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Full braking at Alpha Centauri

Space visionaries might have figured out a way to travel within decades to the nearest star system, Alpha Centauri. But, once we get there, how do we slow down?

The dream of star travel is alive and well. An enormous rectangular photon sail might power the first starships to the Alpha Centauri system, closest star system to Earth. In this artist’s concept, the four red beams emitted from the corners of the sail depict laser pulses for communication with Earth. Image via Max Planck/ Planetary Habitability Laboratory, Univesity of Puerto Rico at Arecibo.

In April of 2016, Russian high-tech billionaire Yuri Milner and others announced the Breakthrough Starshot, a plan to spend $100 million on proof-of-concept studies in the coming years to explore the dream of star travel in our time. The Breakthrough Starshot concept is an exceedingly fast flyby mission to the next-nearest star system, Alpha Centauri, located about 4 light-years or 25 trillion miles (40 trillion km) away. Breakthrough Starshot’s goal is to reach Alpha Centauri with a fleet of nanostarships within 20 years. Let’s assume it turns out to be feasible. Is it possible we could do more than sweep past? Two scientists have now announced a braking method, using the radiation and gravity of the Alpha Centauri stars themselves. If a hypothetical starcraft could be decelerated, it could then, possibly, be rerouted to explore the red dwarf star Proxima Centauri and its newly discovered Earth-like planet Proxima b.

Astrophysicist René Heller of the Max Planck Institute for Solar System Research in Göttingen and IT specialist Michael Hippke worked together on this idea, which they submitted online at arXiv.org in late January 2017. For their study, they didn’t use Breakthrough Starshot’s concept of nanostarships, which are envisioned as weighing about a gram (0.036 ounces) each.

Instead, they considered just one slightly bigger starship, weighing about 0.2 pounds (less than 100 grams). It would be mounted to about a million-square-foot (100-thousand-square-meter) sail. That’s a sail about equal in size to the area of 14 soccer fields.

Heller and Hippke’s concept is for the probe’s sail to be redeployed as its nears the Alpha Centauri system, so that the spacecraft would be optimally decelerated by the incoming radiation from the stars in the Alpha Centauri system. Their statement explained:

The tiny spacecraft would first need to approach the star Alpha Centauri A as close as around four million kilometers [2.4 million miles] … at a maximum speed of 13,800 kilometers per second [8,600 miles per second, or 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-maneuvers [aka gravity assists] have been performed numerous times by space probes in our solar system.

Eclipse coming! Penumbral lunar eclipse February 10-11, 2017

A comparison of the sizes and colors of the stars in the Alpha Centauri system with our sun. Via Wikimedia Commons.

Alpha Centauri A and B are a double-star system, and a third star Proxima – whose location with respect to the other two is indicated here by arrow – might or might not be gravitationally bound to them. Proxima is the closest star to Earth. Image via the European Southern Observatory.

Their concept might work, they determined, but the sacrifice would be the speed of the trip. Breakthrough Starshot originally said it wanted to try to reach the Alpha Centauri system in 20 years, that is, within the span of some of our lifetimes. In Heller and Hippke’s braking scenario, the craft would take longer to reach Alpha Centauri. Hippke said:

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 traveling.

The payoff would be huge, though. Rather than shooting past the system (a flyby), in theory an autonomous, active light sail – as visualized by Heller and Hippke – could settle into a bound orbit around Alpha Centauri A – the largest star of three in the Alpha Centauri system – and possibly explore its planets. However, the two scientists are thinking even bigger. Their statement said:

The sail could be configured so that the stellar pressure from star A brakes and deflects the probe toward Alpha Centauri B [second-largest star in the system], 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 – the smallest of the three stars in the Alpha Centauri system, and the actual closest star to Earth – caused a sensation in August 2016 when an observing campaign called Pale Red Dot, begun in early 2016 under the auspices of the European Southern Observatory (ESO), found a planet orbiting around it. This is Proxima Centauri b, which appears to be only slightly more massive than Earth. It’s in Proxima Centauri’s habitable zone, meaning there’s a potential for liquid water to exist on its surface. Water, of course, is a prerequisite for life as we know it. Heller said:

This finding prompted us to think about the possibility of stopping a high-velocity interstellar lightsail at Proxima Centauri and its planet.

The two astronomers are now discussing their concept with the members of the Breakthrough Starshot Initiative, to whom they say they owe the inspiration for their study. Heller commented:

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.

Read more about Heller and Hippke’s study via Max-Planck

Photo via BreakthroughInitiatives.org.

Artist’s concept of Breakthrough Starshot nanostarship. A 100-gigawatt light beam to propel approximately 1,000 ultra-lightweight nanocraft like this one to 20 percent of light speed. This fleet of nanostarships could reach the three stars of the Alpha Centauri system within about 20 years of launch. Image via BreakthroughInitiatives.org.

Bottom line: Using the Breakthrough Starshot project as their inspiration, two scientists have calculated the feasibility of full braking – and entering orbit – around the stars in the Alpha Centauri system.

Visit Yuri Milner’s Breakthrough Initiatives website

Read about the Alpha Centauri system

Deborah Byrd

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