A laser is a device that emits a beam of coherent light through an optical amplification process. Laser propulsion is any method of propelling a spacecraft that uses the energy of laser beams.
There are two main types of laser propulsion, depending on whether the laser is onboard or off-board the spacecraft. Onboard methods involve the use of lasers as part of a nuclear propulsion system. One method uses the laser to help expel mass from the spacecraft as in a conventional rocket. This is the more frequently proposed method, but is fundamentally limited in final spacecraft velocities by the rocket equation.
Off-board laser propulsion is part of a larger class of propulsive methods known as beamed-energy propulsion where the energy source is a remote (usually ground-based) laser system and separate from the reaction mass. The propulsive energy comes instead from a fixed, high-power laser beam that is directed onto the spacecraft by a tracking and focusing system. It uses photon radiation pressure to drive momentum transfer and is the principle behind solar sails and laser sails. Off-board techniques have been proposed to boost lightweight vehicles either from the ground to orbit, or on interplanetary or interstellar missions. These techniques include laser-powered launching to orbit and laser light sails.
Its great advantage is that it removes the need for the spacecraft to carry its own source of energy and onboard propulsion system. In a conventional chemical rocket both energy and reaction mass comes from the solid or liquid propellants carried on board the vehicle. LPP has the potential to increase the payload and decrease the launch costs in comparison with other conventional methods of producing thrust.
Laser pulse propulsion (LPP) has attracted large prominence due to its non-contact nature and ability to affect the motion of objects ranging from various macroscopic materials to microscopic objects and even individual microspheres.
Light is comprised of photons, which have no mass, but they do have momentum and can transfer it to a sail. As the energies of individual photons are very small, an extremely large sail size is needed for any appreciable acceleration. The major drawback of directed energy propulsion lies in its weak efficiency: the thrust imparted by a radiation beam illuminating some object with power P is of order P/c. Roughly speaking, 1 N of thrust requires an illumination on the sail of at least 300 MW.
Laser thrusters, which exert force through light pressure, usually need extremely powerful lasers to generate tiny amounts of thrust. Powerful lasers developed as part of the United States SDI (Strategic Defense Initiative) program have the potential to launch lightweight spacecraft into low Earth orbit. Tests have already been conducted, by Leik Myrabo of the Rensselaer Polytechnic Institute and other scientists from the United States Air Force and NASA, using a 10-kW infrared pulsing laser at the White Sands Missile Range and an acorn-shaped craft with a diameter of 12.2 cm and a mass of 50 g.
The base of the craft is sculpted to focus the beam from the laser on to a propellant. In tests so far, this propellant has been air, which is heated by the beam to a temperature of 10,000–30,000°C, expands violently, and pushes the craft upwards. A height of 71 m was achieved in an October 2000 trial. To orbit a 1-kg spacecraft will demand a much more powerful, one-megawatt laser and a supply of onboard propellant, such as hydrogen, to take over at altitudes where the air gets too thin.
In recent years, directed-energy (DE) propulsion has been the subject of considerable research and interest. Examples include the Starlight program – also known as the Directed Energy Propulsion for Interstellar Exploration (DEEP-IN) and Directed Energy Interstellar Studies (DEIS) programs – developed by Prof. Phillip Lubin and the UCSB Experimental Cosmology Group (ECG). As part of NASA-funded research that began in 2009, these programs aim to adapt large-scale DE applications for interstellar missions.
There’s also Breakthrough Starshot and Project Dragonfly, both of which emerged from a design study hosted by the Initiative for Interstellar Studies (i4iS) in 2013. These concepts call for a gigawatt-power laser array to accelerate a lightsail and a small spacecraft to a fraction of the speed of light (aka relativistic speeds) to reach nearby star systems in decades, rather than centuries or millennia.
NASA’s Starlight and Breakthrough Starshot programs
In 2016, the Breakthrough Starshot Initiative was initiated to focus on wafer scale spacecraft and interstellar fly-bys to Alpha Centauri with the objective of achieving it before the end of the century.
Directed energy systems may enable interstellar probes as part of human exploration in the not-too-distant future, and they are at the heart of the NASA Starlight program and the Breakthrough Starshot Initiative to enable humanity’s first interstellar missions. The same core technology has many other applications, such as rapid interplanetary travel for high mass missions, including those carrying people; planetary defense; and the search for extraterrestrial intelligence (SETI).
