The 150-year flight to alpha Centauri

Interstellar travel, which for many years has been a major element of science fiction today can become reality — would be money. For only $ 100 million or so a customer may actually buy a new commercial rocket to travel beyond the Solar system. The key here is patience. If such a missile tomorrow to run to the nearest port of destination — potentially habitable exoplanet Proxima b, discovered recently in the triple star system of alpha Centauri at a distance of 4 light years from Earth, the trip will take 80 000 years.

Instead of spending $ 100 million on such a slow transport, in April last year, the entrepreneur billionaire Yuri Milner, said that would have spent the same money to come up with another way to reach the alpha Centauri system within the time not exceeding the limit of human life. A project called Starshot Breakthrough (“a Breakthrough to the stars” — approx. lane) provides a global rejection of the missiles in favor of lighting the sails — the finest mirror surface, which is driven by laser beams and helps to gain a high speed in space. Preliminary plans for this project involve the use of conventional rockets, by which at the beginning 2040-ies orbiting the earth and plan to install thousands of two-metre high light sails weighing only one gram each. The sails will be placed cm chips with built-in cameras, sensors, jet engines and batteries. Each ultralight spacecraft will be a ground-based laser power of 100 gigawatts aimed from earth orbit in the direction of the alpha Centauri system at a rate of 20 percent of the speed of light. In this case, interstellar flight would have taken only 20 years old, and the probe would reach alpha Centauri already in 2060 years.

But such high speeds are expensive. Even the most modest estimates of the project Starshot far exceed the original Milner invested $ 100 million towards developing the project for decades may need 10 billion, maybe more, mostly due to huge spending on the construction of a ground-based laser installation. To do without state assistance and international cooperation, most likely, will not succeed. In addition, the light sails, which have to go through a 20-year journey that will sweep through the Centauri system is so lightning fast that they will have only few seconds to obtain macro photography and other data about Proxima b and any other planets near it. And while the probes will be removed in the interstellar darkness, the light sails will try to pass on precious information to Earth, using laser beams, the capacity of which does not exceed the signal strength of a conventional cell phone.

Slow journey to the stars

Some critics believe this is a lively pursuit of alpha Centauri a poor investment. “After learning about the project Starshot, we considered it wasteful to spend that kind of money on a migratory mission, which will take several decades, and the pictures will be a couple of seconds,” says independent researcher Michael Hippke from Germany. Working with Rene Heller, an astrophysicist from the Institute of Solar system research max Planck in göttingen, Hippke developed an alternative flight program, which, according to him, the scientific benefit is greater, and costs will require less. Instead of building a multibillion-dollar laser system to disperse tiny light sails to relativistic speeds and a single span of Heller and Hippke suggest using only one light from the stars to send at a lower speed larger sails to all three stars in the system alpha Centauri with the option “Parking” orbits. The results of their research will be published in the issue of the journal Astrophysical Journal Letters on 1st February.

The essence of their proposal is to not only use sunlight to speed up our system leaving the light sails, but also the light and gravity of the three stars in the alpha Centauri system at the end of the flight. Heller and Hippke calculated that such a journey under the power of sail surprisingly low density, weighing about 100 grams and occupying an area of 100 thousand square meters (approximately 15 football fields!). This design of sail possible, given the rapid development of the science of materials. Gradually adjusting the angle as you get closer to the stars to capture more of the pressure of radiation of the latter, such a sail may develop enough speed to secure to any orbit within the system.

To reach potentially habitable planet Proxima b, these “photogravitational” auxiliary systems, oddly enough, will require sending the light sails first to the brightest sun-like stars alpha Centauri A and alpha Centauri B, despite the fact that they are located on two trillion miles farther from us than the smaller and dim parent star, the planet Proxima b — Proxima Centauri. This is due to the inhibition caused by high radiation pressure of the stars alpha Centauri A and B, and, therefore, more rapid approximation to the system lighting the sails of any size. But the radiation the twin stars has a limit; if a huge sail Heller and Hippke will reach speeds higher than 4.6 percent of the speed of light, it will simply slip through the system. They suggest that the flight to alpha Centauri A and B will take almost a century, followed by 50 years of travel to the final destination — a stable orbit around Proxima.

“Your journey would take 7 times longer than a 20-year mission Starshot, but you could have spent on thorough research years and even decades, not a few seconds,” says Heller. Comparing the ratio of time on research and time travel in both cases, Heller adds, “Starshot could use for research on the place only one hundred millionth part of the whole mission, and we — about one hundredth, that is a million times more.” In addition, Cycling to start the sails of sunlight, this option eliminates the need for the construction of a multibillion-dollar laser unit.

And yet, they offer 150-year journey can’t start tomorrow. Offer Heller and Hippke, among other things, provides a rare configuration of the stars in the alpha Centauri system, which only happens once in 80 years, when their orbits are in one plane crossing the path of any probe out of our Solar system. Next time it will happen in 2035, but in such a short period of time, not a single sail even closer to the system. Heller and Hippke suggest waiting for the next “alignment” in the year 2115.

According to Heller, sending them sail directly to the star Proxima Centauri would require much lower space velocities due to the weak radiation pressure and the braking ability of the smaller of the two stars, bringing the total flight time would have reached the Millennium.

Patience, please

Hippke considers mission involving several generations with the end point in orbit around alpha Centauri standing expectations, even if her return he will never see. “Amazing photos with these space probes will be to our children and grandchildren. Just imagine alien rivers, volcanoes and perhaps even exotic life!” The choice in favor of the mission length in a century opens up opportunities to explore other nearby bright stars, says Hepke. The large star Sirius, for example, only two times farther than alpha Centauri, but because it shines about 25 times brighter than the Sun, the braking effect of the pressure of radiation is stronger than her, and it is also the fastest approach to her light sails. Anyway, the possibility of the direction of the light sails to orbit many nearby stars implies the natural completion of the next generations in the long term, urgent objectives of the mission Starshot.

Despite all these advantages, AVI Loeb, an astronomer at Harvard University and chair of the scientific Advisory Committee of the project Breakthrough Starshot, I do not believe that this alternative proposal offers real benefits compared to plan Starshot to use the laser system gigawatt class to send to the stars the small sails. “To achieve near-light speed using star light you need a very fine sail,” says Loeb, noting that the lower the pressure of sunlight, the lower the density of the light sails. Hippke and Heller say that, theoretically, their sails can be created from ultra-light high-strength materials such as graphene, but Loeb doubt that the creation of interstellar probe sheet of graphene just a few atoms thick and with an area of 100,000 square meters will be lighter than building a massive laser system. “This surface is on the order of magnitude thinner than the wavelength of light, which it should reflect, and so the reflectivity will be low,” says Loeb. “It is not possible to reduce by several orders of magnitude weight while maintaining stiffness and the reflection coefficient of the material of the sails”. In other words, the graphene sail with an area of 100,000 square meters may be too flimsy for real space travel. In addition, the project plans Starshot is to run not one, but thousands of sails, and even if everyone successfully crossed the interstellar space probe will get only a few seconds for panoramic images, the number will exceed what you could get several consecutive spans.

The biggest, according to Loeb, the problem is that will survive the ambitious plans of the projects with participation of several generations of the inevitable meeting with the frailty of human life. “If you ignore the duration of the trip, you can always use conventional missiles and small losses to get to the alpha Centauri system after 80 000 years,” he says. “But the people who work on the project Starshot, more ambitious. We want to get there in our lifetime”.