Photonic laser propulsion roadmap to laser pushed wafersats to 25% lightspeed

DE-STAR, or Directed Energy System for Targeting of Asteroids and exploRation, is the brainchild of UC Santa Barbara physicist Philip Lubin and Gary B. Hughes. DE-STAR initial objective is to deflect asteroids.

The DE-STAR system could be leveraged for many other uses, such as stopping the rotation of a spinning asteroid and achieving relativistic propulsion.

Tests simulated space conditions. Using basalt — the composition of which is similar to known asteroids — they directed a laser onto the basalt target until it glowed white hot — a process called laser ablation, which erodes material from the sample. This changes the object’s mass and produces a “rocket engine” using the asteroid itself as the propellant. In space, this would be powerful enough to alter its course.

The team simulated a spinning asteroid using basalt to determine whether they could slow, stop and change its rotation direction. They used magnets to spin the basalt and then directed the laser in the opposite direction to slow the rotation.

“Our video shows the basalt sample slowing down, stopping and changing direction and then spinning up again,” said Brashears. “That’s how much force we’re getting. It’s a nice way to show this process and to demonstrate that de-spinning an asteroid is actually possible as predicted in our papers.”

Lab measurements have shown that in terms of thrust, the conversion of laser energy to force through this method is about 100 micronewtons per watt, which works out to 10 kilowatts per newton

According to Lubin, a professor of physics at UCSB, manipulating the speed of a spinning asteroid offers another important possibility in space: the ability to explore, capture and mine asteroids. This is something NASA aims to do with its Asteroid Redirect Mission. The mission — which remains theoretical — is intended to visit a large near-Earth asteroid, collect and return a boulder from its surface and possibly redirect the asteroid into a stable orbit around the moon.

Pictures of Wafer Scale Spacecraft with laser on reflector. Includes fiber optic cables for cloaking
and wafer as the payload. The red depicts the laser light.

Photon Propulsion

In addition, the students explored photon propulsion, which is key to the group’s latest project, DEEP-IN, or Directed Energy Propulsion for Interstellar exploratioN. The DEEP-IN concept relies on photon propulsion, whereby thrust from photons emitted from the laser array could be used to propel a spacecraft. This allows for the possibility of relativistic flight — speeds approaching the speed of light — for the small spacecraft required for future interstellar missions.

The team also tested a photon recycler, a device that reuses photons from the laser by shining them on a reflector cavity. “We have a second mirror at some distance away that bounces the photons back and forth like a ping-pong ball onto the spacecraft reflector.” Brashears said. “In effect, we’re recycling these photons to achieve a force multiplication that allows the vehicle to go even faster. So far, with a simple implementation, we have achieved an amplification factor of five. Much more is possible with refinement. This works as predicted, though implementing it into the full flight system will be complex.”

Laser pushed roadmap

Directed Energy Interstellar Propulsion of WaferSats Getting to 25% of lightspeed will be over 4000 times faster than Voyager 1.

Researchers propose a roadmap to a program that will lead to sending relativistic probes to the nearest stars and will open up a vast array of possibilities of flight both within our solar system and far beyond. Spacecraft from gram level complete spacecraft on a wafer (“wafer sats”) that reach more than ¼ c and reach the nearest star in 15 years to spacecraft with masses more than 100,000 kg (100 tons) that can reach speeds of near 1000 km/s such systems can be propelled to speeds currently unimaginable with our existing propulsion technologies. To do so requires a fundamental change in our thinking of both propulsion and in many cases what a spacecraft is. In addition to larger spacecraft, some capable of transporting humans, we consider functional spacecraft on a wafer, including integrated optical communications, optical systems and sensors combined with directed energy propulsion. Since “at home” the costs can be amortized over a very large number of missions. The human factor of exploring the nearest stars and exo-planets would be a profound voyage for humanity, one whose non-scientific implications would be enormous. It is time to begin this inevitable journey beyond our home.

They assume a slightly futuristic sail with thickness of 1 µm for many cases and 10 µm (thick even for todays sails). Future advancements in sails thickness down to 0.1 µm and below can be envisioned but are NOT assumed. They will only make the conclusions even more optimistic. The density of all sails we consider is about the same, namely ρ ~1,400 kg/m3

Wafer Scale Spacecraft

Recent work at UCSB on Si photonics now allows us to design and build a “spacecraft on a wafer”. The recent (UCSB) work in phased array lasers on a wafer for ground-based optical communications combined with the ability to combine optical arrays (CMOS imagers for example) and MEMS accelerometers and gyros as well as many other sensors and computational abilities allows for extremely complex and novel systems. Traditional spacecraft are still largely built so that the mass is dominated by the packaging and interconnects rather than the fundamental limits on sensors. Our approach is similar to comparing a laptop of today to a super computer with similar power of 20 years ago and even a laptop is dominated by the human interface (screen and keyboard) rather than the processor and memory. Combining nano photonics, MEMS and electronics with recent UCSB work on Si nano wire thermal converters allows us to design a wafer that also has an embedded RTG or beta converter power source (recent LMCO work on thin film beta converters as an example) that can power the system over the many decades required in space. Combined with small photon thrusters (embedded LEDs/lasers for nN thrust steering on the wafer gives a functional spacecraft. While not suitable for every spacecraft design by any means, this approach opens up radically new possibilities. In addition the power from the laser itself can add significant power to the spacecraft even at large distances.

