nuclear propulsions offer a virtually unlimited energy density and superior thrust based on the amount of propellant used. This offers the added benefit of having to carry less propellant, which means the spacecraft could either be smaller or have more room for cargo and other amenities.

Although no NTP or NEP engines have ever been used as part of a spacecraft, several design concepts have been proposed and some have even been built and tested. These have ranged from the traditional solid-core design – such as the Nuclear Engine for Rocket Vehicle Application (NERVA) – to more advanced and efficient concepts that rely on either a liquid or a gas core.

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The key elements of a NERVA solid-core nuclear-thermal engine. Credit: NASA

Despite these advantages in thrust and fuel-efficiency, the most sophisticated NTP concept has a maximum specific impulse of 5000 seconds (50 kN·s/kg). Based on this level of thrust, NASA has estimated that it would take a spacecraft using this method 90 days to get to Mars when it is closest in its orbit to Earth (i.e. when it is at "opposition", which happens every two years).

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But adjusted for a one-way journey to Proxima Centauri, a nuclear rocket would take centuries to reach relativistic speeds and about 1000 years to reach its destination. So while nuclear rockets are a sound means for exploring the Solar System, they are still limited when it comes to interstellar missions.

While the vast majority of experiments took place during the Cold War Era, NASA recently decided to pursue the technology again through their Game Changing Development Program at the NASA Marshall Space Flight Center. To develop NTP mission concepts, they've partnered with the Virginia-based nuclear tech company, BWX Technologies (BWXT).

Nuclear Pulse Propulsion (NPP):
Here is another possible propulsion method that relies on nuclear reactions, but in a much less controlled form. Put simply, NPP consists of a large spacecraft dropping nuclear devices in its wake and detonating them. The shock is then converted into forward momentum by a "pusher" pad.

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The concept was originally proposed in 1946 by Stanislaw Ulam, a Polish-American mathematician who participated in the Manhattan Project. In 1947, he and American physicist Frederick Reines performed the preliminary calculations.

By 1958 and 1963, Ted Taylor at General Atomics and physicist Freeman Dyson from the Institute for Advanced Study at Princeton University came together to spearhead a project to develop NPP. Known as Project Orion, this effort aimed to harness the power of thermonuclear explosions to generate a high specific impulse.

Though hardly elegant by modern standards, the advantage of the design is that it can theoretically achieve relativistic speeds, with some estimates suggesting it could achieve a velocity as high as 5% the speed of light (or 54 million km/h; 33.55 million mph). At this speed, an Orion spacecraft could make it to Proxima Centauri in just under 20 years!

Of course, there's a big downside to this design, which is its size. According to estimates produced by Freeman Dyson in 1968, an Orion spacecraft that used hydrogen bombs to generate NPP would weight 400,000 to 4,000,000 metric tons. And at least three-quarters of that weight consists of nuclear warheads that weight about 1 metric ton each.

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Artist's impression of an Orion spacecraft. Credit: bisbos.com/Adrian Mann

According to Dyson’s most conservative estimates, it would cost roughly $367 billion in 1968 dollars to build the spacecraft itself. Adjusted for inflation, that works out to roughly $2.6 trillion dollars, which accounts for over 75% of the US government's revenue for 2019.

There’s also the problem of all the radiation and nuclear waste this spacecraft would leave in its wake. In fact, it was for this reason that the Project may have been terminated in 1963. It was in this same year that the Partial Test Ban Treaty was passed, which sought to limit nuclear testing and fallout in the planet’s upper atmosphere.

Fusion Rocket:
Another possibility that relies on nuclear power involves rockets that use on thermonuclear reactions to generate thrust. In this case, pellets of deuterium and helium-3 are compressed by electron beams to the point where a fusion reaction occurs. This creates a high-energy plasma that would be focused by magnetic nozzles to generate thrust.

Like nuclear rockets, this concept also has the advantages of fuel efficiency and specific impulse. Exhaust velocities of up to 10,600 km/s (38 million km/h; 23.7 million mph) are estimated, which is far beyond the speed of conventional rockets. Also like nuclear rockets, the technology has been studied extensively and may proposals have been made over the past few decades.

