Achieving interstellar travel has been the dream of countless generations, but the challenges remain monumental. Aside from the vast distances involved, there are also the prohibitive energy requirements and the sheer cost of assembling spacecraft that could survive the trip. Right now, the best bet for achieving an interstellar mission within a reasonable timeframe (i.e., a single person’s lifetime) is to build gram-scale spacecraft paired with lightsails. Using high-power laser arrays, these spacecraft could be accelerated to a fraction of the speed of light (relativistic speeds) and reach nearby stars in a few decades.
There are a handful of major projects, like Breakthrough Starshot, that hope to leverage this technology to create spacecraft that could reach Alpha Centauri in a few decades (instead of centuries). This technology also presents other opportunities, like facilitating communications across interstellar distances. This is the idea recently by a team of researchers led by the Initiative for Interstellar Studies (i4is). In a recent paper, they recommended that a swarm of gram-scale spacecraft could rely on their launch laser to maintain optical communications with Earth.
Today, multiple space agencies are investigating cutting-edge propulsion ideas that will allow for rapid transits to other bodies in the Solar System. These include NASA’s Nuclear-Thermal or Nuclear-Electric Propulsion (NTP/NEP) concepts that could enable transit times to Mars in 100 days (or even 45) and a nuclear-powered Chinese spacecraft that could explore Neptune and its largest moon, Triton. While these and other ideas could allow for interplanetary exploration, getting beyond the Solar System presents some major challenges.
As we explored in a previous article, it would take spacecraft using conventional propulsion anywhere from 19,000 to 81,000 years to reach even the nearest star, Proxima Centauri (4.25 light-years from Earth). To this end, engineers have been researching proposals for uncrewed spacecraft that rely on beams of directed energy (lasers) to accelerate light sails to a fraction of the speed of light. A new idea proposed by researchers from UCLA envisions a twist on the beam-sail idea: a pellet-beam concept that could accelerate a 1-ton spacecraft to the edge of the Solar System in less than 20 years.
Between the exponential growth of the commercial space industry (aka. NewSpace) and missions planned for the Moon in this decade, it’s generally agreed that we are living in the “Space Age 2.0.” Even more ambitious are the proposals to send crewed missions to Mars in the next decade, which would see astronauts traveling beyond the Earth-Moon system for the first time. The challenge this represents has inspired many innovative new ideas for spacecraft, life-support systems, and propulsion.
In particular, missions planners and engineers are investigating Directed Energy (DE) propulsion, where laser arrays are used to accelerate light sails to relativistic speeds (a fraction of the speed of light). In a recent study, a team from UCLA explained how a fleet of tiny probes with light sails could be used to explore the Solar System. These probes would rely on a low-power laser array, thereby being more cost-effective than similar concepts but would be much faster than conventional rockets.
NASA and China plan to mount crewed missions to Mars in the next decade. While this represents a tremendous leap in terms of space exploration, it also presents significant logistical and technological challenges. For starters, missions can only launch for Mars every 26 months when our two planets are at the closest points in their orbit to each other (during an “Opposition“). Using current technology, it would take six to nine months to transit from Earth to Mars.
Even with nuclear-thermal or nuclear-electric propulsion (NTP/NEP), a one-way transit could take 100 days to reach Mars. However, a team of researchers from Montreal’s McGill University assessed the potential of a laser-thermal propulsion system. According to their study, a spacecraft that relies on a novel propulsion system – where lasers are used to heat hydrogen fuel – could reduce transit times to Mars to just 45 days!
It’s no secret that humanity is poised to embark on a renewed era of space exploration. In addition to new frontiers in astronomical and cosmological research, crewed missions are also planned for the coming decades that will send astronauts back to the Moon and to Mars for the first time. Looking even further, there are also ideas for interstellar missions like Breakthrough Starshot and Project Dragonfly and NASA’s Starlight.
These mission concepts entail pairing a nanocraft with a lightsail, which would then accelerated by a directed-energy array (lasers) to achieve a fraction of the speed of light (aka. relativistic velocity). Naturally, this raises a number of technical and engineering challenges, not the least of which is communications. In a recent study, a team of scientists sought to address that very issue and considered various methods that might be used.
In a few decades, the Breakthrough Starshot initiative hopes to send a sailcraft to the neighboring system of Alpha Centauri. Using a lightsail and a directed energy (aka. laser) array, a tiny spacecraft could be accelerated to 20% the speed of light (0.2 c). This would allow Starshot to make the journey to Alpha Centauri and study any exoplanets there in just 20 years, thus fulfilling the dream of interstellar exploration within our lifetimes.
