Can we reach the Proxima Centauri star?

Emergency stop at Alpha Centauri

Space visionaries are considering an interstellar journey to our neighboring star

In April last year, billionaire Juri Milner announced the Breakthrough Starshot Initiative. He plans to invest 100 million US dollars in the development of a nano spaceship that can be accelerated to 20 percent the speed of light in order to reach the Alpha Centauri star system within 20 years. So far, the question of how the bullet should slow down at the target has not been resolved. René Heller from the Max Planck Institute for Solar System Research in Göttingen and his colleague Michael Hippke propose to use the radiation and gravity of the stars. Then the vehicle could even be redirected to the companion star Proxima Centauri and its Earth-like planet Proxima b.

In the current science fiction film Passengers A huge spaceship flies at half the speed of light to the distant planet Homestead II. After 120 years it should reach its destination and the 5000 passengers should settle in the new home. A dream that, according to the current state of knowledge, cannot be technically realized. "With today's technology, we would need almost 100,000 years for such a flight with a small probe," says René Heller.

After all: Milner's Starshot project also looks fantastic, as it is based on a completely new concept: Many probes, which weigh only a few grams and are equipped with a light solar sail, are first brought conventionally to a great height and then with a huge, powerful laser beam from the Earth illuminated from. The pressure of light accelerates the nanoships up to 20 percent of the speed of light in a few minutes and drives them in the direction of the Alpha Centauri star system 4.2 light-years away, where they arrive 20 years later.

Regardless of the technical requirements, Heller and his colleague Michael Hippke asked themselves: “How could one optimize the scientific yield on such a mission?” Such a fast probe covers a distance like that between the earth and the moon in just six seconds. So in no time she would have been racing past the stars and planets in the Alpha Centauri system.

The solution: When it arrives, the probe's sail must be aligned in such a way that the radiation from the stars in the Alpha Centauri system coming towards it slows the vehicle down as far as possible. In Michael Hippke, René Heller, as an astrophysicist involved in the preparations for the exoplanet mission PLATO, found a congenial partner who created the computer simulations.

The two astrophysicists assumed a space probe that weighs less than 100 grams and is equipped with a 100,000 square meter sail. That corresponds to the area of ​​14 soccer fields. With the approach to Alpha Centauri, the braking power increases. The stronger the braking, the more speed can be reduced on arrival and the faster the probe can be at its start in the solar system.

If the tiny spaceship approaches the star up to around four million kilometers (corresponding to five star radii), it is allowed to arrive at a maximum speed of 13,800 kilometers per second (4.6 percent of the speed of light) - at higher speeds the probe would race past the star .

At the same time, the star attracts the probe with its gravity. This effect could be used to distract them on their path; Space probes have carried out such swing-by maneuvers many times in our solar system. "With these orbit parameters, the probe would be on the move for almost 100 years," says Michael Hippke, "about twice as long as the Voyager probes that have been on the road since the 1970s and are still working."

Theoretically, the nanoship could swing into orbit around Alpha Centauri and possibly explore its planets. But Heller and Hippke think even further. You have to know that Alpha Centauri is a triple star system. The two partners A and B orbit each other in a very narrow orbit, while the third, called Proxima Centauri, is 0.22 light-years away.

Now the sail could align itself in such a way that the radiation pressure from star A brakes and deflects the probe so strongly that it reaches Alpha Centauri B after a few days and is thrown again in the direction of Proxima Centauri. It would arrive there after another 46 years - around 140 years after taking off from Earth.

Proxima Centauri caused a stir in August 2016 when astronomers at the European Southern Observatory (ESO) discovered an exoplanet about as massive as Earth and orbiting the star in the habitable zone. This makes it theoretically possible for liquid water to exist on it - an important prerequisite for life, at least on earth.

“This find also animated us to think about possible trajectories to this star with a subsequent park orbit around its planet,” says René Heller. The Max Planck researcher and his colleague propose another change for the strategy of the Starshot project: Instead of a huge energy-guzzling laser, solar radiation could also be used to accelerate a nanoprobe out of the planetary system. "To do this, it would have to approach the sun within about five solar radii so that it can get the necessary thrust from there," says Heller.

The two astronomers are already in contact with the participants in the Starshot project. "Together with the scientists and strategists from Breakthrough Starshot, we are considering whether our new idea could have an impact on Starshot or a follow-up mission," says Heller.

Although the new scenario described is based on a mathematical study and computer simulations, there are already ideas for the required hardware: "The sail could consist of graphene, an extremely thin and light, but mega-tear-resistant carbon film," says René Heller. The film would have to withstand the harsh conditions on the trip and the heat close to the star.

Optics and electronics would have to be tiny. But: If you remove all the parts that are unimportant for the function of a modern smartphone, “only a few grams of function-relevant technology remain.” In addition, the lightweight space sail would have to navigate independently and transmit its measurements to earth by laser. For this it needed energy that it could possibly harvest from the star radiation.

Breakthrough Starshot therefore poses extreme challenges for researchers, which so far can only be solved theoretically. Nevertheless: "Many great visions in human history had to struggle with almost insurmountable hurdles," says Heller. "And now we are approaching an age in which people can leave their own star system and explore extrasolar planets up close."