Optical propulsion with metajet could be used on future spacecraft, but it will take time
Chemical propulsion in space has proven to be reliable so far and has a long history of (more or less) successful applications. It's a solution that has several advantages but also clear limits for missions that reach beyond the Moon and Mars, especially when a crew is involved. NASA is focusing on nuclear propulsion with the Space Reactor-1 Freedom project (nuclear electric), and Russia is also looking in a similar direction. Other solutions are in the works and could arrive in several years. Among these is optical propulsion, particularly that based on metajet.
As mentioned, real applications of this technology are still a long way off, but preliminary studies are nonetheless encouraging. One of the latest updates regarding optical propulsion and metajet comes from a study by Texas A&M University and Northeastern University titled "Optical propulsion and levitation of metajets."
Optical propulsion and metajet for future spacecraft
The idea behind this is not new. It is a technology that could integrate with that of "solar sails," which have already been tested in space but whose capacity remains currently limited regarding transportable mass and thus payload.
The study instead discusses a technology of optical manipulation based on engineered metasurfaces that could be used both in microrobotics and for interstellar sails. One of the challenges, when it comes to moving in space, is managing the movements of a hypothetical spacecraft. With traditional methods, movement was possible laterally or vertically, but not simultaneously.
Researchers sought to combine Newton's second law with the generalized Snell's law using metamaterials. Specifically, it involves developing metajet and thus surfaces that feature nanometer-scale pillars made of silicon oxide with a height of 500 nm. These three-dimensional structures are grouped together, and each group forms a supercell that produces anomalous refraction with a controlled angle. Within a supercell, there can be from 3 to 8 pillars, thereby adjusting both the angle of refraction and efficiency.
As mentioned above, we are still far from a real application of these technologies, especially in aerospace. Laboratory experiments using a 1000 nm pulsed laser showed how metajets moved in the opposite direction to that of the source. Interestingly, the movements occurred both laterally and vertically, overcoming the limits of optical manipulation used so far.
Currently, it seems that the best combination is achieved with a three-pillar structure that has a refraction efficiency of about 78% with a refraction angle of about 40° and a propulsion speed of 7 μm/s (these are still very small devices). Optical power appears to be the fundamental element upon which performance depends, not the size of the device.
The technology related to lasers in space is experiencing a particularly interesting period primarily due to the possibility of surpassing information transmission through radio waves. The technological advancements involving the development of lasers that operate on spacecraft and increasingly precise pointing systems could also be useful in the case of optical propulsion and metajet for real applications. A realistic estimate of when we might see the first larger-scale solutions is still complicated to ascertain, but it will almost certainly take decades. In the meantime, chemical propulsion could be used for missions to the Moon and Mars, coupled with nuclear propulsion (thermal or electric), while further ahead, optical propulsion could open avenues that go beyond the inner Solar System, towards interstellar space.