Studying Proxima b: Tiny Sailing Probes Could Orbit Nearby Exoplanet

[Buster's Uncle]

Studying Proxima b: Tiny Sailing Probes Could Orbit Nearby Exoplanet
By Mike Wall, Space.com Senior Writer |  February 2, 2017 07:08am ET


Tiny spacecraft could actually study Proxima b from orbit rather than just zoom past the newfound, potentially habitable alien world, a new study suggests.

In April 2016, cosmologist Stephen Hawking, several other scientists and billionaire investor Yuri Milner announced Breakthrough Starshot, a $100 million project that aims to develop the technology required to accelerate minuscule, sail-equipped "nanoprobes" to 20 percent the speed of light using powerful ground-based lasers.

The Starshot team's long-term vision involves sending fleets of these sail craft to nearby stellar systems, including Alpha Centauri, a double star that lies just 4.3 light-years from Earth. At 20 percent the speed of light, such probes could theoretically get to Alpha Centauri just 20 years after lifting off.



Artist's illustration of the surface of the Earth-size, possibly habitable alien planet Proxima b.  Credit: ESO/M. Kornmesser


The Starshot project got another tempting target this past August, when astronomers announced the existence of a possibly habitable Earth-size world circling Proxima Centauri — a red dwarf star that some scientists consider to be part of the Alpha Centauri system. (Proxima Centauri lies just 0.22 light-years from the Alpha Centauri pair.)

The original Starshot plan calls for missions to this planet (known as Proxima b), or to any other destination, to be flyby affairs; the nanoprobes would snap photos and collect other data as they hurtle by at breakneck speed. But it doesn't have to be this way, according to the new study, which was led by René Heller of the Max Planck Institute for Solar System Research in Göttingen, Germany.

Heller and co-author Michael Hippke, an IT specialist, performed computer simulations showing that Starshot-like probes could slow down enough at Alpha Centauri to be captured into orbit there. This deceleration would come courtesy of the binary's starlight pressure — which would push back on the nanoprobes' sails, just as outgoing photons would have pushed the spacecraft forward at the beginning of its trek — and the gravitational pull of the Alpha Centauri stars.



The postage-stamp-sized space probes of Breakthrough Starshot could make a flyby of planets around Alpha Centauri within 20 years after launch. See how Breakthrough Starshot could work in our full infographic.  Credit: by Karl Tate, Infographics Artist


Indeed, Starshot probes could use Alpha Centauri "photogravitational assists" to sling themselves into orbit around Proxima Centauri and eventually even work themselves into position around Proxima b itself, Heller and Hippke wrote in the new study, which was published this month in the The Astrophysical Journal Letters.

This strategy would allow the probes to gather much more data than they'd collect during a superfast flyby, which would be over in a matter of hours, the researchers said.

"Photogravitational assists allow visits of three stellar systems and an Earth-sized potentially habitable planet in one shot, promising extremely high scientific yields," they wrote in the study.

There are some catches, however. For example, Heller and Hippke found that this deceleration strategy would likely only work for probes traveling a maximum of 4.6 percent the speed of light — meaning it would take the sail craft about 95 years, rather than 20 years, to get to Alpha Centauri. The journey from Alpha Centauri to Proxima Centauri would take an additional 46 years, the researchers determined.

Furthermore, the probes' sails would have to be enormous to catch enough light to slow down — about 1.1 million square feet (100,000 square meters), or the size of 14 soccer fields, Heller and Hippke said. (This assumes that the sails are made of graphene, or some other material that's similarly lightweight and tough.) The baseline Breakthrough Starshot concept, by contrast, envisions a deployed sail that's no larger than 172 square feet (16 square m).

On the plus side, such a gigantic sail would allow probes to get up to interstellar speeds solely by catching light from the sun, obviating the need for a laser system, Heller and Hippke said.

The researchers aren't advocating their concept as a replacement for Starshot's planned flyby missions. Rather, the two ideas could serve as a sort of one-two punch in interstellar exploration, Heller and Hippke said.

"Our new mission concept could yield a high scientific return, but only the grandchildren of our grandchildren would receive it," Heller said in a statement. "Starshot, on the other hand, works on a timescale of decades and could be realized in one generation. So we might have identified a long-term, follow-up concept for Starshot."


http://www.space.com/35549-proxima-centauri-mission-solar-sail.html

[gwillybj]

Full Braking at Alpha Centauri
by Staff Writers
Gottingen, Germany (SPX) Feb 03, 2017

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. Rene Heller of the Max Planck Institute for Solar System Research in Gottingen 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.

In the recent science fiction film Passengers, a huge spaceship flies at half the speed of light on a 120-year-long journey toward the distant planet Homestead II, where its 5000 passengers are to set up a new home. This dream is impossible to realize at the current state of technology. "With today's technology, even a small probe would have to travel nearly 100,000 years to reach its destination," Rene Heller says.

Notwithstanding the technical challenges, 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. Rene 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.

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 Rene 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," Rene 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."

Interstellar journey: The aim of the Starshot project is to send a tiny spacecraft propelled by an enormous rectangular photon sail to the Alpha Centauri star system, where it would fly past the Earth-like planet Proxima Centauri b. The four red beams emitted from the corners of the sail depict laser pulses for communication with the Earth.
Image courtesy Planetary Habitability Laboratory, Univesity of Puerto Rico at Arecibo.

http://www.spacedaily.com/reports/Full_Braking_at_Alpha_Centauri_999.html