“Our primary focus currently is on very small robotic spacecraft. They won’t carry humans onboard—it’s not the goal for the interstellar portion of our program,” said Lubin. “If humanity wants to explore other worlds outside our solar system, there are no other physically obtainable propulsion options for doing this—with two exceptions.
“One way would be if we could master a technological approach known as antimatter annihilation engines, which are theoretical propulsion systems that generate thrust based on energy liberated by interactions at the level of subatomic particles. But we don’t currently have a way to do that,” Lubin said, “and it involves a number of complexities we do not have a current path to solving.
“The other option is directed energy or photonic propulsion, which is the one we’re focusing on because it appears to be feasible,” Lubin said. In one variant, directed energy propulsion is similar to using the force of water from a garden hose to push a ball forward. Miniscule interstellar spacecraft (typically less than a kilogram and some that are spacecraft on a wafer) can be propelled and steered via laser light, he said. “Miniaturizing spacecraft isn’t required for all of the mission scenarios we’re considering, but the lower the mass of the spacecraft the faster you can go,” Lubin said. “This system scales in different ways than ordinary mass ejection propulsion.”
One of the greatest challenges in validating this photonics concept as it relates to propulsion is the demonstration of the laser power required to accelerate the proposed/hypothetical spacecraft, according to Lubin. Large directed energy systems are not built using a single gigantic laser, but instead rely on beam combining, which involves the use of many very modest power laser amplifiers. “Our system leverages an established typology called ‘Master Oscillator Power Amplifier’ design,” said Lubin. “It’s a distributed system so each laser amplifier “building block” is between 10 and 1000 Watts. You can hold it in your hand. Instead of building a gigantic laser, you combine a lot of small little laser amplifiers that, when combined, form an extremely powerful and revolutionary system.”
One significant challenge for relativistic spacecraft is radiation hardening, because “when we begin to achieve speeds close to the speed of light, the particles in interstellar space, protons in particular, that you plow into—ignore the dust grains for the moment—are the primary radiation source,” said Lubin. “Space isn’t empty; it has roughly one proton and one electron per cubic centimeter, as well as a smattering of helium and other atoms.”
Smashing into those particles can be significant at high speeds because while they may be traveling slowly within their own frame of reference, for a fast-moving spacecraft they make for high-speed impacts. “When you hit them it’s like driving in a rainstorm. Even if the rain is coming down straight from the sky your windshield gets plastered because you’re going fast—and it’s quite a serious effect for us,” Lubin said. “We get enormous radiation loads on the leading edge as the front gets just absolutely clobbered, whereas the rest of the spacecraft that is not the forward edge and facing in different directions doesn’t get hit much at all. It’s an interesting and unique problem, and we’re working on what happens when you plow through them.”
In terms of a timeframe for putting directed energy propulsion technology to work, “We’re producing laboratory demos of each part of the system,” said Lubin. “Full capability is more than 20 years away, although demonstration missions are feasible within a decade.”
Tests start on first design of ‘wafer scale’ spacecraft
Measuring just 90mm in diameter, a prototype of the flat pancake-like craft, more formally known as a Wafer Scale Spacecraft (WSS), was recently launched into the skies above Pennsylvania via balloon, to an altitude of 32 kilometres (105,000 feet), by students at UC Santa Barbara under the guidance of physics professor and experimental cosmologist Philip Lubin.
The test, which was to evaluate the wafercrafts functionality and performance, performed ‘an excellent first flight’ according to Nic Rupert, a development engineer in Lubin’s lab and in its brief venture the wafer collected more than 4000 images of the Earth which it stored in its onboard flash memory. This encouraging initial performance of the programme, funded by NASA NIAC grants, along with generous donations from private backers, paves the way forward for the wafer to “evolve dramatically from here,” added Rupert.
This slim, round disk, no bigger than a flattened muffin case, is being developed to use photonic technologies, I.e lasers, to propel it through space at relativistic speeds – a similar concept that has been proposed for the light sail technology being researched by Breakthrough Initiatives for its Starshot mission. Unlike the light sail however, the wafercraft will be powered by a laser array on Earth in a process known as Directed Energy Propulsion.
Put basically this type of propulsion uses energy beamed to the spacecraft from a remote power plant to provide energy; it can be in the form of microwaves, but in this instance the wafercraft will use higher energy beams of laser light.
