Updated analysis for laser sail mission to Alpha Centauri

There is a 49-page report that has updated analysis on sending a small probe towards the Alpha Centauri system in a 50 year total mission time traveling at an average cruise velocity of 10% of light speed.

The Andromeda Study: A FemtoSpacecraft Mission to Alpha Centauri
Andreas M. Hein, Kelvin F. Long, Dan Fries, Nikolaos Perakis, Angelo Genovese, Stefan Zeidler, Martin Langer, Richard Osborne, Rob Swinney, John Davies, Bill Cress, Marc Casson, Adrian Mann, Rachel Armstrong

They believe a mission could be attempted within the next 10-20 years provided adequate investment is made and also that key laser technology is permitted into the space environment.

The specific probe design down selected has a total spacecraft mass of 280 grams propelled by a 1.1 GW 500nm beam, or a smaller design at 28 grams in mass.

The Initiative for Interstellar Studies team would like to conduct a space mission attempt during the 2017-2020 time-frame which demonstrates the first laser-sail in space using ChipSat technology. They estimate that such a mission cost would be in the range of $250,000+ and this concept is directly on the technology roadmap towards the grander concept described in this report. They are looking for funding to
support this attempt as a way of kickstarting global efforts towards laser-sail propulsion. They want to keep it simple, cheap, do it quickly and to minimize risk by ground validation and physics demonstration.

Proving laser sail in orbit

A tiny laser sail (20 cm x 20 cm) is deployed from a CubeSat. Sail has a total mass of 50 g (sail + ADCS + transmitter). They need to send the position of the sail, and also actively control the sail attitude with some magnetic coils. The CubeSat has an on-board electric micro-propulsion system, so it can chase the sail all the time and provide constant thrust, the distance from the sail is always kept constant, thereby having a constant pointing accuracy requirement. Push it with a 1-2 Watt laser. After 29 days of laser illumination, the required orbit increase of 5 km for the sail
has been achieved.

All components have a TRL of 7 or higher. The CubeSat can be provided by FOTEC, Austria, a company for which i4is has links.

Estimated cost / schedule: The cost can be estimated in less than 1 million US-$. Depending on the funding, this mission can be assembled in 18-24 months.

Alpha Centauri mission discussion

They looked at three different areal geometries for an interstellar 280 gram probe. They looked at three different temperatures for comparison, which includes 600 K, 1,000 K and 1,500 K.

The space-based laser infrastructure would have a beam power output of 15 Gigawatts.

According to Philip Lubin current fibre-fed lasers can have efficiencies near 40%. For example, the DARPA Excalibur program’s laser has a specific power of 5kg/kW with prospects of reaching 1 kg/kW near term. It further assumed that laser efficiency will increase to 70% with the specific mass decreasing to 0.1 kg/kW in 10-20 years.

Graphene based supercapacitors are positioned at the side of the nuclear battery. The battery then loads the supercapacitors, and then they supply power to selective components such as the antenna, OBDH etc. The spacecraft structure has distributed MEMS sensors such as magnetometers etc. The antenna is a phased array, which is folded at the bottom of the spacecraft, approximately 1 meter on each edge when unfolded.

A graphene whipple shield is used at the front of the spacecraft for protecting the spacecraft from the interstellar matter that is incoming. The whipple shield consists of multiple layers of shield material. Each laser is intended to extract energy from the incoming particle. Part of the energy is released via radiation, when a plasma is generated upon impact. The plasma cools down immediately, releasing its energy via radiation during its transit to the next layer in the shield. Hence, the multiple layers of the shield are more effective than a bulk shield. Alternative materials such as Beryllium are also possible. The shield is approximately 1 mm thick and 5 cm to 10 cm wide, depending on the width of the spacecraft. The spacecraft flies in the direction of the graphene shield, once it has completed its acceleration phase. Note that the graphene sail provides additional shielding before it is eroded away.

The spacecraft has actuators. These are MEMS actuators (gyros) that are distributed on the spacecraft for providing attitude control. There are also MEMS propulsion units for desaturating gyro’s occasionally. During the acceleration phase, the spacecraft is attached to the sail via graphene wires. The sail has optomechanical elements for steering. The sail has furthermore a slightly conic shape in order to self-adjust
to the laser beam maximum.

The cost of the laser power generation infrastructure strongly depends on synergies between potential space power satellite systems. However, using near-term ultrathin solar cells with a specific power of 6 kg/kW we expect a cost of development, production, and launch to be in the billions US-$. The advantage of a space-based infrastructure are a factor 10 higher PV power output and laser efficiency compensating for high transportation cost.

Distributed mirror/lens system

To lower the overall mass and simplify the deployment, instead of using solid lenses, mirrors and other optical elements (e.g. an Axicon) a distributed or inflatable system can be imagined. The orbital rainbow study and DARPA’s Membrane Optic Imager Real-Time Exploration (MOIRE) are examples of such concepts. One class of such systems for refractive optics can be described by the “Bruggeman effective medium” approximation. Instead of a space filling medium, particles in a certain size range, with certain shapes and with a certain refractive index are used (e.g. ice crystals) to fill the volume of a prescribed shape by some amount (fill fraction).

Sail Materials

A technology which can significantly improve the performance characteristics of an interstellar sail is the one of dielectric materials. The main benefit of dielectric materials lies in their ability to have their reflecting properties “tuned” at a specific wavelength. By alternating between high index and low index dielectrics, the reflectance at a specific wavelength can be increased close to unity. Their high
emissivity, low absorption and high-temperature properties makes them suitable for high laser intensities.

Minimal Interstellar mission to reach 10AU per year speed

A cubesat with Indium FEEP microthrusterswould have the required specific impulse (4000 s) and the most efficient way of carrying propellant (Indium stored in solid state). Each thruster can provide a nominal thrust of 0.35 mN. A thrusting time of 20 years is needed for a ∆v of 25 km/s. This would be faster than Voyager 17 km/s.

an artist´s concept of the spacecraft (Credit: Adrian Mann); the first CubeSat houses an array of micro-thrusters, the second contains a miniaturized radioisotope thermal generator with affixed radiators, the third contains the payload. In this picture the 1U unit for the Indium propellant reservoir is missing.


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