Spacecraft made of carbon-foam bubbles could zoom from Earth to Alpha Centauri in 185 years, driven solely by the power of the sun, a new study finds.
A swarm of these probes might help discover and study our solar system's mysterious Planet Nine, if this hypothesized world exists, scientists added.
Conventional rockets driven by chemical reactions are currently the leading form of space propulsion. However, they are not anywhere close to efficient enough to reach another star within a human lifetime.
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 about 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 if the probe were headed in the right direction (which it is not).
The problem with all conventional spacecraft thrusters is that the propellant they use 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. This problem becomes exponentially worse the larger a spacecraft gets.
Previous research has therefore suggested that "light sailing" might be one of the only technically feasible methods to get a probe to another star within a human lifetime. Although light does not exert much pressure, scientists have determined that what little it does apply could have a major effect. Indeed, numerous experiments have shown that "solar sails" can rely on sunlight for propulsion, given a large enough mirror and a spacecraft that is light enough.
The $100 million Breakthrough Starshot initiative, which was announced in 2016, aims to launch swarms of microchip-size spacecraft to Alpha Centauri, each of them sporting extraordinarily thin, incredibly reflective sails. The plan has these "starchips" flying at up to 20% the speed of light, reaching Alpha Centauri in about 20 years.
A drawback of the Starshot project is that it requires the most powerful laser array ever built to propel the starchips outward. Not only does the technology to build this array currently not exist, the project's estimated total costs may reach $5 billion to $10 billion.
In the new study, astrophysicists suggested that a cheaper option could involve bubbles made of carbon foam. Probes made of this stuff could make interstellar journeys faster than any rocket while powered solely by sunlight, without the need for a giant laser array, the researchers found.
In order to develop a way for sunlight to propel a light sail to useful interstellar speeds, researchers analyzed previous scientific research looking for strong, lightweight materials. They settled on aerographite, a carbon-based foam 15,000 times lighter than aluminum.
The scientists calculated that a hollow aerographite sphere about 3.3 feet (1 meter) in diameter with a shell 1 micron thick (about 1% the width of an average human hair) would weigh just five millionths of a pound (2.3 milligrams).
If such a sphere carrying 0.035 ounces (1 gram) of payload were released about one astronomical unit (AU) from the sun, sunlight would push it to a speed of up to about 114,000 mph (183,600 km/h) — three times that of Voyager 1. Such a sphere would take about 3.9 years to reach the orbit of Pluto. (One AU is the average Earth-sun distance, which is about 93 million miles, or 150 million km.)
If such a sphere were released about 0.04 AU from the sun — the closest that NASA's Parker Solar Probe gets to our star — the more intense sunlight there would accelerate the spacecraft to nearly 15.4 million mph (24.8 million km/h). It could travel the 4.2 light-year distance between Earth and Proxima Centauri, the closest star to our solar system, in 185 years, the researchers said. The larger the sphere, the faster it could go, or the more payload it could carry. (Proxima Centauri is one of the three stars in the Alpha Centauri system.)
"What I find amazing about our results is the fact that the power output of a star, in our case the sun, can be used to propel an interstellar probe to the nearest stars without the need of an additional onboard power source," study lead author René Heller, an astrophysicist at the Max Planck Institute for Solar System Research in Göttingen, Germany, told Space.com.
"We don't need a billion-dollar ground-based laser array to shoot at a sail in space," Heller said. "Instead, we can use green energy, so to say."
The researchers noted that a few grams of electronics or other payload is not a lot to bring aboard a mission. Still, they argued the payload for these craft would be 10 times the mass of the spacecraft, whereas the payload on chemical interstellar rockets would typically be one-thousandth the weight of the rocket.
The researchers suggested these spacecraft could potentially carry a 32-watt laser weighing only two-thousandths of a pound (1 gram). Analyzing any disruptions from this laser beam might help researchers detect gravitational effects, which might in turn help reveal the presence of worlds otherwise too dark and cold to spot, such as the hypothetical Planet Nine, Heller said.
The scientists estimated that developing a prototype bubble craft might cost $1 million. They calculated each foam ship might then be built for about $1,000 or less, and a rocket launch to deploy and test these craft might cost $10 million.
The biggest caveat of this work right now "is that no one has ever built an aerographite structure larger than a few centimeters, while we need something that's a few meters in size," Heller said. Still, the researchers are in contact with experimentalists who suggest that creating such large structures is possible in principle, he noted.
Another point of caution about this concept is that there is currently no way to control the trajectory of the spheres once they are deployed. "In order to reach a certain target, this needs to be rectified," Heller said.
If onboard electronics and equipment could enable active maneuvering, "then it might be possible to transport small masses — 1 to 100 grams — between Earth and Mars within weeks," Heller said.
The scientists envision conventional rockets bringing the bubble craft to space and then deploying them for sunlight to propel. It remains uncertain how well these bubbles would survive transport.
"One good thing about aerographite is its compressibility," Heller said. "Even after extreme compression, a sample of aerographite can reinflate to its initial state. So if we compress a meter-sized aerographite sail in the laboratory, maybe we can ship it into space and reinflate it there prior to launch. The question is, what happens to its onboard electronics?"
The scientists are now running experiments to test how well aerographite absorbs and reflects light. They detailed their findings online July 7 in the journal Astronomy & Astrophysics.
Follow Charles Q. Choi on Twitter @cqchoi. Follow us on Twitter @Spacedotcom and on Facebook.