4 ms·

Perhaps a really naive question: what can you do with a solar sail in orbit? I've always heard these as mentioned in the context of very long distance trips (h

by abeppu 2y ago

Perhaps a really naive question: what can you do with a solar sail in orbit? I've always heard these as mentioned in the context of very long distance trips (heading away from the sun). But if you're orbiting the earth, how do you take advantage of this? In general, how do you use a solar sail to do anything other than head away from the sun?

teraflop 2y ago
The sail doesn't have to be oriented directly perpendicular to the sun. By angling it, you can gain momentum in other directions, just like a sail on a boat doesn't have to push you directly downwind. When in orbit around earth, thrust in the direction of your orbit (prograde) causes you to gain energy and altitude, and retrograde thrust does the opposite.
- kbelder 2y ago
  Huh, now that I think about this, it makes me realize solar sails might be able to be used to drop material directly into the sun? Angle it for maximum retrograde acceleration in a solar orbit, and twenty years later the cargo impacts the sun? That's normally a very difficult task.
- Alupis 2y ago
  Is there any indication of how much thrust/acceleration they expect from this, and over what time scale?
  - graboy 2y ago
    625 micro-newtons [1]. Total weight of craft is 16kg [2]. That works out to 3.4 m/s gained per day. [1] The "Solar Sail" wikipedia page says 800m x 800m is about 5N. This is 80m^2. [2] https://ntrs.nasa.gov/api/citations/20210017923/downloads/SmallSat_2021_ACS3_overview_20210620_rev_b_for_review.pdf https://ntrs.nasa.gov/api/citations/20210017923/downloads/Sm...
    - Alupis 2y ago
      Interesting data. This[1] seems to suggest that this relatively small solar sail would be able to reach the moon from earth's orbit after ~3 years (I could be horribly misinterpreting this data!). That seems impressive, given how "simple" this method is. However, it strikes me as difficult to use this the deeper into space we go. I would assume the further from the sun you go, the less thrust you receive from the sail. Also, how do you slow down and stop? Maybe you don't... [1] https://spacemath.gsfc.nasa.gov/weekly/5Page26.pdf https://spacemath.gsfc.nasa.gov/weekly/5Page26.pdf
      - db48x 2y ago
        Sure, the amount of acceleration you get decreases as you get further from the nearest star, but that’s ok. Remember that you keep all the velocity you gained along the way, when you’re in space. You can be going pretty fast by the time you turn your sail to face the star at your destination.
- abeppu 2y ago
  > just like a sail on a boat doesn't have to push you directly downwind I'm not a sailor, but doesn't this rely on the force of water on the keel, to avoid being pushed laterally? > When in orbit around earth, thrust in the direction of your orbit (prograde) causes you to gain energy and altitude, and retrograde thrust does the opposite. For station-keeping purposes, when should you prefer a solar sail as versus PV panels and an ion drive?
  - zardo 2y ago
    > I'm not a sailor, but doesn't this rely on the force of water on the keel, to avoid being pushed laterally? The analogy with sailing is actually kind of weak. Since you get elastic collisions with photons on the sail, you can (approach) applying force at 90 degrees from the direction of the light. Unlike with actual sailing, you can never have an "upwind" force component.
- jahnu 2y ago
  My intuition is failing me here. How does that work since there is no body of water like a boat has to push off?
  - zardo 2y ago
    The sail is a mirror. Subtract the momentum vector of the incoming light from that of the reflected light and you have the momentum change applied to the sail.
  - teraflop 2y ago
    It's true that the mechanism is different, but this still works (to a limited extent) even without any external object to transfer momentum to, just using the momentum of the photons themselves. I can't be bothered to draw a diagram, but imagine that the sun is to the "north", and the solar sail is at a 45-degree angle running NE/SW. Incoming photons heading south reflect off the sail and leave to the west. So the photons experience a net momentum change to the NW, pushing the sail to the SE, which is not directly away from the sun.
    - jahnu 2y ago
      Thanks, that makes complete sense!
alnwlsn 2y ago
Raise and lower your orbit, I guess. Any unbalanced force on something in orbit will do one of the two.