Naturally, this plan presents a number of engineering and logistical challenges, one of which involves the transmission of data back to Earth. In a recent study, Starshot Systems Director Dr. Kevin L.G. Parkin analyzes the possibility of using a laser to transmit data back to Earth. This method, argued Parkin, is the most effective way for humanity to get a glimpse of what lies beyond our Solar System.
At the University of California, Santa Barbara, researchers with the UCSB Experimental Cosmology Group (ECG) are currently working on ways to achieve the dream of interstellar flight. Under the leadership of Professor Philip Lubin, the group has dedicated a considerable amount of effort towards the creation of an interstellar mission consisting of directed-energy light sail and a wafer-scale spacecraft (WSS) “wafercraft“.
If all goes well, this spacecraft will be able to reach relativistic speeds (a portion of the speed of light) and make it to the nearest star system (Proxima Centauri) within our lifetimes. Recently, the ECG achieved a major milestone by successfully testing a prototype version of their wafercraft (aka. the “StarChip“). This consisted of sending the prototype via balloon into the stratosphere to test its functionality and performance.
Scientists, futurists, and science fiction writers have been talking about it for over a century, and fans of science fiction and futurists have fantasized about it for just as long. The portable directed-energy weapon that zaps your enemies, rendering them incapacitated or reducing them to a pile of ashes!
The concept has gone through many iterations over the decades, ranging from laser pistols and cannons to phasers. And yet, this staple of science fiction is largely based in science fact. Since the early 20th century, scientists have sought to develop a working directed-energy weapon, based on ideas put forward by many inventors and scientists.
A”death ray” is a theoretical particle beam or electromagnetic weapon that was originally proposed independently during the 1920s and 30s by multiple scientists. From these initial proposals, research into energy-based weapons has been ongoing. While most examples come predominantly from science fiction, several applications and proposals have been produced during the latter half of the 20th century.
During the early 20th century, many scientists claimed that they had created a working death ray. For instance, in September of 1924, British inventor Harry Grindell-Matthews attempted to sell what he reported to be a death ray that could destroy human life and bring down planes at a distance to the British Air Ministry.
While he was never able to produce a functioning model or demonstrate it to the military, news of this prompted American inventor Edwin R. Scott to claim that he was the first to develop a death ray. According to Scott, he had done so in 1923, which was the result of the nine years he spent as a student and protege of Charles P. Steinmetz – a German-American professor at Union College, New York.
In 1934, Spanish inventor Antonion Longoria claimed to have invented a death ray machine which he had tested on pigeons at a distance of about 6.5 km (4 miles). He also claimed to have killed mice that were enclosed in a thick-walled metal chamber.
However, it was famed inventor and electrical engineer Nikola Tesla who provided the most detailed framework for such a device. In a 1934 interview with Time Magazine, Tesla explained the concept of a “teleforce” (or directed energy) weapon which would be capable of destroying entire squadrons of airplanes or an entire army at a distance of 400 km (250 miles).
Tesla tried to interest the US War Department and several European countries in the device at the time, though none contracted with Tesla to build it. As Tesla described his invention in an article titled “A Machine to End War“, which appeared in Liberty Magazine in 1935:
“this invention of mine does not contemplate the use of any so-called ‘death rays’. Rays are not applicable because they cannot be produced in requisite quantities and diminish rapidly in intensity with distance. All the energy of New York City (approximately two million horsepower) transformed into rays and projected twenty miles, could not kill a human being, because, according to a well known law of physics, it would disperse to such an extent as to be ineffectual. My apparatus projects particles which may be relatively large or of microscopic dimensions, enabling us to convey to a small area at a great distance trillions of times more energy than is possible with rays of any kind. Many thousands of horsepower can thus be transmitted by a stream thinner than a hair, so that nothing can resist.”
Based on his descriptions, the device would constitute a large tower that could be mounted on top of a building, positioned either next to shores or near crucial infrastructure. This weapon, he claimed, would be defensive in nature, in that it would make any nation employing it impregnable to attack from air, land or sea, and up to a distance of 322 km (200 miles).
During World War II, multiple efforts were mounted by the Axis powers to create so-called “death rays”. For instance, Imperial Japan developed a concept they called “Ku-Go”, which sought to use microwaves created in a large magnetron as a weapon.
Meanwhile, the Nazis mounted two projects, one which was led by the researcher known as Schiebold that involved a particle accelerator and beryllium rods. The second, led by Dr. Rolf Wideroe, was developed at the Dresden Plasma Physics Laboratory until it was bombed in Feb. 1945. In April of that year, as the war was coming to close, the device was taken into custody by the US Army.
