Last year, famed theoretical physicist Stephen Hawking and Russian billionaire Yuri Milner announced an ambitious plan to launch a tiny spacecraft to the Alpha Centauri system. Of course, such an ambitious plan requires a search for no less ambitious solutions. For example, one of the unresolved problems relates to how a spacecraft moving at one-fifth the speed of light can stop after it reaches its destination. Will he be capable of such a maneuver at all?
A couple of European scientists seem to have found the right answer to this question. In a paper published in The Astrophysical Journal Letters, physicist Rene Heller of the Max Planck Institute and computer scientist Michael Hippke discuss how the radiation and gravity of Alpha Centauri stars could be used to slow a spacecraft. According to scientists, instead of just zipping by, a tiny spacecraft equipped with a light sail can slow down enough to study in detail the triple star system and, possibly, even the Earth-like planet Proxima b located near one of the stars of this system.
Recall that within the framework of the Breakthrough Starshot Initiative Milner plans to invest $ 100 million in the development of an ultra-light autonomous spacecraft with a light sail, which can accelerate to 1/5 the speed of light (about 60,000 km / s). Thanks to this, the robotic probe will be able to reach Alpha Centauri - the closest star system to Earth - in just 20 years, and not in 100,000, as is the case with traditional chemical accelerators.
According to Milner and Hawking's original plan, the tiny probe would be attached to a compact, a few meters in size, a sail of light controlled by a phased array of lasers. The energy generated by these lasers would theoretically be enough to accelerate the tiny probe to speeds much higher than the fastest spacecraft today can show.
Render of the proposed light sail technology
However, this is not the only scheme for implementing this project proposed. According to Heller and Hippke's version, using a larger "photon" sail would eliminate the need to use a laser array. In this case, the probe itself will be only a few centimeters in size and weighing only a few grams. To accelerate and enter interstellar space, the craft will be equipped with several large, but at the same time very light, thin and strong sails. According to the scenario proposed by European scientists, the probe will push the radiation of our Sun towards Alpha Centauri. Upon reaching the required level of inertia, the apparatus will fold down its sails and continue its journey towards the neighboring star system.
Scientists believe that in this case, the probe will be able to develop 4.6 percent of the speed of light and in about 95 years will reach Alpha Centauri. Yes, this is almost five times longer than in the original plan of Milner and Hawking, but in theory it will greatly simplify the task of stopping the probe in the right place.
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“The interstellar journey to the Alpha Centauri system will presumably take place at speeds that will reduce the travel time to less than one thousand, and ideally less than one hundred years. At this speed, the spacecraft will need an incredibly large amount of energy to slow down and reach the desired orbits,”says Heller.
“Using whatever kind of fuel will only complicate the project as a whole. If the ship requires fuel on board, then it itself will be too heavy in this case, which, in turn, will only increase the need for an even larger supply of fuel."
Given these limitations, as well as the lack of a suitable solution at the moment, scientists suggest that the probe in this case will simply sweep past Alpha Centauri, as was the case with the New Horizons spacecraft, which flew past Pluto. But again, if we take into account the difference in speed, the probe, unlike the "New Horizons", will not be able to provide at least some more or less accurate measurements of this stellar system. Fortunately, according to the two scientists, there is an option that, in theory, will not only allow the spacecraft to slow down to acceptable speeds at the desired point, but also conduct a detailed study of the Alpha Centauri system.
“We have found a method to slow down the spacecraft using the energy of the star itself. Light particles can be used to slow down the light sail. In this case, no additional fuel is required on board. And the plan itself as a whole fits into the general concept proposed by the Breakthrough Starshot Initiative."
Animation of "photogravitational capture" by the star Alpha Centauri A
For the success of the implementation, it is necessary to come up with a way in which the apparatus can unfold its sails upon arrival in the system. In this case, the radiation emanating from the system will create the necessary pressure, which will slow down the probe. Thanks to computer simulations, Heller and Hippke calculated that with a probe weighing 100 grams, the sail area would be about 100,000 square meters (about 14 football fields). Upon arrival at the system, the braking power of the radiation from Alpha Centauri on the sail will increase. Computer simulations indicate that there will be sufficient force to effectively slow the craft down. In other words, the same physics that will be responsible for pushing the probe towards the neighboring system will also slow down the vehicle upon its arrival at the desired location.
During the deceleration maneuver, the probe would need to approach Alpha Centauri A by a distance of five stellar radii (that is, a distance equivalent to five radii of this star), or about 4 million kilometers, in order to be captured into its orbit. At this point, the spacecraft will begin to decelerate to about 2.5 percent of the speed of light. However, it is important to note here that if deceleration fails at maximum speed (4.6 percent of the speed of light), the probe will be thrown back into interstellar space.
Every successful journey begins with creating a map. In this case, all the maneuvers of an autonomous space nano-apparatus are shown on its journey to Alpha Centauri A, from which the path to Alpha Centauri B will take only four days. The ultimate mission of the probe could be a 46-year journey to the star Proxima Centauri, the home address of the earthlike planet Proxima b
Upon reaching Alpha Centauri A, the space probe will be captured by its gravity, whose power can be used for further maneuvers. Similar maneuvers, for example, were used to accelerate the Voyager 1 and Voyager 2 probes while they were still inside the solar system. In theory, the autonomous probe could enter the orbit of Alpha Centauri A and look for possible exoplanets. Heller and Hippke also drew up a plan to launch a probe to systems of other stars - Alpha Centauri B (the companion star of Alpha Centauri A) and Proxima Centauri (the distant third star of the system, located 0.22 light years, or 1.2 trillion kilometers) from the generally accepted centers of mass of stars A and B. According to this plan, the flight to Alpha Centauri A will take about a century, then 4 more days will be required to fly to Alpha Centauri B,and then 46 years on the journey to Proxima Centauri.
And yet, the extra time spent, according to scientists, can pay off in full. One of the most memorable discoveries of 2016 was the discovery by astronomers of an earth-like planet near the star Proxima Centauri. Ultimately, the opportunity to "close" to explore this planet may turn out to be one of the most (if not the most) significant events in modern astronomy. Sending the collected data about the planet, given the distance to Earth, will take a little over 4 years. However, so far these are just dreams, because at the moment we do not have systems that would simultaneously be compact enough to fit on a nanoprobe, and at the same time have sufficient power to transmit signals over such distances.
The lack of a suitable transmitter is far from the only problem that must be solved by all means before sending a probe towards a neighboring star system. Equally important is finding a solution and designing a suitable power system for the probe. Nevertheless, researchers are not going to lose optimism, since science is not standing still. For example, it is good news that laboratories have already developed some of the ultralight materials that will be required to implement this project.
“It could take one to two decades to build such an interstellar solar sail,” comments Heller.
The scientist also adds that the surface of the sail should be designed in such a way as to reflect the waves of the blue and red ranges of the visible spectrum, and possibly further beyond them.
“We don’t have the technology yet, but again, over the past few years, science laboratories have made a lot of progress, and researchers have discovered materials that can reflect up to 99.9% of the volume of light.”
Heller and Hippke are set to present their detailed concept to the Breakthrough Starshot Initiative's leadership team at the upcoming Breakthrough Discuss to be held in Palo Alto, America this April.
“We really want to hear from them and hear their views on our proposal, as this group includes, among other things, world experts in the emerging field of interstellar travel research using light sail systems,” says Heller.
NIKOLAY KHIZHNYAK