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Artist's impression of Project Daedalus spacecraft standing next to the Saturn V rocket. Credit: Adrian Mann

Between 1973 and 1978, the British Interplanetary Society conducted a feasibility study known as Project Daedalus. This study called for the creation of a two-stage unmanned spacecraft, based on current knowledge and technology, that would be able to make the trip to Barnard’s Star (5.9 light years from Earth) in a single lifetime.

The larger first stage would operate for 2.05 years and accelerate the spacecraft to 7.1% the speed of light (0.071 c). This stage would then be jettisoned and the smaller second stage would accelerate the spacecraft up to about 12% of light speed (0.12 c) over the course of 1.8 years.

The second-stage engine would then be shut down and the ship would cruise at this velocity for 46 years. All told, the mission would take 50 years to reach Barnard's Star. Adjusted for Proxima Centauri, this same type of spacecraft could make the trip in 36 years.

Of course, the project also identified numerous stumbling blocks as well, most of which are still unresolved. For instance, there is the fact that helium-3 is scarce on Earth, which means it would have to be mined elsewhere else - such as the Moon or the outer Solar System.

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Artist's impression of an Icarus spacecraft. Credit: Icarus International/Adrian Mann

Second, the fusion reaction driving the spacecraft would need to vastly exceed the energy used to trigger the reaction. And while experiments here on Earth have made significant progress, we are still a long way away from the point where the energy produced would be sufficient.

Third, there is the sheer cost of building such a ship. Even by the most conservative estimates, a fully-fueled Daedalus’ spacecraft could weight as much as 60 billion metric tons (66 billion US tons). To put that in perspective, the gross weight of NASA’s Space Launch System (SLS) is just over 30 million metric tons (33 million US tons).

Based on previous estimates, a single launch of the SLS will cost about $5.46 billion USD. Extrapolating for a ship that is 2000 times as massive, you come out with a ballpark figure of $10.92 trillion. In short, a fusion rocket would be both prohibitively expensive and requires a level of fusion reactor technology that is currently beyond our means.

Icarus Interstellar, an international organization of volunteer citizen scientists (some of whom worked for NASA or the ESA) have since attempted to revitalize the concept with Project Icarus. Founded in 2009, the group hopes to make fusion propulsion and other advanced propulsions concepts feasible during the course of the 21st century.

Fusion Ramjet:
Also known as the Bussard Ramjet, this concept shares some similarities with Project Daedalus and Icarus. Here too, the spacecraft would generate thrust by compressing hydrogen fuel to the point of fusion. But unlike other fusion rockets, the Bussard Ramjet was designed to provide its own fuel by scooping it from the Interstellar Medium (ISM).

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Artist's concept of a Bussard Ramjet. Credit: Adrian Mann

This is done via an enormous electromagnetic field, which collects hydrogen as the ship travels. Acceleration forces this hydrogen into a progressively tighter magnetic field, compressing it until thermonuclear fusion occurs and then directing it through nozzles to generate propulsion.

The main benefit of this design is that it does not require any fuel, which eliminates the burden of collecting the enormous amounts needed to power an interstellar journey. Its reaction mass is also the most common element in the Universe (hydrogen), which makes up over 90% of the ISM in terms of mass.

In addition, it also eliminates the single-greatest source of a spacecraft's mass, thus allowing for speeds approaching 4% of the speed. However, in addition to being very costly to build, the concept presents some major issues, not the least of which is drag.

In short, the very interstellar medium a Bussard Ramjet scoops its fuel from would also be what slows it down, and this would become progressively worse as the spacecraft accelerates. In addition, scientists have determined that the interstellar medium is significantly less dense than what was assumed in 1960 when the concept was first proposed by physicist Robert W. Bussard.

Generation Ships:
Another possibility is to forgo the idea of making it to another star system quickly and to develop a spacecraft that will be suitable for a very long haul. In this case, the spacecraft would need to be large enough to accommodate a crew of a few hundred (or few thousand) over the course of multiple generations.

