Space travel visionaries solve the problem of interstellar slowdown at Alpha Centauri

February 1, 2017
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. Credit: Planetary Habitability Laboratory, Univesity of Puerto Rico at Arecibo

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.

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 would have to travel nearly 100,000 years to reach its destination," René 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. René Heller, an astrophysicist 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.

Flight past Alpha Centauri A

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 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 . "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."

Explore further: Image: Hubble's best image of Alpha Centauri A and B

More information: Heller, R., & Hippke, M. (2017) "Deceleration of high-velocity interstellar sails into bound orbits at Alpha Centauri", The Astrophysical Journal Letters, Volume 835, L32, DOI: 10.3847/2041-8213/835/2/L32

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5 / 5 (7) Feb 01, 2017
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."

Um yeah, but space is a bit of a harsh environment compared to the average smombies' pockets. You can't just take the electronics and shoot them, naked, at another star and expect them to survive the trip.

100grams is also pretty ambitious (as it includes the weight of the sail). Graphene weighs 0.77milligrams per square meter. So 100k square meter sail already 77g
If you add any kind of reflective layer this will be WAY over the 100g mark. And you need some structural elements to make sure it doesn't collapse under the pressure of the radiation for accelerating/decelerating it.

The StarChip probe package is envisioned at a few grams with a 'compact laser for data transmission'. However I've not seen anyone mention how such a small laser can transmit data over 4 light years.
1.6 / 5 (7) Feb 01, 2017
Sure, the 100 gram useless piece of space junk will be a $100Mil monument to the dumbass ego. Hopefully no aliens will notice it (only 0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000012 chance they will, as no one lives in Alpha Centauri system).
4 / 5 (2) Feb 01, 2017
What level of drag do you get from this size sail, from the thin interstellar medium?
How do you compensate for the unknown medium and wind particulates drag near proxima?
5 / 5 (2) Feb 01, 2017
What level of drag do you get from this size sail, from the thin interstellar medium?

The idea is probably not to keep it unfurled after the initial push...and unfurl it again when you get close to Proxima Centauri to use it as a "drag chute"
1 / 5 (8) Feb 01, 2017
This is a time to start getting serious about UFOs and ET (Cenos) aliens.


There may even be an ET alien base in Antarctica.


And these same aliens who have been visiting Earth for 4300 years may have given us the Mass of planet Nine (AKA Vulcan).


1 / 5 (1) Feb 03, 2017
Yes, to say the least, this presented concept is problematic. The asteroid belt contains sufficient kinetic energy to send a much larger probe out at as high or higher speed with far greater safety. Why don't they use that? Asteroids are just sitting there waiting for someone clever enough to start bouncing things around, so to speak. The physics is Newtonian for crying out loud.
5 / 5 (3) Feb 03, 2017
The asteroid belt contains sufficient kinetic energy to send a much larger probe out at as high or higher speed exactly is that supposed to work?
4 / 5 (1) Feb 04, 2017
I really have no idea how such a small probe can pack all of the systems needed for this probe to be worth while. How is it generating power, storing it, keeping itself warm. What about redundant systems for when a cosmic particle crashes through the probes electronics? Can the probe receive over the air software updates in order to fix the software glitch the system will no doubt be launched with. Can we really track this probe to sufficient accuracy in order to perform corrective trajectory manoeuvres? What bit rate can you achieve from such a tiny, low powered (where's the power coming from) laser? Is it possible to point lasers so accurate we can hit this probe from Earth, 4 light years away??? Surely bi-directional communication is required. Better not need to update the probe in a hurry, 4 years is some savage communication lag!!!!! Unfurling and furling of the sail repeatedly and probe stabilisation during the process...... GOOD F_cking luck with that!!!!!!
5 / 5 (2) Feb 05, 2017
Building such a small probe and getting it to the Alpha Centauri system might be plausible. Having such a thing communicate back with only a watt or two of laser power seems pretty damn impossible. Receiving such a low powered signal from Mars would be very hard. It takes the huge dishes of the Deep Space Network to receive the ~20w transmissions from the Voyagers at over 100 AU. Proxima is 268,000 AU away. I've been poking around the Centauri Dreams site and others and found ideas such as:
* send a swarm that could form a huge antenna by synchronizing transmissions
* send a stream of craft that could act like repeaters across the interstellar void
* use the lightsail as a semaphore, reflecting a VERY POWERFUL laser beamed from Earth.
These all seem to have serious problems too, but one concept blew my mind. (cont)
5 / 5 (1) Feb 05, 2017
http://www.centau...?p=35486 describes the idea of getting probes positioned to utilize the gravitational lens effect of the Sun and Alpha Centauri. The effect begins at about 550AU from the sun, corresponding to the point at which incoming EM rays from objects on the opposite side of the sun just skim the suns surface and arrive at the focal point. Moving further out corresponds to rays focusing from a ring further above the suns surface. Putting a probe along that focal line at Alpha Centauri (about the same mass, hence distance) would allow communications with astonishingly little power. Anyone who has been in a rotunda and whispered between acoustic focal points across a large noisy space will be familiar with the effect. (cont)
5 / 5 (1) Feb 05, 2017
Put one at the gravitational lens of the Sun, the other at the lens of the other star. At this point, things get wild. The minimum transmitted power drops to less than 10-4 watts. You're reading that right — one-tenth of a milliwatt is enough to create error-free communications between the Sun and Alpha Centauri through two FOCAL antennas. Maccone's paper assumes two 12-meter FOCAL antennas. StarShot envisions using its somewhat smaller sail as the antenna, a goal given impetus by these numbers.
One still has the problem of relaying in to Earth from 550 AU. (Remember, the Voyagers have only reached the 100 AU neighborhood.) Probes in the Alpha Centauri system would have to collect data, then get to a lensing point or deal with the relay issues to another probe, at least not over non-interstellar distance.
5 / 5 (2) Feb 05, 2017
Well, all of these semaphore ideas (light ofr gravitational) are based on the probes being rather stationary with respect to proxima centauri. However the idea is to shot them there at 0.2c and just have them do a flyby. I fail to ee how these two ideas mesh. There's no way to slow them down that much at the destination.

Even the swarm-antenna idea doesn't quite work as at the speed and how they are being sent they would only remain in a viable configuration for a tiny amount of time.
not rated yet Feb 05, 2017
Most of the ideas have the problem of detecting a signal coming from near the overwhelming output of the star. (Same reason we struggle to see exoplanets.) Ideas such as the swarm involve maneuvering which would be hard for a tiny object with barely any fuel.

This article is about a much slower speed mission which I think might be impractical based on limited attention span of civilization. Also the possibility that we'll have much better propulsion systems before the probe could get there - the probe could find itself being passed by tourist ships on the way to the same destination. ;)

The gravitational focus concept is fascinating because it implies low power interstellar communications are possible. Alas, still limited to lightspeed.
5 / 5 (4) Feb 05, 2017
With slow missions I see another issue:

You can graph the time it takes at which a probe (or manned craft) could get there. You can also graph the time it takes each time we double the capabilities of our thrust systems. By those graphs it currently makes no sense to launch, because technological advance will make a craft that is launched *later* arrive there earlier.

We shouldn't be concerned with how we get information back with technology we could make now. We should look to how to get information back at a time when we are close to the break-even point of "travel time vs. tech advance"

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