Humanity has the means to launch probes into orbit around nearby stars. But do we have the necessary patience?
Interstellar travel, which has been a staple of science fiction for many years, could become a reality today - if only there were money. For as little as $ 100 million or so, a customer can actually purchase the latest commercial rocket and travel outside the solar system. Patience is key here. If such a rocket is launched tomorrow to the nearest port of destination - the potentially habitable exoplanet Proxima b, recently discovered in the triple star system Alpha Centauri at a distance of 4 light years from Earth - the flight will take 80,000 years.
Instead of spending $ 100 million on such a slow transport, billionaire entrepreneur Yuri Milner said last April that he would spend the same money to come up with another way to get to the Alpha Centauri system in a time frame that does not exceed the limit of human life. The project, called Breakthrough Starshot, seeks to move away from rockets globally in favor of a light sail, the thinnest mirrored surface propelled by laser beams to accelerate through space. The preliminary plans for this project provide for the use of conventional rockets, through which, already in the early 2040s, thousands of four-meter light sails weighing only one gram each are planned to be installed in earth orbit. The sails will contain centimeter chips with built-in cameras, sensors, jet engines and batteries. Each ultralight spacecraft will be directed from Earth orbit towards the Alpha Centauri system using a 100 gigawatt ground laser at 20 percent the speed of light. In this case, the interstellar flight would take only 20 years, and the probes would have reached Alpha Centauri in the 2060s.
But these high speeds cost a lot of money. Even the most modest estimates of the Starshot project far exceed Milner's initial $ 100 million - the project could require $ 10 billion over decades, or more, mainly due to the huge costs of building a ground-based laser facility. Most likely, it will not be possible to do without government assistance and international cooperation. In addition, the light sails, which will survive the 20-year voyage, will sweep through the Centauri system so lightning fast that they will only have a few seconds to obtain macro photographs and other data on Proxima b and any other planets near it. And while the probes move away into interstellar darkness, the light sails will try to transmit precious information to Earth using laser beams,the power of which does not exceed the signal strength of a conventional cell phone.
A slow journey to the stars
Some critics regard this fussy pursuit of Alpha Centauri as a bad investment. “When we heard about the Starshot project, we found it wasteful to spend that kind of money on a flyby mission that would take several decades and take a few seconds to take pictures,” says independent researcher Michael Hippke from Germany. Working with Rene Heller, an astrophysicist at the Max Planck Institute for Solar System Research in Göttingen, Hippke developed an alternative flight program that he said would bring more scientific benefits and cost less. Instead of building a multi-billion dollar laser system to accelerate the tiny light sails to near-light speed and fly them once, Heller and Hippke propose using starlight alone to send larger sails at a lower speed to all three stars in the Alpha Centauri system with the ability to "park" in orbits. Their findings will be published in the February 1st issue of the Astrophysical Journal Letters.
The essence of their proposals is to use not only sunlight to accelerate the light sails leaving our system, but also the light and gravity of the three stars of the Alpha Centauri system at the end of the flight. Heller and Hippke calculated that such a trip could be done on a mind-bogglingly low density sail, weighing about 100 grams and covering an area of 100 thousand square meters (which is about 15 football fields!). This sail design seems feasible given the rapid development of materials science. By gradually adjusting the angle as it approaches the stars to capture more pressure from the latter, such a sail can develop sufficient speed to anchor in any orbit within the system.
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To reach the potentially habitable planet Proxima b, such "photogravitational" auxiliary systems, oddly enough, would require sending a light sail first to the bright sun-like stars Alpha Centauri A and Alpha Centauri B, despite the fact that they are located two trillion kilometers further away from us than the smaller and fainter parent star of the planet Proxima b - Proxima Centauri. This is due to the deceleration caused by the high radiation pressure of the Alpha Centauri stars A and B, and, therefore, a more rapid approach to a light sail system of any size. But radiation from twin stars has a limit; if Heller and Hippke's huge sail reaches a speed higher than 4.6 percent of the speed of light, it will simply skip past the system. They estimate that the flight to Alpha Centauri A and B will take almost a century.followed by another 50 years of travel to its final destination - a stable orbit around Proxima.
“Your journey would take 7 times longer than a 20-year Starshot mission, but you could spend years and even decades on thorough research, not a few seconds,” says Heller. Comparing the ratio of research time to travel time in both cases, Heller adds, "Starshot could use only one hundred millionth of the entire mission for on-site research, and we could use about one hundredth, or a million times more." In addition, by using sunlight to launch the sail, this option eliminates the need to build a multi-billion dollar laser unit.
And yet, their proposed 150-year journey cannot begin tomorrow. Heller and Hippke's proposal, among other things, provides for a rare configuration of stars in the Alpha Centauri system, which happens only once every 80 years, when all their orbits are in the same plane, crossing the trajectory of any probe from our solar system. The next time it will happen in 2035, but in such a short period of time, no sail can even come close to the system. Heller and Hippke suggest waiting for the next such "alignment" in 2115.