The first recorded example of this concept comes from American rocketry-pioneer Robert H. Goddard, for whom NASA's Goddard Space Flight Center is named. In his 1918 essay, “The Ultimate Migration”, he described an “interstellar ark” of cryogenically frozen passengers leaving the Solar System after the death of the Sun.

Goddard recommended that, in the event that atomic energy could not be harnessed to power such a spacecraft, that a combination of hydrogen and oxygen should be used, as well as solar energy. He envisioned that these would be sufficient to get the ship up to speeds of 4.8 to 16 km/s (3 to 10 mi/s) - roughly 57,936 km/h (36,000 mph).

He also envisioned that the crew would be kept in stasis with the pilot being awakened at intervals to steer the ship:

"This will, of course, necessitate a very large apparatus, initially, unless solar energy can be used over a considerable time, to get up speed, either by passing through the Solar System from end to end, crossing as far from the Sun as possible, or spiralling outward until sufficient speed has been obtained.

"The pilot should be awakened, or animated, at intervals, perhaps of 10,000 years for a passage to the nearest stars, and 1,000,000 years for great distances, or for other stellar systems. To accomplish this, a clock operated by a change in weight (rather than by electric charges, which produce too rapid effects) of a radiation substance, should be used... This awakening would, of course, be necessary in order to steer the apparatus, if it became off its course."

See Also: Megastructures: A Sign of Larger Than Life Aliens

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Artist's concept of an Enzmann starship, a fusion rocket concept proposed in 1964. Credit: Rick Sternbach

This was followed by Konstantin E. Tsiolkovsky (often hailed as the "father of spaceflight"), who wrote of a “Noah’s Ark” in his essay “The Future of Earth and Mankind” in 1928. In Tsiolkovsky's version, these spaceships would be self-sufficient and crews would be kept in wakeful conditions until they reached their destination thousands of years later.

In 1964, Dr. Robert Enzmann proposed the most detailed concept for a generation ship to date. Known as "Enzmann Starship", the proposal called for a deuterium-powered generation ship that would measure 600 meters (2000 feet) in length and support an initial crew of 200 people (with room for expansion).

While this concept frees up planners from coming up with a rapid means of transportation, it presents a series of downsides. These were raised in a series of studies (2017-2019) conducted by Dr. Frederic Marin of the Astronomical Observatory of Strasbourg using tailor-made numerical software (called HERITAGE).

For instance, in the first two studies, Dr. Marin and colleagues conducted simulations that showed that a minimum crew of 98 (max. 500) would need to be coupled with a cryogenic bank of sperm, eggs, and embryos in order to ensure genetic diversity and good health upon arrival.

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The interior of a self-contained space colony. Credit: Rick Guidice

In the third study, Dr. Marin and another group of scientists determined that the ship carrying them would need to measure 320 meters (1050 feet) in length, 224 meters (735 feet) in radius, and contain 450 m² (~4,850 ft²) of artificial land to grow enough food to sustain them.

Like many other concepts presented here, a ship of this size would be prohibitively expensive to build. It would also be very challenging to see to the needs of a crew that numbered in the hundreds. Beyond food and water, there is also the matter of maintaining their psychological health.

There is also the possibility that while this ship is taking centuries or longer to travel to another star system, other, faster means of propulsion will have been invented. So while a multi-generational crew is traveling to a nearby star, another mission will be able to overtake it.

Directed-Energy Propulsion:
Solar sails (or light sails) have long been considered to be a cost-effective way of exploring the Solar System. Rather than relying on rockets or propellant, a sail relies on radiation pressure from a star to push a large reflective surface to high speeds. The obvious benefit of such a system is that it is lightweight and requires no fuel.

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Artist's impression of Breakthrough Starshot. Credit: Breakthrough Initiatives

But in the case of interstellar missions, the sail would need to be accelerated by focused beams of directed energy (i.e. lasers or microwaves) to reach a velocity where it could make it to a nearby star system in a reasonable amount of time. The concept was originally proposed by Robert Forward in 1984, who was a physicist at the Hughes Aircraft’s Research Laboratories at the time.

