Laser propulsion could satisfy our spacecraft’s need for speed

There are many wonderful places we would love to visit in the universe, and probably countless more that we haven’t even seen or heard of yet. Unfortunately… they are all so far away. A trip to Mars at best takes about six months with current technology, while if you want to visit Alpha Centauri, it’s four light years away!

When it comes to leaping those great distances, conventional chemical rocket technology just doesn’t cut the mustard. It turns out, however, that lasers could hold the key to reducing space travel times!

45 days to Mars on the laser

Laser thermal propulsion is a relatively simple concept and could make our spacecraft travel through our celestial neighborhood faster than ever before. A powerful laser beam fired from Earth targets a large heat exchanger aboard the craft, through which a propellant is pumped. As the heated propellant expands, it is eventually vented from a nozzle in the same manner as in a traditional rocket. It is also similar to the concept of nuclear thermal propulsion, but instead of using heat from a nuclear reaction, it relies on externally supplied laser energy.

A recent paper suggests that such a propulsion system could operate at a specific impulse of around 3000 seconds. It is basically a measure of the thrust an engine develops per mass of fuel. At 3000 seconds, a laser thermal propulsion system could be said to be at least 12 times more fuel efficient in terms of thrust than the solid rocket boosters (SRBs) of the Space Shuttle.

A rendering of a laser thermal powered spacecraft. Note the large reflector and flare nozzle on the back. Credit: Duplay, 2022, DC BY 4.0

This allows a laser thermal propulsion to achieve much greater velocity changes with less fuel, giving a space mission the ability to send payloads farther and faster. Calculations show that with an idealized mission plan, a payload of around 1000 kg could be sent to Mars in just 45 days, much faster than the usual 6-7 months possible in typical chemical-fueled missions. .

A conceptual laser thermal propulsion space mission to Mars. Credit: Duplay, 2022, DC BY 4.0

The technology involved is complex, as you’d expect. A large laser array with a power of the order of 100 MW would be required for the mission. The spacecraft itself would be launched out of the atmosphere on a conventional chemical rocket, after which it would separate and reveal a large inflatable parabolic reflector. The ground-based laser would then fire for up to an hour, using adaptive optics to counter the effect of Earth’s atmosphere on the beam. The spacecraft’s parabolic reflector would then focus energy on a chamber to heat the hydrogen propellant which would be expelled from a nozzle at high speed, providing thrust.

If desired, the spacecraft could be designed to release the payload capsule on its way to Mars, with the laser thermal propulsion unit separating and returning to stable Earth orbit for resupply. This has the advantage that the propulsion system itself could be used multiple times in quick succession to transport payloads far beyond Earth.

Such a system has one major flaw that stands out. While a laser on Earth is used to accelerate the spacecraft to high speed, there is no corresponding laser array on Mars that can slow the craft down upon arrival. Using chemical propulsion is also not a practical way to slow down, as it would take up far too much of the useful payload of the craft. Instead, the researchers determined that a very careful aerodynamic braking maneuver in the Martian atmosphere could be used to slow down an oncoming craft. However, this is a delicate operation that must be executed flawlessly to ensure success.

All in all, such a system could easily be developed in the short term. Although no one has a 100MW laser array, modern fiber optic laser technologies mean that such a power figure is not outside the realm of possibility. Similarly, much work would be required to create a reliable laser thermal spacecraft and ground system capable of sending payloads in useful directions in space not only limited by the relative positions of the spacecraft and the ground laser.

Ride the laser to the stars

If you want to get as far as our nearest star, Alpha Centauri, you’ll have to travel even faster. Even going at the speed of light, it would take four years to get there. So a probe intending to travel that far would want to get as close to that speed as possible to get there in a reasonable amount of time.

Laser sails may well hold the answer to this problem. They rely on the concept of photon radiation pressure, where light hitting a surface actually creates pressure and pushes it. They are called sails because the concept is exactly the same as a sailboat of centuries past. Instead of fabric and wind, however, a laser sail substitutes advanced nano-materials and powerful laser light.

An artist’s conception of the laser sail idea. Such a craft could potentially carry a gram-scale payload at relativistic speeds, down to about 0.2c. Credit: Breakthrough Initiatives

Recent research suggests that a laser sail on the scale of a few meters could propel a craft weighing one gram at speeds of up to 0.2 times the speed of light. This would allow Alpha Centauri to be reached in about 20 years, rather than the tens of thousands of years it would take with conventional rockets.

The concept would require the use of a sail made from extremely thin sheets of materials like aluminum oxide, silicon nitride and molybdenum disulfide. Measuring thousands of times thinner than a sheet of paper, the sail should be strong enough not to tear, and also be able to dissipate heat so as not to melt under the power of the laser propelling it.

Advanced sail nano-structuring would be the key to achieving this goal. The idea is to produce a sail with high reflectivity to maximize acceleration due to photon pressure, while maintaining high thermal emissivity to keep the sail cool enough not to melt. With a 100 GW laser array firing at the sail, that’s no small feat. Much like a conventional sail on a sailboat, the material would be allowed to swell under the pressure of incoming light. This greatly reduces the risk of tears.

At best, the sail could only carry a tiny payload of a few grams. It is hoped that advanced manufacturing methods could create a microprocessor, cameras and communications hardware for the probe that would be able to communicate over the vast distances between Alpha Centauri and Earth.

It’s a bold plan, which could allow space research to tackle subjects further away than ever before. However, the challenges ahead are great. The requirement for extremely powerful laser arrays exceeds our current capabilities, and the question of materials has yet to be resolved. Additionally, any message sent by a probe to Alpha Centauri would take four years to return to Earth, so communication issues arise as well.

Either way, the research conducted so far by Breakthrough Initiatives shows that laser sail concepts aren’t necessarily science fiction. With the right investment and development, they could prove to be a useful method of propulsion for research vessels one day in the future.

Conclusion

Unlike other seemingly science fiction technologies, such as ion thrusters, these laser propulsion methods are still far from being used in real space missions. There are huge challenges to overcome, and it’s also worth thinking about any unlucky birds or other wildlife that find themselves in the beam of a megawatt or gigawatt class laser.

However, if we are to open the heavens, it will take more than our existing technology can do. So these projects, or perhaps other whimsical new ideas, could one day take us far beyond our own solar system.

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