May 10, 2016

Starshot Breakthrough Initiative for laser pushed interstellar nanocraft

Breakthrough Starshot aims to demonstrate proof of concept for ultra-fast light-driven nanocrafts, and lay the foundations for a first launch to Alpha Centauri within the next generation. Along the way, the project could generate important supplementary benefits to astronomy, including solar system exploration and detection of Earth-crossing asteroids.

Breakthrough Starshot is a $100 million research and engineering program aiming to demonstrate proof of concept for light-propelled nanocrafts. These could fly at 20 percent of light speed and capture images of possible planets and other scientific data in our nearest star system, Alpha Centauri, just over 20 years after their launch.

Nextbigfuture covered the project last month when it was announced. Here is more information from the Breakthrough Initiative website.

Since 2010, Draper and Cornell University have collaborated on research into spacecraft that could be reduced to the size of a postage stamp and dubbed “ChipSats.” While ChipSats are small and inexpensive to launch, they face challenges far different from those of larger spacecraft and require a completely different approach to space missions.

Due to their tiny size, ChipSats experience disturbances in space in a different manner from large spacecraft. Much like a dinghy is greatly affected by waves that cannot move an oil tanker, the importance of small environmental forces, such as solar radiation pressure and aerodynamic drag, is magnified for ChipSats. This represents a challenge for completing the journey to Alpha Centauri and pointing precisely to send data back to Earth. But it is also an opportunity for developing new guidance and control approaches that take advantage of the environment.

Draper is addressing these issues as it develops ChipSats for a proposed mission to explore Europa, with funding from NASA’s Innovative Advanced Concepts (NIAC) branch. “Traditional spacecraft mission architectures minimize risk while maximizing redundancy,” said Brett Streetman, who leads Draper’s ChipSat work. “The small size and low cost of ChipSats offer an opportunity to accept far more risk, as you only need a small percentage of them to survive the mission, so you can pack critical space exploration capabilities into a tiny package.”

The Europa mission uses a CubeSat that could scan the Jovian moon to detect areas that may have the thinnest ice and then send hundreds of ChipSats to look for signs of life below the surface. Draper is applying expertise in areas including spacecraft mission design, micropackaging, and transitioning university research into operational prototypes as it develops ChipSats. Field tests of workable prototypes could begin as early as 2019, potentially helping pave the way for the systems that could be used in the Alpha Centauri concept decades later.

Nanocraft interstellar concept

The nanocraft concept, combining light beamer, lightsail and StarChip, is by far the most plausible system for launching a realistic mission to Alpha Centauri within a generation. The key elements of the proposed system design are based on technology either already available or likely to be attainable in the near future under reasonable assumptions.

Gram-scale StarChip components | 4 photon thrusters
Sub-gram scale 1 watt diode lasers are currently widely available at very low costs. The manufacturing trend has seen power double for the same mass every two years. It is anticipated this trend will continue for these devices for some time.

KickSat was a small-satellite (femtosatellite) project inaugurated in early October, 2011, to launch a large number of very small satellites from a 3U CubeSat. The satellites have been characterized as being the size of a large postage stamp. and also as "cracker size". The mission launch was originally scheduled for late 2013 and was launched April 18, 2014.

Kicksat reached its orbit and transmitted beacon signals that were received by radio amateurs, telemetry data allowed the prediction of the orbit and the reentry on 15 May 2014 at about 01:30 UTC. Due to a clock reset, however, the femtosatellites were not deployed but burned up inside the KickSat mothership.

Gram-scale StarChip components | 4 cameras
Sub-gram-scale 2 megapixel cameras are currently widely available at very low costs. The trend has been a doubling of pixels for the same mass every two years.

Arxiv - Robustness of Planar Fourier Capture Arrays to Color Changes and Lost Pixels

Gram-scale StarChip components | 4 processors
Sub-gram scale microprocessors are currently widely available at very low costs. The trend has been a doubling of processor count for the same mass every two years. It is anticipated that these devices will continue this trend for some time

Gram-scale StarChip components | Battery
Battery design is one of the most challenging aspects of the mission. Currently under consideration for the energy source onboard are plutonium-238, which is in common use, or Americium-241. 150mg has been allocated for the mass of the battery. This includes the mass of the radioisotope and the ultra-capacitor.

Gram-scale StarChip components | Protective coating

A protective coating is required for the dust collisions and the erosions caused by atomic particles in the interstellar medium.

Lightsail | Integrity under thrust

To inform the study, a beamer in the 100 Gigawatt class was considered. If, for example, 10^-5 of the energy is absorbed by a 4 meter by 4 meter sail, it will be heated by about 60kW per square meter, which is roughly 60 times more than sunlight illumination on Earth. This will heat the material but not melt it.