Much like a solar sail, an interstellar light sail benefits from the fact that it carries no fuel, but also the fact that laser energy does not dissipate with distance nearly as much as solar radiation does. So while it would take time to accelerate a laser-driven sail, it would be able to accelerate over long distances.

In 1999, Geoffrey A. Landis conducted a Phase I concept study for the NASA Institute for Advanced Concepts (NAIC) on the possibility of a laser-pushed lightsail. The concept would consist of a sail composed of dielectric thin films that would be accelerated up to speeds of up to 30,000 km/s or 10% the speed of light (0.1c).

Such a spacecraft, he argued would be ideal for "fast-transit missions to the outer planets, Kuiper and Oort cloud missions, and interstellar precursor missions." Used for a mission to Proxima Centauri, a spacecraft of this type would be able to make the journey in about 43 years.

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An artist’s illustration of a light-sail powered by a radio beam (red) generated on the surface of a planet. Credit: M. Weiss/CfA

Another version was proposed in a 2000 study by Robert Frisbee, a director of advanced propulsion concept studies at NASA’s Jet Propulsion Laboratory. According to his variation, a lightsail measuring 320 km (200 miles) in diameter could be accelerated to 0.35c and would make the journey in just over 12 years.

Meanwhile, a sail measuring about 965 km (600 mi) in diameter could be accelerated to 0.47c and make the journey in under 9 years. However, according to Frisbee’s own study, the lasers would require a steady flow of 17,000 terawatts of power – close to what the entire world consumes in a single day!

In recent years, the concept of the lightsail has been updated and new variants proposed. In April 2016, Russian billionaire Yuri Milner launched Breakthrough Starshot, a program dedicated to creating a lightsail-driven “wafercraft” (dubbed the StarChip) that would make the journey to Proxima Centauri within our lifetime.

By leveraging advancements made in miniaturization, the spacecraft itself would be no larger than a smartphone or credit card and would be studded with tiny sensors, a guidance and navigation system, and tiny thrusters. This craft would then be towed by a large light sail that would be accelerated by an Earth-based laser array.

With a maximum velocity of about 60,000 km/s (37,282 mps) – or 20% the speed of light (0.2 c) - the craft would be able to make the journey to Proxima Centauri in just 20 years. As Prof. Abraham Loeb, the Frank B. Baird Professor of Physics at Harvard University and the Chair of the Starshot Advisory Committee said in a recent interview with SciTech Europa Quarterly:

"The only method that seemed potentially feasible when we first started to consider this challenge was light sail technology, where, rather than carrying fuel, you use a powerful laser to push against a lightweight sail. We established the perimeters for this about six months later, finding that we would need to use a very powerful laser (some 100 gigawatts in power) which will be delivered to the sail for a few minutes. If the sail weighs roughly a gram, and the payload (including the camera, navigation device, and communication device) weighs roughly a gram, then it is possible to reach a fifth of the speed of light by pushing on the sail, which is a few meters in size, with the laser."

A similar mission concept is known as Project: Dragonfly, which would also rely on a laser-driven lightsail to achieve relativistic speeds. But unlike Starshot, the Dragonfly concept calls for a significantly heavier spacecraft to allow for more scientific instruments to be included. The spacecraft would also carry a magnetic sail to slow it down upon arrival in Proxima Centauri.

Both of these concepts were the result of the same conceptual design study, which was hosted by Initiative for Interstellar Studies (i4iS) in 2013. Dragonfly and Starshot were the top two designs and were presented with two other finalists at a workshop held at the British Interplanetary Society in July 2015.

The Dragonfly concept won and the team behind it (from the Technical University of Munich) then launched a Kickstarter campaign to raise money for their design. Meanwhile, the design submitted by the team from the University of California, San Diego, evolved into the design for Breakthrough Starshot.

Theoretical Methods:

Beyond ideas that rely on readily-available or tested technology, there are also those that would incorporate methods that are either unproven or are still in the realm of the theoretical. It is these methods, by and large, that proponents of crewed interstellar spaceflight have their hopes pinned on, mainly because there exist no affordable means to send human beings to the nearest star within a single lifetime... yet!