The wafercraft includes integrated optical communications, optical systems, sensors and a camera and when combined with directed energy propulsion, it would be capable of speeds around 20 percent of the speed of light, if not more. With a large enough laser array, you’d be at Alpha Centauri in something like 20 years, says Rupert – this is a fraction of the time that current rocket systems would take to reach our nearest star system.
The initial design of the wafercraft started off with commercially available components and conventional printed circuit board (PCB) construction techniques. The next iteration is to improve upon the scale by shrinking them to around 35mm x 35mm x 6.1mm and weighing just a paltry 10.25 grams.
Although these diminutive devices would appear to quash the need for CubeSats, this is not the end for this blossoming box technology that is just beginning to see its debut in deep space missions. Once perfected, these wafercrafts could then be mass manufactured at an incredibly low cost and packed into CubeSats and launched in “swarms” to collect vast amounts of data on whatever object is being targeted.
And their uses are potentially endless; mitigating space debris, boosting Earth-orbiting satellites, planetary defence against wayward asteroids and comets or even remotely powering distant solar system outposts noted Lubin. “Some of the more interesting, short-term ones would involve interplanetary missions,” he said.
The group working on the wafercraft are a long way off yet from firing one off in the direction of our nearest exoplanet, Proxima Centauri b, but it is aiming for a suborbital first flight next year.
The U.S. Army is developing a system in which a laser shot from the ground can power up a military drone mid-flight, according to New Scientist. The key is hitting a photovoltaic cell on the drone, which then converts the light from the laser into electricity. The Army hopes to be able to do this from up to 500 meters (.31 miles) away.
The Army still has several hurdles to overcome before its drone-powering laser system is ready for the battlefield, though. The biggest one: not melting the drone. See, any laser energy that isn’t converted to electricity by the drone’s photovoltaic cell becomes heat — heat that could do serious damage to a drone. The Army’s research team is working on a way to ensure excess heat can dissipate without causing any damage to the military drone. Making sure the beam only hits where it’s supposed to will be critical for making that happen.
Spacecraft built from graphene could run on nothing but sunlight
Yongsheng Chen of Nankai University in Tianjin, China, and his colleagues have been investigating a “graphene sponge“, a squidgy material made by fusing crumpled sheets of graphene oxide. While cutting graphene sponge with a laser, they noticed the light propelled the material forwards. That was odd, because while lasers have been used to shove single molecules around, the sponge was a few centimetres across so should be too large to move.
The team placed pieces of graphene sponge in a vacuum and shot them with lasers of different wavelength and intensity. They were able to push sponge pieces upwards by as much as 40 centimetres. They even got the graphene to move by focusing ordinary sunlight on it with a lens.
Graphene sponge could be used to make a light-powered propulsion system for spacecraft that would beat solar sails. “While the propulsion force is still smaller than conventional chemical rockets, it is already several orders larger than that from light pressure,” they write.
“The best possible rocket is one that doesn’t need any fuel,” says Paulo Lozano of the Massachusetts Institute of Technology. He thinks a graphene-powered spacecraft is an interesting idea, but losing electrons would mean the craft builds up a positive charge that would need to be neutralised, or it could cause damage.
Currently, aside from China, the European Space Agency (ESA) is also researching the idea of graphene solar sails, in collaboration with the Graphene Flagship, a €1 billion EU research initiative. The experiment in which GrapheneX, a team of PhD-students from TU Delft, tested graphene as a material for future solar sails has been carried out successfully. During the five runs of the experiment in ESA’s drop tower in Bremen, the team observed direct motion of their graphene samples under the influence of the laser.
The Graphene Flagship is a pan-European research initiative dedicated to developing new technologies based on graphene, the single-atom-thick allotrope of carbon with excellent electrical, mechanical, thermal and optical properties.
Solar sails are typically made from aluminium. Materials World quoted Cartamil-Bueno as leader of the so called GrapheneX team: “There are several advantages to graphene and one drawback. Graphene has better mechanical properties, most importantly superior strength with very little mass – being only one atom thick. However, momentum transfer is more efficient when the material is more reflective, which is not the case with graphene. Most light travels through it, but a big part of it is absorbed.” Cartamil-Bueno stated “This can be resolved in the same manner as a standard solar sail, by coating it with aluminum.”
