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. This alignment must be produced when the target star system (Alpha Centauri) has the correct configuration with respect to all planetary and stellar bodies in the intervening space, such that the flyby occurs within 2 AU of the target planet. Using on-board photon or others thrusters, the nanocraft will have the ability to make some modest mid-course corrections, on the order of 1-2 AU.

The task of pointing the array is dominated by the problem of keeping the sail on the beam. This problem is defined by the width of the sail and the distance to it. As an example, for meter-scale sail size the launch distance is on the order of a few million km. The pointing accuracy required for beam stability at this distance is on the order of a milliarcsecond. There are several mitigation approaches that could be used to counter these effects. A model of the atmosphere, calibrated with radar, laser beam, and optical measurements in real time, would enable the required beam precision to be achieved. Targets such as Alpha Centauri are bright star systems that will inform pointing requirements.

Monitoring the laser beam output provides the information needed to form the beam. The Starshot system would be very different than a conventional telescope, and specialized to its purpose. For example, most ground-based telescopes, such as the Keck telescope, point to within a few arcseconds and can track in a closed loop mode to better than 100 milliarcseconds. For the purposes of Starshot, a significant improvement on this precision is required. However, the beam synthesis inherent in the phased array system provides considerable fine-pointing capability, supplemented by closed loop tracking of the beacon on the spacecraft.

Feb 02, 2017 18:58 Breakthrough Initiatives Posted on: Breakthrough Initiatives

RE:
"Jan 09, 2017 10:26 michael.million@sky.com Posted on: Centauri Dreams
Here is an interesting article on MEM's reaction wheels, the bearings appear to be an issue though, perhaps as mentioned ball bearings or could we use a standing laser wave between the reaction wheel disc spindle to support it. And standing waves to support the disc during start up and then they are removed with a few used to read the reaction direction to aid navigation.

https://escies.org/download/webDocumentFile?id=7218";

Answer:
Thank you for your consideration. It is not clear to me that we need reaction wheels. We were considering photon thrusters. These are 10 Milliwatt lasers. With 3 of these we can change our attitude given enough time. It will be at least 10 years before we begin to address these problems.

- Pete Klupar, Breakthrough Starshot

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