Antimatter Rockets:
Put simply, antimatter is composed of antiparticles, which have the same mass but opposite charge as regular particles. When collisions between matter and antimatter particles occur, they annihilate each other and release a significant amount of energy.

For the sake of an antimatter rocket, collisions between hydrogen and antihydrogen atoms would be harnessed. These reactions produce as much energy as a thermonuclear bomb, along with subatomic particles known as pions and muons.

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Artist's concept of an antimatter rocket. Credit: NASA/MSFC

In one version of this concept, these particles (which travel at one-third the speed of light) would be channeled by a magnetic nozzle to create thrust. In other versions, similar to what is achieved with nuclear propulsion, the reactions can be used to heat propellant (like hydrogen) or generate electricity to create thrust.

The most advantages to this class of rocket are its comparatively low rest mass and considerable energy density. Pound for pound, it is the most powerful and fuel-efficient concept ever proposed. In fact, the mutual annihilation of a half pound of hydrogen and antihydrogen particles would unleash more energy than a 10-megaton hydrogen bomb.

With this kind of energy density in a lightweight package, it would be rather easy (at least in theory) to achieve relativistic velocities. According to a report by Dr. Darrel Smith & Jonathan Webby of the Embry-Riddle Aeronautical University in Arizona, an antimatter rocket could reach 0.5 the speed of light and reach Proxima Centauri in a little over 8 years

It is for this reason that NASA’s Institute for Advanced Concepts (NIAC) has researched the technology as a possible means for long-duration missions (such as missions to Mars and other locations in the Solar System). Unfortunately, when it comes to interstellar missions, the amount of fuel it would take to make the trip (and hence the cost) is multiplied exponentially.

From Point A to Point B: How Do We Achieve Interstellar Flight?
Artist’s concept of the Vacuum to Antimatter Rocket Interstellar Explorer System (VARIES). Credit: Adrian Mann

According to report prepared by Frisbee for the 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, a two-stage antimatter rocket would need over 815,000 metric tons (900,000 US tons) of fuel to make the journey to Proxima Centauri in approximately 40 years.

While that is certainly respectable, the cost of manufacturing that much antimatter would be enormous. In 1999, NASA estimated that it would take $62.5 trillion to produce a single gram of antihydrogen. In 2006, for the sake of a NASA-funded study, Dr. Gerald Smith of Positronics Research estimated that it could be done for $25 billion a gram.

At present, the total amount of antimatter that has been created using particular accelerators is less 20 nanograms. As a possible solution, proposed by Richard Obousy of Icarus Interstellar in a 2012 study, is to build a vessel that can create antimatter which it could then store as fuel.

This concept, known as the Vacuum to Antimatter-Rocket Interstellar Explorer System (VARIES), would rely on large lasers (powered by enormous solar arrays) which would create particles of antimatter when fired at empty space. Like the Ramjet concept, this proposal solves the problem of carrying fuel by harnessing it from space.

Some studies performed by the NIAC have explored whether or not it might be possible to use magnetic scoops to collect naturally-occurring antimatter in radiation fields - like the Van Allen Belt around Earth, the radiation belts of gas giants, etc. However, these concepts still require the creation of starships that would be prohibitively expensive to build using today's technology.

Alcubierre Warp Drive:
Proposed by Mexican physicist Miguel Alcubierre in 1994, this proposed method was an attempt to find a means of FTL travel that would not violate Einstein’s theory of Special Relativity. In short, the concept involves stretching the fabric of space-time in a wave, which would (in theory) cause the space ahead of an object to contract while the space behind it would expand.

An object inside this wave (i.e. a spaceship) would then be able to ride this region, known as a “warp bubble” of flat space. This is what is known as the “Alcubierre Metric”, which (when interpreted in the context of General Relativity) would allow a warp bubble to appear in a previously flat region of spacetime and move away, effectively at speeds that exceed the speed of light.

Since the ship is not moving within this bubble but is being carried along as the region itself moves, conventional relativistic effects such as time dilation would not apply. Hence, the rules of space-time and the laws of relativity would not be violated in the conventional sense.

