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.

Comments (13)

  1. troy buckley:

    The hazard of collision between the micro ships whilst correction in vector is carried out seems to be important. Would there be any on board collision avoidance system such as that used on aircraft?

  2. Marc Baetens:

    There could be an nteresting alternative over earth based lasers. By having the lasers in space, powered by solar energy there are significant advantages:
    - the sail could be at close range making the aiming a lot easier
    - the cooling of the laser can be done by using the cold of space
    - having several lasers at wider ranges may improve the possibility of directing the sail
    - there's no interference with the atmosphere

  3. Mariano Usandivaras:

    Recién leí el artículo y coincido con Marc. Hasta me imagine los mismo problemas y soluciones al modelo de los láseres. Pero creo que el problema es la puntería. Imagínese que un error de puntería en un milímetro a un objetivo de distancia de millones kilómetros , entonces le estamos errando por mucho. Por otro lado imagínese que haya que hacer un cambio de rumbo , por más pequeño que sea , y de vuelta estamos con el problema de la puntería de los láser.
    Por otro lado fuera de la tierra evitamos otro problema el de la rotación de la misma. Por el tema de los láseres en posición fija.
    Por último creo que los láser debieran ir adosados a la nave, se me ocurre. Lo que plantea muchísimos otros problemas a resolver
    Saludos

  4. michael.million@sky.com:

    Translated for Mariano Usandivaras

    I recently read the article and I agree with Marc . So I figured the same problems and solutions to the model of lasers. But I think the problem is the aim . Imagine a pointing error in a millimeter to a target of million kilometers away , then you 're wandering through a lot. On the other hand imagine you have to make a change of direction , however small it is, and we're back to the problem of the aim of the laser. On the other hand off the ground we avoid other problems the rotation thereof . On the issue of lasers in a fixed position .
    Finally I think the laser should be attached to the ship , it occurs to me . This raises many other problems to solve
    regards

  5. Breakthrough Initiatives :

    Excellent questions. Thank you.

    There is essentially no possibility of collision between the spacecraft as they are only expected to be launched every day or every few days.

    As for the question of pointing to the precision required over the long accelerations distances (typically few million km) we need an excellent feed back and feed forward system to point the beam onto the sail. There is a laser beacon on the spacecraft that is critical to aid in our pointing but even this will be challenging when we get to light travel times during the initial acceleration phase since the feed back time is slow. This is one of the challenges we face but the placement of numerous beacons in the atmosphere as well as on the spacecraft and mothership (launcher) and the tracking of the target from the mothership aid us in this challenge.

    – Prof. Philip Lubin, Breakthrough Starshot

  6. michael.million@sky.com:

    It might be a good idea to have one guidance laser added, the laser is responsible for preparing the sail for acceleration, using the laser light or even microwaves to give rotational momentum to the sail as well as power its computer systems for pointing and beam centring. If we had dielectric gratings on the sail that are sensitive to the oncoming guidance light we could have them reflect the light towards the centre to tension the sail and slightly off to one side to give a rotational component for stabilization. Once the sail is rotating and ready for acceleration all the other lasers surrounding the sail then converge on the sail at the same time, the guidance laser can be connected light wise to the other laser power units for greater accuracy and control.

    https://str.llnl.gov/str/pdfs/09_95.2.pdf

  7. Nathan Bemis:

    I posted an idea that may help at the: Launch | Keeping beam pointed on meter-scale lightsail section. It's quite long because of additional ideas proposed with links.

  8. Breakthrough Initiatives:

    RE:
    "Sep 09, 2016 20:54michael.million@sky.comPosted on: Centauri Dreams
    It might be a good idea to have one guidance laser added, the laser is responsible for preparing the sail for acceleration, using the laser light or even microwaves to give rotational momentum to the sail as well as power its computer systems for pointing and beam centring. If we had dielectric gratings on the sail that are sensitive to the oncoming guidance light we could have them reflect the light towards the centre to tension the sail and slightly off to one side to give a rotational component for stabilization. Once the sail is rotating and ready for acceleration all the other lasers surrounding the sail then converge on the sail at the same time, the guidance laser can be connected light wise to the other laser power units for greater accuracy and control. https://str.llnl.gov/str/pdfs/09_95.2.pdf";

    Answer:
    Thanks for your interest these are very exciting ideas. The mass of the sail needs to be kept low so this is very difficult. We will have to study many alternatives. For a recent study of the stability of sail riding on the laser beam, see https://arxiv.org/pdf/1609.09506.pdf

    - Avi Loeb, Breakthrough Starshot

  9. Breakthrough Initiatives:

    RE
    "Oct 16, 2016 21:41Nathan BemisPosted on: Breakthrough Initiatives
    I posted an idea that may help at the: Launch | Keeping beam pointed on meter-scale lightsail section. It's quite long because of additional ideas proposed with links."

    Answer:
    See response on the section your posted.

  10. michael.million@sky.com:

    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





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