Australian researchers design laser propulsion system for interstellar travel, reported in June 2021
Australian researchers reported having designed a propulsion system in June 2021 for an interstellar mission to shoot a spacecraft off to one of our closest stellar neighbours – using 100 million lasers. The research team at the Australian National University (ANU) are answering the call of the Breakthrough Starshot project for an ultra-light spacecraft that uses “light sail” technology. The ambitious mission aims to send this interstellar traveller zooming across tens of trillions of kilometres of space to reach Alpha Centauri, the second-closest star to the Sun – within just 20 years.
For reference, the spacecraft currently furthest away from Earth is Voyager 1, which is only around 22 billion kilometres away, just outside our solar system – and it was launched in 1977. Even with the conventional spacecraft technology we have today, it would take more than 100 lifetimes to reach the nearest stellar systems.
“Ordinary chemical propulsion, such as that which took us to the moon nearly 50 years ago to the day, would take nearly one hundred thousand years to get to the nearest star system, Alpha Centauri,” Lubin said. “And even advanced propulsion such as ion engines would take many thousands of years. There is only one known technology that is able to reach the nearby stars within a human lifetime and that is using light itself as the propulsion system.”
The paper, published in the Journal of the Optical Society of America B, outlines the design concept: a giant array of millions of lasers down on Earth, acting together as one to illuminate the “sail” of a spacecraft and send it speeding off on its interstellar mission. Robert Ward, the physicist who founded the ANU node of the project, says the lasers’ coordination is key.
“The Breakthrough Starshot program estimates the total required optical power to be about 100 GW – about 100 times the capacity of the world’s largest battery today,” Ward explains. “To achieve this, we estimate the number of lasers required to be approximately 100 million.” One of the main challenges the researchers faced was how to measure how much each laser “drifted”.
“We use a random digital signal to scramble the measurements from each laser and unscramble each one separately in digital signal processing,” says co-author Paul Sibley. “This allows us to pick out only the measurements we need from a vast jumble of information. We can then break the problem into small arrays and link them together in sections.”
The design concept requires not only an enormous number of lasers on the ground, but also one in space – a “guide laser” will be placed on a satellite in orbit around the Earth and will act as a conductor for the ground-based show. By measuring subtle changes in the atmosphere and beaming back the information, it will correct the path of the lasers down on Earth so they won’t be distorted by the planet’s atmosphere.
“Once on its way, the sail will fly through the vacuum of space for 20 years before reaching its destination,” Bandutunga speculates. “During its flyby of Alpha Centauri, it will record images and scientific measurements which it will broadcast back to Earth.”
But this paper just outlines a design – next, the team have to start building. “The next step is to start testing some of the basic building blocks in a controlled laboratory setting,” Bandutunga says. “This includes the concepts for combining small arrays to make larger arrays and the atmospheric correction algorithms.”
Laser Propulsion For Earth-Mars
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!
The research was led by Emmanuel Duplay, a McGill graduate and current MSc Aerospace Engineering student at TU Delft. He was joined by Associate Professor Andrew Higgins and multiple researchers with the Department of Mechanical Engineering at McGill University. Their study, titled “Design of a rapid transit to Mars mission using laser-thermal propulsion”, was recently submitted to the journal Astronomy & Astronomy.
Direct Laser Propulsion For Earth-Mars in 30 Days and Eventually 2.5 Days reported in April 2021
Limitless Space Institute gave out nine interstellar space-related grants for about $1 million to 2 million each. Phil Lubin’s direct laser propulsion group received a grant and they will use to prove out their directed laser propulsion system at the 10-20 watt level. They are also working on components at the kilowatt to tens of kilowatt level. The plan would be to scale to tens of kilowatts and work towards power beaming to the moon for experiments. In 5-10 years, they hope to reach megawatt levels.
If they reach 500 megawatts then direct laser propulsion could be used for 30 day transit times to Mars. When the system scales to the gigawatt or tens of gigawatt level with a matching deceleration and launching system on Mars this would enable 2.5 day transit times between Earth and Mars. Increasing or decreasing mass by 10,000 times changes the speed by 10 times. A 100-gram package could sent ten times faster than a 1000 kilogram package using the same size laser array and power system. You could send a 1000 kilogram mission to Mars in 30 days or rapidly deliver an urgent package of 100 grams to Mars in about 3 days.
This system for urgent delivery of tiny packages with 1G acceleration and deceleration would be feasible in the 10-20 year time frame.