On January 7th, 1943, engineer and inventor Nikola Tesla died in his room at the Hotel New Yorker in Manhattan. A story quickly developed that within his room, Tesla had scientific paper in his possession that provided the most detailed description yet for a death ray. These documents, it was claimed, had been seized by the US military, who wanted them for the sake of the war effort.
Examples in Science Fiction:
Ray guns, and other examples of directed-energy weapons have been a common feature in science fiction for over a century. One of the first known examples comes from H.G. Wells seminal book, War of the Worlds, which featured Martian war machines that used “heat rays”. However, the first use of the term was in The Messiah of the Cylinder (1917), by Victor Rousseau Emanuel.
Ray guns were also a regular feature in comic books like Buck Rogers (first published in 1928) and Flash Gordon, published in 1934. In Alfred Noyes’ 1940 novel The Last Man (released as No Other Man in the US), a death ray developed by a German scientist named Mardok is unleashed in a global war and almost wipes out the human race.
The concept of the blaster was introduced by Isaac Asimov’s The Foundation Series, which were described as nuclear-powered handheld weapons that fired energetic particles. In Frank Herbert’s Dune series, energy weapons take the form of continuous-wave laser projectors (lasguns), which are rendered obsolete by the invention of “Holtzman shields”.
According to Herbert, the interaction of a lasgun blast and this force field results in a nuclear explosion which typically kills both the gunner and the target. Further examples of death rays can be found in just about any science fiction franchise, ranging from phasers (Star Trek) and laser blasters (Star Wars) to spaceship-mounted beam cannons.
In terms of real-world applications, many attempts have been made to create directed-energy weapons for offensive and defensive purposes. For instance, the development of radar before World War II was the result of attempts to find applications for directed electromagnetic energy (in this case, radio waves).
In the 1980s, U.S. President Ronald Reagan proposed the Strategic Defense Initiative (SDI) program (nicknamed “Star Wars”). It suggested that lasers, perhaps space-based X-ray lasers, could destroy ICBMs in flight. During the Iraq War, electromagnetic weapons, including high power microwaves were used by the U.S. military to disrupt and destroy the Iraqi electronic systems.
On March 18th, 2009 Northrop Grumman announced that its engineers in Redondo Beach had successfully built and tested an electric laser capable of producing a 100-kilowatt ray of light, powerful enough to destroy cruise missiles, artillery, rockets and mortar rounds. And on July 19th, 2010, an anti-aircraft laser was unveiled at the Farnborough Airshow, described as the “Laser Close-In Weapon System”.
In 2014, the US Navy made headlines when they unveiled their AN/SEQ-3 Laser Weapon System (or XN-1 LaWS), a directed-energy weapon designed for use on military vessels. Ostensibly, the purpose of the weapon is defensive, designed to either blind enemy sensors (when set to low-intensity) or shoot down unmanned aerial vehicles (UAVs) when set to high-intensity.
Then is what is known as “Active Denial Systems”, which use a microwave source to heat up the water in the target’s skin, thus causing physical pain. Currently, this concept is being developed by the US Air Force Research Laboratory and Raytheon – a US defense contractor – as a means of riot-control.
A Dazzler is another type of directed-energy weapon, one which uses infrared or visible light to temporarily blind an enemy. Targets can include human beings, or their sensors (particularly in the infrared band). The emitters are usually lasers (hence the term “laser dazzler”)and can be portable or mounted on the outside of vehicles (as with the Russian T-80 and T-90 tank).
An example of the former is the Personnel Halting And Stimulation Response rifle (PHASR), a prototype non-lethal laser dazzler being developed by the US Air Force Research Laboratory’s Directed Energy Directorate. Its purpose is give infantry or other military personnel the ability to temporarily disorient and blind a target without causing permanent damage.
Blinding laser weapons were banned by treated under the UN Protocol on Blinding Laser Weapons, which was passed in 1995. However, the terms of this protocol do not apply to directed-energy weapons that inflict only temporary blindness.
We’ve come a long way since the term “raygun” became a household name. At this rate, who knows what the future will hold? Will Tesla’s dream of a Death Ray ever come true? Will we see directed-energy satellites put in orbit, or handheld lasers becoming the mainstay of armed forces and space explorers? Hard to say. All we can be sure of is that the truth will likely be stranger than the fiction!
In 2015, Russian billionaire Yuri Milner founded Breakthrough Initiatives with the intention of bolstering the search for extra-terrestrial life. Since that time, the non-profit organization – which is backed by Stephen Hawking and Mark Zuckerberg – has announced a number of advanced projects. The most ambitious of these is arguably Project Starshot, an interstellar mission that would make the journey to the nearest star in just 20 years.
