A decelerating gravity slingshot and solar pressure could be used to slow an interstellar solar sail travelling up to 4.6% of lightspeed

In April last year, billionaire Yuri Milner announced the Breakthrough Starshot Initiative. He plans to invest 100 million US dollars in the development of an ultra-light light sail that can be accelerated to 20 percent of the speed of light to reach the Alpha Centauri star system within 20 years. The problem of how to slow down this projectile once it reaches its target remains a challenge. René Heller of the Max Planck Institute for Solar System Research in Göttingen and his colleague Michael Hippke propose to use the radiation and gravity of the Alpha Centauri stars to decelerate the craft. It could then even be rerouted to the red dwarf star Proxima Centauri and its Earth-like planet Proxima b.

* a very light sail about 300 meters on a side for a 10 gram probe can approach a target star system to with 5 solar radii and slow down and then swing to the next star where it would fully stop and go into orbit

Heller and his colleague Michael Hippke wondered, “How could you optimize the scientific yield of this type of a mission?” Such a fast probe would cover the distance from the Earth to the Moon in just six seconds. It would therefore hurtle past the stars and planets of the Alpha Centauri system in a flash.

The solution is for the probe’s sail to be redeployed upon arrival so that the spacecraft would be optimally decelerated by the incoming radiation from the stars in the Alpha Centauri system. René Heller, an astrophysicist working on preparations for the upcoming Exoplanet mission PLATO, found a congenial spirit in IT specialist Michael Hippke, who set up the computer simulations.

The two scientists based their calculations on a space probe weighing less than 100 grams in total, which is mounted to a 100,000-square-metre sail, equivalent to the area of 14 soccer fields. During the approach to Alpha Centauri, the braking force would increase. The stronger the braking force, the more effectively the spacecraft’s speed can be reduced upon arrival. Vice versa, the same physics could be used to accelerate the sail at departure from the solar system, using the sun as a photon cannon.

The tiny spacecraft would first need to approach the star Alpha Centauri A as close as around four million kilometres, corresponding to five stellar radii, at a maximum speed of 13,800 kilometres per second (4.6 per cent of the speed of light). At even higher speeds, the probe would simply overshoot the star.

During its stellar encounter, the probe would not only be repelled by the stellar radiation, but it would also be attracted by the star’s gravitational field. This effect could be used to deflect it around the star. These swing-by-manoeuvres have been performed numerous times by space probes in our solar system. “In our nominal mission scenario, the probe would take a little less than 100 years – or about twice as long as the Voyager probes have now been travelling. And these machines from the 1970s are still operational,” says Michael Hippke.

Theoretically, the autonomous, active light sail proposed by Heller and Hippke could settle into a bound orbit around Alpha Centauri A and possibly explore its planets. However, the two scientists are thinking even bigger. Alpha Centauri is a triple star system. The two binary stars A and B revolve around their common centre of mass in a relatively close orbit, while the third star, Proxima Centauri, is 0.22 light years away, more than 12,500 times the distance between the Sun and the Earth.

The sail could be configured so that the stellar pressure from star A brakes and deflects the probe toward Alpha Centauri B, where it would arrive after just a few days. The sail would then be slowed again and catapulted towards Proxima Centauri, where it would arrive after another 46 years − about 140 years after its launch from Earth.

In order to keep the weight down, the sail would have to be just a few atoms thick. That means it would be orders of magnitude thinner than the wavelength of light that it aims to reflect, and so its reflectivity would be low. “It does not appear feasible to reduce the weight by so many orders of magnitude and yet maintain the rigidity and reflectivity of the sail material,” Loeb says.

Hippke acknowledges the problem. “The issue of producing an extremely thin material with high surface reflectivity seems to be a very challenging exercise,” he says. However, he can see solutions coming over the horizon. A one-atom thick coating of silicon would boost the reflectivity of the graphene sail enormously, he points out, and silicon-based metamaterial monolayers are now being designed.

At a distance of about 4.22 ly, it would take about 100,000 years for humans to visit our closest stellar neighbor Proxima Centauri using modern chemical thrusters. New technologies are now being developed that involve high-power lasers firing at 1 gram solar sails in near-Earth orbits, accelerating them to 20% the speed of light (c) within minutes. Although such an interstellar probe could reach Proxima 20 years after launch, without propellant to slow it down it would traverse the system within hours. Here we demonstrate how the stellar photon pressures of the stellar triple α Cen A, B, and C (Proxima) can be used together with gravity assists to decelerate incoming solar sails from Earth. The maximum injection speed at α Cen A to park a sail with a mass-to-surface ratio (σ) similar to graphene (7.6 × 10^−4 gram m^−2) in orbit around Proxima is about 13,800 km per second (4.6% c), implying travel times from Earth to α Cen A and B of about 95 years and another 46 years (with a residual velocity of 1280 km s−1) to Proxima. The size of such a low-σ sail required to carry a payload of 10 grams is about 100,000 square meters = (316 meters)2. Such a sail could use solar photons instead of an expensive laser system to gain interstellar velocities at departure. Photogravitational assists allow visits of three stellar systems and an Earth-sized potentially habitable planet in one shot, promising extremely high scientific yields.