ISRO Chief K Sivan admitted to the fact that LPSC or Liquid Propulsion Systems Center is developing LASER propulsion. “It is not just the aircraft, the target is to develop light craft using laser propulsion for interplanetary mission. With such light craft, powered by high speed laser propulsion, it will be possible to go to Mars in four to eight minutes and the challenge is for LPSC to play a lead role in developing that,”
Experiment with graphene as a material for solar sails a success
Testing graphene’s space-propulsion potential, the team of PhD students from TU Delft participated in ESA’s Drop Your Thesis! campaign, which offers students the chance to perform an experiment in microgravity at the ZARM Drop Tower in Bremen, Germany. To create extreme microgravity conditions, down to one millionth of the Earth’s gravitational force, a capsule containing the experiment was catapulted up and down the 146 metre tower, leading to 9.3 seconds of weightlessness.
The goal of the experiment was to measure the vertical displacement of the sail by radiation of different lasers. We simulated how a solar sail would move in space through a vacuum.” Despite some technical problems, the ESA-backed experiment was a success and may indeed lead to the first craft to reach the star system Alpha Centauri. However, as the experiment is not completely accomplished, Cartamil-Bueno is in discussions with the ESA to resume the project.
The experiment was the result of a collaboration between the GrapheneX team at TU Delft, the Graphene Flagship research initiative and the European Space Agency (ESA). The TU Delft Space Institute also supported the project. “Graphene as we know has a lot of opportunities. One of them, recognised early on, is space applications, and this is the first time that graphene has been tested in space-like applications, worldwide,” said Prof. Andrea Ferrari (University of Cambridge, UK), Science and Technology Officer of the Graphene Flagship.
The GrapheneX team designed and built an experiment to test graphene for use in solar sails, using free-floating graphene membranes provided by Flagship partner Graphenea. The idea was to test how the graphene membranes would behave under radiation pressure from lasers. In total, the experiment ran five times over 13-17 November 2017. Despite technical problems at the start of the week, the team successfully observed direct motion of the graphene under the influence of the laser, and are now closely analysing the results.
China to use graphene, light to propel spaceships
China Academy of Launch Vehicle Technology (CALT), the rocket development arm of the Chinese space programme, revealed that it has designed a graphene composite film suitable for use in light-propelled spacecraft. The idea of graphene propulsion was first put forward in a paper published in 2015 by researchers from Nankai University, Tianjin.
The United States, Europe and Japan have been developing spacecraft with solar sails made with polyamide film, but the thrust generated in this way is said to be relatively weak. Now Chinese researchers are developing sails made from graphene, which can withstand temperatures over 800 degrees Celsius (1,472 degrees Farenheit).
Previous research conducted by Professor Chen Yongsheng with the Tianjin-based Nankai University, showed graphene could be driven by various light sources including sunlight, and that, in the vacuum conditions of outer space, the thrust generated is 1,000 times higher than that of polyamide film.
There were also reports in previous years that commercial graphene-based supercapacitors could be chosen for use in spacecraft. Skeleton Technologies, a Germany-based technology firm, reportedly devised a unique process in 2015 to produce “curved graphene” from silicon carbide, which it used in their line of commercially available supercapacitors. The company stated that their products could deliver twice as much energy storage capacity and five times more performance power compared with other traditional ultracapacitors. The high degree of purity of curved graphene also ensures up to two times higher tolerance, and four times lower resistance.
China completes design of graphene composite film for light propulsion
China Academy of Launch Vehicle Technology (CALT), the rocket development arm of the Chinese space programme, revealed that it has designed a graphene composite film suitable for use in light-propelled spacecraft.
This is part of CALT’s research on graphene-based spacecraft propulsion, a new technology that converts light into electrical energy. The method utilizes a technology similar to the solar sail, which was already tested by Japan’s space agency JAXA during its IKAROS mission to Venus. Unlike the solar sail, however, the graphene sail will not use thin-film solar cells, but will instead be covered with graphene film, a two-dimensional material known for its strength and conductivity.
Like the solar sail, graphene-based propulsion will use radiation pressure, making use of solar energy for propulsion. However, according to CALT, research in China has shown that graphene can be up to 1000 times more effective.
Said Song Shenju, from CALT’s R&D centre, “Graphene propulsion will revolutionze the design of propulsion systems, and will open yet another door for humanity to explore outer space. However, the technology is still in its development phase and is still a long way from a prototype.”
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