From Point A to Point B: How Do We Achieve Interstellar Flight?
Artist's impression of the Alcubierre Drive at work. Credit: NASA/Les Bossinas

Assuming that a spacecraft could be outfitted with an Alcubierre Drive system, it would be able to make the trip to Proxima Centauri in less than 4 years. So when it comes to theoretical interstellar space travel, this is by far the most promising technology, at least in terms of speed.

Unfortunately, the concept also presents a number of theoretical problems. For starters, there are currently no known methods for creating a warp bubble in a region of space that does not already contain one. In addition, extremely high energies would be required to create this effect, and there is no known way for a ship to exit a warp bubble once it has entered.

Attempts at development currently include ongoing investigations by the Tau Zero Foundation, which is continuing the work of NASA's Breakthrough Propulsion Physics Project (BPP) which was founded in 1996 and discontinued in 2002.

In 2012, NASA’s Advanced Propulsion Physics Laboratory (aka. Eagleworks) began investigating the concept and developed an interferometer to detect spatial distortions produced by the Alcubierre metric. The team lead – Dr. Harold Sonny White – described their work in a NASA paper titled Warp Field Mechanics 101. To date, their research has been inconclusive.

From Point A to Point B: How Do We Achieve Interstellar Flight?

Artist's impression of a black hole (labeled). Credit: ESA/Hubble, ESO, M. Kornmesser

Black Hole Spacecraft:
Here's where things get really interesting! For decades, scientists have understood that in addition to altering the curvature of spacetime, black holes also take in matter and emit energy in the form of Hawking radiation. If properly harnessed, this would make a tiny artificial black hole an extremely powerful power source.

The concept was first introduced by science fiction author Arthur C. Clarke in this 1975 novel, Imperial Earth while a similar idea was presented by Charles Sheffield in his 1978 short story, “Killing Vector”. In both cases, Clarke and Sheffield describe how advanced civilizations could extract energy from rotating black holes to meet their energy needs.

The concept was explored more recently in a 2019 study by Dr. Louis Crane and Shawn Westmoreland of Kansas State University. In their proposal, a tiny black hole would be created using a gamma-ray laser that would create a black hole that is roughly a billion tonnes in mass.

It goes without saying that the ability to create and harness an artificial black hole is far beyond anything humanity is capable of. However, there is also a proposed method for harnessing the gravitational power of black holes to accelerate a spacecraft. It's known as the Halo Drive, a concept proposed by Prof. David Kipping, the leader of Columbia University’s Cool Worlds lab.

This method is a modified version of a “Dyson Slingshot“, an idea was proposed by physicist Freeman Dyson (who also proposed the Dyson Sphere). In his 1963 book, Interstellar Communications (Chapter 12: “Gravitational Machines“), Dyson described how spacecraft could slingshot around compact binary stars in order to receive a significant boost in velocity.

Kipping's version of this "gravitational machine" considered how black holes – especially binary pairs – could constitute an even more powerful gravitational slingshot. This is based in part on the recent work by the Laser Interferometer Gravitational-Wave Observatory (LIGO), which has indicated that there could be as many as 100 million black holes in the Milky Way galaxy alone.

This concept also comes with challenges, which include building a spacecraft capable of surviving being accelerated by an event horizon, as well as the tremendous amount of precision needed. On top of that, there’s the challenge of reaching a black hole, which could take millennia.

Conclusion:

When it comes right down to it, there is no simple or easy method for interstellar space travel. But when assessing the available options, the question - "can it be done?" - does have a simple enough answer. As with anything having to do with long-term plans for space exploration, the answer is "yes, but..."

Depending on the method, they are either too slow to send a crewed mission to another star in a relatively brief amount of time, they are prohibitively expensive, they would require technology that doesn't yet exist, or they are based on theoretical principles that remain unproven.

So really, depending upon your preferred method of interstellar travel, the answer is either, "yes, but how long are you willing to wait?", or "yes, but how much are you willing to spend?" Once you figure that out, you can pretty much accomplish anything.

Further Reading:

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