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

Jun 06, 2017 23:54 khokolateKke@gmail.com Posted on: Breakthrough Initiatives

It might be possible to bypass some of the precision problems by using relay satellites between the grounded lasers and the craft. Because the relays wouldn't have the size limitations of the sail, they could collect the beams over a larger area and redirect them to the craft. From my basic understanding of optics I know that due to conservation of etendue the idea of "re-focusing" an unfocused beam is problematic. However, that assumes a static, nonreactive system. Also, I'm not sure if the beam passing through the relays would give it any net thrust...

Jun 12, 2017 13:52 Robert Clark Posted on: Centauri Dreams

The difficulty of focusing onto a small sail is another reason why it should be investigated doing space self-assembly to form a larger spacecraft. I mentioned this also in the thread:

GRAM-SCALE STARCHIP COMPONENTS | 4 PROCESSORS.
http://breakthroughinitiatives.org/index.php?controller=Forum&action=viewforum&id=19&page=3

Having a larger spacecraft allows us to collect more science, and we don't have the technical problem of having to miniaturize everything.

The problem is though small craft have a good area to volume ratio, and with volume as a proxy for mass, a good area to mass ratio. This is important for getting a high acceleration due to light pressure.

However, one way to maintain the high area to volume(mass) ratio for a large craft would be to keep the large craft very thin, just as thin as the small craft. The large mass is obtained by making the areal dimension large.

Bob Clark

Jun 29, 2017 14:44 Robert Clark Posted on: Centauri Dreams

Yet another possibility for getting the large scale structure for the sail or the transmitting mirror/lens is by making it inflatable. For instance Project Echo in the sixties used a metallized balloon satellite 40 meters across to bounce radio signals off of.

Bob Clark

Jul 15, 2017 03:44 Breakthrough Initiatives Posted on: Breakthrough Initiatives

RE:
Jun 06, 2017 23:54 khokolateKke@gmail.com Posted on: Breakthrough Initiatives

Answer:
It may be a good idea to have some optical elements of the laser system in space, though this presents a number of engineering difficulties. In addition to being able to adjust the pointing of the beam, as you have suggested, it may be possible to take advantage of “photon recycling” (multiple bounces of the beam).

- Zac Manchester, Breakthrough Starshot

Jul 15, 2017 03:45 Breakthrough Initiatives Posted on: Breakthrough Initiatives

RE:
Jun 12, 2017 13:52 Robert Clark Posted on: Centauri Dreams

Answer:
The ability to deploy or assemble a larger sail on orbit is not really a limiting factor. There are many good engineering reasons for keeping the sail smaller, including material strength and structural rigidity concerns. The larger the sail is, the more difficult it will be to control and keep in one piece.

- Zac Manchester, Breakthrough Starshot

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