Comment by mabbo | Hacker News Reader

Comment by mabbo | Hacker News Reader

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My understanding of what's going on here is that the light sail isn't aiming or changing at all. It's just a big sail, in orbit.

When it's moving away from the sun, the sail is being pushed "forward", which adds to it's velocity- this increases the apogee on the opposite side of the orbit. When it's moving towards the sun, the sail is being pushed "backwards", lowering it's current velocity and reducing the perigee. As the perigee comes lower, it spends less time in that end of the orbit, where it's gaining velocity, and when it's apogee is high it spends more time in that end of the orbit, where it's losing velocity. It's not a winning strategy I'm afraid.

But imagine! Imagine if we had a slightly different solar sail. One that could retract and expand at will, via solar powered electric motors. Then we could have it open it's sails when the "wind" is favorable, then close them when it's not. It could slowly but surely escape earth's gravity well, and gracefully travel around the solar system. A big enough one might even be able to accelerate right out of the solar system.

I didn’t work on Lightsail, but the article implies that it does aim the sail. It isn’t expanding/contracting, but it does change orientation using the reaction wheels. Basically, they turn the sail broadside to catch the sunlight to raise the orbit, and turn it on edge when they don’t want the extra drag.

This particular satellite won’t ever leave orbit because the sail can’t overcome even the tiny amount of atmospheric drag. If you want to achieve escape velocity with a sail you’ll need a bigger sail and a higher orbit to start out. Right now it’s only feasible for small satellites and we’ve been able to achieve escape velocity using rockets for decades.

Eventually, I do envision a competition to see who can get a payload to Mars with the least amount of fuel. Kind of like the fuel efficiency challenges that college engineering departments participate in today.

Hypermiling in space is an awesome concept. Would be so cool to have the solar race competition evolve from cars to spacecraft.

I have to admit, I got the idea from the beginning of the book Abaddon’s Gate. Solar sails might not be required but I think there’s a future in people plotting slingshot trajectories and sending packages around the solar system as fast and as cheap as possible. One day it will be manned missions, the space equivalent of people that fly hot air balloons around the world.

The novels of the James S. A. Corey authors are very inspiring given our recent space achievements. A highly recommended sci-fi series.

And a couple years after that, Amazon Prime parcels...

> a bigger sail and a higher orbit

But a bigger sail would increase the atmospheric drag!

But the higher orbit would reduce the drag. You could probably get away with the same size sail, but you’ve got to start high enough that the sail’s tiny thrust over its 50% (ish) duty cycle can overcome the drag present in 100% of the orbit. Once you’re high enough a bigger sail would mean lifting the orbit faster.

higher orbit will reduce drag

A minor note: the LightSail team says the perigee is lowering due to atmospheric drag not due to the sun slowing it down:

> The perigee, or low point of its orbit, has dropped by a similar amount, which is consistent with pre-flight expectations for the effects of atmospheric drag on the spacecraft.

That's not to say the solar pressure isn't potentially slowing it down too, but it sounds like the main driver is drag in this case.

I wonder if it'd be feasible to just have the sail be a one way mirror? Transparent on the "front".

There are no known materials that act as a one way mirror. We don't expect to find any such material. With a one way mirror it would be easy to lower global entropy.

However, one side reflective/white one absorbing black is possible. I guess the weight of extra paint is not attractive, compared to attitude control which you need anyway.

EDIT: this matters because full 180° reflection is like "bouncing" the photon to opposite velocity, can up to double exchanged momentum vs absorption. (This is why reflective mylar is nice)

Solar sails are intended for interplanetary travel in heliocentric orbit. Providing a continuous radial low thrust, resulting in an expanding spiral trajectory. In this application, the wind is always favorable.

This is a proof of concept test vehicle in low earth orbit, it isn't going anywhere.

You can try tilting a sail in 2-d simulation: http://wry.me/t/gravity/gravity.html (I don't promise I got the light-pressure math right, but it least this shows maneuvering without needing to extend and retract it.)

Nice simulation. I think I managed to reach escape velocity :-)

> retract and expand at will

I think LightSail 2 already does just that, though, but possibly not very efficiently.

From the article:

> LightSail 2's single momentum wheel, which rotates the spacecraft broadside and then edge-on to the Sun each orbit to turn the thrust from solar sailing on and off.

I don't think you'd even need to retract the sail - just structure it differently.

There's an omni-directional wind turbine (https://newatlas.com/2018-dyson-award-winner-o-wind-turbine/...) which sort of encapsulates what I'm thinking of.

I'm not too sure about how easy it would be to use a non-planar sail to redirect photons in the direction required to constantly build forward momentum - but a photonic "wind" turbine used to power a quantum dot emitter array might be able to do something.

Changing orientation is likely easier than retracting the sail. Even just rotation per orbit could bias the acceleration gain.

However, I suspect the goal was kept as modest as possible to increase odds of success and lower costs.

Gravity is exponential, which makes lifting fuel off of Earth a losing strategy for long-distance travel. Anything that is fueled by something already in space seems incredibly promising.

Newton's approximation of gravitational force is: G = gMm/d^2

The variable here is d (the distance between the centres of mass) - gravity is inverse quadratic (https://en.wikipedia.org/wiki/Inverse-square_law), not exponential.

I suspect they were referring to the tyranny of the rocket equation.

They have 3 perpendicular coils that can orient the spacecraft relative to Earth's magnetic field. The reaction wheels are for fine tuning the attitude.

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