7.03.2022

Laser sailing could be a game changer for exploration.

In this illustration, a low-power laser (red cone) on Earth could be used to shift the orbit (red lines) of a small probe (grey circle), or propel it at rapid speeds to Neptune and beyond. (Image credit: Adapted from Nano Letters 2022, DOI: 10.1021/acs.nanolett.1c04188)

Miniature spaceships the size of cellphones could fly across the solar systemusing sails propelled by lasers, which would allow the tiny spacecraft to reach much faster speeds — and, potentially, much more distant destinations — than conventionally powered rockets, a new study finds. 

Current spacecraft usually take years to make trips within the solar system; for example, NASA's New Horizons probe took nearly 10 years to reach Pluto.  

In theory, spacecraft using conventional rockets would need thousands of years to complete an interstellar voyage. For example, Alpha Centauri, the nearest star system to Earth, lies about 4.37 light-years away — more than 25.6 trillion miles (41.2 trillion kilometers), or more than 276,000 times the distance from Earth to the sun. It would take NASA's Voyager 1 spacecraft, which launched in 1977 and reached interstellar space in 2012, about 75,000 years to reach Alpha Centauri even if the probe were headed in the right direction, which it's not.

The problem with all rocket thrusters is that the propellant they carry with them has mass. Long trips require a lot of propellant, which makes spacecraft heavy, which, in turn, requires more propellant, making them heavier, and so on. 

Previous research has suggested that "light sailing" might be one of the only technically feasible ways to get a spacecraft to another star within a human lifetime. Although light does not exert much pressure, scientists have long suggested that what little pressure it does apply could have a major effect. Indeed, numerous experiments have shown that "solar sails" can rely on sunlight for propulsion if the spacecraft is light enough and has a large enough sail. 

Indeed, the $100 million Breakthrough Starshot initiative, announced in 2016, plans to launch swarms of microchip-size spacecraft to Alpha Centauri, each of them sporting extraordinarily thin, incredibly reflective sails propelled by the most powerful lasers ever built. The plan has them flying at up to 20% the speed of light, reaching Alpha Centauri in about 20 years.

A major challenge Starshot faces is building the lasers needed for propulsion. It calls for a ground-based laser array on the order of 0.4 square miles (1 square kilometer) and as powerful as 100 gigawatts, which would be by far the most powerful laser ever made on Earth.

In the new study, the researchers suggest that a more humble ground-based laser array — one that's 3.3 to 33 feet (1 to 10 meters) wide and 100 kilowatts to 10 megawatts in power — could still prove useful by sending tiny probes across the solar system, propelling them to much faster speeds than rocket engines could.

"Such lasers can be built already today with a relatively small investment," study senior author Artur Davoyan, a materials scientist at the University of California, Los Angeles, told Space.com. "We do not need to wait till a 100-gigawatt laser becomes available."

Going interstellar on a reasonable timescale imposes more constraints than voyaging within the solar system. For instance, Starshot aims to send probes to another star within a human lifetime, so its spacecraft are designed to be extraordinarily lightweight — each just 0.035 ounces (1 gram) or so — to fly as fast as possible given the amount of energy they receive. 

Laser sails for interplanetary voyages, by contrast, do not have to be as lightweight. The scientists envision spacecraft for such trips ranging up to 3.5 ounces (100 g) or so — a mass "comparable with that of a typical cell phone," Davoyan said.

Whereas Starshot faces mass constraints that make it challenging to fit all the needed spacecraft systems and instruments into a single platform, a 3.5-ounce probe "can easily be equipped with all the needed components, including spectrometers, accelerometers, particle detectors, cameras and so on — all the key ingredients to conduct a proper scientific mission in far reaches of space," Davoyan said.

Furthermore, because a laser array can launch more than one probe, it could potentially send a fleet of tiny probes, each with different equipment, to a destination. "For example, one may be a magnetometer probe, another equipped with a camera, the third serving as a particle detector," Davoyan said. "We foresee that many small probes can be sent to really different destinations to do breakthrough science."

In addition, because interplanetary voyages do not require the kind of powerful lasers needed with Starshot, they also do not require large sails with the kind of extraordinary material properties needed to withstand the many demands of interstellar flight, such as not vaporizing under the light of such a powerful laser. The researchers suggested that silicon nitride or boron nitride sails about 4 inches (10 centimeters) wide should suffice for flights within the solar system.

"Our work is a first step to fast and low-cost interplanetary and deep space missions," Davoyan said. "We see that a new model for space exploration can emerge, where individual users, which typically do not have access to space, could now spend just a few thousand dollars and launch a real deep space mission."

Laser arrays on the order of 100 kilowatts are already under development by the U.S. military; in 2020, for instance, the U.S. Navy's littoral combat ship USS Little Rock received a 150-kilowatt laser. Furthermore, the cost of high-power lasers is rapidly dropping every year, driven by the need for optical telecommunications, with 1-kilowatt lasers available for less than $10,000, Davoyan noted.

"Rough estimates show that [a] 1-megawatt laser beamer could be constructed with less than $100 million, which is far less than most of NASA's missions," Davoyan said. "Importantly, once built, the beamer can be used and reused to launch multiple probes in different directions. Essentially, the laser beamer is an initial capital investment and, once built, serves as a launchpad. The mission cost then consists of producing probes, which, with the use of mass manufacturing, can be on the order of $100, launching probes to orbit for less than $100 per probe and then operating a mission during its useful lifetime. Therefore, overall the laser-driven approach offers very low cost for space exploration."

The scientists estimated that a 0.035-ounce laser sail with a 4-inch sail driven to speeds of about 112,000 mph (180,000 km/h) could reach Mars in 20 days, compared with the 200 days for NASA's Perseverance rover; Jupiter in 120 days, compared with five years for NASA's Juno probe; Pluto in less than three years, compared with 10 years for NASA's New Horizons craft; and 100 times the distance of Earth from the sun in 10 years, compared with nearly 30 years for NASA's Voyager 1 spacecraft.

"The fact that we can change the way space is being explored already today with a minimal investment is truly energizing," Davoyan said. "Such an approach allows almost everyone to develop and launch their own mission — something that was not possible before. It would be really exciting to see an undergraduate student sending their own science probe to, say, Jupiter."

The scientists now hope to test and prototype their ideas. "We are also partnering with industry and government to move further some of the designs and ideas we have," Davoyan said. "We believe we can make a real difference in the future of space exploration." 

The scientists detailed their findings online Jan. 31 in the journal Nano Letters.

Quelle: SC