They present a new method of decelerating interstellar light sails from Earth at the α Cen system using a combination of the stars’ gravitational pulls and their photon pressures. This sailing technique, which we refer to as a photogravitational assist, allows multiple stellar fly-bys in the α Cen stellar triple system and deceleration of a sail into a bound orbit. In principle, photogravitational assists could also allow sample return missions to Earth. The maximum injection speed to deflect an incoming, extremely light and tensile sail (with properties akin to graphene) carrying a payload of 10 grams into a bound orbit around Proxima is about 4.6 % c, corresponding to travel times of 95 years from Earth. After initial fly-bys at α Cen A and B, the sail could absorb another 1280 km s−1
upon the arrival at Proxima, implying an additional travel time between α Cen AB and Proxima of 46 years.

Arrival at Proxima with maximum velocity could result in a highly elliptical orbit around the star, which could be circularized into a habitable zone orbit using the photon pressure near periastron. The time required for such an orbit transfer is small (years) compared to the total travel time. Once parked in orbit around Proxima, a sail could eventually use the stellar photon pressure to transfer into a planetary orbit around Proxima b. In a more general context, photogravitational assists of a large, roughly 100000 square meters= (316 meters)2 -sized graphene sail could
(1.) decelerate a small probe into orbit around a nearby exoplanet and therefore substantially reduce the technical demands on the onboard imaging systems;
(2.) in principle allow sample return missions from distant stellar systems;
(3.) avoid the necessity of a large-scale Earth-based laser launch system by instead using the sun’s radiation at the departure from the solar system;
(4.) limit accelerations to about 1000 g compared to some 10, 000 g invoked for a 1 m2 laser-riding sail; and
(5.) leave of the order of 10 grams for the sail’s reflective coating and equipment.

These benefits come at the price of a yet to be developed large graphene sail, which needs to be assembled or unfold in near-Earth space and which needs to withstand the harsh radiation environment within 5 radii of the target star for several hours. This technical challenge, however, could be easier to tackle than the construction of a high-power ground-based laser system shooting laser sails in near-Earth orbits.

Arxiv – Deceleration of High-velocity Interstellar Photon Sails into Bound Orbits at α Centauri

Proxima Centauri caused a sensation in August 2016 when astronomers at the European Southern Observatory (ESO) discovered an exoplanet companion that is about as massive as the Earth and that orbits the star in its so-called habitable zone. This makes it theoretically possible for liquid water to exist on its surface – water being a key prerequisite for life on Earth.

“This finding prompted us to think about the possibility of stopping a high-velocity interstellar lightsail at Proxima Centauri and its planet,” says René Heller. The Max Planck researcher and his colleague propose another change to the strategy for the Starshot project: instead of a huge energy-hungry laser, the Sun’s radiation could be used to accelerate a nanoprobe beyond the solar system. “It would have to approach the Sun to within about five solar radii to acquire the necessary momentum,” Heller says.

The two astronomers are now discussing their concept with the members of the Breakthrough Starshot Initiative, to whom they owe the inspiration for their study. “Our new mission concept could yield a high scientific return, but only the grandchildren of our grandchildren would receive it. Starshot, on the other hand, works on a timescale of decades and could be realized in one generation. So we might have identified a longterm, follow-up concept for Starshot,” Heller says.

Although the new scenario is based on a mathematical study and computer simulations, the proposed hardware of the sail is already being developed in laboratories today: “The sail could be made of graphene, an extremely thin and light but mega-tough carbon film,” René Heller says. The film would have to be blanketed by a highly reflective cover to endure the harsh conditions of deep space and the heat near the destination star.

The optical and electronic systems would have to be tiny. But if you were to remove all the unnecessary components from a modern smartphone, “only a few grams of functional technology would remain.” Moreover, the lightweight spacecraft would have to navigate independently and transmit its data to Earth by laser. To do so, it would need energy, which it could harness from the stellar radiation.

Breakthrough Starshot therefore poses daunting challenges that have so far only been solved theoretically. Nevertheless, “many great visions in the history of mankind had to struggle with seemingly insurmountable obstacles,” Heller says. “We could soon be entering an era in which humans can leave their own star system to explore exoplanets using fly-by missions.”

Previously there was a NASA study looking at using a close gravity slingshot to accelerate a solar sail

An advanced solar sail could theoretically reach about 13% of the speed of light if it could withstand high temperatures and was ultrathin and performed a gravitational slingshot move around the sun that passed within a couple of solar radii. The materials for such a sail do not yet exist (at least in sufficient quantities).

NASA has a page that describes gravity assists

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