Put the Brakes on Photonic Propulsion

Micah Knight/Managing Editor

A new form of spacecraft propulsion burst onto the scene last week: Photonic Propulsion. Last Monday, various news sources touted headlines such as “Novel Laser Technology to Propel Humans to Mars in Three Days!” (Economic Times), “NASA Photonic Propulsion: Could We Travel to Mars in 3 Days?” (The Inquisitr), and “Mars in Three Days? NASA Touts New Propulsion System” (Fox News). But those headlines were misleading at best. Humans are not getting to Mars in three days by photonic propulsion in this century. Here’s why:

What is photonic propulsion anyway? It’s like a solar sail, but powered by some man-provided ‘wind.’ In the newly-proposed case, the power source is a giant laser array shooting the solar sail. Solar sails use “solar wind”, particles emitted by the sun, to push a spacecraft. In 1984, Robert Forward proposed an interstellar spacecraft propelled by laser-pushed lightsails, which is precisely the same concept as photonic propulsion. Why is this idea going viral on the news now, thirty years after it was published? On February 11, NASA 360 posted a video on Youtube titled “Going Interstellar,” in which professor Philip Lubin explains the idea of photonic propulsion, and its potential, in two minutes. In the video, an excerpt from a talk he gave at the NIAC (National Institute of Aerospace) conference in Seattle in Oct. 2015, he talks about how to get spacecraft to the relativistic speeds (a speed a significant fraction of the speed of light) that particle accelerators can get atomic particles to with electromagnetic acceleration. He states in the video, “the SLS, when it will take off, will have a power off the launch pad between 50 to 100 Gigawatts. It turns out, to get to relativistic speeds with the spacecraft we’re talking about [a one-gram spacecraft] you need basically the same power level, and for about the same amount of time. It takes about ten minutes to get to orbit with the shuttle, it takes us ten minutes to get to 30% the speed of light… just using different technology. We could propel a 100-kilogram robotic craft to Mars in a few days. If you want to push something like shuttle class, it takes you roughly a month to get there.”

Photo Courtesy: Photonics.com / An illustration of the working principle of a photogenically propelled spacecraft.

Photo Courtesy: Photonics.com / An illustration of the working principle of a photogenically propelled spacecraft.

A 50 Gigawatt (GW) or more powerful laser is an impossible order with current technology. The most powerful lasers currently are on the order of kilowatts – one thousand times less powerful than is necessary for photonic propulsion applications – and aren’t active for more than several seconds. 50 GW is approximately enough power to supply 8.3 to 17 million households, according to WolframAlpha, or is approximately 1/9 of the electric power consumption of the US in 2008. A laser of that magnitude with the ability to put out that much power is decades in the future. Furthermore, if such a laser was stationed on Earth, atmospheric lensing or disturbances would wreak havoc with the beam, and the rotation of Earth would make such a laser need to track the object it was propelling, and it would be unusable for a significant portion of most days because of the Earth’s orientation. Putting the laser in orbit would mitigate the atmospheric effects, but brings up issues of its own, the least of which being hoisting that much mass and that large of a power source to Earth orbit.

Furthermore, laser beams spread out over distance. Once the craft began accelerating and getting further and further from the Earth, the force on the spacecraft would drop dramatically, making the photonic propulsion less and less efficient.

In his presentation at the NIAC conference (viewable at https://livestream.com/viewnow/niac2015seattle), Lupin went into more detail concerning the system – the statement that they could get a craft to 30% the speed of light is for a “wafer-scale spacecraft (~1 gram)”. For the 100-kg robotic craft to Mars in three days, it would “be going greater than the galactic escape velocity, about 1200 km/s” relative to the sun when it reached The Red Planet.

However, when the craft got to Mars, it would need to slow down to orbit the planet. To orbit Mars, a spacecraft needs to have a velocity of approximately 6 km/s relative to Mars, or 25 km/s relative to the sun. To change the velocity from 1200 km/s to 25 km/s is, again, impossible. The fastest man-made object was the Helios 2 probe, with a velocity of approximately 70 km/s relative to the sun, a speed achieved only by multiple fly-bys of planets to slingshot it to a higher speed. The fastest any spacecraft has been propelled by rockets was the New Horizons spacecraft, which escaped Earth at a velocity of approximately 16 km/s. To slow a spacecraft that much is far beyond any current human capabilities.

Therefore, the only way to slow the spacecraft going to Mars in three days would be to have another such laser array at Mars to decelerate the spacecraft to an orbital insertion velocity. That would require another 50+ GW laser to be at Mars – either in orbit or on the planet, which could only be transported there after humans were well established on Mars.

Man’s current big goal is to go to Mars, and NASA is hoping to send a manned mission there within the next few decades. Currently, the travel time is several months, which NASA is hoping and trying to reduce as much as possible. While photonic propulsion is a cool concept, the possibility of implementation is far in the future. An article on NASA’s website (“Is Warp Drive Real?”) states that “NASA is not pursuing interstellar flight”, but is continuing to advance ion propulsion for future deep space missions. Spaceflight near the speed of light is still a dream, and will be for many, many years.