Laser Sail

The laser sail is both similar to and fundamentally different than a solar sail. For small sails, even with low powers the flux can easily exceed 100 MW/m2 or 10000 Suns. This requires a very different approach to the sail. For the small reflectors we propose using a pure dielectric reflection coating on ultra-thin glass or other material. Spherical (bubbles) sails are an option for testing. The loss in fiber optic quality glasses allows loss in the ppt(10-12)/μm (of thickness) which is even better than we need. This is an area we need to explore much more. The flux at the tip of high power single mode fiber optic exceeds 10 TW/m2, higher than we need. Rather than the typical 1/4 reduced wavelength anti reflective (AR) dielectric coating, we will need to design a 1/2 wave reflection coating for the sail.

Power Generation

Thermoelectrics show the greatest promise for energy for the wafercraft in the near future. Plutonium-238, the traditional fuel source for radioisotope thermoelectric generators (RTGs) produces 560 mW/g of heat in its pure form, and 390 mW/g of heat in its fuel pellet form (Plutonium Dioxide). Current RTG technology (NASA’s Multi-Mission RTG, or MMRTG for short) has an electrical conversion efficiency of 6-7%. Assuming 6.5% efficiency from thermal to electrical conversion we get about 25 mw (electrical)/g. In order to generate 5 mw we need about 0.2 g.

Stirling engines are much more efficient with about 30-50% efficiency for the temperatures we can get BUT there are no chip scale Stirling engines and no 20-100 year lifetime Stirling engines currently exist even without the extreme requirement of chip level system. MEMS equivalents may be possible but this is a research item yet to be explored. This leaves open the possibility of much more power (by a factor of 5-10) that may be achieved. This would greatly expand our data rates as well as sensor suite possibility for the small systems

G-Forces on Small-Scale Spacecrafts

Possibly the greatest benefit of our wafer scale design is the high speeds our spacecraft can reach. We have discussed the ability of our ship to be accelerated to about 0.25c in ten minutes. This is an acceleration of roughly 10,000 g’s, an acceleration that could put a formidable strain on our delicate wafer. However, this may not be as big of a problem as it may seem. Many present day weapons systems incorporate electronic components into their artillery shells to correct trajectory mid flight. During launch, these electronic components must be able to sustain accelerations of at least 10,000 g’s, sometimes ranging to even higher than 15,000 g’s depending on the system. Upon muzzle exit, the artillery shells are subject to substantial pressure changes, resulting in significant shocks and vibrations. Our spacecraft would not be subject to such volatile environment, as our acceleration takes place over a period of at least ten minutes, rather than a fraction of a second. Many methods have been successfully developed to house electronic components during launch, most involving some sort of shock absorbing material such as foam or gel. It is reasonable to think that a similar method could be used for our spacecraft.

Photon recycling for larger thrust and efficiency
The efficiency of the photon drive can be improved by reusing the photons reflected by the spacecraft reflector in an effective optical cavity mode to get multiple photon reflections. This is known as photon recycling. It is not a new concept but may be of some use for some of our applications.

Free space phase control over large distances during the acceleration phase will be a critical enabling capability. This will require understanding the optics, phase noise and systematic effects of our combined onboard metrology and off-board phase servo feedback. Reflector stability during acceleration will also be on the critical path as will increasing the TRL of the amplifiers for space use. For convenience we break the roadmap into several steps.

One of the critical development items for space deployment is greatly lowering the mass of the radiators. While this sounds like a decidedly low tech item to work on, it turns out to be one of the critical mass drivers for space deployment. Current radiators have a mass to radiated power of 25 kg/kw, for radiated temperatures near 300K. This is an area where some new ideas are needed. With our current Yb fiber baseline laser amplifier mass to power of 5kg/kw (with a likely 5 year roadmap to 1 kg/kw) and current space photovoltaics of less than 7 kg/kw, the radiators are a serious issue for large-scale space deployment.

Mass and speed with 100 GW laser

1 gram     24% of lightspeed
10 grams   14% of lightspeed
100 grams   7.8% of lightspeed
1 kg        4.3% of lightspeed 
10kg        2.4% of lightspeed
100kg       1.4% of lightspeed
1000kg      0.77% of lightspeed
10 tons     0.43% of lightspeed
100 tons    0.24% of lightspeed