June 13, 2017

By laser drive into the depths of the universe

					Laser drive

Humanity has always dreamed of traveling to distant galaxies. In the hope of realizing this dream, researchers are currently working on miniature satellites with laser drives and minuscule optics.

At “only” 4.34 light years away, Alpha Centauri is our nearest star system. But even this—in the scale of the universe—tiny distance is insurmountable for rockets with today’s drive technology: They would have to travel for many thousands of years. Nevertheless, the breakthrough initiative of Russian billionaire Yuri Milner and British physicist Stephen Hawking is considering concrete travel preparations. The initiative has donated USD 100 million to develop the technical basis for a trip to Alpha Centauri. If at all, it will be possible to get there only by utilizing photonic solutions.

The initiative places no restraints on the research. Suggested solutions are welcome. To date their approach is for microsatellites weighing just a few grams being driven from the earth by near-infrared lasers with cumulated power in the three-digit gigawatt range. To increase the chances of success, it is planned to have a mother ship in orbit that successively releases thousands of tiny high-tech packages into the high-energy light beam, where they will be accelerated to one-fifth of light speed. The energy would be transferred to them via wafer-thin energy sails consisting of just a few layers of atoms. Once accelerated, the satellite chips will take 20 years to reach the distant galaxy, where they will use miniature cameras to collect image data and transmit it back to the earth by laser. Even at light speed, this data transmission would take almost 4.5 years.

Manifold technological challenges

Numerous technological breakthroughs will be necessary before the vision can come true. Philip Lubin from the University of California in Santa Barbara outlines these challenges in a 73-page “Roadmap to Interstellar Flight”. They include the task of precisely aligning the lasers in spite of manifold atmospheric influences. Adaptive optics will have to do this in real-time. A material must also be found for the sail, capable of withstanding the laser energy while reflecting it to the maximum extent. It will also have to be unaffected by erosion from the dust and corrosive gases in space. Here the initiative is hoping for advances in nanotechnology. In addition to the question of the material, the exact shape of the sails remains to be clarified. The energy of movement must not be able to tear them, nor may turbulence influence their course.

There are other questions relating to microcameras, batteries, control systems and communication lasers. Certainly Moore’s law drives the miniaturization of microsystems. But will the technology be robust enough to survive the breathtaking speed of the 20-year flight? And will it be possible to reduce the flight speed at the end of the journey sufficiently that recordings and images will be meaningful? From a scientific point of view it would also be desirable to equip the large number of satellites with different imaging systems for recording in visible and invisible wavelength ranges.

Laser drives also at the German Aerospace Center

There are many hard nuts to crack before the Breakthrough computer animation becomes reality. Another question is how soon laser drives will be accepted by the space industry. NASA is also researching this technology, as is the German Aerospace Center (DLR), because rocket drives with bundled light are considered an efficient alternative to current solid-state technology. Because hardly any fuel needs to be transported—within the earth’s atmosphere it is sufficient to use lasers to trigger plasma ignition in the air—we have an interesting relationship between mass and payload. In future, small satellites in particular could be carried into orbit by high-energy light. Or more precisely: use the pressure waves from plasma detonation to ignite earth-bound lasers in combustion chambers with the form of parabolic mirrors. Small amounts of fuel could also be included in the chambers to increase the thrust.

As yet the whole idea is at the early stage of research. In addition to pulsed laser sources with high-quality beams with a power requirement of approximately 1 MW per kilogram of starting mass, DLR also needs active optics and tracking systems that can reliably direct the laser beam into the combustion chamber regardless of turbulence and atmospheric influences. The talk here is of distances up to 1,000 km—which is almost tangible proximity compared to the light years of the Alpha Centauri Mission.