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Conventional fuel-based rockets will likely take us to Mars and probably enable us to reach even more distant destinations in the solar system. However, we will not realistically be able to travel outside the Solar System with the use of rocket fuel.
Photonic propulsion is an existing technology that could be another way to send probes and manned spacecraft to distant areas of the Solar System and beyond. A spacecraft driven by photonic propulsion could accelerate to much greater speeds than those achieved by rocket propulsion, about 30 percent the speed of light according to experts. By comparison, Voyager 1 is moving at 0.0007 percent of the speed of light.
Photonic propulsion of spacecraft would be based on firing laser light at a large reflective sail attached to a craft. Although the photons in the laser don't have mass, they do have energy and momentum. They would transfer a small quantity of kinetic energy to the reflective sail, driving a craft forward.
A similar phenomenon occurs every day on our own skin. Light from the sun disturbs the atoms in our skin via their heat and momentum; however, the shifts are so small that they are imperceptible. The effects are minimal because the particles are dispersed; photons strike your skin in an arbitrary, divergent pattern.
By contrast, the laser of a photon propulsion system would send off a billion, billion photons crammed tightly together at a single point, exerting significant pressure referred to as radiation pressure.
In the frictionless void of space, prolonged pressure could hypothetically push a spacecraft to about 30 percent of the speed of light. After being launched into orbit with a standard rocket, the probe could unfurl a large 'light sail' that would be hit by a powerful laser, possibly based on Earth.
Solar, or ‘light’, sails are comprised of extremely-reflective, ultra-light and ultra-thin materials that basically form a huge mirror. In space, radiation pressure is translated to acceleration, which can be elevated by increasing the surface area of a sail, likely to thousands of square meters in size. With enough time, a spacecraft driven by a solar sail will accelerate to much greater speeds than those achieved by rockets.
The idea of photonic propulsion isn’t completely new, having been conceived by the 1920s by Russian researchers. It has been considered largely a theoretical concept until recently because the masses of 20th-century spacecraft were just too large to move via photonic propulsion.
However, the dawn of semiconductor technologies and the miniaturization of electronics has finally made the concept more feasible. As a result of modern electronics, a probe powered by photonic propulsion could effectively carry enough lightweight equipment to take pictures, perform chemical studies and get various data on celestial bodies.
The decreased size of components also means a light sail would not have to be absurdly large.
Roadblocks on the Path to Photonic Propulsion
Needless to say, there are a significant number of obstacles to be cleared before a photonic propulsion system can be used with a spacecraft.
It would require an unprecedented amount of infrastructure that to shine a strong enough laser from Earth to push a 100 kg spacecraft. Scientists have estimated that such a laser would need to supply an amount of power equal to that generated by a Space Shuttle launch, around 50 to 100 gigawatts.
Also, traveling at such high speeds would make the spacecraft’s handling very tricky. Just slowing the spacecraft down after it reaches top speed also appears to be a big challenge and staying away from collisions with debris could also be difficult.
It will probably be decades before a useful photonic propulsion is implemented. However, the fact the technology seems to be feasible is exciting enough to pay attention.
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