This concept involves an ultra-light nanocraft that would rely on a laser-driven sail to achieve speeds of up to 20% the speed of light. Naturally, for such a mission to be successful, a number of engineering challenges have to be tackled first. And according to a recent study by a team of international researchers, two of the most important issues are the shape of the sail itself, and the type of laser involved.
As they indicate in their study, titled “On the Stability of a Space Vehicle Riding on an Intense Laser Beam“, the team ran stability simulations 0n the concept, taking into account the nature of the wafer-sized craft (aka. StarChip), the sail (aka. Lightsail) and the nature of the laser itself. For the sake of these simulations, they also factored in a number of assumptions about Starshot’s design.
These included the notion that the StarChip would be a rigid body (i.e. made up of solid material), that the circular sail would either be flat, spherical or conical (i.e. concave in shape), and that the surface of the sail would reflect the laser light. Beyond this, they played with multiple variations on the design, and came up with some rather telling results.
As Dr. Elena Popova, the lead author on the paper, told Universe Today via email:
“We considered different shapes of sail: a) spherical (coincides with parabolic for small sizes) as most appropriate for final configuration of nanocraft en route; b) conical; c) flat (simplest) (will be seen to be unstable so that even spinning of craft does not help).”
What they found was that the simplest, stable configuration would involve a sail that was spherical in shape. It would also require that the StarChip be tethered at a sufficient distance from the sail, one which would be longer than the curvature radius of the sail itself.
“For the sail with almost flat cone shape we obtained similar stability condition,” said Popova. “The nanocraft with flat sail is unstable in every case. It simply corresponds to the case of infinite radius of curvature of the sale. Hence, there is no way to extend center of mass beyond it.”
As for the laser, they considered several how the two main types would effect stability. This included uniform lasers that have a sharp boundary and “Gaussian” beams, which are characterized by high-intensity in the middle that declines rapidly towards the edges. As Dr. Popova stated, they determined that in order to ensure stability – and that the craft wouldn’t be lost to space – a uniform laser was the way to go.
“The nanocraft driven by intense laser beam pressure acting on its Lightsail is sensitive to the torques and lateral forces reacting on the surface of the sail. These forces influence the orientation and lateral displacement of the spacecraft, thus affecting its dynamics. If unstable the nanocraft might even be expelled from the area of laser beam. The most dangerous perturbations in the position of nanocraft inside the beam and its orientation relative to the beam axis are those with direct coupling between rotation and displacement (“spin-orbit coupling”).”
In the end, these were very similar to the conclusions reached by Professor Abraham Loeb and his colleagues at Starshot. In addition to being the Frank B. Baird, Jr. Professor of Science at Harvard University, Prof. Loeb is also the chairman of the Breakthrough Foundation’s Advisory Board. In a study titled “Stability of a Light Sail Riding on a Laser Beam” (published on Sept, 29th, 2016), they too examined what was necessary to ensure a stable mission.
This included the benefits of a conical vs. a spherical sail, and a uniform vs. a Gaussian beam. As Prof. Loeb told Universe Today via email:
“We found that a parachute-shaped sail riding on a Gaussian laser beam is unstable… We show in our paper that a sail shaped as a spherical shell (like a large ping-pong ball) can ride in a stable fashion on a laser beam that is shaped like a cylinder (or 3-4 lasers that establish a nearly circular illumination).”
As for the recommendations about the StarChip being at a sufficient distance from the LightSail, Prof. Loeb and his colleagues are of a different mind. “They argue that in case you attach a weight to the sail that is sufficiently well separated from the parachute, you might make it stable.” he said. “Even if this is true, it is unclear that their proposal is useful because such a configuration is rather complicated to build and launch.”
These are just a few of the engineering challenges facing an interstellar mission. Back in September, another study was released that assessed the risk of collisions and how it might effect the Starshot mission. In this case, the researchers suggested that the sail have a layer of shielding to absorb impacts, and that the laser array be used to clear debris in the LightSail’s path.
When Milner and the science team behind Starshot first announced their intention to create an interstellar spacecraft (in April 2016), they were met with a great deal of enthusiasm and skepticism. Understandably, many believed that such a mission was too ambitious, due to the challenges involved. But with every challenge that has been addressed, both by the Starshot team and outside researchers, the mission architecture has evolved.
At this rate, barring any serious complications, we may be seeing an interstellar mission taking place within a decade or so. And, barring any hiccups in the mission, we could be exploring Alpha Centauri or Proxima b up close within our lifetime!