by abeppu 2y ago

teraflop 2y ago

The sail doesn't have to be oriented directly perpendicular to the sun. By angling it, you can gain momentum in other directions, just like a sail on a boat doesn't have to push you directly downwind. When in orbit around earth, thrust in the direction of your orbit (prograde) causes you to gain energy and altitude, and retrograde thrust does the opposite.

kbelder 2y ago
Huh, now that I think about this, it makes me realize solar sails might be able to be used to drop material directly into the sun? Angle it for maximum retrograde acceleration in a solar orbit, and twenty years later the cargo impacts the sun? That's normally a very difficult task.
Alupis 2y ago
Is there any indication of how much thrust/acceleration they expect from this, and over what time scale?
- graboy 2y ago
  625 micro-newtons [1]. Total weight of craft is 16kg [2]. That works out to 3.4 m/s gained per day. [1] The "Solar Sail" wikipedia page says 800m x 800m is about 5N. This is 80m^2. [2] https://ntrs.nasa.gov/api/citations/20210017923/downloads/SmallSat_2021_ACS3_overview_20210620_rev_b_for_review.pdf https://ntrs.nasa.gov/api/citations/20210017923/downloads/Sm...
  - Alupis 2y ago
    Interesting data. This[1] seems to suggest that this relatively small solar sail would be able to reach the moon from earth's orbit after ~3 years (I could be horribly misinterpreting this data!). That seems impressive, given how "simple" this method is. However, it strikes me as difficult to use this the deeper into space we go. I would assume the further from the sun you go, the less thrust you receive from the sail. Also, how do you slow down and stop? Maybe you don't... [1] https://spacemath.gsfc.nasa.gov/weekly/5Page26.pdf https://spacemath.gsfc.nasa.gov/weekly/5Page26.pdf
    - db48x 2y ago
      Sure, the amount of acceleration you get decreases as you get further from the nearest star, but that’s ok. Remember that you keep all the velocity you gained along the way, when you’re in space. You can be going pretty fast by the time you turn your sail to face the star at your destination.
abeppu 2y ago
> just like a sail on a boat doesn't have to push you directly downwind I'm not a sailor, but doesn't this rely on the force of water on the keel, to avoid being pushed laterally? > When in orbit around earth, thrust in the direction of your orbit (prograde) causes you to gain energy and altitude, and retrograde thrust does the opposite. For station-keeping purposes, when should you prefer a solar sail as versus PV panels and an ion drive?
- zardo 2y ago
  > I'm not a sailor, but doesn't this rely on the force of water on the keel, to avoid being pushed laterally? The analogy with sailing is actually kind of weak. Since you get elastic collisions with photons on the sail, you can (approach) applying force at 90 degrees from the direction of the light. Unlike with actual sailing, you can never have an "upwind" force component.
jahnu 2y ago
My intuition is failing me here. How does that work since there is no body of water like a boat has to push off?
- zardo 2y ago
  The sail is a mirror. Subtract the momentum vector of the incoming light from that of the reflected light and you have the momentum change applied to the sail.
- teraflop 2y ago
  It's true that the mechanism is different, but this still works (to a limited extent) even without any external object to transfer momentum to, just using the momentum of the photons themselves. I can't be bothered to draw a diagram, but imagine that the sun is to the "north", and the solar sail is at a 45-degree angle running NE/SW. Incoming photons heading south reflect off the sail and leave to the west. So the photons experience a net momentum change to the NW, pushing the sail to the SE, which is not directly away from the sun.
  - jahnu 2y ago
    Thanks, that makes complete sense!

alnwlsn 2y ago

Raise and lower your orbit, I guess. Any unbalanced force on something in orbit will do one of the two.