Using fully dielectric sails, we can reduce the absorption to less than 10-9 for optimized materials.

Two possible approaches to mitigate the heating challenge have been identified:
1. High reflectivity
2. Low absorption

Physical Review B - Perfect dielectric-metamaterial reflector

Lightsail | Structure
Building a skeleton structure that will be able to hold the sail in shape during launch, be resilient to the interaction with the interstellar medium and potentially be able to modify the shape of the sail, is a major challenge given the gram-scale mass constraint.

Lightsail | Stability on the beam
Beam shape and lightsail structure should be optimized for stability during the launch phase. In this period, on the order of 10 minutes, an illumination energy of order 1TJ is delivered to the sail.

Light Beamer | Cost
The estimated cost of the laser array is based on extrapolation from the past two decades, and the prospects of mass production to reduce the associated cost.

Light Beamer | Phase
In order to test the feasibility of the system, the case of a meter-scale sail was examined. For example, to focus the light beam on a 4 meter X 4 meter sail across an acceleration distance of 2 million kilometers requires a focusing angle of 2 nano-radians (0.4 milliarcseconds), which is the diffraction limit for a kilometer-scale light beamer operating at a wavelength of 1 micron.

Light Beamer | Atmosphere
The atmosphere introduces two effects: absorption (or ‘reduction of transmission from unity’), and loss of beam quality (or ‘blurring of the beam spot’). The transmission of the atmosphere at a wavelength of 1 micron is extremely good, exceeding 90% at high altitude ground-based sites.

Launch Site | Power Generation and Storage
Power generation and storage at the launch site is challenge. Developing a site with adequate infrastructure to generate the energy at a high altitude site is difficult.

Launch | Precision pointing for a meter-scale lightsail
The light beamer must focus a spot smaller than the sail onto the sail, as it orbits 60,000km above the Earth’s surface.

Launch | Keeping beam pointed on meter-scale lightsail
There are a number of effects that make this task difficult. These include beam instabilities, laser mode issues, differential forces on the sail, differential heating of the sail, and instabilities in the atmosphere induced by the energy of the beam.

Launch | Precisely determining orbital position of an exoplanet
In order to bring a nanocraft to within 1AU of a planet in a system like Alpha Centauri, accurate locations of all the bodies near the path of flight would be required.

Launch | Range safety and objects in beam path
The radiative flux on an object such as a bird, airplane, or spacecraft moving through the beam would be about the same as the output energy flux at the beamer, or 100 kw/m2 – about two orders of magnitude above sunlight on Earth.

Launch | Potential collision of nanocraft with planet
Breakthrough Starshot has no intention of colliding any nanocraft with any object in space. Even though an accidental collision between a nanocraft and another object is a remote possibility happens, the resulting effects must still to be examined.

Cruise | Interstellar dust
Based on estimates of the density of dust in the local interstellar medium, over the course of a journey to Alpha Centauri each square centimeter of the frontal cross-sectional area of the StarChip and lightsail would encounter about 1,000 impacts from dust particles of size 0.1 micron and larger. However, there is only a 10% probability of a collision with a 1 micron particle, and a negligible probability of impact with much larger particles.

Cruise | Interplanetary dust
Since the trajectory to Alpha Centauri would take the nanocrafts away from the ecliptic plane of the solar system, there would be much less impact from solar system dust than from interstellar dust. Little is currently known about the dust content in the Alpha Centauri star system.

Cruise | Interstellar medium and cosmic rays
The mean free path and Larmor radius of interstellar plasma particles is far greater than the size of the nanocraft, meaning that they would impact the nanocraft walls independently rather than forming a bow shock.

Flyby | Pointing camera at target planet
During an encounter with an exoplanet, the nanocraft’s camera would need to rotate in order to image the target.

Communication | Pointing transmitter towards earth
Finding the Earth should be reasonably straightforward, given its proximity to the Sun, which would be bright from the vantage of Alpha Centauri. The on-board star tracker would also be useful, as would locking onto the Starshot laser system.

Communication | Sending images with laser using sail as antenna
Images of the target planet could be transmitted by a 1Watt laser onboard the nanocraft, in a ‘burst mode’ which uses the energy storage unit to rapidly draw power for the power-intensive laser communications mode. Upon approach to the target, the sail would be used to focus the laser communication signal.

Communication | Receiving images with light beamer array
Recent advances by groups at MIL Lincoln Labs and the Jet Propulsion Laboratory have demonstrated that it is possible to detect single photons emitted by lasers from very large distances.

Policy | Light beamer and relativistic-speed nanocrafts

SOURCES - Draper, Breakthrough Initiatives, wikipedia

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