Back in April, Russian billionaire Yuri Milner and famed cosmologist Stephen Hawking unveiled Project Starshot. As the latest venture by Breakthrough Initiatives, Starshot was conceived with the aims of sending a tiny spacecraft to the neighboring star system Alpha Centauri in the coming decades.
Relying on a sail that would be driven up to relativistic speeds by lasers, this craft would theoretically be capable of making the journey is just 20 years. Naturally, this project has attracted its fair share of detractors. While the idea of sending a star ship to another star system in our lifetime is certainly appealing, it presents numerous challenges.
Assessing the risks of interstellar travel, this paper addresses the greatest threat where relativistic speed is concerned: catastrophic collisions! To put it mildly, space is not exactly an empty medium (despite what the name might suggest). In truth, there are a lot of things out there on the “stellar highway” that can cause a fatal crash.
For instance, within interstellar space, there are clouds of dust particles and even stray atoms of gas that are the result of stellar formations and other processes. Any spacecraft traveling at 20% the speed of light (0.2 c) could easily be damaged or destroyed if it suffered a collision with even the tiniest of this particulate matter.
“To evaluate the risks, we calculated the energy that each interstellar atom or dust grain transfers to the ship along the path of the projectile in the ship. This acquired energy rapidly heats a spot on the ship surface to high temperature, resulting in damage by reducing the material strength, melting or evaporation.”
In short, the danger of a collision comes not from the physical impact, but from the energy that is generated due to the fact that the spaceship is traveling so fast. However, what they found was that while collisions with tiny dust grains are very likely, collisions with heavier atoms that can do the most damage would be more rare.
Nevertheless, the damage from so many tiny collisions will certainly add up over time. And it would only take one collision with a larger particle to end the mission. As Dr. Hoang explained:
“We found that the ship would be damaged by collision with heavy atoms and dust grains in the interstellar medium. Heavy atoms, mostly iron can damage the surface to a depth of 0.1mm. More importantly, the surface of the ship is eroded gradually by dust grains, to a depth of about 1mm. The ship may be completely destroyed if encountering a very big dust grain larger than 15micron, although it is extremely rare.”
In terms of damage, what they determined was that each iron atom can produce a damage track of 5 nanometer across, whereas a typical dust silicate grain measuring just 0.1. micron across (and containing about one billion iron atoms) could produce a large crater on the ship’s surface.
Over time, the cumulative effect of this damage would pose a major risk for the ship’s survival. As a result, Dr. Hoang and his team recommended that some shielding would need to be mounted on the ship, and that it wouldn’t hurt to “clear the road” a little as well.
“We recommended to protect the ship by putting a shield of about 1 mm thickness made of strong, high melting temperature material like graphite.” he said. “We also suggested to destroy interstellar dust by using part of energy from laser sources.”
These projects, which are being funded by NASA, seek to harness the technology behind directed-energy propulsion to rapidly send missions to Mars and other locations within the Solar System in the future. Long-term applications include interstellar missions, similar to Starshot.
In all cases, directed-energy technology is being proposed as the solution to the problems posed by space travel. In the case of Starshot, these include (but are not limited to) inefficiency, mass, and/or the limited speeds of conventional rockets and ion engines.
As Professor Lubin told Universe Today via email, he and his colleagues are in general agreement with the research team and their findings:
“The recent paper by Hoang et al revisits the section (7) in our paper “A Roadmap to Interstellar Flight” that discusses our calculation for the effects of the ISM on the wafer scale spacecraft. Their general conclusion on the effects of the gas and dust collisions were essentially the same as ours, namely that it is an issue, but not a fatal one, if one uses the spacecraft geometry we recommend in our paper, namely orient the spacecraft edge on (like a Frisbee in flight) and then use an edge coating (we use [Beryllium], they use graphite).”
“As for the sail interactions with the ISM we recommend either rotating the sail so it is edge on (lower cross section) or ejecting the sail after the initial few minutes of acceleration as it is no longer needed for acceleration. However. as we desire to use the sail as a reflector for the laser communications we prefer to keep it, though a secondary reflector could be deployed later in the mission if necessary. These detailed questions will be part of the evolving design phase.”
Indeed, there are many safety hazards that have to be accounted for before any mission to interstellar space could be mounted. But as this recent study has shown – with which Professor Lubin agrees – they are not insurmountable, and a mission to Alpha Centauri (or, fingers crossed, Proxima Centauri!) could be performed if the proper precautions are taken.
Who knew the future of space travel would be every bit as cool as we’ve been led to believe – complete with lasers and shielding?
And be sure to enjoy this video from NASA 360, addressing directed-energy propulsion: