Ground-Based Lasers Could Accelerate Spacecraft to Other Stars

An artist's illustration of a light-sail powered by a radio beam (red) generated on the surface of a planet. The leakage from such beams as they sweep across the sky would appear as Fast Radio Bursts (FRBs), similar to the new population of sources that was discovered recently at cosmological distances. Credit: M. Weiss/CfA

The future of space exploration includes some rather ambitious plans to send missions farther from Earth than ever before. Beyond the current proposals for building infrastructure in cis-lunar space and sending regular crewed missions to the Moon and Mars, there are also plans to send robotic missions to the outer Solar System, to the focal length of our Sun’s gravitational lens, and even to the nearest stars to explore exoplanets. Accomplishing these goals requires next-generation propulsion that can enable high thrust and consistent acceleration.

Focused arrays of lasers – or directed energy (DE) – and lightsails are a means that is being investigated extensively – such as Breakthrough Starshot and Swarming Proxima Centauri. Beyond these proposals, a team from McGill University in Montreal has proposed a new type of directed energy propulsion system for exploring the Solar System. In a recent paper, the team shared the early results of their Laser-Thermal Propulsion (LTP) thruster facility, which suggests that the technology has the potential to provide both high thrust and specific impulse for interstellar missions.

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Tiny Swarming Spacecraft Could Establish Communications with Proxima Centauri

Swarm of laser-sail spacecraft leaving the solar system. Credit: Adrian Mann

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.

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A Novel Propulsion System Would Hurl Hypervelocity Pellets at a Spacecraft to Speed it up

Graphic depiction of Pellet-Beam Propulsion for Breakthrough Space Exploration. Credits: Artur Davoyan

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.

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Laser-Powered Sails Would be Great for Exploring the Solar System too

Swarm of laser-sail spacecraft leaving the solar system. Credit: Adrian Mann

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.

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Lasers Could Send Missions to Mars in Only 45 Days

Swarm of laser-sail spacecraft leaving the solar system. Credit: Adrian Mann

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!

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What’s the Best Way to Communicate With an Interstellar Probe When it’s Light-Years Away From Earth?

An artist's illustration of a light-sail powered by a radio beam (red) generated on the surface of a planet. The leakage from such beams as they sweep across the sky would appear as Fast Radio Bursts (FRBs), similar to the new population of sources that was discovered recently at cosmological distances. Credit: M. Weiss/CfA

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.

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How Will we Receive Signals From Interstellar Probes, Like Starshot?

Credit: Breakthrough Initiatives

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.

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Prototype of a Future Interstellar Probe was Just Tested on a Balloon

Credit: UCSB

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.

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What is the Death Ray?

Death Ray

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.

Definition:

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.

The Death Star firing its superlaser. Image Credit: Wookieepedia / Lucasfilm
Directed-energy weapons, like the Death Star’s superlaser, are a common feature in science fiction. Credit: Wookieepedia / Lucasfilm

History:

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).

 Century Magazine photographer Dickenson Alley) of Tesla sitting in his Colorado Springs laboratory with his "magnifying transmitter" generating millions of volts. The 7-metre (23 ft) long arcs were not part of the normal operation, but only produced for effect by rapidly cycling the power switch.[117]
Photograph of Tesla sitting in his Colorado Springs laboratory with his “magnifying transmitter” generating millions of volts. Credit: Wikipedia Commons/Century Magazine/Dickenson V. Alley
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.

Dresden, 1945, view from the city hall (Rathaus) over the destroyed city Deutsche Fotothek?
Dresden, 1945, view from the city hall (Rathaus) over the destroyed city. Credit: Wikipedia Commons/ Deutsche Fotothek?

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.

War of the Worlds
H.G. Wells’ 1898 novel about a Martian invasion, War of the Worlds, featured alien machines using heat rays to spread havoc. Credit: Henrique Alvim Correa (1906)

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.

Modern Development:

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.

An artist's concept of a Space Laser Satellite Defense System. Credit: USAF
An artist’s concept of a Space Laser Satellite Defense System. Credit: USAF

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).

The personnel halting and stimulation response rifle (PHASR) is a prototype non-lethal laser dazzler developed by the Air Force Research Laboratory's Directed Energy Directorate, U.S. Department of Defense. Credit: USAF
The personnel halting and stimulation response rifle (PHASR) is a prototype non-lethal laser dazzler developed by the Air Force Research Laboratory’s Directed Energy Directorate, U.S. Department of Defense. Credit: USAF

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!

We have written many articles about the directed-energy and lasers for Universe Today. Here’s Telescope’s Laser Pointer Clarifies Blurry Skies, The Challenges of Lasers in Space, Don’t Want Aliens Dropping By? Engage Laser-Cloaking Device, Could a “Death Star” Really Destroy a Planet?, and Finding Aliens May be even Easier than Previously Thought.

If you’d like more info on the Death Ray, check out this article from Rense.com.

We’ve also recorded an entire episode of Astronomy Cast all about the Death Ray. Listen here, Episode 36: Gamma Ray Bursts.

Source:

What’s the Most Stable Shape for an Interstellar Lightsail?

Project Starshot, an initiative sponsored by the Breakthrough Foundation, is intended to be humanity's first interstellar voyage. Credit: breakthroughinitiatives.org

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.

The researchers include Elena Popova of the Skobeltsyn Institute of Nuclear Physics in Moscow; Messoud Efendiev of the Institute of Computational Biology (ICB) at the German Research Center for Environmental Health (GmbH); and Ildar Gabitov of the Skoltech Center for Photonics and Quantum Materials in Moscow. Combining their expertise, they conducted a study that examined various stability models for this proposed mission.

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.

A phased laser array, perhaps in the high desert of Chile, propels sails on their journey. Credit: Breakthrough Initiatives.
A phased laser array, perhaps in the high desert of Chile, propels sails on their journey. Credit: Breakthrough Initiatives

“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.

These conclusions echoed a similar study produced by Professor Phillip Lubin and his colleagues. A professor at the University of California, Santa Barbara (UCSB), Lubin is also one of the chief architects of Project Starshot and the mind behind the NASA-funded Directed Energy Propulsion for Interstellar Exploraiton (DEEP-IN) project and the Directed Energy Interstellar Study.

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!

Further Reading: arXiv