Sending their sails directly to Proxima Centauri, Heller said, would require much lower cosmic speeds due to the weak radiation pressure and stopping power of the lesser of the two stars, bringing the total flight time to a full millennium.
Patience please
Hippke sees a multi-generational mission with an endpoint in orbit around Alpha Centauri worth the wait, even if he never sees her return. “Our children and grandchildren will receive amazing photos from these space probes. Just imagine alien rivers, volcanoes, and perhaps even exotic life! Choosing a century-long mission opens up opportunities to study other nearby bright stars as well, Hippke says. The large star Sirius, for example, is only two times farther than Alpha Centauri - but since it shines about 25 times brighter than the Sun, its inhibitory effect from radiation pressure is stronger, and this will ensure a faster approach of light sails to it. Howbeit,The ability to send light sails into orbit around many nearby stars suggests a natural conclusion by next generations in the long term to the immediate goals of the Starshot mission.
Despite all these advantages, Avi Loeb, an astronomer at Harvard University and chairman of the scientific advisory committee for the Breakthrough Starshot project, is not convinced that this alternative proposal offers real advantages over Starshot's plan to use a gigawatt-class laser to send small sails to the stars. … “A very thin sail is needed to reach near-light speed using starlight,” says Loeb, noting that the lower the pressure from sunlight, the lower the density of the light sail should be. Hippke and Heller say that, in theory, their sails could be made from ultra-light, high-strength materials such as graphene, but Loeb doubts that creating a sheet of graphene for an interstellar probe several atoms thick and 100000 square meters will be easier than building a massive laser facility. "Such a surface is an order of magnitude thinner than the wavelength of light that it should reflect, and therefore its reflectivity will be low," says Loeb. "It is not possible to reduce the weight by several orders of magnitude while maintaining the rigidity and reflection coefficient of the sail material." In other words, a 100,000 square meter graphene sail may be too flimsy for real space travel. In addition, the plans of the Starshot project include launching not one, but thousands of sails, and even if each probe that successfully crossed interstellar space receives only a few seconds for panoramic images, their number will exceed what could be obtained during several successive flights."Such a surface is an order of magnitude thinner than the wavelength of light that it should reflect, and therefore its reflectivity will be low," says Loeb. "It is not possible to reduce the weight by several orders of magnitude while maintaining the rigidity and reflection coefficient of the sail material." In other words, a 100,000 square meter graphene sail may be too flimsy for real space travel. In addition, the plans of the Starshot project include launching not one, but thousands of sails, and even if each probe that successfully crossed interstellar space receives only a few seconds for panoramic images, their number will exceed what could be obtained during several successive flights."Such a surface is an order of magnitude thinner than the wavelength of light that it should reflect, and therefore its reflectivity will be low," says Loeb. "It is not possible to reduce the weight by several orders of magnitude while maintaining the rigidity and reflection coefficient of the sail material." In other words, a 100,000 square meter graphene sail may be too flimsy for real space travel. In addition, the plans of the Starshot project include the launch of not one, but thousands of sails, and even if each probe successfully crossing interstellar space receives only a few seconds for panoramic images, their number will exceed what could be obtained during several successive flights.and therefore its reflectivity will be low,”says Loeb. "It is not possible to reduce the weight by several orders of magnitude while maintaining the rigidity and reflection coefficient of the sail material." In other words, a 100,000 square meter graphene sail may be too flimsy for real space travel. In addition, the plans of the Starshot project include launching not one, but thousands of sails, and even if each probe that successfully crossed interstellar space receives only a few seconds for panoramic images, their number will exceed what could be obtained during several successive flights.and therefore its reflectivity will be low,”says Loeb. "It is not possible to reduce the weight by several orders of magnitude while maintaining the rigidity and reflection coefficient of the sail material." In other words, a 100,000 square meter graphene sail may be too flimsy for real space travel. In addition, the plans of the Starshot project include launching not one, but thousands of sails, and even if each probe that successfully crossed interstellar space receives only a few seconds for panoramic images, their number will exceed what could be obtained during several successive flights.000 square meters may be too flimsy for real space travel. In addition, the plans of the Starshot project include launching not one, but thousands of sails, and even if each probe that successfully crossed interstellar space receives only a few seconds for panoramic images, their number will exceed what could be obtained during several successive flights.000 square meters may be too flimsy for real space travel. In addition, the plans of the Starshot project include the launch of not one, but thousands of sails, and even if each probe successfully crossing interstellar space receives only a few seconds for panoramic images, their number will exceed what could be obtained during several successive flights.
The biggest challenge, according to Loeb, is whether the ambitious multi-generational project plans will survive the inevitable encounter with the frailty of human life. “If you ignore the length of the trip, you can always use conventional rockets and get to the Alpha Centauri system with low losses in 80,000 years,” he says. “But the people who are working on the Starshot project are more ambitious. We want to get there in our lifetime."
Lee Billings